Reset your expectations of solar sails. They are a fast and free way to travel to any point in the Solar System, as many times as you want, any time of the year. Solar sails can carry passengers and they have a nearly unlimited number of uses. You just have to... think big! Solar Sails Art by Stepan Polivanov. Sunlight is abundant. The energy needs of any activity can be met by simply collecting enough of it. All of human civilization, for example, could be powered by the perfectly using the sunlight passing through an 86 km square near Earth. There’s more than enough sunlight to power space travel. How convenient that we can just harvest what we need to get anywhere in the solar system! The usual approach is to use photovoltaic devices. They convert sunlight into electricity, which can then power electrical devices directly. They’ve had a long history: the first solar panels in space were launched with Vanguard 1 in 1958. Those panels produced 1 Watt in total. Today, the largest solar-powered installation in orbit is the International Space Station. Its eight arrays could generate up to 240 kW when new. Accumulated damage from micrometeorites and radiation will necessitate their gradual replacement though. The latest development is the large-scale use of electric thrusters. The DART mission used the NEXT-C thruster at a 3.5 kW power level, and thousands of Hall-effect thrusters are deployed with the SpaceX Starlink constellation. However, solar panels are not the only way to make use of solar power. Solar sails directly convert sunlight into thrust. Their efficiency usually exceeds 90% and they’re very resistant to degradation. A spacecraft equipped with these sails can accelerate endlessly without requiring a drop of propellant. How is this possible? How much acceleration can be achieved? How far can we push this technology? Let’s dive into these questions in the following sections. Light and Speed  Space sailing! By Jett Furr. The way a solar sail works is pretty simple. Light has no mass but it has momentum. When it strikes a surface, it generates radiation pressure proportionate to its intensity. If the surface is reflective, that pressure can nearly be doubled.  An equation describes radiation pressure: Radiation Pressure = Light Intensity x (1 + Reflectivity) / C Radiation Pressure is in Pascals (Pa). Light Intensity is in W/m^2. Reflectivity is a figure between 0 and 1. C is the speed of light in m/s. In the vacuum near Earth’s orbit, the intensity of sunlight is 1361 W/m^2. A surface with a reflectivity of 0.9 (90% of sunlight is reflected) would feel a radiation pressure of about 1361 x (1 + 0.9) / (3 x 10^8) = 8.6 x 10^-6 Pa or 8.6 microPascals.  You can convert radiation pressure into thrust if you know the area of the reflective surface. Radiation Thrust = Radiation Pressure x Area Radiation Thrust is in Newtons (N). Radiation Pressure is in Pascals (Pa). Area is in m^2.  A square solar sail that’s 10m on each side would have an area of 100 m^2. Following the previous example, it is feeling a pressure of 8.6 microPascals. The total thrust it generates is 8.6 x 10^-6 x 100 = 8.6 x 10^-4 or 0.86 milliNewtons. That’s not a lot of thrust; it’s equivalent to the weight of 3 grains of rice.  From thrust, we can work out acceleration if we know the mass of the sail. The mass of a sail is related to its area: usually a sail with 10x the area is also 10x heavier. We’ll therefore use the area density. Solar Sail Acceleration = Radiation Thrust / (Area Density x Area) Solar Sail Acceleration is in m/s^2. Radiation Thrust is in Newtons (N). Area Density is in kg/m^2. Area is in m^2. A typical area density for today’s solar sails is 10 grams per square meter, or 0.01 kg/m^2, as found on the IKAROS sail. A 100 m^2 sail would therefore mass 1 kg. The acceleration we can expect from 0.86 milliNewtons of thrust is 0.00086 m/s^2, or 0.086 milligee. It would take this sail nearly two minutes to gain 1 m/s. In more familiar terms: its “0-100 km/h” time is 9 hours! If we combine all the previous equations, we can write: Sail Acceleration = Light Intensity x (1 + Reflectivity) / (C x Area Density) Notice how ‘Area’ disappears from the equation. A bigger sail will not accelerate faster. Improving reflectivity does not affect acceleration much as the difference between a good 90% reflectivity and an incredible 99.999% reflectivity is small. That leaves two factors that really affect acceleration: Light Intensity and Area Density. Light Intensity varies between 64 MW/m^2 at the surface of the Sun to 0.873 W/m^2 near Pluto. Increasing that figure means getting closer to the Sun, but that might not always be where you’re headed. Area Density is the only contributor to sail acceleration that we can really affect.  In the example we used, we ended up with a solar sail that accelerates about four times slower than a snail because it only had access to Earth-levels of sunlight and had a 0.01 kg/m^2 mass per area. If we attach a payload to that sail, it will accelerate even more slowly. Angles relevant to a solar sail. What’s more, conventional solar sails can only reflect sunlight back in the direction it came from or off to the side. For a spacecraft trying to spiral out of its current orbit then into a new one, this is a problem. It will have to angle its solar sail to produce sideways thrust to increase its orbital velocity. The efficient angle is 35° to the Sun. At this angle, the sail’s ‘effective thrust’ is actually 82% of maximum as it is not fully facing the Sun. The result is a space vehicle that takes months to years to accomplish the most minor of maneuvers, let alone interplanetary transfers that require deltaV changes in the thousands of m/s. This is currently acceptable for small probes that can fit inside rocket upper stages as secondary payloads.  But can we do better? Less is More Lightsail 2's deployment. The material that today’s solar sails are made of is aluminized Mylar supported by extensible trusses. It is only a few micrometers thick (7.5 micrometers in the case of IKAROS) and manages to reflect 90% of sunlight. 2 micrometer thick aluminized PEN being spooled and handled manually The main struggle is getting a reflective surface that can be packaged like origami into a very small volume, survive the vibrations of a rocket launch, then reliably deploy out in space into a rigid structure. Solar sail deployment Also, since solar sails so far have been of modest size, the mass of secondary equipment like control actuators or deployment mechanisms take up a proportionally larger fraction of total mass. Simply making larger sails will improve their area density. They also need features to prevent tears from ripping through the entire sail, or to keep edges of the sail material from curling inwards.  The first step to improving area density is to take current materials and improve them. The DLR/ESA 60m sail is an example of a realistic near-term goal. Reducing Mylar sheet thickness to just 3 micrometers could reduce area density to 4.2 g/m^2. Add in the mass of the aluminium coating and the graphite booms and you achieve 5.3 g/m^2. The Encounter sail is even larger, and projected Mylar sheets of just 1 micrometer thickness, for an overall area density of 1.9 g/m^2.  Another avenue to reduce area density is to cut away solid support structures in favour of other ways to keep the sail to its intended shape. Inflatable booms can be lighter than graphite rods. Rotating sails can be lighter still - they can use centripetal forces to both extend the sail and to keep it under tension. These result in circular sails, or even ‘heliogyros’ where the sail is divided into ‘blades’ kept under tension like a helicopter’s rotor. Controlling their motion is more complicated though. A heliogyro sail catching up to Halley's Comet. If we put together the thinner materials and innovative configurations, we are likely to achieve sails with 1 g/m^2 area density. This would grant them accelerations on the order of 0.86 milligee, likely to be 0.5 milligee with payload included. That’s enough to get a spacecraft from geostationary orbit to Earth escape in about a week. But we can do even better. A better material than Mylar can be considered. Carbon fibers supporting an aluminium layer can achieve 0.5 g/m^2 or better. At some point, we can imagine removing the supporting layer entirely and relying instead on the metal reflector alone. This layer can be very thin: as low as 0.14 micrometres in this proposed example. It is a way to achieve sails with an area density of 0.33 g/m^2, rising to 0.45 g/m^2 when including support booms.    Nanotechnology can come to our aid.  Sub-wavelength metasurfaces allow solar sails to remain reflective while having gaps smaller than the wavelengths of sunlight. It is similar to how the metal screen in your microwave oven’s door reflects the 2.45 GHz (12.2 cm) wavelengths inside but lets visual wavelengths (0.4 to 0.7 micrometers) through. A solar sail can use the same concept to become mostly empty space while remaining reflective. The shortest wavelength we want to capture would be 0.4 micrometers, so we could use a quarter-wavelength grid spacing of 0.1 micrometers (so it would be 75% empty). That would reduce the sail mass four-fold, down to 0.1 g/m^2.  More advanced sail configurations. Theoretically, we can shave down the reflective surface down to the absolute minimum thickness that can interact with sunlight. This seems to be 10 nanometers for Aluminium. A sail of that thickness would have an area density of 0.027 g/m^2. If we further apply the sub-wavelength gaps, we could bring it down to the level of 0.01 g/m^2. That would enable accelerations on the order of 87 milligee. But it comes with many downsides. This sort of ultra-thin surface is very weak (tensile strength of 0.225 Pa) so while it would survive the radiation pressure of the Sun, it might not handle any other sort of disturbance. It would also become vulnerable to the ambient space environment: solar wind, interplanetary dust, radiation damage and so on. Radiation damage to solar sail surfaces. Better performance can be achieved if we abandon reflective sails and use diffraction instead. The many ways a diffractive sail can manipulate light. Diffractive sails let sunlight pass through them while bending the angle at which it emerges, which produces a force at the opposite angle. If they bend sunlight to the right, they feel recoil from the left. Diffraction gratings made with very thin transparent materials like silica, or reflective strips made of aluminium spaced by about a micrometer, can do the job. There are few details on how such a grating could be constructed, but they are often directly compared to conventional sails in terms of area density.   Diffractive sail thrust. The most interesting feature is that control of their optical properties (switching the angle of the grating, for example) allows them to redirect that sunlight in any direction, and therefore control the direction of their thrust. Conventional sails can only accelerate away from the Sun or at an angle away from it. Refractive sails would work more like an engine in that they can choose with direction to accelerate. They can do this with over 90% of the energy they gather, as described here. This allows the sail to always point at the Sun for maximum efficiency while thrusting sideways or even towards it. Advanced control would even allow them to discard control thrusters or control gyroscopes for further mass savings. A 'photon thruster' sail. Another interesting configuration for solar sails is the ‘photon thruster’. It’s essentially a set of two mirrors working together: one large collector and a smaller mobile mirror to redirect the light in any direction. The advantages are a much wider range of maneuvers that become possible, and since the collector mirror can always face the Sun directly, there is no loss of efficiency from angling the thrust. The downside is increased complexity and the additional weight of the mobile mirror. There are even more speculative types of solar sail, but their applications are limited.  ‘Black sails’, for example, absorb sunlight. They do not reflect or redirect the sunlight, so they only accelerate away from the Sun (try using the equations above with R=0). It’s useful for well timed exploration of the outer solar system or interstellar missions, but not much else. The upside is that they can use the thinnest and strongest materials available: carbon nanomaterials. Their very low density works in their favour. Aerographite, for example, has a density of just 0.18 kg/m^3. A 1 mm layer of it is opaque and could create a solar sail with an area density of 0.18 g/m^2. It is unclear whether it remains opaque at 0.1 mm thickness, which translates to 0.018 g/m^2. The potential acceleration would be as high as 0.25 m/s^2. A 3mm wide graphene light sail under testing At the material limit, we have a single layer of graphene. It’s just one atom thick (0.345 nanometers) and its area density would be 0.00077 g/m^2. However, pure graphene only absorbs 2% of sunlight while letting the rest through. That limits its acceleration to 0.12 m/s^2 near Earth. It has been suggested that adding a layer of Molybdenum disulfide (MoS2) can increase the absorption to 37% of sunlight. The area density of a graphene/MoS2 bilayer would increase to 0.004 g/m^2 allowing its potential acceleration to become 0.4 m/s^2. It’s also possible to create black sails with area density below the material limit, by using dusty plasmas. These sails would magnetically confine a plasma that can hold charged particles within itself. Those particles would absorb sunlight. Since plasma sails can grow to immense sizes of several tens of kilometers in diameter, with very little mass within them, they effectively constitute solar sails with area density as little as 0.001 g/m^2 or less. Few studies have focused on dusty plasmas or their characteristics, so there is no data on their sunlight-absorbing properties. We therefore can’t guess at their potential acceleration. The Sails Let’s design four solar sails to use as a reference for the rest of this post. They’re meant for increasing payloads and more demanding missions with each technology level. Note that we’re adding 20% to the area density to account for all the other equipment a sailcraft needs to navigate. Characteristic Acceleration is the acceleration the sail manages when receiving full illumination at Earth sunlight levels (1361 W/m^2).  Modern Sail Area Density: 10 g/m^2 Sunlight Interaction: 190% Area: 100 m^2 Sail mass: 1.2 kg Payload: 1 kg Total Mass: 2.2 kg Characteristic Acceleration: 0.00039 m/s^2 This Modern Sail is of the type we have already launched into space. 190% efficiency is thanks to 90% reflection nearly doubling the momentum it gains from sunlight. It’s made of Aluminized Mylar several micrometers thick, held up by 10 m wide square rigging, propelling a microsat-sized payload. Due to its tiny total mass, several of these can be sent into orbit by the smallest of launchers. Advanced Heliogyro Sail Area Density: 2 g/m^2 Sunlight Interaction: 190% efficiency thanks to reflection Area: 50,000 m^2 Sail mass: 120 kg Payload: 100 kg Total Mass: 220 kg Characteristic Acceleration: 0.0019 m/s^2 This sail is made of six blades rotating around a central truss. Thanks to its Mylar layer being thinned to 1 micrometer, its area density is much improved over the Modern Sail. It’s small enough to fit into the secondary payloads of a regular rocket like a Falcon 9 on a geostationary transfer orbit mission.  Nanofilm Sail Area Density: 0.1 g/m^2 Sunlight Interaction: 180 % Area: 4,000,000 m^2 Sail mass: 480 kg Payload: 1000 kg Total Mass: 1480 kg Characteristic Acceleration: 0.022 m/s^2 This sail uses a reflective surface that’s just a tenth of a micrometer thick, further lightened with sub-wavelength holes. Carbon nanotube struts hold it up into a 2000 m wide square. Since some shorter wavelengths pass through its holes, it has a reduced sunlight interaction efficiency. At this scale, it is matching the useful payloads of deep space probes while requiring only a fraction of the initial mass. Launching it up from the ground is a dubious proposition though. Diffractive Meta-Sail Area Density: 0.01 g/m^2 Sunlight Interaction: 90% Area: 2,500,000,000 m^2 Sail mass: 30,000 kg Payload: 10,000 kg Total Mass: 40,000 kg Characteristic Acceleration: 0.26 m/s^2 This sail is 50 km wide. Despite its immense size, it’s still only the equivalent of the payload of a single heavy lift rocket today. The sail surface is a nanostructured diffraction grating about 10 nanometers thick, actively controlled by piezoelectric actuators to be able to give complex shapes to the sunlight that passes through it. Its sunlight interaction efficiency is 90% as it does not reflect the light but merely redirects it. This is a sail large enough to transport humans on interplanetary missions. However, it is certainly too fragile to be launched atop a rocket or even folded before deployment. The whole structure would have to be assembled in orbit.  Solar trajectories The most important aspect of solar sail travel is the sail’s acceleration relative to the local gravity.  A sail with very low acceleration spiralling out of LEO to escape velocity In Low Earth Orbit, local gravity is quite similar to the 1g found on the surface. Solar sail acceleration would be much much smaller than the local gravity. The Modern Sail would have an acceleration of 0.039 milligees, for example.  In this environment, solar sails can only make very gradual spiralling trajectories as they attempt to raise or lower their orbit. A useful approximation for how long this takes is to divide the difference between the initial and destination orbits’ velocities by the sail’s acceleration.  Let’s try it. The orbital velocity at 1000 km altitude is 7350 m/s. The orbit velocity at 35,800 km (geostationary) altitude is 3075 m/s. The difference is 4275 m/s. The Modern Sail would take 4275/0.00039 = 10,961,538 seconds or about 4.16 months to complete the maneuver. Realistically, it will take a while longer as we have not accounted for the time spent in Earth’s shadow, the inefficiencies from accelerating sideways to the sunlight and the effects of atmospheric drag, which can be significant even far above the atmosphere. The Advanced Heliogyro Sail with its 0.0019 m/s^2 acceleration would do it in 26 days.  This approximation holds true so long as sail acceleration is far below local gravity.  In the following table, epsilon E is the ratio between the spacecraft’s acceleration and local gravity. The third column is the ratio between the deltaV needed to reach escape velocity, and the initial orbital velocity. And in the fourth column, we have the number of orbits needed to complete the maneuver.  When E is very low (10^-5), we have to expend practically as much deltaV as our current orbital velocity to escape into deep space. In this specific example, your deltaV requirement is 95.75% of your current orbital velocity. It also takes an incredible number of orbits to complete this sort of maneuver (over 50,000!). As E increases, you need less deltaV and less orbits to reach escape velocity. When E is a hundredth (10^-2) of local gravity, you only need to spend 76% of current orbital velocity to escape and it takes just 51 orbits to do this. While deltaV savings don’t mean much to a solar sail, they do translate into additional time savings.  However, you will notice that solar sails with the performance levels we find interesting will achieve accelerations comparable to or exceeding local gravity. Consider what happens as we move away from Earth. At an altitude of 100,000 km, the local gravity is 0.035 m/s^2. At 400,000 km, or 1 lunar distance, it is only 0.0024 m/s^2. Compare these values to the accelerations that advanced solar sail designs can achieve. The Advanced Heliogyro Sail manages 0.79 times the local gravity at 1 lunar distance. That’s a ‘thrust to weight’ ratio we typically find on fighter jets! In other words, high performance sails can pull off acrobatic maneuvers, albeit in slow motion from our perspective. Throughout the rest of the solar system, we have this useful chart: At 1 AU from the Sun, solar gravity is 0.0059 m/s^2. At 5.2 AU (Jupiter’s orbit), it falls to 0.00022 m/s^2. Note that the Sun’s gravity and the intensity of sunlight follow the same inverse square scaling with distance (D^-2). Intensity of sunlight determines a sail’s acceleration. Therefore, a solar sail will always maintain the same ratio of acceleration to gravity, regardless of how far it is from the Sun. We call this the sail’s beta B or ‘lightness ratio’. For example, if a sail manages an acceleration of 0.001 m/s^2 at 1 AU, which is six times lower than solar gravity, then it will achieve 0.000037 m/s^2 near Jupiter, which is also six times lower than solar gravity over there.  