Monday 10 October 2022

Hypervelocity Tether Rockets

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


  1. Another source of waste heat that might be significant is the suspension damping the vibrations in the tethers. Perhaps you could damp them with some sort of linear motor and extract a bit of the power as electricity instead of just converting it all to heat, but there would still be some. Or perhaps having the counterweight tether also release a mass half a rotation later would help remove most of the vibration.

    Still, I imagine the vibration issues from the tethers briefly being unbalanced thousands of times per as they drop masses would be one of the main challenges. It might lead to fatigue issues, or power limits based on the max temperatures the tether material can with stand.

    1. I imagine we could design some sort of suspension mechanism that uses a magnetic pulse to push on the tether at the exact moment to counter the recoil of the propellant release. That way the tether never actually moves!

  2. Saucer shapes are good at lateral acceleration—thus Pye-Wacket. I have two interests.

    The Bola concept:

    Two bagged asteroids are connected by tethers, tightened up to produce artificial gravity—so that more conventional methods of mining might be brought to bear.

    The Flyby Rotorvator Skyhook:

    This would be backspun so as to lift very heavy payloads into orbit in one go. Rocket assist only to reduce strain on the tethers—-all done over the open ocean. Space platform yanked up, asteroid hunk rotates down in its place at near zero velocity.

    1. The Bola concept could work but asteroids are a bad target: most are literal dust piles that fall apart at the slightest push, so they would disintegrate under any useful level of gravity.

    2. Maybe not the metal ones. How about two tethered statites whose surface-anchored lines intersect each other in an X? The simple act of having the statite sails tack toward or away from each other gives you a scissor lift to space.

  3. Way cool. I suggest naming these devices after baseball pitchers.

    1. I'm sure someone could end up with a USSF Babe Ruth one day.

  4. Great article as always.
    I have a few suggestions that may improve the performance of this system / solve some issues.
    I suggest turning the tether into a disc. This would allow for continuous matter ejection, solving the problem of the vibration mentioned above by Massimo.
    As for the ejection of propellant, I would suggest getting inspired by home printer technology. One could perhaps use something similar to inkjet nozzles to precisely release microdroplets of propellant within a controlled time window. For solid propellants, LASER printers may provide a good template, due to their ability to release microscopic amounts of toner in a very precise and fast way.
    One problem that you may not have thought of, is the temperature dependence of the mechanical properties of the tether material. When operating in space, the tether may be exposed to a wide range of temperatures, which may impact its tensile strength, especially if it is composed of a polymer. I would suggest solving the problem the following way: make the “flying saucer”-like enclosure of the tether out of radiators. If the radiators operate at 60°C, as you suggest in your article, then the tether sandwiched in between will reach a thermal equilibrium slightly under that temperature. This also may kill two birds with one stone: by doubling the function of the tether enclosure as a radiator, we are saving mass.
    One weird quirk of the propellant feeding mechanism is: the easiest way to feed liquid propellant is probably by running a single (very thin) pipe down the length of the tether, turning it into a centrifugal pump. However, one must consider that propellants such as water or carbon dioxide turn back into a solid at the high hydrostatic pressures experienced at the end of the pipe. To solve this, one could perhaps have the propellant be vaporized by the waste heat of the motor and condense at the end of the tether. This would eliminate the column of liquid which creates the hydrostatic pressure. It would also help to keep the tether at a constant temperature and turn it into an additional radiator. Additionally, this allows the propellant to be used as a heat sink.
    As for the use of the tether for energy storage, I am not too sure of its utility. While I have not run the math, I suspect that the energy density of the tether will barely be competitive with modern Li-ion batteries and will be dwarfed by fuel cells. The main advantage I see with storing energy in the tether, is that it allows to “charge up” the tether at a lower power input. In your first example, you use a tether mass of 22.8kg for a 12,843kg spacecraft. Lowering the power allows to reduce the mass of the non-tether components and increase the mass of the tether. One could spin up the tether at a lower power level, over longer periods of time, and release small bursts of high thrust for Oberth maneuvers (without waste additional waste heat during the maneuver). I think this is what you suggest in the article? The key here is the lack of waste heat when energy is used from storage. It allows for all non-tether components to be reduced, while keeping the high peak thrust.

