NASA Studies Aerogel Fission Fragment Rocket 200X Better ISP Than Chemical

NASA NIAC phase 1 studies development of a nuclear fission fragment rocket engine (FFRE) that is exponentially more propellent efficient than rocket engines currently used to power today’s space vehicles and could achieve very high specific impulse (>100,000 sec) at high power density (>kW/kg).

Fission Fragment rockets are several proposed nuclear rocket designs made since the 1980s with the highest potential theoretical top speed. The fission-fragment rocket is a rocket engine design that directly harnesses hot nuclear fission products for thrust, as opposed to using a separate fluid as working mass. The designs can, in theory, produce very high specific impulse while still being well within the abilities of current technologies. The new aerogel design proposal seems to be the most buildable version yet. It should be smaller, cheaper and simpler to make.

Here is a video from 5 years ago on fission fragment rockets.

A previous proposal by Rodney L. Clark and Robert B. Sheldon theoretically increases efficiency and decreases complexity of a fission fragment rocket at the same time over the rotating fibre wheel proposal. In their design, nanoparticles of fissionable fuel (or even fuel that will naturally radioactively decay) are kept in a vacuum chamber subject to an axial magnetic field (acting as a magnetic mirror) and an external electric field. As the nanoparticles ionize as fission occurs, the dust becomes suspended within the chamber. The incredibly high surface area of the particles makes radiative cooling simple. The axial magnetic field is too weak to affect the motions of the dust particles but strong enough to channel the fragments into a beam which can be decelerated for power, allowed to be emitted for thrust, or a combination of the two. With exhaust velocities of 3% – 5% the speed of light and efficiencies up to 90%, the rocket should be able to achieve over 1,000,000 sec Isp.

Current proposed designs for Fission Fragment Rocket Engines are prohibitively massive, have significant thermal constraints, or require implementing complex designs, such as dusty plasma levitation, which limits the near-term viability. Ryan Weed and his team propose to develop a small prototype low-density nuclear reactor core and convert the nuclear energy stored in a fissile material into a high velocity rocket exhaust and electrical power for spacecraft payloads.

The key improvements over previous concepts are:
1. Embed the fissile fuel particles in an ultra-low density aerogel matrix to achieve a critical mass assembly
2. Utilize recent breakthroughs in high field, high temperature superconducting magnets to constrain fission fragment trajectories between moderator elements to minimize reactor mass.

The aerogel matrix and high magnetic field (B>20T) allows for fission fragments to escape the core while increasing conductive and radiative heat loss from the individual fuel particles. NIAC work will provide detailed mission analysis of fast transit to SGL for direct imaging and high-resolution spectroscopy of a habitable exoplanet at a distance of up to 100 light years. The FFRE propulsion system could provide delta-V to reach the SGL in less than 15yrs and provide the slowdown and maneuvering capability at SGL. The telescopes would act as a single pixel detector while traversing the Einstein Ring region, building an image of the exoplanet with enough resolution to see its surface features and signs of habitability.

Fission Fragment Concepts Review

Theoretically, there was the idea that a mature fission fragment rocket capability could have a 200 gigawatt system enabling a 10% of lightspeed travel to Alpha Centauri. The 200 gigawatt system is 200 times larger than most commercial nuclear fission energy reactors and it is about double the power of all nuclear fission reactors in the USA currently and about 40% of the nuclear fission reactors on earth.

The ultimate potential is there but the NASA NIAC study is lets make something much smaller and simpler.

16 thoughts on “NASA Studies Aerogel Fission Fragment Rocket 200X Better ISP Than Chemical”

  1. Just be careful to look in the rear view mirror when you hit full throttle, “With exhaust velocities of 3% – 5% the speed of light”!

  2. At the risk of displaying severe Dunning-Kruger effect, I have an idea. I commented before on what I see a very desirable fission reactor for space applications,

    So here’s a picture I drew.

    The basic idea is you use a molten salt with nuclear material in it held in a fuel tank, reactor/nozzle. The fuel tank, reactor/nozzle is spun to keep the molten reactor fluid in the container when heated up to a fluid state. In the picture, blue is the proton accelerator, yellow is the fuel and red the particles accelerating out the back. The advantages I see to this is long trusses can be built in space very lite. This means you could use electrostatic forces of low power and very long particle acceleration paths, and low stress on the truss, to end up with very high energies to start the fission. The reactor fluid itself could be used to capture neutrons, so reduce shielding requirements.

    Possible foibles.
    In the link on accelerator-driven subcritical reactor (ADSR) they use tungsten or lead as a spallation material for neutrons. Could Thorium fluoride salts be used? Or could the reactor fuel be lead mixed with uranium like Russian reactors? The goal I would think would be to have the fuel mixture act also as the spallation material for neutrons.