Small lightness ratios leads to very long travel times. A ratio of 0.015, which is an acceleration of around 0.00008 m/s^2 near Earth, would mean an annoying long transfer time of 1647 days to Mars. That’s 4.5 years. Increasing the lightness ratio dramatically shortens travel times. It makes solar sails act more like powerful rockets. A lightness ratio of 0.1 is enough to take on fast trajectories to Mars. 176 days is about 6 months, which is shorter than the usual 9 month Hohmann transfer chemical rockets can manage. And unlike regular rockets, a solar sail can make its way from Earth to Mars or back without having to wait for any transfer windows. Another study shows that a Mars mission can take less than 120 days using sails with a lightness ratio as low as 0.2. However, that involved meeting Mars with a velocity of 5-12 km/s. An Advanced Heliogyro sail would make up that velocity gap in 70-170 days. The other option, as the study suggests, is aerocapture into Mars orbit.  We can reasonably assume that sails in that performance category can complete trips to the Outer Planets at least as fast as minimum energy Hohmann transfers: 2.7 years to Jupiter, 6 years to Saturn. Solar sails are relatively quick even with low performance levels. But if we increase performance to a lightness ratio of 0.5, we would enable a solar sail to directly move away from the Sun and accelerate onto a solar escape trajectory that eventually resembles a straight line into interstellar space.  We have this trajectory for a sail with a lightness ratio of 0.6, where it takes 2.8 years to reach Jupiter: The same paper finds an 11.5 year trajectory to Saturn with a lightness ratio of 0.5: And we know solar sails can do even better. At the upper end of their performance, they will act like advanced propulsion systems, such as nuclear-electric thrusters or fusion rockets. Their lightness ratio exceeds 1, so they can effectively ‘cancel out’ the Sun’s gravity and pick up straight line trajectories to wherever they want. Metaphorically, they’re unleashed to go wherever they please.  The trajectories possible with different lightness ratios. If a solar sail with a lightness ratio of exactly one exits Earth’ sphere of influence, it will start to travel in a straight line with Earth’s orbital velocity and tangent to Earth’s orbit at that point. If it is timed correctly, it will drift at 29.78 km/s straight towards its target, whether it be Mars, Jupiter or beyond. That is enough to reach those planets in 88 days and 243 days respectively. With a lightness ratio exceeding one, a solar sail can take that initial orbital velocity and add to it. It can shorten the trip by many months. The Nanofilm Sail has a lightness ratio of 3.73. It can keep accelerating towards a target like Jupiter, crossing the 4.2 AU minimum distance to that planet in 103 days. At the end of the trip, it is travelling at 85.8 km/s. The Diffractive Meta-Sail has a lightness ratio of 44. It completes that same trip to Jupiter in 37 days, reaching a velocity of 253.1 km/s by the end of it. Those same sails can reach Saturn in 203 days and 66 days respectively. A solar sail cannot accelerate into the Sun. Of course, just zipping past your destination isn’t always desirable. A sail can just turn itself edge-on to the Sun to stop accelerating away from it and let gravity take over. Solar gravity is however very weak. The sail performance doesn’t really matter in that case: the trip always takes about 630 days (1.72 years). For Saturn it is 1660 days (4.5 years). Caution! These are only rough calculations that have to be taken with a grain of salt, as they are basically assuming the sail makes a straight line for its target and stops at the end with near-zero velocity. A realistic trajectory, such as one that tries to reduce the relative velocity during the encounter with Jupiter (13 km/s) or Saturn (9.7 km/s) will be different and might take a bit longer. Still, they are much faster than minimum energy Hohmann transfers, and they can be launched any time during half the year where Earth and the target planet are on the same side of the solar system. Now, there are tricks to make those trips shorter.  From "Multiple-satellite-aided capture trajectories at Jupiter using the Laplace resonance" Jupiter and Saturn have large moons that can help capture a solar sail into an orbit around the gas giants. Gravity assists can change the velocity of a passing spacecraft by a maximum of twice the planet or moon’s orbital velocity. The Moon, for example, orbits Earth at about 1 km/s, so theoretically it could grant a velocity change of up to 2 km/s. Lunar Gravity Assists have been calculated to provide 1.5 km/s of deltaV savings for capturing asteroids into Earth orbit from a single pass. Earth's Moon used for gravity assists and slingshots. We can therefore expect proportional deltaV savings from other large moons. Jupiter has Io, which orbits at 17.3 km/s. A Nanofilm Sail that only has to slow down to 17.3 km/s before reaching Jupiter could shorten its trip time to 312 days (-50%). If Enceladus and its 12.6 km/s orbital velocity can be used to capture into Saturn, then the Nanofilm Sail’s trip is reduced by 51% to 814 days. Again, gravity assists are much more complicated than this but we do get a rough estimate for what sort of benefits we get from them. A solar sail at Saturn Returning from the gas giants is not easy. It takes a very long time to maneuver far from the Sun, and even in the best case afterwards, only solar gravity can pull the sails inwards. For example, the Nanofilm sail has an acceleration of 0.000813 m/s^2 near Jupiter. It would take it around 185 days to cancel out Jupiter’s orbit velocity and start falling straight into the Sun. From a distance of 5.2 AU, it would ‘fall’ for 740 days until it is at a 1 AU distance from the Sun. Intercepting Earth is done near the Sun, using maneuvers that take up to 25 additional days. The total mission time is therefore 950 days or 2.6 years. That’s only slightly faster than a Hohmann transfer. Using a better sail can shorten the 185 day and 25 day portions of the trip, but does nothing to the 740 day freefall period. Inner solar system missions don’t have to cover as much distance as missions to the gas giants, but they do have to contend with targets that have much higher orbital velocities: Venus at 35 km/s and Mercury at 47 km/s. Neither of them has a useful moon for gravity assists either. Capture into those planetary systems is therefore quite challenging. A Mercury sample return mission. For low performance solar sails, we expect to see slow spiralling trajectories. Thankfully, their acceleration increases as they get closer to the Sun. On the way to Venus, a solar sail’s acceleration nearly doubles. The average acceleration is 145% of its characteristic acceleration. Near Mercury, a solar sail is zipping around at 6.7 times its characteristic acceleration. A sail’s average acceleration is 383% its Earth-normal acceleration on the way to Mercury.  We have here data on Venus transfers with a sail of characteristic acceleration of 0.1, 1 or 2 mm/s^2. That’s lightness ratios of 0.017, 0.17 and 0.34. We see that increasing performance brings the transfer time closer to the Hohmann minimum-energy trajectory that takes 146 days to Venus. Mercury remains a challenging target for low performance sails. One way around that is a flyby of Venus to shorten the travel time to the innermost planet. We can see significant benefits for sails with lightness ratio below 0.1. Venus flyby on the way to Mercury. Another paper studies interplanetary trajectories for a diffractive sail. The range of characteristic accelerations is 0.1 to 1 mm/s^2. Surprisingly, it takes about the same amount of time to reach Venus as it does Mercury; about 200 days.  To Venus: To Mercury: High performance sails with excellent lightness ratios have similar troubles with the inner planets as they do returning from the outer planets. It’s the ‘cannot fall inwards faster than gravity’ problem. Even if they cancel out all of their orbital velocity, they can only be pulled inwards by the relatively weak solar gravity.  At Earth’s orbit of 1 AU, solar gravity is 0.0059 m/s^2. Near Venus’ 0.72 AU, it is 0.0113 m/s^2. Mercury is the closest to the Sun at 0.39 AU and experiences 0.0387 m/s^2 of solar gravity.  We can use a table to calculate the position of an object falling into the Sun every 1 hour. It should take about 42 days to fall from Earth to Venus, accumulating about 26 km/s of velocity by the end of it. Falling from Earth to mercury takes 57 days, reaching 52.5 km/s. Falling all the way into the Sun takes 65 days, which matches theoretical results. The direct trajectory that gives the minimum possible travel time from Earth to Venus or Mercury for a high performance solar sail is the following: cancel out Earth’s orbital velocity (29.78 km/s), fall to the desired altitude, cancel out the accumulated fall velocity, accelerate to the target’s orbital velocity.  For a Nanofilm sail travelling to Venus, the Earth maneuver takes 15.7 days. Falling towards Venus takes 42 days. Cancelling out the 26 km/s inwards velocity and gaining the planet’s 35 km/s orbital velocity take together (26,000+35,000)/(0.022 x 1.91): 16.8 days. The total trip is 58.5 days before optimizations. A high-tech Diffractive Meta-sail travelling to Mercury would cancel out Earth’s orbital velocity in 1.3 days. It then falls for 57 days. Removing the 52.5 km/s inwards velocity and catching up to Mercury orbiting at 45 km/s can be done in one intense 97.5 km/s maneuver. It is made more manageable by the 3.83x boost to sail acceleration nearer the Sun, so it only takes  (97,500)/(0.26 x 3.83): 1.1 days. The total trip duration is 59.4 days, with 96% of the time spent just falling towards the Sun. Returning from Venus or Mercury to Earth is like to be even faster.  'Sundiver' maneuvers with a sail You may have noticed how much faster sails get when they approach the Sun. The fact is exploited fully with the SunDiver maneuver (also called a solar photonic assist), where sails get to within 0.1 AU of the Sun for a massive 100x increase to their characteristic acceleration. Even a sail with a modest lightness ratio of 0.3 can reach Neptune in 933 days thanks to a single SunDiver maneuver. It greatly speeds up travel to the far reaches of the Solar System. Combined with the Oberth effect from swinging around so quickly near the Sun, you can get sails with modest characteristic acceleration to incredible velocities exceeding 400 km/s. Characteristics of an aerographite spherical sail At the upper limit, using carbon nanomaterials like Aerographite, we can push solar sails to 2% of the speed of light. Of course, there is no way to stop these sails afterwards. They can fly past planetary targets or simply head out into interstellar space.  'Pole sitter' statite Finally, high performance solar sails, especially designs with a lightness ratio equal or exceeding one, have the notable ability to just… stop. They can hover in any place. This turns them into ‘statites’; stationary satellites. These are useful if you want to place an observatory or telescope in an exact position or lay down a fusion highway.  A solar sail following a 'displaced orbit' A high performance sail could also enter ‘short orbits’ that only partially cancel out gravity near a planet, or follow complex trajectories (neither lines nor orbits) that take them between arbitrary points in space, like sitting exactly halfway between the Moon and Earth or above a specific crater on a spinning asteroid. Since they need no propellant, they can keep this up indefinitely.  Hybrid Braking So far we have talked about solar sails exclusively using sunlight for propulsion. This doesn’t have to be the case. ‘Hybrid sails’ use a mix of propulsion systems. OKEANOS combines a solar sail with thin film photovoltaic cells. For example, the payload of a solar sail can maneuver independently. Several sailing missions propose dropping a capsule during a flyby of Mars so that it can aerobrake to the surface. This saves on having the solar sail fight the planet’s gravity all the way down to a low orbit and then back out to escape velocity. A payload could also detach and use its own thrusters to go down to a planet, then rendezvous with the sail to return to interplanetary space. This can be the case with a lander with in-situ refuelling that lands nearly empty then returns to orbit nearly empty again.  A solar sail aerocapture into Saturn. The large area of a solar sail also makes it very useful for aerocapture. The Modern Sail has an average mass per area of 0.022 kg/m^2. The Mars 2020 mission that landed the Perseverance rover used an aeroshell with a mass per area of about 95 kg/m^2. This means the solar sail can slow down 4320x faster in Mars’ thin atmosphere. Aerocapture into Mars orbit with a sail. Or more realistically: it can brake from interplanetary velocities into a Mars orbit using the thin gases present high above Mars. The same is possible with the more advanced sails in the upper atmospheres of the gas giants. Whether the sails can survive the maneuver without being damaged is another question.   A simple hybrid sail. A “hybrid sail” is a more interesting design where the solar sail serves double-duty as part of another propulsion system. The obvious use case is a solar collector for a photovoltaic system. The huge reflective surfaces of a solar sail interact with large amounts of solar power but only convert a tiny fraction of it into useful kinetic energy. The Nanofilm Sail and its 4,000,000 m^2 surface area collects 5.44 GW of sunlight near Earth. If only a tiny fraction of that power is converted into electricity and fed into an ion thruster, we would get useful thrust levels throughout the solar system. It is most useful when braking towards the outer planets like Jupiter or Saturn. In that situation, the solar sail is still handling hundreds of megawatts of power but it still takes months to slow down into a capture orbit. If the Nanofilm Sail sacrificed half of its payload to a 400 kW solar-electric propulsion system weighing 200 kg (including a 9000s Isp thruster) and had 488 kg of propellant, it would be able to provide 9 Newtons of thrust anywhere. It would be enough to slow down by 20 km/s within 34.5 days. That’s an average of 0.0067 m/s^2 or 8.3x the photon-only acceleration near Jupiter. The major benefit is that when combined with gravity assists from a moon like Io, the sail only needs to slow down to (20+17.3): 37.3 km/s. That saves a massive amount of time. The usual 343 day trip that’s been reduced to 235 days with the gravity assist is further reduced to 159 days with the electric thruster. An even greater benefit is to be had near Saturn. At that distance, the sail is still capturing 59.3 MW of power but the photon-only acceleration falls to just 0.00024 m/s^2. The electric thruster would provide 28x that much acceleration. And it would mean the sail only needs to slow down to (20+12.6):32.6 km/s, which shortens the 748 day trip to 364 days! Similar calculations can be made for the other sails. There may be trade-offs where adding an engine and propellant to the payload lowers the sail’s characteristic acceleration but shortens the overall trip. You only need to find the right balance. This tends to favour high power density (W/kg) propulsion systems with very high specific impulse. However, do be wary of the fact that many propulsion systems cannot easily be scaled down. A Modern Sail with 1 kg of payload might find room for cold-gas micro-thrusters but probably not for an electromagnetic nozzle. The sails themselves are very fragile so an unshielded nuclear reactor is not a good idea either.   The ultimate hybrid sail is probably one where the sail material serves as a photonic thruster near the Sun, and then a second stage is used for braking for which the sail material is consumed as propellant.  A plasma thruster - VASIMR - could theoretically use aluminium gas as propellant. Imagine a plasma thruster that can turn solar sail material into a cloud of ions, ready to be shot out of a nozzle by magnetic fields. Apply this idea to the Diffractive Meta-Sail design. The sail alone represents 75% of the total mass of the vehicle. Its massive 2,500,000,000 m^2 area can capture 3.4 TW of sunlight near Earth, decreasing to 125.6 GW near Jupiter and 9.23 GW near Uranus. Instead of redirecting 90% of that sunlight into empty space to produce photon thrust, it could focus it instead onto an advanced photovoltaic receiver behind the sail. A 200 MW propulsion system (30% efficiency) would never need more than 666 MW of Sunlight, or 7% of the sail area to function. We can therefore consume 93% of the sail material as propellant and still have enough area to power the plasma thruster. The overall mass ratio is 3.3. The electric propulsion system would occupy half the payload capacity (5 tons) if we assume a fantastic power density of 40 kW/kg, achievable with fully superconducting designs. If the plasma thruster has a specific impulse of 12,000s, we’d get an average acceleration of 0.13 m/s^2 and a total of 140.5 km/s of deltaV.  In other words, a Diffractive Meta-Sail that swaps out half its payload for a high-Isp propulsion system could shorten its 112 day trip to Jupiter to less than 44 days. It would use its solar sail to accelerate all the way out to 2.1 AU, reaching a peak velocity of 210.8 km/s, before braking, then switching to its electric thruster for the last 12.5 days. The final maneuver is a capture around Io at 17.3 km/s.  Could we have just relied on the electric thruster alone? Probably not. The solar sail provided a ‘free’ 263.8 km/s of deltaV - more if we count the climb out of Earth’s gravity. An electric-only mission would have to provide a full (263.8+140.5): 404.3 km/s of deltaV to complete the same mission. Even with 12,000s Isp, that entails a mass ratio of 31, which is totally impractical.   The fission sail concept uses a very thin layer of uranium. Fission events are started by antimatter or neutrons from a nuclear reactor. There are even more options for combining propulsion systems with a solar sail. Solar thermal thrusters would provide a quick burst of thrust but not much deltaV. The backside of a solar sail could be coated with radioactive materials, turning it into a two-sided fission sail. The electric sail rides the solar wind instead of sunlight. The sail could be electrically charged, turning it into an electric sail that rides the solar wind. That resource is available everywhere in the solar system and doesn't have the same drop-off as sunlight. We can imagine a craft that starts as a solar sail, then consumes its sail materials and spins them back out as a web of charged elements, creating a much larger electric sail.   Riding more than sunlight A massive 'pinwheel' helio-gyro with human-sized payload capacity. In previous ToughSF posts, we frequently mention how well space infrastructure pairs with various propulsion systems. The same goes for solar sails. A simple example would be space tugs. They would move solar sails out of the deep gravity wells of planets, saving them the weeks of spiralling outwards to deep space. Or, they could ‘catch’ sails coming in from interplanetary trajectories, which would be especially useful for settlements around Jupiter or Saturn waiting for cargo deliveries.  