    1. Thank you.

      A disk would indeed maximize thrust. I think it would be most beneficial for lower velocity disks where the tether mass is only a small percentage of the total propulsion system mass (so increasing it barely affects average kW/kg but multiplies thrust several times).

      Having the protecting enclosure for the tether act as radiators is a great idea.

      I would not worry about water becoming solid at the tether tips. At 1 million g, a 1 mm height of water would experience a pressure of 9.81 MPa. That is far below the GPa pressures needed to turn water solid at room temperature.

      For your final suggestion, yes that is what I suggest, in less detail.

  5. There is a better electric motor made for electric planes at 47 kW/kg: SPM 236e Aero.

    In other news I have completely lost interest in space. The Limits to Growth and Tragedy of the Commons are here to collect their toll. There is absolutely no way human civilization could survive this. Apollo missions were done at peak civilization, when GDP and population growth were at record high. Space colonies will stay bong dreams.

    1. Dark.
      Also wrong.
      We are just at the beginning of the Space Renaissance.
      Starlink has proven to be the first "killer app", practically printing money while also requiring so much launch capacity that costs are gonna be pushed down an order of magnitude.

    2. Do you have a reference for this 'SPM 236e Aero' engine's performance?

  6. This is an amazing article and I hadn't heard of SpinLaunch before this, so thanks for that.

    I was wondering this type of spacecraft would be well-suited for voyages beyond Jupiter, like to Saturn and Uranus?

    1. If you have a power source which can operate that far from the Sun (basically a nuclear reactor) then yes, it is great for travel to the Outer Solar System.

  7. Great work as usual.

    If you are using turbine to convert solar or nuclear energy into electric would it be possible to skip conversion to electric part and have turbine power tether directly?

    Could pair of tethers be used for impact fission and fusion? Seems like fission wouldn't be much of a problem given velocities, but if memory serves me right fusion requires either precision impacts of finely manufactured pellets or velocities in excess of several hundreds km/s.

    1. That's an interesting idea. A 'direct drive' rotating tether with no electrical components could definitely work.

      A pair of tethers could ignite impact fusion if you use specialized collapsing bullets like those described in 'gun fusion' ( to reduce the impact velocity requirements to a level tethers could achieve.

  8. Hello Matter Beam,

    It sounds as if the technology for tethers is nearer-term than that for macrons. Is that correct?
    If both technologies were fully developed, when would it be most efficient to us one or the other?

    Thank You

    1. Yes, that's correct. Tethers are a technology that's already been demonstrated in space while macron accelerators are only specialized lab tools.

      If they are fully developed, then both would be used in different applications. For smaller deltaV requirements or if you don't have powerful reactors, tethers would be the better propulsion system.

  9. Much appreciated.

  10. I found an interesting twist on this concept on the Selenian Boondocks blog today, it was an old post about using rotating tethers to boost the exhaust velocity of rockets.

    It uses the same concept of flinging propellent from the tip of a rotating tether once per rotation, except it also had a rocket engine at the tether tip that further accelerate the propellent, so the exhaust velocity is the tether tip velocity plus the engine exhaust velocity. I suppose this could be considered another type of staged tether engine, where instead of the second stage being another rotavator, its a small rocket.

    Of the two a simple monopropellant thruster might honestly be an easier second stage to add to the end of a tether since it avoids the trickiness of transferring propellent across a rotating joint. The tether already acts as a giant centrifugal pump so the engine could just be a solid state reaction chamber and a nozzle.

    Another option the author considered a few years later was moving the propellent control valves to the housing and have them inject propellent droplets into the path of the tether, where they would slam into a cup shaped nozzle on the tether tip. The heat and pressure of impact would cause the fuel to react. The exploding fuel would then leave the nozzle with both the tip velocity and the nozzles exhaust velocity. The main benefit of this method is that it moves the propellent valves and plumping off of the tether so they are no longer subjected to huge amounts of g's. The tether is now just a cable with a metal cup at the end. Or a solid disk with multiple cups lining the rim.