    Could the reactor bubble like boiling water throwing the reactor fuel out of the rotating container? Higher rotating speeds would maybe help this to not happen. In fact I’m not so sure how the Fission Fragment Rocket would not do something similar. Neutron embrittlement, eventually breaking off pieces of the aerogel and flinging them out the rocket. If the aerogel could work without this happening, then I would think a molten salt would also work.

    Another problem would be that all the lighter elements would be forced to the top causing the accelerated protons to not interact with the active part of the fuel. A possible fix would be to have another container higher, towards the center of the rotating fuel/nozzle, in which fuel is pumped from the far outer part of the container,(where the heavy elements would separate to). Any excess from the pump could just spill over into the main fuel holding tank.

    If you could use Thorium I would think this would be a great advantage. Start with a not so radioactive material when launched and react it in the drive.

    If this looks like it will work, I would like credit by calling it “Sam’s particle accelerated fission rotorocket”

    • I forgot to add I would rally like to see criticism of this idea, other than go kill yourself. Is it even in the ballpark of working?

  3. Glad to see this topic still stirs up some discussion 🙂
    We are planning a fission fragment extraction demo (@3T) using an MRI during off hours, Al vacuum chamber.. starting with alphas then Cf source to test FF range in various aerogels, damage, particle distribution, etc.
    Thanks for the write up Brian! – Ryan W.
    Ps alpha centauri and SGL are very different missions.. ISP for direct FFRE is probably too high for Mars , but too low for AC

  4. Glad to see this topic still stirs up discussion 🙂
    We are planning a fission fragment extraction demo (@3T) using an MRI during off hours, Al vacuum chamber.. starting with alphas then Cf source to test FF range in various aerogels. Thanks for the write up Brian! – Ryan w.
    Ps alpha centauri and SGL are very different missions.. ISP for direct FFRE is probably too high for Mars , but too low for AC

  5. Ryan Weed is the positron dynamics guy. Guess he’s moving on from antimatter.

    I’m kinda way too lazy to tabulate things for a mixture of aerogel matrix and unknown uranium/carbide/oxide spatial density, but I’m going to guess it would need to be quite large.
    The stopping power of aerogel for heavy ions is significantly greater than air, which will stop an alpha particle in a few centimeters. Not really seeing promising physics here aside from the ‘COTS’ 20T magnet /s.

    • Actually, these aerogels run to about a sixth or less of the density of air. The only reason they don’t float better than Helium is that the pores end up full of air.

      But you’d probably want to use something with a smaller critical mass than U235. (52kg.)

      Neptunium 236 would get you down to 7 Kg, and it’s still got a decent half life, (150K years) so it can be handled.

      Curium 247 also has a critical mass of about the same, and the half life is like 15 million years. I’d probably go with that, assuming it could be sourced.

      Then you wouldn’t rely on the aerogel to contain all the critical mass, you’d have a ring of fissile material around it to bring it to criticality.

      • Heavy ions won’t travel centimeters in the rarest gel…. stopping power is a very strong function of charge; 100MeV fission products have like +40 electric charge and are traveling much slower than 5MeV +2 charged alphas. A graphite pebble bed reactor has 0.05g HALEU/cc, is well moderated and still needs to be meters across the major dimensions to ‘buckle’ or support multiplication.

        A ring of fissile isotopes never before refined in quantity, arranged in a ring, will make 99% of power in the ring and drive the working volume with a small fraction of the isotropic neutron leakage. The ring is actually a shape employed for criticality safety in order to avoid criticality. The ring geometry would drastically increase fissile requirements.

        All that and we still have to assume the materials hold together under use… AFAIK the gel is kinda delicate.

        TLDR: it’s a lame idea. Hope Mr. Weed gets his funding tho. Fools and their money.

  6. Sounds promising; The dusty plasma approach is probably best for engines that are going for a high mass ratio, as you can swap out the propellant dust as you go. But this approach would be easier to pull off for low mass ratios.

  7. The one I really like is the salt water rocket, about 3%U235. Really dangerous, but it could be done.
    So the aerogel rocket doesn’t have a radioactive exhaust plume?

    • Depends. If you use a fission fragment rocket based on sustaining a chain reaction, yeah, the fragments are typically also radioactive. But you can do a fission fragment rocket based on radioactive decay that emits beta particles, like Strontium 90, and get a non-radioactive exhaust.

      Much lower ISP, though, with that approach.

      It’s basically a non-issue, though, given that you’re exposed to radiation just being in space.

  8. Anything that helps get telescopes to the distance we need to gravitationally lens the sun,I’m for. Superconducting usually means heavy, even with the new ReBCo tapes, as they use Km of tape for the magnets of fusion devices.
    I do love nuclear rockets. L:et’s go!

    • I like the “terrascope” idea. Use the atmosphere of the earth to focus the light. Performance is less, but the light detectors are in a location that is *much* easier to reach than the solar gravitational lens.

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