There is no reason why a solar sail couldn't also work as a laser sail (with low intensity beams) Another well-described piece of space infrastructure is beamed power. Laser beams especially work very well with solar sails. Most sails would be able to receive a large range of laser wavelengths owing to the fact that they are supposed to reflect most solar wavelengths from UV to infrared. Lasers could boost solar sails away from Earth, or help them brake into capture orbits around the gas giants. The beam energy can be used to directly provide thrust via radiation pressure, or indirectly by being directed in hybrid sails that have photovoltaic cells to power electric thrusters (and similar designs). We’ve mentioned already how reducing the braking deltaV can drastically shorten trip times.  A Skyhook is a low orbit rotating tether. Rotating tethers are another classic example. They can fling solar sails into higher orbits or catch them on the way down. If a solar sail is too fragile to survive the g-forces from swinging around a tether, it could detach its payload and the two would go separate ways: the payload making a short trip down a rotating tether and the sail performing long spirals to park in another orbit.  One unique pairing between tethers and sails is the ‘self-winding tether’. In this previous post, we explained how tethers are two-way momentum exchange devices. They absorb momentum from incoming payloads, store it, and then spend momentum to launch payloads to higher orbits. An imbalance between incoming and outgoing momentum has to be compensated by using rocket engines or electrodynamic propulsion. But what if there wasn’t an ambient magnetic field to exploit or enough propellant to keep up with the demand for momentum? The solution would be a solar sail. It could carry a dumb weight up to a high altitude then ‘drop it’ down to the tether. The tether absorbs its momentum. The sail then spirals down to the tether’s orbit, picks up the weight again and carries it back up for another drop. This process can be repeated indefinitely, with nothing expended except free sunlight. Solar sails can also enable mass stream propulsion. Basically, many sails in a row, all working together like a train, carry a number of masses each. The sails head out on a long loop that takes them far away from where another spaceship is sitting. They then accelerate back in, picking up free velocity from sunlight. At the last moment, they all drop their masses and divert to the side. The masses continue in a stream up to a high velocity rendezvous with the spaceship. If the spaceship has a sort of receiver/nozzle like a pusher plate, magnetic nozzle or other such device, it can ‘ride’ the mass stream without having to spend any energy or propellant of its own. The mass stream effectively concentrates the energy gathered by solar sails over a long period into short bursts. A fleet of solar sails, perhaps bringing a mass stream up to speed. Mass stream propulsion has been described in previous posts. Essentially, it strips a spaceship down to just a payload and a nozzle. The nozzle converts the mass stream into thrust. Since the spaceship does not have propellant, reactors, radiators or any such equipment, it is very lightweight. Thrust comes externally without being limited by mass ratios, and it can be used to both accelerate and brake. Combined, we get a way to accelerate hard up to incredible velocities for rapid trips around the solar system. The high velocity masses would be consumed in a magnetic nozzle like this one. If sails use the SunDiver maneuver, we get sails up to 400 km/s and beyond. They would release (or become, with sacrificial micro-sails) mass streams that drift to a rendezvous with a mass-stream-rider spaceship, which would also accelerate to those velocities. It’s enough for an Earth to Saturn trip in less than 37 days. Braking would be handled by pre-positioned mass streams orbiting the destination. Best of all, the energy to drive all these mass streams is gathered for free from the Sun. There would be no need for giant railguns positioned at both the departure and destination points! Another feature of solar sails can be exploited to maximum effect. Sails with a lightness ratio better than one can position themselves anywhere as statites. This means that rotating tethers or laser beams and relay mirrors can be placed wherever they are needed. A statite could release a string of masses at the right place to act as a slow mass stream that a fast spaceship can brake into. There’s a huge range of creative uses for equipment or mass that can be moved freely to any point in the Solar System.  Sailing prospects So, where and how could Solar Sails be used? We’ve clearly shown how solar sails are not necessarily slow or restricted to small payloads. They can get to places quickly and are very flexible in their uses. Solar sailing should become commonplace in the future. Solar sails are most likely to be used to send low-cost probes to the rocky planets first. Then they would find use as ‘asteroid hoppers’ travelling from asteroid to asteroid as long their control electronics keep working. ‘Moon Tour’ missions where they survey the many bodies orbiting Jupiter and Saturn would come later. These sorts of missions make use of sails’ zero-propellant travel capability. What happens next is more speculative.  The 'Lunar Flashlight' uses the sail as large light source to explore the Moon. Nuclear propulsion in the future may be fast, but it costs fuels like uranium or tritium. Solar sailing is nearly free once the sail is built. There will always be a niche for ‘free’. Sails could be used to return precious metals from asteroids back to Earth. They can redirect whole asteroids, given enough time - for either Earth's defense or other operations. Only a 'free' propulsion system could make use of large masses of otherwise low value resources, like icy comets They could also shuttle supplies from Earth to Mars on a predictable schedule, all year round. With the SunDiver maneuver and aerocapture, they could be the first missions to actually enter orbit around Uranus or Neptune.  Mining with sails. Once we master solar sail technology and produce better lightness ratio designs, the options expand greatly. Uncrewed missions could use gigantic solar sails to carry payloads anywhere in the solar system. Passengers would use mass streams accelerated by solar sails to get to distant locations within days or weeks. Hybrid sails would bridge the gap for payloads that don’t need to get to places so quickly, or could be used when transport infrastructure is not available.  A sail on a 'non-Keplerian' orbit. Beamed power infrastructure to keep colonies alive around icy moons could rely on mirror relays held in position by statites. We might even see huge sail ‘motherships’ that have an onboard laser to move around a fleet of smaller dedicated laser sails. Meanwhile, statites sitting over the Sun’s poles read solar activity and warn everyone else of approaching solar storms within minutes.  The possibilities are numerous. Don’t dismiss solar sailing!

Rotating tethers can reach incredible velocities when they are built out of high strength materials. With some design features, they can greatly surpass the exhaust velocities of chemical or even nuclear rockets. They can become propulsion systems with impressive performance... and might look like the classic 'saucer' spaceship. How would they work? What performance could they achieve? Rotating Tethers Cover art by Mack Szbtaba. Rotating tethers are a fascinating topic that have been treated in depth by previous posts on ToughSF, such as using them to extract energy from planetary motion or make space travel much shorter.  Two SpaceX Starships in a 1500m tether formation spun to generate artificial gravity. In summary, a tether made of high strength-to-weight ratio material can withstand enormous forces while remaining lightweight. If spun in a circle, usually many kilometers wide, it can support a load on one end as long as it is supported by a counter-weight on the opposite side. The tip velocity achievable before the tether breaks from centrifugal force will reach several kilometers per second. It can be boosted even further if the tether is tapered: wider at the base and thinner towards the tip. With this technique, tethers made of mass-produced materials like Kevlar can cover a significant fraction of orbital velocity, making it good enough to be used to build a skyhook. Skyhook principle of operation. The important factor here is how heavy of a tether we need to handle a certain payload mass spinning at a certain velocity. First we need to work out the characteristic velocity of a tether, which depends on its material properties: tensile strength and density. Characteristic velocity = (2 * Tensile Strength / Density)^0.5 Characteristic velocity in m/s Tensile Strength in Pascals Density in kg/m^3 For Kevlar, the values we have are 3,620,000,000 Pa and 1,440 kg/m^3. Kevlar’s characteristic velocity is 2242 m/s Then we need to find the ratio between the tether’s tip velocity and the characteristic velocity, which we’ll simply call the Velocity Ratio VR. VR = Tip Velocity / Characteristic Velocity If our tether is spinning at 3300 m/s, then the VR is 3300/2442 = 1.351 Finally we get to the Tether Mass Ratio. It is the ratio between the tether mass and the payload mass it can handle.  Tether Mass Ratio (TMR) = 1.772 * VR * e^(VR^2) A tether with a VR of 1.351 will have a Tether Mass Ratio of 1.772 * 1.351 * e^(1.351^2) = 14.85. It means that a 1485 kg Kevlar tether can handle a 100 kg payload at its tip while spinning at 3300 m/s.  The HASTOL concept relied on 3250 m/s tethers.  The Tether Mass Ratio is square-exponential. It climbs extremely rapidly with increasing VR. Doubling the tip velocity to 6600 m/s, for example, raises the Tether Mass Ratio of a Kevlar tether to 7122. Now a 712.2 ton tether is needed for the same 100 kg payload; a nearly 48x increase. As a consequence of this scaling relationship, large rotating tethers are optimized for velocities only slightly above their material’s characteristic velocity. Then some safety margin has to be added on top. It is not practical to have a 10 ton capsule matched with a tether of several thousand tons. Hundreds of launches would be needed to justify the presence of the tether.  Large tethers also have some additional complications limiting their performance, such as the need to add multiple redundancy against micro-meteorite strikes and shielding against solar radiation that would otherwise degrade their materials. All of these measures cut into the mass actually dedicated to supporting a payload. Hoytether multiple redundant tether lines. But that is not the only way to use tethers. We can design them for an entirely different role. Higher Velocities It is possible to imagine much smaller tethers, perhaps a few meters across, spinning at much higher velocities. They would be completely enclosed in a protective container. The idea of a smaller, faster tether launching objects is not new. In fact, it is being worked on at full scale by alternative launch companies like SpinLaunch today. The idea is that we can increase tether velocity to many kilometers per second, then release small masses from the tether tips. This can be water or dust grains or whatever can flow down the tether’s length. Their release generates recoil in the opposite direction: that’s thrust. Momentum is lost with each release, though it can be regenerated by an electric motor that spins the tether.  Counter-rotating tethers ejecting water for propulsion. If we mount a tether like this on a spacecraft, it can be used as a rocket engine as propellant exiting in one direction and thrust produced in the opposite direction. As long as two counter-rotating tethers are used, there is no torque. Essentially, they become an electric thruster with an ‘exhaust velocity’ equal to the tether tip velocity.  There are many advantages. The tethers can use nearly any propellant they can pipe to their tips. Whether it is dust gathered from an asteroid’s surface, nitrogen scooped up from the edge of Earth’s atmosphere or water derived from a lunar mining operation, it can all go in the propellant tanks with minimal processing. That means there is no need to haul a chemical factory with you to every landing site in the Solar System. An orbital gas scoop. The tether itself should be practically frictionless and have nearly 100% efficiency. It operates mechanically (no electric currents or coolant flows) so it should produce negligible heat even at extreme power outputs, which are in turn limited only by its RPM.  A frictionless magnetic bearing is necessary to enable high efficiency rotating tethers. A tether rocket compares favourably in many ways to existing technology like Hall effect thrusters or MPD thrusters. They do not have to pay the energy penalty to ionize their propellant, nor do they have the pulsed energy storage concerns of mass drivers (railguns, coilguns). Further advantages will be described later in this post.   These tethers can be spun to very high velocities at the expense of impressive mass ratios. The g-forces exerted at their tips would be immense, but it is acceptable as their payloads won’t be fragile spacecraft. Also, since they are on a much smaller scale, it becomes much more affordable to build them out of the best materials available. For example, Toray’s polyacrylonitrile fiber T1100G with a characteristic velocity of 2,796 m/s or new UHMWPE fibres (Dyneema) being tested to a characteristic velocity of 2900 m/s.  These may seem like tiny gains over the characteristic velocity of widely available Kevlar, but remember that the Tether Mass Ratio is square-exponential. Small improvements lead to huge decreases in tether mass. Here is a table of the performance we can get: All of these materials make it possible to achieve tether tip velocities exceeding the best performance of chemical rockets (460s Isp or 4512 m/s) with a moderate mass ratio. Kevlar struggles when going faster than that. T1100G or UHMWPE can get us 7500 m/s exhaust velocity with a Tether Mass Ratio in the thousands. An exhaust velocity exceeding that of nuclear thermal rockets (1000s Isp or 9810 m/s) is achieved with T1100G at TMR 2.27 million and UHMWPE at TMR 0.89 million. A Tether Mass Ratio in the millions sounds extreme but consider it in these terms: a tether of 1 ton mass would be handling 1 gram of propellant at its tip. If it is 1 meter in radius, and the tip velocity is 10,000 m/s, then it makes a complete rotation 1591 times a second 95,460 RPM). It is not so extreme: commercial hard-drive disks spin at 7200 RPM and ultracentrifuges manage 100,000 RPM. We could compare at them to uranium gas centrifuges spinning at 90,000 RPM.  Rows of uranium gas ultracentrifuges. If this 1m long tether releases a 1 gram drop of water every time it completes a rotation, it will have a mass flow rate of 1.59 kg per second. Thrust is propellant flow rate times exhaust velocity, so multiplying that figure by 10,000 m/s gives us a thrust of 15.9 kN. Thrust power is equal to half the thrust times exhaust velocity, which in this case is 0.5 * 15,900 * 10,000 = 79.5 MegaWatts! Let’s try to design two realistic Hypervelocity Tether Rockets, one with T1100G aiming for an exhaust velocity of 6000 m/s which is ideal for travel between the Earth and Moon, and another using slightly more advanced UHMWPE aiming for 10,000 m/s which is better for interplanetary travel. The g-forces at the tether tips will exceed 1,000,000g, which is troublesome as there would have to be some moving part that controls the flow of propellant that can open and close thousands of times a second. A piezoelectric poppet valve that can open and close 2000 times a second. Putting as many components as possible on the external container (control electronics, magnetic actuators) rather than on the moving tip could help. Lunar Tether Rocket The Toray T1100G material is selected because you can order spools of it right now. The individual fibres have a tensile strength of 7000 MPa and a density of 1790 kg/m^3. With its characteristic velocity, 6000 m/s tip velocity means a Tether Mass Ratio of 380. Why 6000 m/s? Because it allows a rocket to make the 8400m m/s deltaV trip from Low Earth Orbit to Low Lunar Orbit and back with a propellant mass ratio of 4 (that’s 3 kg of propellant for each 1 kg of empty rocket). That is modest for an upper stage of a launch vehicle, let alone a lunar transfer stage. The tether here can have a length of 3.67 m. It would rotate at 15,607 RPM. If it aims to shoot off 10 grams of water with each rotation, then it will have a mass flow rate of 2.6 kg/s. The tether itself will mass 3.8 kg but we can bump that up to 5.7 kg to add a 50% safety margin. A counter-weight doubles that value to 11.4 kg. It will feel 60 Newtons of recoil with each release, which seems like it can easily be handled by a suspension mechanism. To counter torque effects, we must add a second tether rotating in the opposite direction, which adds another 11.4 kg for a total of 22.8 kg. Average thrust from both tethers is 31.2 kN. Thrust power is 93.6 MW.   This power can be delivered by a high power density megawatt-scale electric motor. An example of this today would be the H3X HPDM-3000 that manages 2.8 MW of output with a power density of 12.7 kW/kg. It is already meant to be stacked in multiple units. 93.6 MW of power would need to be delivered by 7370 kg of these electric motors. The motors are 94% efficient, so there’s 5.97 MW of waste heat to consider. The motors operate at 60°C, so 4282 m^2 of double-sided radiator panels are needed to handle their waste heat. This may need 4282 kg of 1 kg/m^2 radiator panels based on carbon fibre heat pipe technology.   In total, this propulsion system masses 11,675 kg. If we add a 10% mass margin for equipment like water pumps, tether container walls, coolant pipes, we arrive at a total mass of 12,843 kg. The tethers are by far the smallest component, representing only 0.178% of the mass total. Toray T1100G Tether Rocket Performance Tip velocity = 6000 m/s Total Mass = 12,843 kg  Thrust = 31.2 kN Thrust-to-weight ratio = 0.247 Average power density = 7.3 kW/kg If you add a power supply, propellant tanks, structural components and a payload, you get the rough draft of an Earth-Moon spaceship. The Hypervelocity Tether Rocket here far exceeds the performance of most electric propulsion systems you could slot into its place on such a spaceship. Aerojet Rocketdyne’s Hall thrusters struggle to reach 0.26 kW/kg. NASA’s more advanced electric thrusters aim for up to 4 kW/kg, but at a reduced efficiency of 60 to 85%. They are superior in terms of specific impulse, but that is not particularly needed in cis-lunar space.  Interplanetary Tether Rocket Now we look at a 10,000 m/s UHMWPE tether. It will be more advanced but still within the realm of ‘near future technology’. Tether Mass Ratio is 891,437.  The tether is short: 0.95 m in radius. It spins at 100,000 RPM. The amount of propellant released with each rotation is 1 gram. That means a tether mass of 891.4 kg and a mass flow rate of 1.67 kg/s. With counter-weights and a second counter-rotating tether, the tether assembly adds up to 3566 kg. We bump this up to 5349 kg for a 50% safety margin. The average thrust produced from the two tethers is 33.4 kN. Thrust power is 167 MW.  Fully superconducting electric motors can reach astounding kW/kg values At this power level, it is sensible to switch superconducting devices. NASA’s 2035 goals for turboelectric propulsion on aircraft uses high temperature superconductors to achieve 40 kW/kg at 99.99% efficiency. The electric motor mass would only need to be 4175 kg. The waste heat produced at 65 Kelvin would be 16.7 kW. A superconducting design. A 201 kW Stirling cryocooler of 300 W/kg, would raise the temperature to 300 Kelvin (30% of Carnot efficiency) and 670 kg of equipment. The radiators to handle the final heat load (16.7 + 201 * 0.7 = 157.4 kW) add another 171 kg.  In total, this propulsion system masses 10,365 kg. If we add a 10% mass margin as before, we arrive at a total mass of 11,401 kg.  UHMWPE Tether Rocket Performance Tip velocity = 10,000 m/s Total Mass = 11,401 kg  Thrust = 33.4 kN Thrust-to-weight ratio = 0.298 Average power density = 14.65 kW/kg This design has even higher performance and better specific impulse. It is well suited for missions to Mars. Its performance is somewhat comparable to a solid-core nuclear thermal rocket using liquid hydrogen, as it has the same exhaust velocity but it does not need bulky cryogenic propellant tanks or a full electrolyzing ISRU plant to refuel it. If solar or beamed power is available, it could do away with nuclear technology altogether and still achieve comparable performance.  