    The main issue I see with the second method is wear in the cups, though perhaps they could be readily replicable, or even deliberately made of an ablative material to act as additional propellent. This would limit the length of a burn before you had to despin and replace the solid ablative propellent on the disk, but would dramatically simplify the pluming and control due to not having to pump the fuel inside the tether while under spin. And if you had pairs of rotors, the energy of despinning one for refueling could be used to respin its freshly fueled twin. Since this craft is already assumed to be dominated by power generation mass rather than engine mass, having multiple tethers for countering torques and eliminating refueling down time wouldn’t be too much of a burden.

  11. Fascinating article, as usual.
    Tether thrusters as you describe them here would basically be two counterrotating flywheels which made me think:
    How much of a problem would this engine pose for attitude control during burns. This still is a fairly low thrust engine and therefore would need to fire for long periods, probably with a very large and not very well balanced spacecraft attached to it. Would the wheels resist rotation in a specific direction?
    if the wheels are "vertical" with their axis of spin being "horizontal" then pitch would be free and easy but it seems like they would pose alot of resistance to roll and yaw.
    Cool concept and now i want it as a robonaut or thruster in High Frontier 4 All :P .

    1. The rotating tethers can act as Control Moment Gyroscopes and Reaction Control Thrusters simultaneously.
      To turn perpendicular to their direction of spin, you can 'twist' against the wheels and the recoil force will push you sideways in the opposite direction. To turn parallel to the direction of spin, just eject propellant at a slight angle up or down.

  12. I recall seeing a variation of this idea a long time ago as part of Gerald K O'Neil's space colonization project. The rotors were mounted on "mass catchers" that were at the Lunar L2 point to gather the bags of mined lunar regolith for processing.

    Since a lot of the project revolved around what we now call IRSU, the mass catcher would have been made from a fiberglass material derived from melted lunar basalt, and the spinning arms would also likely be made from this basalt glass material. There was no way the performance would match some of the ships being described. but then again, we are describing the equivalent of a cargo ship for carrying bulk ores. I don't recall any numbers for ISP, thrust or anything else.

    Glad to see you're back


    1. I'm sure using tethers as a mass driver for propulsion has occurred to many people before, but I doubt anyone thought of raising the Tether Mass Ratio into the millions!

      Bulk ores as both propellant and final payload could work. They would be cheap enough to justify throwing away 80% of every load shipped.

  13. Just a quick question (I suspect I know the answer already but...) Additional mass would be added but could high speed rotating disks with tubes drilled through them radiating out from the core achieve similar thrusts while reducing the chances of catastrophic tether failure.

    1. Yes, you could use whole disk instead of a single tether. Thrust-to-weight ratio would be identical.

  14. Put two of these engines together, 90 degrees apart, near the center of mass, and you can rotate along 2 planes by clutching to them like a flywheel. In addition, if you leave the sides clear, you can decelerate at 100% thrust and translate left/right and up/down at 50% max thrust simply by changing where the tethers let their load go.

    All you need now is a flywheel/RCS thrusters to allow you to rotate in the third plane and you have pretty darn maneuverable spacecraft (if your assumptions on weapons and thruster power make maneuver relevant).

    That's on top of what looks like getting nuclear engine Isp without the hassles of building a nuclear engine - a normal, regular, well understood reactor is all you need - along with being able to trade exhaust velocity for exhaust mass to shift between 'efficient-acceleration' and 'high-acceleration' modes. Now, instead of needing to shove nuclear engines and the reactor off some place far away (both heavy items) and shield them from each other, you just need to deal with the reactor alone.

    With the icing on top that you can use pretty much *anything* for reaction mass.

    Another possibility - a hubless tether. Where the launcher arm is attached to the hub and the motor spins the hub. Now the launcher arm is under compressive force rather than tension but you have way less torque required out of the motor to get the whole thing spinning since your spinning it from the rim instead of the hub. Probably going to be higher mass I would imagine - but that's ok if you want to use it as a flywheel to store energy or turn against.