Neither of these designs are optimized. There could be further performance gains to be had from selecting a better tip velocity or cooling solution. For example, the propellant water could first be used to cool the electric motors to save on the mass of radiators needed. Or, we could employ several tethers to multiply the thrust the engine could produce without having to also increase RPM or tip velocity.   Staging tethers on tethers Rockets get around the problem of exponential mass ratio by using staging. Tethers can employ the same strategy. Instead of placing a payload on the tip of a tether, another smaller tether can be attached. Each tether would spin independently of each other, and at the right moment, their tip velocities would add up.  Here is an example with Kevlar: We want a tip velocity of 10,000 m/s. As we calculated previously, this would require an impractical tether with a Tether Mass Ratio of over 139.1 million. If we instead break it down into tethers of 5,000 m/s velocity, and stage them tip-to-tip, we would obtain stages with a mass ratio of 240. Two stages would add their tip velocities to 10,000 m/s and multiply their mass ratios to 240 x 240 = 57,600. This is obviously much lower than one huge tether.  There is very little literature available on this idea. The closest concept is the Tillotson Two-Tier Tether, as depicted here. There will be challenges to designing a two-stage tether for use as a rocket. There’s the issue of transferring propellant between the tethers, which could be very troublesome if you want solid particles as propellant. Designing a rotating joint that can work smoothly when under high g-forces can’t be easy. Then there’s the difficulty of restoring momentum to the second-stage tether. A second-stage tether also needs its own counter-weight, which could double the overall mass ratio. But, if all these challenges can be solved, then we would get much more impressive tether rockets. Here is a table for two-stage performance: The same material selection as in the previous section is given a second stage so that the total Tether Mass Ratio for both stages reaches 500, 50,000 and then 500,000. The final ratio is doubled to account for the second stage tether’s counterweight. In this arrangement, even Kevlar exceeds 11 km/s tip velocity. UHWPE manages 13.1 km/s with a final tether ratio of 1 million.  Let’s update the two tether rocket designs with staged tethers: Toray T1100G Two-Stage Tether Rocket Performance Tip velocity = 7430 m/s Total Mass = 12,843 kg  Thrust = 25.2 kN Thrust-to-weight ratio = 0.2 Average power density = 7.3 kW/kg We maintained the 380 final tether mass ratio from the Toray 1100G tether rocket. However, with two stages, we get an exhaust velocity of 7.43 km/s. Thrust power from the electric motor is identical so the thrust-to-weight ratio has to fall to 0.2. UHMWPE Two-Stage Tether Rocket Performance Tip velocity = 10,000 m/s Total Mass = 6095 kg  Thrust = 33.4 kN Thrust-to-weight ratio = 0.56 Average power density = 27.4 kW/kg The UHMWPE tether rocket aims for the same tip velocity, but with two stages the final Tether Mass Ratio (x2) can fall from 891,437 to just 7128. The tether assembly is reduced from 5349 kg to 42.7 kg, raising the overall thrust-to-weight ratio and average power density significantly.  Note that for both of these designs, we are only calculating the mass of the engine - the part that converts electrical power to thrust. A complete spaceship would have to include an electrical generator, be it an onboard reactor, solar panels or a laser-photovoltaic receiver. In a realistic study, you will find that high engine power densities means the average power density of the propulsion module of a spaceship approaches that of the power generating section alone. The overall performance of a spaceship won’t improve much if you have a terrible power generator (0.2 kW/kg solar panels) but excellent engines (20 kW/kg).  Solar-electric spacecraft with football fields of photovoltaic panels might not benefit much. Two-stage tether tip velocities means we obtain a propulsion system that can make shorter interplanetary trips. 1200 seconds of specific impulse means that a spaceship that’s 75% water (a mass ratio of 4) has 16.3 km/s of deltaV. It can start in Low Earth Orbit and arrive in Low Mars Orbit in 88 days, or complete a trip to Io’s orbit around Jupiter in 1.73 years instead of the usual Hohmann transfer of 2.73 years. This is without the assistance of aerobraking and with the ability to quickly load up on propellant at the destination for the return trip.  A relatively quick trip from Earth to Jupiter. Theoretically, a third tether stage is possible. It would push the potential performance of tether rockets well into the domain of electric thrusters (2016s Isp with UHMWPE) while retaining the upper hand in thrust-to-weight and power density. However, the problems mentioned above would all be exacerbated.  Carbon extraordinaire So far we have restricted ourselves to materials available in bulk today. Better materials exist; we only need to learn how to manufacture them in large quantities. The most promising of these are carbon nanomaterials: nanotubes and graphene. Carbon nanotubes are being grown right now, up to lengths of 50 centimeters. Graphene flakes are regularly added to epoxy resins and nanocomposite materials to enhance their strength. In the future, we could see them being produced in much larger quantities, enough to use for tethers.  In order of difficulty of manufacture, we have multi-walled carbon nanotubes, single-walled carbon nanotubes and then graphene. Here are their ‘perfect’ properties: The characteristic velocity of these materials can exceed 10 km/s. When used in a tether with a Tether Mass Ratio (TMR) of 100, they can achieve tip velocities approaching 20 km/s. In a TMR 10,000 tether, they approach 30 km/s and they can push beyond 60 km/s with a TMR of 1 million. That’s better than what most electric thrusters are capable of today. Of course, it is unlikely we will be able to form tethers of several meters in length with zero defects, errors or safety margins using these materials in the near future. The strength of a single perfect fibre is reduced when it has to be bundled with many other fibres, bringing down the ‘engineering strength’ to about half of the maximum with no other factors involved. Even at their weakest, carbon nanotubes far surpass other materials. If we assume that a half of the theoretical maximum could be achieved in bulk quantities, the tip velocities we would actually achieve would be reduced by 42%. Then, we could apply staging. A two-stage hypervelocity tether rocket with specific impulse of 2000 to 4800s seems achievable with these materials. The overall power density of the rocket is difficult to estimate because access to carbon nanomaterials would also affect the weight of components like electric motors or radiator panels. The final design could easily exceed 100 kW/kg. It does mean that the performance of the power generating source becomes critical to good overall performance. Even a nuclear reactor with radiators and a turbine that we consider excellent today at 10 kW/kg would become a performance bottleneck when paired with a 100 kW/kg carbon nanotube tether rocket.  Mechanical Rocketry What’s it like to use hypervelocity tether rocket engines? The radiators are tapered to fit inside the reactor's shadow shield, with the water tanks serving as extra shielding.  They can simply be mounted on spacecraft and used to travel by throwing propellant out. It would look rather weird: they have no nozzles, only need small propellant tanks and their most distinguishing feature might look like a wheel... or if the tethers are placed internally, the whole spaceship might be configured like a disk. Not aliens, a spaceship with equatorial tether-rockets (and fancy lighting)! Meaning, your diamond hard science fiction can have fully justified 'flying saucers' roaming the Solar System. The tethers can thrust in different directions by selecting a different firing port for their exhaust. A disk-shaped spaceship with firing ports along its rim can accelerate in any direction. It just has to take care not to aim its exhaust at nearby objects.  Docking might have to be done entirely using secondary propulsion (RCS thrusters). Water can drill holes through asteroids, space stations and other spacecraft when shot out at 10 km/s. Over long distances, it would disperse into harmless mist but at short distances it would be dangerous. Dust or other solid particle propellant would not disperse and would remain dangerous forever. Their use in the Outer Solar System or between asteroids might be justified by the vast distances involved, but not in cluttered low planetary orbits, especially if exhaust velocity is less than escape velocity (the dust would circle back around).  Spaceship pilots might need to pay attention to how long it takes for their tethers to reach operational RPM. Thrust would not be instantaneous, which makes delicate or urgent manoeuvres troublesome.  Thrust levels can be adjusted by firing more or less frequently. Theoretically, the tether can be spun down to a lower tip velocity to allow for more propellant to be fired with each rotation. The potential thrust would increase exponentially as the tether velocity is decreased. However, the other critical component in a tether rocket is the electric motor. Its output is tied to its RPM, so spinning slower might also mean less watts from the motor. The solution to this is a gearbox… but the practical details of building a MW-scale 100,000 RPM set of gears are best left to people in the future. It should be noted that electric motor power does not have to exactly match the thrust power of a tether rocket. The spinning mass of a tether can be considered a type of flywheel, so it can store energy. Energy can be accumulated gradually by a small motor (which enables some mass savings), then released quickly from the tether. This is most useful for spacecraft that aim to raise their orbit via multiple short burns at the periapsis of their orbit. It maximizes the contribution of the Oberth effect and was used by Rocketlab’s Photon stage for the CAPSTONE lunar mission. It’s possible to rely on rotating energy storage alone for propulsion. An asteroid mining spacecraft could land on a target, hollow it out for raw materials, build flywheels-tethers out of the leftovers and spin them up before leaving. Those tethers would then eject pieces of asteroid dust for propulsion until their energy ran out. RAMA proposed this architecture but with a different way of converting stored energy into thrust (using catapult sling arms). In fact, asteroid mining is one of the best applications of tether rockets. The ability to use any propellant, the decent exhaust velocity (for an electric rocket) and the ability to store energy then release it quickly combine to make tether rockets ideal for asteroid hopping spacecraft. The deltaV for travelling between asteroids can be very low, which suits the tether rocket perfectly. An asteroid mining spaceship. Perhaps the ring sections could be tether-rockets... Sunlight may be too weak to keep a powerful motor running continuously in the asteroid belt, so slowly accumulating energy into a flywheel is a good option to have.  Being able to use asteroid dust as propellant means the mining ships can hop to very ‘dry’ targets without worrying about the availability of water to refuel themselves. The tether itself could be made of locally sourced materials, such as glass or basalt fibres that exhibit ‘good-enough’ characteristic velocities of 1.5 km/s to 2 km/s. Glass fibre tethers would be larger and heavier than carbon nanotubes, but that’s actually an advantage if they double as energy storage flywheels. Manufacturing basalt fibres. This creates a ‘low performance’ niche for tether rockets. They could excel here as well as they do in the ‘high performance’ role with super-materials and extreme tip velocities. Other Applications Beyond simple use as rockets, hypervelocity tethers can have a variety of further applications. Drilling and excavation A high pressure water drill. A series of high velocity impacts concentrated onto a small area can serve as an efficient drill. Water or dust at 10 km/s can overcome the mechanical strength of practically any material, so what the target is made of does not matter. The impacts can be tuned to bore a hole through a target, or create shockwaves that fracture it into smaller pieces for easy excavation. One idea is to have the spinning tether first serve as a rocket to bring a spaceship close to an asteroid, then become part of mining equipment to dig into the asteroid’s surface and expose the dense core potentially loaded with precious metals. Just make sure to anchor the tether well! Mass Streams 'Pellet beam' propulsion. A tether could launch those pellets. The hypervelocity tether can be used as a mass driver to shoot a series of projectiles to propel other spacecraft. This is known as mass stream propulsion. The spacecraft riding these mass streams only need a device to catch the projectiles - it can be as simple as an ablative pusher plate or as complex as a magnetic nozzle that drops solid targets into the path of the mass streams and pushes off the resulting plasma explosions. Either way, the riders are unburdened by propellant, reactors or radiators, so they can have fantastic acceleration. Mass drivers are usually fixed structures that do not have to worry about their weight, so the tethers can aim for extreme mass ratios. A two-stage T1100G tether with a TMR of 100,000 per stage would have a tip velocity of 17.5 km/s. Spacecraft riding these mass streams could achieve a good fraction of this velocity, perhaps 16 km/s. More mass streams headed in the opposite direction would be waiting for them at their destination for braking. Together, they enable fast interplanetary travel.   Railguns or coilguns could also be used as mass drivers, but they are usually much less efficient and take up a lot more room than tethers.  Stealth Drive Dark, non-radiating and doesn't even leave a trail of hydrogen behind it. You might imagine that a hypervelocity tether would make for a good weapon. It could drill through any target and its firing rate would allow for enough shots to ensure hits at long range. However, this is unlikely. Hypervelocity tethers have no barrel, so they are inaccurate. It would be difficult to put them in a turret. Their large rotating mass means they act like a gyroscope that resists turning. The way the tether mass scales with projectile mass means that only the smallest projectiles are possible. That removes the option of using ‘smart’ guided projectiles with sensors and RCS thrusters to track a target as these may have a minimum mass of several hundred grams.  Worse, they would be extremely vulnerable to battle damage. A small cut on the tether might lead to it completely disintegrating… inside your spaceship.    So spinning tethers are a bad weapon. Does that mean they have no military use? There is one final advantage that comes into play. The exhaust of a tether rocket can be cryogenically cold. The entire launch process does not release any heat. Even the electric motor can be of a superconducting design bathed in liquid helium at <4 Kelvin. So long as you have access to electrical power, the tether rocket can be a completely stealthy propulsion system.

Fusion technology today relies on expensive, building-sized equipment for ignition, or the help of an already powerful fission detonation. What if we could do away with both? Fusion power without the need for fissiles, but also small enough to be launched into space. It is possible, and eventually it will be practical. Let’s look at how that would work and its implications.  The lead image is artwork commissioned from the talented Daemoria on the ToughSF Discord. It features a spacecraft powered by an Orion-type nuclear pulse propulsion system refueling using the ices of an asteroid deep in the Outer Solar System. Click to zoom in! Too big to launch The point of convergence of all the National Ignition Facility's 192 lasers.  Fusion research today focuses on igniting small quantities of deuterium and tritium using the concentrated energy of lasers, magnetic fields, plasma jets or particle beams. This puts the fuel in conditions far more intense than the core of our Sun, which is enough to ignite the nuclear reaction. However, the total amount of energy being handled is not all that great. The latest record-breaking fusion attempt at the National Ignition Facility added 1.8 MegaJoules of energy in the form of a laser pulse to a tiny gold Hohlraum containing a few milligrams of frozen fuel. Only 150 kiloJoules was actually absorbed by the fuel. From this, the fusion fuel yielded 1.3 MJ, or 8.6 times the input.  The energies involved here are equivalent to the kinetic energy of a small truck at highway speeds or the heat released by burning about 50 milliliters of gasoline. Even if we include the total electrical input of the NIF facility during the attempt, 422 MJ (mainly due to the ridiculously low 0.8% efficiency of the lasers), then we are talking about equivalent to the kinetic energy of a medium-sized passenger jet on takeoff or the explosives in a Mark 82 bomb. It is more than we usually encounter in everyday life, but within reach with a little effort. The full NIF facility houses 7680 xenon flash lamps and 3072 glass slab lasers. The NIF cost $3.5 billion to build and spans at least 300 meters. It probably weighs thousands of tons. All just to deliver 150 kJ to a tiny ball of DT. Sure, a more efficient laser and a more compact arrangement of the components could be used, but it is clear that existing fusion technology cannot fit inside the size and mass constraints of modern space launch capabilities. Even the upcoming SpaceX Starship, a superheavy lift vehicle, can only accommodate 100 ton payloads that are less than 8 meters wide. There is a gap of several orders of magnitude between the two.  So how do we move fusion technology into space? Stars in small boxes There is an easy path and a hard path to placing fusion technology in space. We are on the hard path. It involves progressing our current technological development of ignition methods to the point where the equipment needed for fusion ignition becomes lightweight and manages an input-to-output energy ratio (the fusion gain factor) by two orders of magnitude. For example, we could look at the Gradient Field Imploding Liner concept. This design pushes 50 tons of payload to Mars using a 1.2 GW fusion drive. It uses a novel method for ignition (an imploding lithium liner shot through a magnetic coil of over 20 Tesla) that produces a fusion gain factor of 982. After adding up the mass of the equipment needed to generate electricity from the fusion reaction (to power the ignition process) and radiators to remove waste heat, it ends up with a fantastic power density of over 10 kW per kg.  A single Starship launch of 100 tons would be able to deliver a reactor with an output of 1 GigaWatt if fusion technology achieved that performance. That’s enough to tend to the needs of over a million people.  However, these advances are a long way away. It will require immense effort and research investment over the course of several decades to even come close to these figures. What about the easy path? The 15 Megaton yield Castle Bravo test. Fusion reactions have been produced easily and in small packages since the 1950s in thermonuclear bombs. The shortcut here is to create the necessary conditions for igniting fusion fuel using the awesome power of another nuclear reaction: fission. It is much easier to extract energy from unstable uranium or plutonium isotopes. It can be as simple as bringing enough of these substances together in one place. The only challenge that remains is to channel that energy into the fusion fuel - an idea first proposed by Enrico Fermi that resulted in the Teller-Ulam design that used the radiation from a fission stage (the primary) to implode a fusion stage (the secondary). From a physics perspective, it is very elegant: it turns a hard problem (igniting fusion) into two easy problems (igniting fission, then transferring the energy).  From a practical perspective, it is terrifying. Any plane or rocket that could lift a few hundred kilograms had its destructive capability upgraded to levelling an entire city. The W56 warhead weighs only 272 kg but manages a yield of 1.2 million tons (megatons) of TNT.  The incredible yield-to-weight ratios of nuclear warheads. ICBMs have carried these thermonuclear warheads into space, but not into orbit. These missiles cannot achieve orbital velocity, but only because it is not necessary and not because it is impossible. Their deltaV capability is about 6 to 7 km/s and they would need an additional stage to achieve the necessary 9 km/s for Low Earth Orbit. Incidentally, this is how we got the Soyuz rocket; by adding an extra stage to the R-7 ICBM. Thermonuclear weapons have been tested in space. The most famous example is the Starfish Prime shot. A W49 warhead with a yield of 1.4 megatons was detonated at an altitude of 400 km. The Starfish Prime test of 1962. A naïve calculation would find that a SpaceX Starship could be filled with W56 warheads and hold a combined yield equivalent to 441 megatons of TNT. The previous 1 GW reactor would have to work for 58 years to match the energy these warheads could release in microseconds.  It is not so straightforward though. Thermonuclear warheads have many downsides that prevent them from being an acceptable fusion technology in space. The first is their minimum size. The fusion reaction must be initiated by a fission reaction, which requires a critical mass of fissile material. In the smallest warheads, this is brought down to a few kilograms, resulting in a minimum yield of roughly 42 GJ or 10 tons of TNT. A warhead at this scale is extremely wasteful in its use of fissile material. The smallest design that actually liberates a good fraction of its potential energy would release 4,200 GJ or 1000 tons of TNT. Funnily enough, it obtains this from the same amount of fissile material but with a much larger and more complex compression scheme. A fusion stage on top would need to release a multiple of this yield (10 to 20 times more) to be worth its inclusion.  A propulsion system that uses thermonuclear bombs would have trouble if it were hammered by pulses with a yield equivalent to tens of thousands of tons of TNT. A nozzle or pusher plate that receives this blast would be immense, and the suspension system needed to translate the pulses into a continuous acceleration would bring us back to the building-sized equipment we are trying to avoid in the first place.  The second is their need for fissile material. It is in fact the biggest problem with producing thermonuclear warheads. Today, it means that they need a highly controlled substance, which is enriched uranium or plutonium. It is expensive, difficult to manufacture, easily weaponizable and dangerous if accidentally dispersed. Political considerations and social fears have already prevented the launch of much milder nuclear propulsion system, in the form of Nuclear Thermal Rockets, and ruled out designs like the Orion nuclear pulse propulsion rocket by international law.  Even in a fictional setting or alternate-future where these concerns are minimized, there is still the logistical problem of sustaining the use of these materials. The Midnite mine. Uranium is only found in high concentrations on Earth thanks to the action of the terrestrial water cycle. Dry surfaces like the Moon or small bodies like asteroids have their uranium dispersed within them at concentrations similar to the primordial composition of our Solar System. Instead of mining rich veins for uranium at 200,000 parts per million, settlers on Venus or Ceres would be sifting through vast quantities of rock to extract less than 2 parts per million. Map of uranium on the Moon. That’s 5 grams per cubic meter of rock. Worse, only 0.7% of this uranium is of the desired U235 isotope, so only 35 milligrams of enriched material would go towards the thermonuclear warhead. The rest would have to go through a laborious burnup and transmutation process inside breeder reactors. If the minimum critical mass is about 2 kilograms, then over 57,000m^3 of rock would need to be processed for each thermonuclear pulse. A rocket that uses these pulses for propulsion may need thousands of pulse units to complete a trip… it is clearly unsustainable! Deuterium/Hydrogen ratios in the Solar System The fusion fuel is a minor concern in comparison. Deuterium is abundant in all waters of the solar system at 312 parts per million (0.312 grams per kg), and can be higher in the outer solar system. Deuterium concentration was 3 times higher in the samples returned from the comet 67P/Churyumov-Gerasimenko than on Earth. It can be melted out of the ices of a comet and separated by electrolysis. Tritium is trickier to obtain, but it can be manufactured out of lithium, which is a rather common element. It decays with a half-life of 12 years but with the speed of fusion propulsion, most trips will be completed well before then. Helium 3 is very rare in comparison, but obtaining it is still possible from the lunar surface or by scooping up the atmospheres of Venus or the gas giants. Filtering gases is a much easier task than digging through kilometers of rock after all.  Going by the abundance of their fuels, we would want to use Deuterium-Deuterium fusion, then Deuterium-Tritium, then Deuterium-Helium 3.   Pure Fusion A hemispherical implosion test device. The solution is to find a way to use a simple non-nuclear energy source, and concentrate it in a way that can ignite a fusion reaction but without the need for complex or heavy machinery to serve as an intermediary. Fusion, without the ‘dirty’ fissile aspect. This is the ‘pure fusion’ concept that has long been on the minds of scientists since the first fusion bomb was tested. It found renewed interest ahead of and following the Comprehensive Test Ban Treaty in 1996. Some of the methods for achieving pure fusion ignition, especially by Soviet and then Russian scientists, were tested in the 1990s and 2000s in collaboration with LANL. It might be because they feared that they might not have access to the multiple billion dollar investment needed to pursue conventional ignition research. More recent concepts have appeared too. Interest in them has waned since fusion research has become a well funded international effort, like JET and NIF. 'The Gadget' from the Manhattan project. This is a prickly topic to discuss with any nuclear scientist today. The design of a pure fusion device overlaps significantly with that of a regular nuclear warhead. Discussing this topic in detail with the general public generally goes against the rules they have to follow to retain their security clearances. They might inadvertently reveal facts or figures they are not allowed to share, even for far off speculation like this. It is wise to not test their patience. Nuclear weapons after all threaten human civilization on one hand, and offer absolute protection against invasion or loss of sovereignty on the other. Aggressive posturing by small and otherwise weak states like North Korea is only possible because they have incredible destructive power at their disposal. The proliferation of nuclear weapons weakens the protection they offer to existing holders while increasing the risk that they are deployed by someone who doesn’t have much to lose. Anything that threatens to share nuclear power to a wider group is therefore taken very seriously. Pure fusion technology could be considered to be one such proliferation concern. The creation of nuclear weapons that circumvent the most effective anti-proliferation control, which is access to fissile material, could destabilize the relations between nuclear states. Global annihilation would come closer. More specifically, it is a restriction on the enrichment of uranium from 99.3% U238 into >90% U235 (or into Pu239). Uranium gas centrifuges for U235 enrichment. Natural uranium cannot be made into a bomb, and it is regularly shipped around the world by the hundreds of tons to feed nuclear reactors. It would be practically impossible to restrict access to it. ‘Reactor grade’ uranium, which is enriched to less than 5% U235, won’t work either. Climbing up to ‘weapons grade’ is a long and arduous process that requires gas centrifuges that take up several football fields and many megawatts of electricity. The machinery is delicate and needs trained personnel to run… even moderate damage or a cyber attack can take them down. India's Bhabha Atomic Research Centre reactor. The other route, which is to operate a reactor specifically designed to produce Plutonium 239, is also difficult to hide, but it has been successful in the past.  Pure fusion ignition does not need enriched uranium. There is discussion around how the technology could destabilize the current nuclear arms balance, especially since the Comprehensive Test Ban Treaty left open the door to conventional ignition research and therefore there is a legal ground for the development of alternate ignition schemes. However, as we will calculate later, pure fusion devices cannot result in weapons with the same destructive potential as actual nuclear warheads. They might have an effect on warfare at the tactical scale but not really at the strategic level.  Still, there is a real possibility that these designs will be developed seriously in the future, for military purposes or not. They have advantages that are not useful today but might be critical for a space settlement at the edge of the Solar System. Looking into these pure fusion concepts can help inform us about their future potential in propulsion, energy generation and elsewhere. We will look at two plausible concepts for igniting a pure fusion device. The first is Magnetized Target Fusion using explosive-driven flux generators. The second is Multi-Stage High Explosive-driven Implosion Fusion. To these documented concepts we will add invented variants based on other speculative technologies that have been demonstrated in some way or another.  Magnetized Target Fusion using Explosive-driven Flux Generators A helical explosive-driven flux generator design for the MAGO experiments. Explosive-driven Flux Generators are able to convert the chemical potential of a high explosive (HE) into a powerful magnetic pulse. This is done by first creating a strong magnetic field by running an electrical current from a small capacitor through a number of conducting disks (Disk Explosive Magnetic Generator or DEMG) or coils (Helical Explosive Magnetic Generator or HEMG). The detonation of a high explosive compresses these conducting structures into a smaller and smaller volume, which magnifies the electrical current and multiplies the initial magnetic field to several hundred tesla. These steps can be staged, with the magnetic field produced by the first compression being multiplied again by a second compression.  The Tsar Bomba was developed at the Russian VNIIEF. Experiments at the Russian VNIIEF (All-Russian Scientific Research Institute of Experimental Physics) demonstrated a 20 to 25% conversion of high explosive energy into magnetic energy, with electrical currents on the order of 100 MegaAmperes producing magnetic fields of 200 Tesla strength. It should be noted that actual efficiency is likely much higher (1.5x times higher, so in the 30-40% range) but only a fraction of the total output is delivered at a useful rate, as explained in the Efficiencies section in this document. There is also an explanation that these results are from designs that did not really require high explosive-to-magnetic efficiency, and that instead of 70% is possible with end-initiated coaxial generators. A DEMG with 3 modules, containing disks a meter wide, was shown to deliver 100 MJ of energy and an electrical current of 256 MA, and it is possible to stack 25 of these modules and maybe more.  DEMGs tested at the VNIIEF. These powerful magnetic pulses can be used to drive Magnetized Target Fusion (MTF). In this ignition scheme, fusion fuel is first heated into a ‘warm’ plasma, and then it is rapidly compressed by imploding a spherical metal shell (the liner). The shell implodes because of the powerful magnetic pulse we have created using a flux generator. It achieves a substantial velocity of several tens of kilometers per second, enough to raise the pressure and temperature of the plasma trapped inside to fusion ignition conditions. Almost all the fusion energy that is then released is absorbed by the metal shell, causing it to vaporize and expand as a plasma explosion, which can be redirected for thrust or absorbed to generate electricity. MTF has been demonstrated successfully several times with actual fusion neutrons being detected. The biggest current project aiming to use MTF is General Fusion. General Fusion's piston-compressed MTF scheme. It has many advantages over achieving fusion using conventional means. The pressure it can achieve far exceeds anything a tokamak can manage by using static (non-pulsed) magnetic fields, which really helps push fusion fuel particles together. The implosion velocity is much lower than the several hundreds of km/s that need to be achieved at the NIF or most other inertial confinement fusion schemes and it receives that energy far more efficiently than could be managed by a laser or particle beam blasting away at a pellet of frozen fusion fuel. However, it has its own set of challenges and far less investment in its development than the other ignition methods.  For our purposes, we are looking at the following chain of events: HE -> Flux Generator -> Metal Liner -> Fusion Ignition -> Fusion Output Each arrow has a certain efficiency figure associated with it. The only source of energy input is the high explosive, and the only source of energy output is from the fusion reaction. There are some small steps we are omitting here, like losses to electrical switching or the initial heating of the fusion fuel, but they are far smaller (kJ scale) than the energies involved in the main steps (MJ scale). The objective is to have a far greater fusion output than the HE energy input. The MAGO plasma chamber. The VNIIEF’s MAGO project (MAGnitnoye Obzhatiye or magnetic compression) found that if the metal liner had a kinetic energy of 65 MJ and imploded at 20 km/s, it could get 8.9 milligrams of deuterium-tritium plasma pre-heated to 1 million Kelvin to undergo fusion and release 1 GJ of energy. Deuterium-Tritium reactions have an output of 340 TeraJoules per kilogram. The full potential of the 8.9 milligrams of fuel is 3.03 GJ. This means that the implosion got 33% of the fuel to undergo fusion (also called the burnup ratio). The result is a ‘fusion gain’ of 16x. They based these results on experiments with 200 MJ flux generators creating >1000 Tesla fields adding up to 25 MJ into the metal liners. If we assume that 25% of the high explosive’s energy can be converted into magnetic energy, and that 60% of the magnetic HE is around 5 MJ/kg for denser compositions like ‘PBX 9501’, so working backwards, it would take 86.6 kg of HE to deliver 433 MJ as energy input, that gets converted into 108.25 MJ of magnetic energy, which results in 65 MJ of metal liner kinetic energy. The final output is 1000 MJ, giving a return on energy investment of 2.3 times. Component weights for a DEMG-powered pure fusion device. Other estimates in this document’s appendix B suggest that a multi-stage device with a plasma chamber would fit 320 kg of HE inside 3400 kg of equipment to be able to deliver 100 MJ to a metal liner that compresses up to 30 milligrams of DT fuel. The fusion output is 10 GJ, which is a 33% burnup ratio. The performance of the flux generators is pessimistic, with only 6% of the 1600 MJ chemical potential in the HE actually being delivered to the plasma chamber. That means a return on energy investment of 6.25 times. The majority of the mass is dedicated to a 2000 kg DEMG device. In the footnotes, it is explained as a necessarily conservative estimate, far greater than the minimum amount of copper wires needed for simply conducting the electrical current. In fact, it seems like the masses of all the explosive flux generators have been estimated by multiplying the mass of the explosive they contain by a factor 10.  There are few other figures to rely upon for further speculation. Nonetheless, we can put together the data we have to obtain a ‘reasonable’ MTF design that is powered by high explosives. We’ll call this the Early EMG-MTF device. Early EMG-MTF Total mass: 1600 kg HE mass: 100 kg HE energy: 500 MJ HE-to-magnetic efficiency: 25% Magnetic energy: 125 MJ Magnetic-to-kinetic efficiency: 60% Liner kinetic energy: 75 MJ DT fuel: 22.5 milligrams DT burnup: 33% Fusion output: 2.52 GJ Average energy density: 1.57 MJ/kg This design is admittedly not very powerful. 2.52 GJ of fusion output might sound like a lot, but it is only a 5 times return on energy invested. It is also important to look at the average energy density of the device. It is much less powerful than the same mass of simple HE, so it would be a terrible weapon and even worse propulsion system - for comparison, a mixture of hydrogen and oxygen in a rocket engine has an average energy density of 15 MJ/kg. It actually compares poorly to lithium-ion batteries, which is laughable for a thermonuclear reaction. Comparison of the huge structures need to provide an electrical pulse with capacitors or high explosives. Technology is expected to improve. If we conceived of this technology today instead of in 1998, we should hope to get better results. This can include the use of stronger materials, aluminium conductors instead of copper wires or even high temperature superconductors, better HE compositions and perhaps a different explosive flux generator design that comes closer to the 70% HE-to-magnetic efficiency mentioned previously. These would all lead to a lighter device. It is unlikely to fall below 2x the weight of the explosives, because the HE needs to push against something to transfer its momentum efficiently, but a reduction from 10x to 5x the weight is plausible.  More explosive flux generator configurations. Today’s MTF schemes also aim for much higher fusion gain ratios. Tricks to improve the efficiency of the reaction, such as turning the initial warm fuel plasma into a field reversed configuration that is self-containing and prevents heat losses by touching the imploding metal liner too early, can be used.  General Fusion’s initial Acoustic MTF concept had pistons compressing a plasma, with 14 MJ being delivered to the plasma in the final step. This was enough to release 704 MJ of fusion energy, which is a fusion gain of 50 times. We can work out that they use 10 milligrams of fusion fuel with each shot, and that the burnup ratio they assume is 20%. The Fusion Driven Rocket's magneto-inertial ignition concept. John Slough’s Fusion-Driven Rocket uses a type of Magnetized Target Fusion where the metal liner is made of lithium and receives a kinetic energy of 2.8 MJ. In return, it provides a fusion gain of 200. This is far above the fusion gains mentioned previously. There are hotspot ignition schemes that can attain fusion gain ratios in the thousands by starting a burn wave in a much larger quantity of fuel, but let’s not be excessively optimistic.   If we assume that these promises will be fulfilled, then we can guess at the performance of an EMG-MTF built to an advanced technology standard.  Advanced EMG-MTF Total mass: 500 kg HE mass: 100 kg HE energy: 500 MJ HE-to-magnetic efficiency: 70% Magnetic energy: 350 MJ Magnetic-to-kinetic efficiency: 60% Liner kinetic energy: 210 MJ DT fuel: 150 milligrams DT burnup: 33% Fusion output: 16.8 GJ Average energy density: 33.66 MJ/kg We get a much more interesting device. It is 6.7 times more powerful than HE on its own and exceeds the performance of any chemical reaction. But even these improved figures are nowhere near the power of a conventional nuclear warhead which manages energy densities on the order of 10,000,000 MJ/kg.  