    1. The problem with that last hubless tether suggestion that most materials are far weaker under compression than under tension. This severely reduces their strength-to-weight ratio, which in turn exponentially increases the Tether Mass Ratio.

  15. I am totally uneducated in physics so pardon my ignorance, but am curious how can tethers be unsuitable as weapons if they are too inaccurate with their propellant but still be accurate enough to function as mass drivers, striking and propelling spacecraft from mass distances?

  16. long distances, I should say

  17. This type of propulsion system could allow for human-powered spacecraft. The performance won't be the best but an athletic human can produce about 2,000 watts of power in short bursts. Keep in mind that human-powered aircraft and even helicopters have been done before. With an athlete applying force to a gear and or pully system connected to counter-rotating tethers, thrust can be generated. But not much, cyclists can only pedal at just over 100 rpm in short bursts so a high gear ratio is needed to spin the tethers at sufficient speeds. I'm sure this concept can be vastly improved upon, but I just thought of an interesting application for this particular mode of propulsion.

    1. Sure, a human could move a spacecraft, but remember than even the most moderate of orbital changes require energies that humans cannot reasonably deliver.

      For example, a 1000 m/s deltaV will require at least 500 kJ for each kg of a spaceship. A 75 kg human inside the tiniest 1000 kg spaceship would need at least 537.5 MJ to make this move. That's Tour de France level of effort on a 300 Watt bike for 20.7 days.

    2. Good points, and yeah I was envisioning something closer to a spacecraft equivalent to a bicycle. Something that can take people from one part of a large artificial habitat to another, or simply for recreational purposes. Looking at my previous post I didn’t make that clear so that’s on me. Thanks for the response though, happy holidays.

  18. Sorry if this is not the right way to contact you but NASA's 2023 Phase 1 NIAC selections are out and there are 2 ideas I thought you might like to comment on. The first (as a propulsion system or a potential weapon) is a proposal for Pellet Beam Propulsion . The second is for a new type of Bimodal Nuclear Thermal Propulsion (with wave rotor 'topping' cycle).

    1. Hi! I am mostly active on twitter and the ToughSF Discord these days.
      I have seen the NIAC selections. I haven't looked into the pellet beam propulsion much but I did comment on the wave-topper rocket as being too good to be true.
      Thread with calculations:

  19. A bit of a late comment, but I remember reading this article last year, a couple months ago a complimentary use for a tether popped into my head. After watching the Blue Origin & Virgin Galactic launches, it occurred to me that a pair of tethers might work for achieving low earth orbit.
    First a ground-based launch tether imparting vertical speed to get payload above the Karman line, then a second space-based tether to impart the angular momentum needed for orbital insertion or to change perigee or apogee.
    An interesting side benefit of storing angular momentum in LEO would be that collecting space debris might become an economical method to 'mine' angular momentum of space junk.
    Collect space-debris moving at orbital velocity, us a tether to transfer orbital momentum from to simultaneously put the payload into orbital velocity and deorbit the debris so that it burns up.

    1. That might work but it would be difficult... if you had two tethers and evenly divided the launch to orbit between them, you'd need the ground tether to deliver ~1 km/s upwards velocity and 3.6 km/s horizontal velocity, and the space tether would need another 3.6 km/s horizontal velocity meeting the payload at the top of its arc.

      That's close to the limit of performance of existing materials without serious tapering. Also, you'd be subjecting the payload to a passage through the lower layers of the atmosphere at Mach 13!

      Then, there's the space junk momentum. The first trouble is that it's already mostly moving in the same direction you are trying to launch rockets. So I don't see where you can extract momentum. Second, there's just not that much of it up there. Yes, there's over 10,000 tons in space, but most of it is active and not junk.

    2. Agreed, hypervelocity is a bit of a problem for spin launches. I'm reminded that a major cause of meteor breakup is that the atmosphere can't get out of the way fast enough, so you end up with a vacuum-tunnel behind the reentering body, and adiabatically created hyper-compressed plasma ahead, creating incredible stress because of the pressure differential.
      Guess it would be fun to consider a 'project blumbob' pre-payload (perhaps an expendable ablative counterweight, released just before a payload to create a transient low-pressure tube for the payload to follow.