Multi-Stage High Explosive-driven Implosion Fusion This approach attempts to ignite a fusion reaction by imploding the fuel without using a flux generator as an intermediary. High explosives press directly against a metal sphere to cause it to implode into fusion ignition conditions. Normally, this is impossible. HE is powerful and their detonation velocity ranges from 7 km/s to over 10 km/s. The Gurney Equations state that they can push a plate of metal (called a flyer in this situation) up to a third of their detonation velocity, so 2.3 to 3.3 km/s. The UTIAS explosive-drive implosion of a hemispherical chamber. However, some ignition schemes get around this by concentrating the energy of the high explosive shockwaves in some manner. This was demonstrated by using a Voitenko compressor to send a shockwave into a hemispherical chamber filled with deuterium gas. Fusion neutron were successfully produced and detected. The theoretically simple collapsing spherical chamber. Even more effective (in theory) is use explosives to surround a 1m wide sphere of metal and get it to implode into a tiny 0.1 cm-sized volume. This 1000x decrease in volume would bring the initial inward velocity to several thousand km/s and multiply the internal pressure by tens of millions of times, enough to ignite a fusion reaction. Tests have successfully demonstrated 1 MJ-scale detonations imploding metal spheres and hemispheres and causing some fusion reactions to occur. However, they used 20 cm wide spheres and tried to explain how scaling up their designs will not provide much improvement.  Rayleigh-Taylor instabilities forming. The tiniest imperfections in the sphere or the explosive would be magnified as the sphere’s size decreases and would cause the compression to fail. Rayleigh–Taylor instabilities would also cause the smooth surface of the metal sphere to bubble over into a turbulent storm that isn’t very effective at compression fusion fuel. Mitigating these imperfections involves scaling up the sphere to tens of meters in width, and therefore surrounding it with thousands of tons of HE. Not a great solution either.  Instead, what we could do is perform a more moderate implosion, and then convert the energy into another form that can do more work on compressing the fusion fuel. Two methods are documented.  Winterberg's magnetic booster concept. The most complicated method involves the use of a ‘magnetic booster’. The metal sphere that the HE will implode is given an electrical current, which produces a magnetic field. The sphere is also filled with low density fusion fuel in the form of a gas and at its center is a special target. The initial implosion takes place at a velocity of 5 to 8 km/s, depending on the initial size of the metal sphere. Near the end, the walls are closing in at over 20 km/s. This is enough to raise the temperature within the fuel gas to millions of Kelvin. Not enough for ignition, but enough to get the special target to work. The implosion also multiplies the initial magnetic field into something of massive strength. A diagram of this mag-booster concept. The special target is the magnetic booster and a fuel pellet surrounded by ablative material in a small closed chamber next to it. The magnetic booster is a Z-pinch device, basically a number of coils connected to a capacitor and surrounding a conductive tube. The circuit is open, so there is no electrical current. At the final stage of the metal sphere’s implosion, the circuit is closed. Current runs through the coils and creates a small magnetic field. This does nothing on its own, but it does react to the massively strong magnetic field that surrounds it. The interaction of the fields causes a similarly massive electrical current to start running through the conductive tube. This causes the Z-pinch effect, which exerts enormous pressure on the tube and causes it to collapse. This collapse causes the remains of the tube to radiate heat. This comes in the form of energetic UV and X-rays. Penetrating radiation digs into the adjacent chamber that has held the fuel pellet safe so far. The ablative layer surrounding the fuel pellet vaporizes. The reaction force of the vaporized gases forces the fuel pellet inwards, in turn bringing it to fusion ignition conditions. You may have noticed the similarities between this ‘magnetic booster’ and the steps taken by the Teller-Ulam design of a thermonuclear warhead to turn the energy released by a fission primary into X-rays that then cause a fusion secondary to implode and ignite. The ignition of the tiny fuel pellet raises the temperature of all the gases compressed within the metal sphere. It creates a much larger fusion reaction, which could then be used to ignite even larger quantities of fusion fuel… if we were not tired yet of the great complexity and number of steps involved so far.  The complete propulsion system. Winterburg gives us some estimates for the performance of this pure fusion device. It would be a 20 cm wide metal sphere, about a millimeter thick and weighing 40 kg, surrounded by a 10 cm thick layer of HE. The explosive is assumed to be Octol, which has a density of 1700 kg/m^3 and an energy density of 5.3 MJ/kg. This layer is itself contained inside a 10 cm thick iron sphere (the tamper) that weighs 800 kg. The iron is the single biggest contributor to the device’s mass. Its job is to contain the 70 MJ high explosive detonation for a maximally efficient implosion.  The total mass of the device is 853 kg, rounded up to 1000 kg by Winterberg. The fusion reaction within it releases 400 GJ of energy. Most of it is in the form of neutrons, but the iron sphere does an excellent job at absorbing them all. We can call it the Magnetic Booster Implosion Fusion device or MBIF. Here is the summary: Winterberg MBIF Total mass: 1000 kg Tamper mass: 800 kg HE mass: 53 kg HE energy: 70 MJ DT fuel: 2.53 grams DT burnup: 50% Fusion output: 400 GJ Average energy density: 400 MJ/kg This is an incredible performance, blowing away even the best assumptions for the Advanced EMG-MTF. We can attribute this to the much larger quantity of fuel that gets heated to ignition conditions and the elimination of the heavy flux-generator equipment. Still, this is nowhere near the power of a conventional nuclear warhead. A Winterberg pure fusion design, this time relying on compressed 'super-explosives'. Winterberg’s original conception of a ‘mini-nuke’ had a metal sphere collapsing to the point where it radiates in the X-ray wavelengths and causes another ablative stage to compress fusion fuel to the point of ignition, without the need for a complex ‘magnetic booster’. It might reduce the number of steps needed to achieve fusion, at the cost of tightened tolerances on how smooth the metal sphere is and how evenly the HE detonates. These advantages would be seen during the manufacturing stage and not in the actual performance.  Another method attempts to improve on the design offered by Winterberg but combining it with more recent techniques. Finn van Donkelaar suggests that a staged HE accelerator using overdriven detonations can do away with the imploding spheres and heavy iron tamper. It is a less rigorous treatment of the topic, but it does have some interesting figures to offer. There are four steps: acceleration of metal plates (flyers), piston-compression of deuterium-tritium gas followed by a spherical implosion, and finally a fuel pellet surrounded by ablative material that undergoes the final compression. The same principles as those for creating EFPs are used here.  The HE is separated into disks lined up behind metal plates (called flyers). The first HE stage is ignited and it pushes a flyer to 3 km/s. This flyer hits the back of the second stage, creating a shockwave. This second stage adds its own velocity to its own flyer, allowing for flyer velocities greater than what is possible with a single stage - a solution very similar to one adopted by rockets to overcome the deltaV limitations of a single stage. Explosives act differently when compressed due to a shockwave. The shockwave has an additional effect. It causes a sudden compression of the material it passes through. Compressed matter has a higher density and therefore a greater speed of sound. The compression also causes the chemical composition to ignite. Theoretically, the travelling wave will pick up more energy from this combustion, causing it to compress more HE even harder, which again increases the speed of sound and allows it to reach higher velocities. The result is an 'overdriven' detonation velocity superior to the ordinary uncompressed detonation velocity. The combined effects of staging and overdriven explosion velocity would allow flyer plates to achieve 8-12 km/s.  The final flyer hits a converging section that focuses its energy on a ‘cup’. That cup acts like a piston travelling down a tube that contains DT gas before meeting a ‘bowl’. The temperature at this point has increased to 9500 K. The cup and bowl then meet to form a sphere that undergoes its own implosion that forces the fusion fuel into a volume a thousand times smaller. Temperatures reach millions of Kelvin, providing the X-ray radiation needed to make the surface of the fuel pellet surrounded by ablative material explode and finally achieve ignition. The fusion reaction in the fuel pellet provides the spark that gets the rest of the fuel gas to react. We have some performance figures, but with few details. A scaled up device would mass 1600 kg in total, have a length of 2.5m and a width of 0.4m, and yield an output of 8,368,000 MJ. Energy density is 5,230 MJ/kg. The amount of fusion fuel consumed is between 50 and 100 grams, depending on assumptions about burnup ratio. We can call it the Staged Overdriven Accelerator Fusion device. SOAF device Total mass: 1600 kg DT fuel: 50 grams DT burnup: 50% Fusion output: 8.37 TJ Average energy density: 5.23 GJ/kg This performance figure is ridiculously high, and it speaks to the true potential of fusion technology. And yet, it is about 1900 times weaker than a thermonuclear warhead. Other ways to spark the fire There are even more ways to get fusion reactions without needing any fissile material or heavy equipment. They are, however, even more speculative. A SMES device using niobium-tin coils. One example is to use Superconducting Magnetic Energy Storage (SMES) devices. SMESs pushed to the limits of the tensile strength of the materials holding them together can manage impressive energy densities. The quenching process allows them to release their stored energy nearly instantaneously too. Using the maximum strength-to-weight ratio of modern mass-produced materials, such as the 7 GPa strength at 1790 kg/m^3 density of Toray T1100G carbon fibers, would be able to store 3.9 MJ/kg. This is less energy than the 5 MJ/kg of dense explosives like RDX. However, SMES output their energy in the form of electricity, allowing it to be converted into magnetic energy with near-perfect efficiency, and at extremely rapid rates. They also greatly reduce the mass of copper conductors and various magnetic coils needed as they can pass huge currents through small wires (assuming the wires are also superconductors). In effect, 1 kg of Toray 1100G-backed SMES is worth 1.4 to 3.1 kg of HE due to increased efficiency. It would be even better in practice as SMES do not need to explode or push against something to operate (so no need for a heavy tamper), so they can allow for even greater mass savings. At their best, SMES backed by more advanced materials, such as carbon nanomaterials, could exceed 50 MJ/kg while retaining the efficiency benefits over HE. Superconducting materials applied to other parts of an explosive flux generator could result in the following device: SMES-EMG-MTF Total mass: 200 kg SMES mass: 100 kg SMES energy: 5000 MJ SMES-to-magnetic efficiency: 99% Magnetic energy: 4950 MJ Magnetic-to-kinetic efficiency: 80% Liner kinetic energy: 3960 MJ DT fuel: 2.83 grams DT burnup: 33% Fusion output: 320 GJ Average energy density: 1.6 GJ/kg This would bring it more in line with the performance of the staged HE accelerator. Of course, applying SMES technology to the SOAF device itself would bring performance to an even greater level. Simulation of a shear-flow-stabilized Z-pinch, one of the most promising approaches. There are even more ways to use the energy of a large explosion. The flux generators could exploit their ability to produce electrical currents in the hundreds of mega-amperes to drive a large Z-pinch. This could be used to directly compress a metal liner around a fuel pellet, as in the HOPE Fusion propulsion approach (an MTF version was also designed). In that design, 333 MJ is delivered to the specially shaped fuel target, and in return, 1 GJ of fusion energy is released. This energy gain ratio of just 3x is too slim to work with HE, but an improved concept could allow it.  An explosive-driven railgun. Or, the electrical current could be used to power a short but extremely high acceleration electromagnetic gun. It would be connected by long wires to the EMG so the debris from its remains do not damage the accelerator. Whether it is a coilgun or a railgun, a projectile velocity of 20 km/s could be achieved before the current falls off. This is enough to start the multi-staged compression cycle proposed here for low velocity fusion ignition. It would be even easier to use the electrical discharge from SMES, although that raises the difficult question between throwing away empty SMES or installing the equipment to recharge them. The Wilderness Orion The application that stands out the most for these pure fusion devices is in the domain of space propulsion.  A pure fusion device could be used to create a large plasma explosion. A magnetic nozzle or pusher plate could be used to turn that fusion energy into thrust, similarly to the various nuclear pulse propulsion designs.  To estimate the performance of these devices as rockets, we use the method described in a previous blog post. This equation is most useful: Plasma RMS velocity = (2 * Energy Density)^0.5 Plasma RMS (Root Mean Square) velocity is in m/s. Energy density is in J/kg We can turn this into an exhaust velocity by including an efficiency figure for how good a nozzle is at turning an expanding plasma into an exhaust stream. Exhaust velocity = Nozzle efficiency * (2 * Energy Density)^0.5 Exhaust velocity is in m/s. Nozzle efficiency is a ratio. We’ll use 90% (0.9) for the following calculations. Energy density is in J/kg The energy density we use here is that of the entire device. This is because we must assume that the fusion reaction and its X-rays, charged particles, neutrons and other products are all fully absorbed into the device’s mass and converted into heat.  For the Early EMG-MTF design, we get Energy Density = 1,570,000 J/kg. With a nozzle efficiency of 90%, we calculate an exhaust velocity of 1594 m/s. That’s a specific impulse (Isp, or exhaust velocity divided by 9.81) of 162 seconds, which is worse than most cold gas thrusters. No spaceship is going to bother with that. The Advanced EMG-MTF and its 33.66 MJ/kg is much more interesting. We calculate an exhaust velocity of 7384 m/s. That’s an Isp of 752s. This is better than any chemical thruster and comparable to a low performance solid-core nuclear thermal rocket or a solar thermal thruster restricted by poor materials.  The Winterberg MBIF manages 400 MJ/kg. That results in an exhaust velocity of 25,455 m/s. An Isp of nearly 2600s is better than most high-thrust electric thrusters and is only matched by advanced gas-core nuclear rockets. Performance reaches another level once energy density is measured in GJ/kg. The SMES-EMG-MTF would get us 5,200s Isp and the SOAF design manages an even higher 9.400s. Even the most advanced electric thruster would struggle to meet this performance level. For the higher specific impulses, you would want a magnetic nozzle to handle the plasma, as shown in this beautiful piece by Seth Pritchard. This is not to say that high specific impulse is the only thing to aim for. Like other forms of nuclear pulse propulsion, a rocket that drops pure fusion devices into its nozzle also gets very high thrust. More thrust can be delivered by simply sending out these devices to explode more frequently behind the spaceship. All the ignition energy is contained inside the devices, so there is no major rate limit to how often they can be used. Drop a single 1 GJ device per second, and the drive power is 1 GW. Drop ten of them, and it becomes 10 GW. This is most similar to the original Orion design and its Outer Space Treaty-violating nuclear pulse units. The Advanced EMG-MTF dropped at a rate of 1 per second would get you a drive power of 16.8 GW and a thrust (with 90% nozzle efficiency) of 4.1 MegaNewtons.  The main interest in these devices is how they free space propulsion from the need to obtain fissile material from Earth, while also providing a level of performance unmatched by chemical or solar energy. Fusion fuels can be found in any patch of ice in the solar system. High explosives are composed of nitrogen, oxygen, carbon and hydrogen. The red-coloured ices on some comets and icy moons is due to organic compounds, as we can see in this Viktus Justinas piece. Various volatiles like ammonia and carbon dioxide can be found on the surfaces of comets or icy moons. It is not a good idea to research exactly how they are made, but turning those raw materials into the H2N2O2 nitroamide building blocks for C3H6N6O6 cannot be more complex than the processes needed to resupply life support systems. A potential obstacle is the need for metals like copper to create conductors and coils. It is the 25th most abundant element in the Solar System, which might not sound like a lot, but you might expect to find 1 kg of copper for every 1724 kg of iron. A metal-rich asteroid like 16 Psyche or 21 Lutetia would contain 10^18 to 10^19 kg of iron. Roughly, we would expect a near-limitless supply of 10^14 to 10^15 kg of copper. Similar ratios would exist on the surfaces of Mars and the Moon. 3D printing and ISRU are key to NASA's future plans. 3D printing of metals and laser cutting of the HE can create the structures needed to implode the fusion fuel. It should be of similar difficulty as printing solar panels, and NASA already considers that a press-to-print process in the near future. This is the origin of the Wilderness adjective: pure fusion devices allow for ‘wilderness refuelling’ or In-Situ Resource Utilization, the same way chemical rockets can manufacture new fuel out of any mass of water they encounter.    How would these devices look like on a spaceship? Let’s draft two designs for pulse propulsion spacecraft. The first one, the ‘Mars Circuit’ spaceship, aims to travel from Earth to Mars and back, and the second one, the ‘Saturn Circuit’ spaceship, will jet around the outer Solar System.  The Mars Circuit spaceship uses the Advanced EMG-MTF devices. It is a 100 ton spaceship carrying onboard power generation, radiators, life support system, habitation spaces and everything else needed for drifting through interplanetary space. It also has a payload bay that can fit 100 tons. Behind it is a magazine stack of fusion devices. The stack is 35 tons while empty. For a Mars mission, it is filled with 5064 units of half-size (250 kg) versions of the Advanced EMG-MTF devices, totalling 1266 tons. These provide a specific impulse of 753s. Utilizing them is a propulsion system of 108 tons. A USAF Orion with its pulse unit magazines highlighted. This system includes a pusher plate, suspension arms and structural support that can handle 2 MN thrust per pulse. It is directly modelled on the propulsion section of the 10m USAF Orion design (although it would be overbuilt by modern standards). It can drop one pulse unit every 0.8s. Average thrust would be 2.5 MN.   Here is the summary for this spaceship: Mars Circuit spaceship Payload: 100 tons Dry mass: 243 tons Propellant mass: 1266 tons Total mass: 1609 tons DeltaV: 11.4 km/s Acceleration: 0.16g (full) to 0.74g (empty) This is not a zippy ship that can just take straight lines to its destination. It does however have enough deltaV to complete fast 120 day trips to Mars. It curves out of Low Earth Orbit and gently slows down into an orbit around Mars, without aerobraking. All neutrons are absorbed within the EMG-MTF units so this is not a radioactive hazard to its surroundings and won’t be ‘hot’ after use. It can directly approach space stations or other spacecraft, like the vehicles that will take the payload down to the Martian surface. Fresh pulse units can be manufactured entirely out of the resources available from the moons Phobos and Deimos. Within the 1266 tons of propellant, there would only be 37.8 grams of fusion fuel.  The Saturn Circuit spaceship is much larger and goes much faster by exploiting the power of SMES-EMG-MTF devices. It has 500 tons of onboard equipment, which include comfortable living spaces and a fully self-contained manufacturing facility. Payload capacity is 100 tons. Its magazine stack is filled with 100 kg pure fusion devices that contain 0.566 grams of fusion fuel and output 63.5 GJ thanks to SMES technology that stores 10 MJ/kg. Each unit provides a specific impulse of 3632s and a thrust of 3.56 MN. The average temperature of the plasma created by the use of each fusion device is 600,000 K. The Mini-Mag Orion. This allows it to be harnessed by a magnetic nozzle at the rear of the spaceship. A 40 ton propulsion system (based on that of the Mini-Mag Orion) drops a total of 20,000 of these units at a rate of 1 per second. Here is the summary for this spaceship: Saturn Circuit spaceship Payload: 100 tons Dry mass: 560 tons Propellant mass: 2000 tons Total mass: 2660 tons DeltaV: 49.6 km/s Acceleration: 0.14g (full) to 0.55g (empty) This spaceship can really build up speed. Starting in Low Earth Orbit, it stops at Mars in 38 days, orbits Jupiter after 6 months or gets to Saturn in 1 year. It is not the fastest craft conceivable at that technology level, but it can be relied upon to connect the furthest planets without any initial infrastructure or external support. Even its longest trips are short enough that the 12 year half-life of tritium is not really a concern. It does all this using just 11.3 kg of fusion fuel so carrying an excess isn’t difficult.  At 3632s Isp and technically unlimited thrust, made possible by detonating pulse units more frequently or just using larger plasma explosions, there is a clear opening for high performance spacecraft with military potential. The Orion Battleship, a 4000 ton design equipped with 20 Megaton nuclear missiles and naval guns. The combination of wilderness refueling and high performance makes wandering fleets, or more likely pirates, a realistic possibility. Stealth also becomes more effective if you do not need to heat up a nuclear reactor or ignite a fusion core to start maneuvering. Superbombs It is obvious that pure fusion devices have a real potential as weapons.  But by now, we hope that the numbers we have arrived at make it clear that they have nowhere near the destructive potential of existing nuclear warheads. They are thousands to hundreds of thousands of times weaker than a thermonuclear device initiated by a fission primary. An F-35A testing the deployment of a B61 thermonuclear bomb. A B61 nuclear bomb with a yield of 300 kilotons of TNT can easily be carried by any aircraft with a hardpoint capable of more than 324 kg. Matching its performance with the wildest SOAF design would mean a warhead with a mass of  235 tons. It would barely fit inside the payload limits of the An-225, the largest cargo plane in the world. Using the Early EMG-MTF design would require 800,000 tons to reach that yield. That’s closer to the weight of all the US Navy’s nuclear aircraft carriers… combined! The destructive radius of a 2000 lb bombs. It does not mean that there would be no consequences to the development of pure fusion devices. A plausible design with an energy density of 30 MJ/kg would be six times more powerful than simple HE. Real weapons are about 40% to 60% filled with HE, so it is practically a 12x increase in destructive potential. It would be a ‘superbomb’. By another comparison, the effect of a 907 kg (2000 lb) bomb could be matched by that of a 75 kg (165 lb) pure fusion device. Warfare at the tactical scale has already known a significant shift in the effectiveness of bombs with the introduction of precision guidance systems. It allows large and bulky loads, like a Vietnam-era B-52D Stratofortress bay filled with 66 of the US Air Force’s 340 kg (750 lbs) bombs, to replaced by a precision strike by a JDAM-equipped GBU-12 at 227 kg (1000 lbs), of which fighter jets can carry several. A Super Hornet with a full bomb loadout. Superbombs would cause another change in loadouts. The F/A-18 Super Hornet could be carrying 3600 kg of bombs and 1800 kg fuel for a long range strike mission. It would rely on other aircraft to protect it with their air-to-air missiles, and yet more to guide its munitions using equipment like Litening pods. With 30 MJ/kg Superbombs, its loadout could instead be 360 kg of bombs, 1800 kg of fuel and 3240 kg distributed between missiles, electronic warfare equipment, targeting pods or even more fuel. A single fighter could replace an entire squadron. It might even be able to hide its bombs inside internal bays to be able to maintain a stealthy outline, like an F-35B, while delivering the same power as an F/A-18 bristling with weapons.  An MQ-9 Reaper drone equipped with precision-guided Mk 82 bombs.  Or, the expensive jets could be replaced by small drones, each only having to hold a few hundred kg of munitions. Pure fusion devices would make delivering destruction to far away targets even cheaper and easier.  A side-effect of the development of pure fusion devices is the access to ‘neutron bombs’. These are weapons that intentionally leak the radiation produced by the fusion reaction instead of trying to absorb it to maximize the amount of energy that becomes heat. The intention is to deal a lethal effect via penetrating radiation out to a further radius than the blast effect can manage. The Early EMG-MTF device with its 2.52 GJ output would have a blast radius of 36 meters. An Advanced EMG-MTF yielding 16.8 GJ increases this radius to 68m. If these were converted into neutron bombs, they would deliver a lethal dose of radiation out to 272 meters and 512 meters respectively. It is enough to depopulate multiple entire city blocks. These radii are only reduced by about 50% when concrete walls stand in the way. Another consequence is that tank armor becomes much less useful. Today, a nuclear warhead that can kill a tank crew by radiation has to be close enough to destroy the tank itself by blast effect anyway. In this case, a near miss with a small superbomb is enough to deliver a lethal dose. It is unlikely that the neutron effect can be scaled up to many kilometers (which would empty an entire city center with one hit) as air absorbs and scatters the neutrons after some distance, but it is still enough to create a frightening change of priorities during battle.  An invading force could hit populated areas with neutron bombs and rid them of any inhabitants, whether they are innocent civilians or potential defenders. They could then move in and easily hold it. No siege involved, no prolonged cries of the oppressed on social media and news channels. Just a single action that hands an entire city and its economic value, infrastructure and factories, mostly undamaged. Offensive actions would be immensely profitable. Defenders would have to pay an even higher price for letting any missile through their defenses.   The general result would be a gradual evolution of the state of warfare. Nothing as drastic as the invention of the nuclear weapon, far from disrupting the balance between nuclear-armed states, and not worthy of proliferation fears. Significant enough however to change what military planners worry about or aim for. Conclusion Pure fusion devices are still a thing of the future. But, we must start considering the potential consequences of their development today. If their arrival is expected and regulated, we could open up human exploration of the Solar System like never before with spaceships untied from the rest of civilization for years.  But if we are unprepared, or we dismiss their potential effectiveness, then we could end up with yet another shift of warfare towards greater destruction at lower cost.

There has been much discussion about converting the SpaceX Starship to use nuclear propulsion. It would allow for a great increase in specific impulse and a massive extension of mission capabilities. But is it actually worthwhile? The image above is modified from BocaChicaGal’s photo. Nuclear thermal rockets do indeed have impressive performance. Their specific impulse is up to three times greater than chemical rockets, they produce comparable amounts of thrust and they could be designed to accept a variety of propellants, from CO2 to ammonia. They can reduce travel times in space, push around much larger payloads and get refuelled with whatever fluid is available at their destination. Full scale mockup of the NERVA engine. For these reasons, they are lauded as the best way to accelerate human exploration and expansion into space.  They are not a new technology either. The idea to use nuclear energy to propel spacecraft dates back to 1944. Serious testing has been done on nuclear rockets, with ground tests in 1955 and functional gigawatt-scale rockets firing for several minutes by the 1970s. The Strategic Defense Initiative rekindled studies into nuclear propulsion with Project Timberwind, a program that ran until 1991 and resulted in modern designs that were even more capable.  We are now in the midst of another revival of this technology. Millions have been awarded to BWXT, General Atomics and Ultra Safe to restart the development of nuclear rockets.  Naturally, there have been calls to combine the capabilities of nuclear rockets with the other great aerospace development of our time, which are reusable rockets and their champion; SpaceX’s 9m wide Starship and its SuperHeavy booster. Then, we would acquire the ability to send even heavier payloads to orbit and beyond. The modern nuclear rocket The BWXT design. The design and performance targets for nuclear propulsion has shifted considerably over the last few decades. The initial efforts in the 1970s were straightforward in the desire for maximum power and thrust. The reactors from Project Rover. Project Rover, for example, resulted in the Phoebus-2A reactor that managed an output of 4000 MW for 12 minutes during a test run. As a fully developed engine, it would have managed 5000 MW and 825s of specific impulse. It would have held 300 kg of enriched uranium and had a relatively cool core temperature of 2300 K (although the goal was 2500 K). Total mass was 9300 kg, meaning it had an impressive power density of 537 kW/kg. The Pebble Bed nuclear engine. The ultimate version of this sort of maximum power engine would be the Pebble Bed reactors from the 1980s Strategic Defence Initiative era Project Timberwind. The largest version aimed to operate at 3000K to achieve a specific impulse of 1000s). 12,000 MW of power served to produce 2451 kN of thrust. Power density was a whopping 1450 kW/kg.  More recent nuclear thermal rockets have taken a different direction. Each rocket is smaller, aiming for a thrust level of around 100 kN. Multiples are used to ensure reliability during operation. Core temperatures are reduced to prolong engine life. With no hope of being used within an atmosphere, they are vacuum-only designs with a reduced focus on power density. Instead, additional capabilities such as the ability to produce electricity continuously are featured.  A Copernicus Mars vehicle equipped with three SNREs. An example of this would be the ‘Borowski SNRE’ NERVA-derived design that produces 111 kN of thrust. When operated at 2800K, it achieves a specific impulse of 925s. Power density is about 152 kW/kg (504 MW for 3300 kg). An even further diminished version of the SNRE is to be expected in the near future. Highly enriched uranium (enrichment level 93%) will be replaced by HALEU fuel, which has an enrichment level of 20% at most. Five times more fuel in total would be needed to complete the same missions as with highly enriched uranium fuel, or a greatly increased amount of neutron reflector is necessary to surround the reactor core to achieve the same power output. Either way, the power density will suffer.  An additional aspect of nuclear thermal rocket development needs to be addressed: the choice of propellant. Nearly all tests and designs focus on the use of liquid hydrogen as it has the potential to deliver the highest specific impulse. However, other propellants have been considered, especially in the context of ISRU where spacecraft are refueled with whatever is available at their destination. Most significant for our purposes is methane as propellant. It is six times denser than liquid hydrogen, can be stored at 100K, which is compatible with liquid oxygen, and it can be produced using water and carbon dioxide. At high temperatures, it breaks down into hydrogen and carbon, turning it from a 16 g/mol molecule into a 3.25 g/mol plasma. That is how it achieves a specific impulse only mildly lower than what is achievable using liquid hydrogen. Zubrin lists its specific impulse as 606s when heated to 2800K, or 625s at 3000K.   Nuclear Starship A nuclear-powered Starship would not be a complete overhaul of the design. A depiction by smallstars. It will still be a 50m tall steel tube that launches atop the SuperHeavy booster, using vacuum-optimized engines fed by large propellant tanks, and a set of smaller gimballed engines optimized for landing, with flaps to handle reentry. Dry mass in the final version will be 120 tons and about 30 tons of propellant is reserved for landing.  It might be unsurprising to you that we cannot simply bolt on nuclear rockets to the Starship and expect everything to work. Special modifications have to be made to accommodate the new propulsion system, ranging from new attachment points to control software, but we will focus on the most impactful one: radiation shielding. General configuration of a nuclear Starship. The shape of the Starship is not well adapted to handling the radiation from a nuclear rocket. There are large flaps extending to the sides that could scatter radiation back into the crew compartment at the top. Retracting them when the nuclear rockets are in use would be a good idea. Designs which were meant to be nuclear from the start also usually place their reactor or nuclear rocket far from the main body of the spaceship, on the end of a long boom or tapered propellant tanks. Radiation released from a fission reaction spreads as a sphere in all directions - if it is placed further away, the main body of the spaceship intercepts a smaller fraction of it.  The ideal rear end of a nuclear-propelled spacecraft, based on the RNS. The fraction of radiation that cannot be avoided is handled using radiation shielding, with different layers meant to absorb different types of radiation. It is placed as close as possible to the reactors or engines to create the widest shadow of protection, which is why they are also called shadow shields. Illustration of the shadow shield concept. A fission reaction mainly produces fission fragments, gamma rays and neutrons. Fission fragments are heavy ions that do not travel very far. Gamma rays are penetrating photons that are best absorbed by a dense material. Neutrons are high velocity subatomic particles with no charge; they are best dealt with using a material that contains as much hydrogen as possible, like water.  We want to use as little shielding mass as possible.  The densest elements are the best protection against gamma rays, with tungsten (W) being an ideal choice (lead would melt too easily and depleted uranium is not practical).  Lithium Hydride (LiH) is the most mass-efficient protection against neutrons. Boron Carbide (B4C) is 20% heavier than LiH for the same protection, but it melts at 3036K is a very strong ceramic, which is ideal for surfaces exposed to reentry heating.  It must be noted that some radiation protection is already built-in. The beryllium or graphite reflector within the nuclear reactor prevents some radiation from leaving. The 30 tons or more of landing propellant, especially methane, is effective at absorbing neutrons too and will always be present while in space. A much larger load of propellant will be drained as the nuclear propulsion is used, representing several meters of shielding. Furthermore, there will be a 25 meter separation between the engines and the crew compartment, so only 1/625 of the radiation is actually intercepted.  Different estimates for the shielding required place the total at about 440 kg/m^2, which corresponds to 2 cm tungsten plus 2 cm of Boron Carbide. The engines themselves are about 1 meter wide, whether they produce 500 MW or 5000 MW of power, so the shielding to be added to each engine is about 345 kg. Perhaps this estimate is optimistic, but we can rely on all the previously mentioned protections to make up for any deficiency. Consider also that the effectiveness of radiation shielding is not linear but improves exponentially with its thickness - it is easy to adjust protection levels.  Finally, there would be significant changes to be made to supply propellant to nuclear rocket engines. We would extend them to take up the volume currently occupied by liquid oxygen. We won’t be changing the total volume of propellant tanks available to us, for a fair comparison with other versions of the Starship. If we selected liquid hydrogen, we would need specially designed tanks with insulation and active cooling. If we feed the nuclear rockets with liquid methane, we can use the same type of propellant tanks as exists today.  The propellant tanks dedicated to landing would also have to be changed. A nuclear Starship is expected to have a heavier dry mass, so more propellant is needed to land it, which means larger tanks.  With all these modifications in mind, let’s dive into the numbers. Performance analysis We will calculate the performance of a SpaceX starship equipped with nuclear propulsion. Two nuclear rockets are considered: a 150 kW/kg near-term design operating at 2800K, and a 1000 kW/kg far-term design operating at 3000K. They will either replace all engines, or just the three vacuum Raptor engines. We will consider the use of traditional hydrogen propellant as well as the methane alternative, and either try to match the accelerations possible with chemical rockets or aim for a lower performance.  To begin, let’s begin by breaking down the existing Starship design.  It has three vacuum Raptors massing 1.87 tons each, three sea-level Raptors massing 1.11 tons each. These are based on their TWR figures. This leaves 111 tons of dry mass in the form of propellant tank walls, the thermal protection system, the reentry fins and other structures, for a total of 120 tons. There are four propellant volumes: two main tanks and two landing tanks. The main methane tank contains 614.33 m^3 of propellant, containing 268.5 tons of liquid methane at 437 kg/m^3 density. The main oxygen tank contains 798.4 m^3 of propellant, containing 983.6 tons of liquid oxygen at 1232 kg/m^3 density. The methane landing tank contains 13.14 m^3 of propellant, containing 5.7 tons of liquid methane at 437 kg/m^3 density. The oxygen landing tank contains 14.56 m^3 of propellant, containing 17.9 tons of liquid oxygen at 1232 kg/m^3 density. The total propellant mass is 1252.1 tons. 23.6 tons are reserved for landing.  This is only possible thanks to subcooled propellants with increased density. The Starship is meant to carry 100 tons of payload into Low Earth Orbit. With this payload, a full fuel load and its dry mass, it masses 1495.7 tons (a figure very close to the 1500 tons that SpaceX reported to the FAA). It enters orbit by expending all 1252.1 tons of propellant held in its main tanks, leaving it with 243.6 tons. Its mass ratio is 6.13. After releasing its payload, it comes in for reentry massing 143.6 tons. Alternatively, it can be refilled in orbit. The Starship is first accelerated by the SuperHeavy booster, which has a dry weight of 200 tons and holds 3300 tons of propellant. We assume it holds 300 tons of propellant in reserve (enough for a 3150 m/s deltaV boostback plus landing, similar to the Falcon 9 booster), it provides 3150 m/s of deltaV to the Starship at staging.  When the Starship stages off the SuperHeavy booster, all engines fire and produce 13,200 kN of thrust. This gives the Starship an initial TWR of 0.9, but by the time it has exhausted its main tanks, it has a TWR of 5.5.  A Starship with both sea-level and vacuum Raptor engines. The vacuum Raptors have a specific impulse (Isp) of 380s. The sea-level Raptors have an Isp of 350s. The deltaV for a Starship climbing to orbit with a 100 ton payload is 6492 m/s, assuming an average 365s Isp from all engines being used. With this 3150 m/s boost from the SuperHeavy booster, this is enough to get into orbit.  It should take about 207 seconds for the Starship to use up all its main tank propellant with six Raptors running at full power. If the Starship refills back up to its full 1495.7 ton mass in orbit, it will have 6765 m/s of deltaV from only using the main tanks with the vacuum Raptors, plus 380 m/s of deltaV from its landing tanks. That’s a total of 7145 m/s! However, about 800 m/s needs to be reserved to land the Starship and its payload on another celestial body, so only 6345 m/s is available for interplanetary maneuvers.  150 kW/kg, hydrogen propellant Using hydrogen-propelled nuclear thermal rocket engines requires the greatest modification of the Starship, but the least engine development. Our first option is to replace the vacuum Raptor engines with 150 kW/kg nuclear engines that provide 900s of Isp. The three sea-level Raptors with their landing tanks are preserved but not used during the climb to orbit. The main tanks are replaced by a hydrogen tank of 1412 m^3. Normally, liquid hydrogen has a density of just 70 kg/m^3, so it would contain only 98.9 tons of propellant. We generously assume that subcooled liquid hydrogen at 15 Kelvin with a density of 76 kg/m^3 is available, meaning this tank holds 107.3 tons instead. Three hydrogen-propelled nuclear rockets are sized to produce 1523 kN of thrust using 6.72 GW of power. With radiation shielding, they mass 45.2 tons each, totalling 135.6 tons and producing a combined 4570 kN of thrust.   We also need to expand the landing reserves to 64.7 tons to accommodate for the heavier dry mass at landing, mainly due to the hefty engines.  This nuclear Starship has an initial mass of 100 ton payload + 135.6 ton nuclear engines + 107.3 ton liquid hydrogen + 3.33 ton sea-Level Raptors + 64.7 ton landing reserve + 111 ton other structure, for a total of 522 tons. This is a lot lighter than the original Starship, and manages the same initial TWR of 0.9, but it does us no good. Its deltaV is 2032 m/s, mainly because it carries very little hydrogen propellant and so its mass ratio is only 1.258.   The SuperHeavy booster can accelerate this lighter nuclear Starship to 4703 m/s before staging. The deltaV adds up to 6734 m/s, which falls far short of the 9200 m/s typically required to reach a Low Earth Orbit. Achieving orbit is actually impossible for this vehicle. Worse, all of the additional dry mass due to the heavy nuclear engines means its center of gravity is at the bottom of the vehicle - that means it will flip over backwards during reentry! If we remove all payload and replace the entire fairing volume of 933 m^3 with an expanded hydrogen tank containing an additional 70.9 tons of hydrogen propellant, we get the following total: 135.6 ton nuclear engines + 178.2 ton liquid hydrogen + 3.33 ton sea-Level Raptors + 64.7 ton landing reserve + 111 ton other structure equalling 492.8 tons. Onboard deltaV rises to 3985 m/s. The Superheavy booster can add an increased 4777 m/s. It is still short of the 9200 m/s needed to reach orbit. Removing anything more, such as reducing the landing propellant reserves or using smaller nuclear engines, just means the Starship fails earlier or later.  Hydrogen propellant with weak nuclear thermal rocket engines is a losing combination. 1000 kW/kg, hydrogen propellant Different DUMBO designs with 1 to 5 MW/kg. We now replace the weak engines with the 1000 kW/kg powerhouses from decades past. An improved 1000s Isp is available.   As before, we replace the three vacuum Raptor engines with three nuclear thermal rockets sized to produce 3568 kN of thrust in total. They add up to 18.6 tons now, including shielding, and have an output of 17.5 GW. A necessary modification is to reduce the payload volume by 450 m^3 to accommodate more liquid hydrogen. It would bring the total propellant mass up to 141.5 tons, just enough to help it make orbit while carrying the full 100 ton payload. The landing reserve also needs to be increased to 34.3 tons. The initial mass of the Starship becomes 100 ton payload + 18.6 ton nuclear engines + 141.5 ton hydrogen + 3.33 ton sea-Level Raptors + 34.3 ton landing reserve + 111 ton other structure, for a total of 408.7 tons. Its initial TWR is 0.91 and it has a deltaV of 4186 m/s. The SuperHeavy booster propels this even lighter nuclear Starship to 5009 m/s at staging, allowing for a total deltaV of 9195 m/s.  It is still hard to justify the existence of this nuclear Starship. It has less deltaV than the original Starship, and it cannot increase it much by sacrificing payload capacity.  A trip that starts in Low Earth Orbit and ends with a landing on the lunar surface requires 5.9 km/s of deltaV, to be provided by both the nuclear rockets and then the landing engines. This is only possible if the payload was reduced to 34.5 tons. This configuration can only land on the lunar surface by sacrificing some payload. This reduction in capability comes on top of halving the fairing volume available. The same is true for reaching Mars: it must either take a slower trajectory or reduce its payload capacity. Furthermore, it imposes the need for three separate sets of ISRU machinery, for oxygen, methane and hydrogen, if it is to be refuelled on the lunar or martian surface for a return trip. Liquid hydrogen is the most energy-intense propellant to produce, which is an additional complication.  To add to all these deficiencies, a new problem arises: the SuperHeavy Booster would destroy itself. Having a Starship stage that is too lightweight means the SuperHeavy booster reaches extreme velocities that a boostback burn cannot sufficiently reduce, and without any form of thermal protection, it could become too damaged to land itself. Preventing this means reserving more propellant for the boostback burn, but this in turn means the Starship stage is released at a lower velocity. For hydrogen-propelled nuclear Starships that already struggle to reach orbit, it becomes unworkable.   In short, a hydrogen propellant nuclear Starship is not saved by better engines.  150 kW/kg, methane propellant Internal configuration of the advanced KANUTER design. Previous calculations using hydrogen propellant revealed how volume-limited the Starship design was. There was no room for the bulky liquid hydrogen, and getting to orbit meant sacrificing the payload mass and volume advantages that the Starship is built around. These could be solved by using denser liquid methane as propellant for the nuclear propulsion system. The Isp will be lower, but the mass ratios become so much better that more deltaV is available overall. Now, let’s remove the three vacuum Raptor engines and the main tanks. In their place we add 150 kW/kg methane-propelled nuclear rockets delivering 600s of Isp and a single large propellant tank containing 617 tons of liquid methane. We also need to expand the landing tanks to 79 tons. The nuclear engines are sized to output 28.5 GW and deliver 9682 kN of thrust each. They mass 191 tons together. The initial mass of the Starship becomes 100 ton payload + 191 ton nuclear engines + 617 ton methane + 3.33 ton sea-Level Raptors + 79 ton landing reserve + 111 ton other structure, for a total of 1,101.3 tons. Its initial TWR is 0.9, as required, and it has a deltaV of 4834 m/s. The deltaV is not better due to the ridiculously large engines needed to achieve a sufficient TWR. The Superheavy booster is only able to accelerate this Starship configuration to 3623 m/s, bringing the total to 8457 m/s, which is far short of reaching orbit. Orbit is only possible by reducing the payload to 20 tons. Alternatively, we can bring 100 tons to orbit by sacrificing 450 cubic meters of payload volume to an expanded methane propellant tank. Of course, this payload will have to be very dense to fit inside the remaining volume… and the TWR will drop to 0.76! Again, we have an unworkable nuclear Starship. Reduced payload mass or reduced payload volume are the only way to reach orbit. The mass of the engines is overwhelming. In this case, they are 63% of the Starship’s empty mass.  The only advantage of this configuration is perhaps the high amount of deltaV within the Starship stage. It is comparable to the deltaV of the original chemical design, so it can perform the same missions. But getting barely the same performance by going nuclear is not what we want.  1000 kW/kg, methane propellant Time to try the most promising combination. Powerful engines and denser propellant. 625s Isp rockets with an output of 8 GW each yield 2610 kN of thrust. They mass 8.4 tons with their radiation shielding, for a total of 7828 kN of thrust and 24 tons of mass. The same-sized methane tank holds 617 tons of propellant. The landing reserve is expanded a bit to 36 tons. This gives the more powerful nuclear Starship an initial mass of 100 ton payload + 24 ton nuclear engines + 617 ton methane + 3.33 ton sea-Level Raptors + 36 ton landing reserve + 111 ton other structure, for a total of 891.3 tons. From this we get the first actually interesting result so far. The deltaV of the powerful nuclear methane stage is 7209 m/s. It is finally higher than that of the original chemical configuration! The SuperHeavy booster provides another 3944 m/s, for a total of 11,153 m/s.  There are two ways to use this improved performance: increase the payload or reach for harder missions.  This nuclear Starship can carry 245 tons of payload to orbit if it increased its engine thrust to 9128 kN (engine mass would increase to 29 tons). It means that the number of missions to deliver a certain payload amount to any destination is more than halved, and also that the number of refuelling missions needed to get one Starship filled up and ready to go from LEO drops from a dozen to just three. There’s also the option to only partially fill the Starship. It can perform the orbital mission (9200m/s total deltaV) when loaded with only 323 tons of propellant instead of the full 617 tons. Lunar missions become much easier. The original chemical Starship could take up to 215 tons from Low Earth Orbit and land it on the Moon (5930 m/s total, with 800 m/s covered by the landing engines) but it would have to stay there. It cannot return from the lunar surface to Earth’s surface. If it had no payload, it could go to the Moon and insert itself back to Low Earth Orbit, but what’s the point in that? The methane nuclear Starship can perform a one-way mission with 271 tons of payload, if it could land using its nuclear rockets. That figure is reduced to 234 tons if it landed on the lunar surface using Raptor engines. What’s more interesting is that it could take a reduced payload to the Moon and return on its own to its launchpad on Earth. The 138 ton dry mass nuclear configuration departs LEO with 25 tons of payload, 617 tons of methane propellant in the main tanks and 36.7 tons of methane-oxygen in the landing reserve. It heads to the moon and lands there, consuming 520 tons of main tank methane (5930 m/s). It then unloads its payload and then heads back to Earth with a 2700 m/s maneuver. After aerobraking, it lands using sea-level Raptor engines. A great win for reusability! Mars missions benefit as well. The methane-propelled nuclear Starship has access to 7209 m/s of deltaV it can use for interplanetary maneuvers. The usual 120 day trip is reduced to 88 days or less. Even under the perfect alignment of planets that allow the Starship to perform the shortest possible 65 days trips, the nuclear version can shave off nearly two weeks days and bring it down to 52 days.  As with the lunar missions, this additional performance opens up more options. For example, the nuclear Starship can load up on 165 tons of payload instead of 100 tons, while performing the same trips.  The chemical Starship could potentially load up with 465 tons of payload and slow-boat it to Mars on a minimum energy trajectory. This nuclear Starship can do the same with 495 tons of payload, limited mostly by the huge landing propellant reserves it needs.  Or, it could reduce its payload capacity to aim for even more deltaV and even faster trips. With reduced payload, it could widen the Mars launch window by several months, and often be able to go to Mars and return (with refueling on the surface) before the two planets move too far apart.  So will going Nuclear be worth it? The short answer is no. We’ve gone through and calculated the performance of different nuclear Starship configurations and only found one that has advantageous performance. It is also the one that is least likely to exist in the near future. No large nuclear thermal rocket is being developed today, and no testing of nuclear rockets with methane propellants has ever been performed. The current efforts will revive decades-old hydrogen-propelled nuclear rockets at a scale completely unsuited for the Starship. Elon Musk is unlikely to fund the development of the necessary technology, especially as it does not his vision for how SpaceX should operate. He wants to build a modest number of launch vehicles that are reused as much as possible. This is how a very low cost per launch is achieved. If there is a deficiency in performance, more launches and not better Starships are the solution. More expensive nuclear designs with a small performance advantage, mainly in the form of fewer launches, go against this philosophy. Kiwi-A being tested in open air. This comes on top of the various difficulties of developing nuclear rockets compared to chemical rockets. It wouldn’t be possible to return to work a week later if a test model explodes on the stand, which is completely antithetical to the way SpaceX operates. There’s no ‘move fast and break things’ when the US government swoops in every time things go wrong. That is, if they give Elon Musk access to enriched uranium. Or if they allow large-scale testing outside of close government supervision in the first place. Another problem is radiation. Nuclear rockets are safe to handle on the ground without radiation shielding or many precautions, especially when loaded with Low Enrichment Uranium. They only ignite after staging off the Superheavy Booster, far off the ground, so they do not pose a radiation threat to the launch site. If there is an accident upon launch, the uranium could be dispersed, but it is not dangerous - it is safe enough to touch (but don’t eat it)! The challenges and safety risks of a nuclear payload in the (early) Space Shuttle. The problem comes after the ignition of the nuclear rockets. The fuel becomes intensely radioactive. After shutdown, up to 1% of the maximum power output keeps getting released. That’s several megawatts in this case. It falls off rapidly, but radiation levels near the engine would remain lethal for days and harmful for weeks. Remember, it is unshielded around the sides and rear, so there is no protection for someone coming from those angles. NASA estimates that a nuclear rocket engine returns to its ‘safe’ state after a month.  Some nuclear reactor designs needed to be cooled by cold propellant for several hours after use. Rapid reuse becomes complicated. Discharging the payload to orbit and then reentering means the nuclear engines are still ‘hot’ after landing. Even if the landing itself is performed with chemical Raptor engines rather than with active nuclear engines, the residual radioactivity means that any ground crew would need to be fully protected, the refueling facilities will all have to be shielded and adding a new payload then stacking it back on top of a Superheavy Booster without contaminating them become very difficult tasks.  Even in space, where we don’t mind irradiating the empty environment, there are issues. Approaching the International Space Station becomes impossible unless the Starship ‘cools down’ for a month in orbit. Docking maneuvers between a Starship and the craft meant to refuel it have to be done along a narrow corridor between each ship’s radiation shadows. Moon landings take place about 3 days after departure from orbit and the use of the main engines. Nuclear rockets would still be ‘hot’ by then and dangerous to any astronaut approaching from the surface. They would have to land far away from any lunar bases, and rely on shielded rovers to transfer payload across the Moon’s surface across a safe distance. The lack of any air to grant free radiation shielding means this safe distance will be very large. It is less of an issue for Mars missions. Even the shortest missions take more than 2 months and this gives enough time for the nuclear rocket engines to become safe again. Landing is done with chemical rockets, so the Starship is safe to approach once the Martian surface. But this is necessarily a smaller number of missions compared to the Earth-Earth or Earth-Moon missions.  And finally, there’s the ISRU. Martian Starships return to Earth after being refilled by propellant produced by CO2 and water found locally. Vast fields of solar panels or fission reactors produce electricity to crack these molecules and reform them into oxygen and methane. A methane nuclear Starship needs nearly three times as much methane than a chemical Starship. It needs no oxygen, but that is a byproduct of the reaction that produces methane anyway - it is not a true saving. Three times more methane means that ISRU facilities have to be three times bigger or refuel three times less Starships, a hefty penalty.  Can the Nuclear Starship be saved? It is possible to envision a nuclear Starship in the far future. Someone else decides to develop the necessary methane-propelled propulsion technology. Perhaps the basic Starship is adapted to carry more propellant volume, increasing overall mass ratios and making full use of the increased exhaust velocity. And maybe large launch facilities are constructed for billions of dollars to refuel radioactive Starships on the ground, like those once proposed for handling nuclear-powered bombers during the Cold War. The GE Beetle, designed to handle radioactive B-36 bombers. But it is more likely that none of these things happen. Huge performance gains could be had by specializing the chemical Starship for Lunar or Martian missions. These would never land back on Earth’s surface, but they perform their own missions far better than a multi-purpose Starship ever could. And let’s not forget that the final dry mass of the Starship will be lower than 120 tons. SpaceX has released (and tweeted) estimates as low as 60 tons for the final uncrewed version. At the extreme, we have the Starship Lite, stripped of all aerodynamic features, payload fairing and landing systems. It would have a deltaV of 12.7 km/s, thanks to a dry mass of just 40 tons.  If we need the full performance advantage of nuclear propulsion, we should design a spaceship that is intended for it from the get-go. It never lands, only going from orbit to orbit, so there is no need for heat shielding, flaps, high thrust engines, thick steel structure or aerodynamic shaping requirements. Without these constraints, it can instead utilize huge hydrogen tanks and a lightweight structure made of aluminium or carbon composites. Low pressure rockets with 1300s of Isp would be available, as there is never a need for high thrust.  A 30 ton craft with 10 tons of nuclear propulsion, 263 tons of hydrogen propellant and 100 tons of payload would have 13,500 m/s of deltaV, enough to get to Mars in 100 days and brake into a low orbit. It is fast and economical and closer to the current vision for nuclear-powered transportation than a Starship conversion.