Aerogel Core Fission Fragment Rocket Engine | Hacker News Reader

Aerogel Core Fission Fragment Rocket Engine

nasa.gov

42 pointshamilyon217 comments

For those wondering, SGL is a "solar gravitational lens," a point about 542AU from the Sun (75 hours away at the speed of light) where one could park a relatively simple telescope to observe exoplants using the sun as a lens.

https://en.wikipedia.org/wiki/Solar_gravitational_lens

Its quite brilliant... if you can get a detector out there and maneuver it. Especially if you can slow it down once it gets there.

Fission fragment rockets are also an old idea, and a good one: https://www.projectrho.com/public_html/rocket/enginelist2.ph...

> All of the other nuclear thermal rockets generate heat with nuclear fission, then transfer the heat to a working fluid which becomes the reaction mass. The transfer is always going to be plagued by inefficiency, thanks to the second law of thermodynamics. What if you could eliminate the middleman, and use the fission heat directly with no transfer?

> That what the fission fragment rocket does. It uses the hot split atoms as reaction mass. The down side is that due to the low mass flow, the thrust is minuscule. But the up side is that the exhaust velocity is 3% the speed of light! 9,810 kilometers per second, that's like a bat out of hell. With that much exhaust velocity, you could actually have a rocket where less than 50% of the total mass is propellant (i.e., a mass ratio below 2.0).

Last I heard of it you dont need to slow it down, the concept starts working at SGL distance, but it works fine beyond that. In any case at that distance you need a chain of communication relays which themselves can carry telescopes pointing at the sun.

> 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.

I'll be dead before any results, but how great Nasa are seriously considering reducing such a sci-fi thing to practice.

More details in a poster here[0], a video presentation[1], and a paper[2].

The basic idea is to more efficiently use fission energy. When atoms split in fission, they fly apart VERY FAST. If these could be directed out, fission energy could more or less directly be converted to thrust rather than being inefficiently converted to heat and then thrust. This higher efficiency could result in smaller faster spacecraft, the proposed reference design for carrying a capsule to Mars in 90 days is surprisingly small[0]. It also has a combination of relatively high thrust(80 KN) and Isp(2000s).

Now that's if it works. Fission fragment rockets have been very challenging to make, because it's really hard to actually get the fast moving fragments out and direct them. The fission fuel has to be very thin, so the fragments can escape and high magnetic fields need to be used to direct them out. But so far this approach to fission fragment rockets appears relatively promising.

[0]https://custom.cvent.com/216E523D934443CA9F514B796474A210/fi...

[1]https://cvent-production-cs-creative-prod-us-east-1.s3.amazo...

[2]https://www.researchgate.net/publication/373793565_Radiation...

Fogbank lives!

FYI one theory is that the plutonium used in h bombs is embedded in aerogel. That way it’s stable yet easily implodable.

This came to light a few years ago when it came out that the DOD forgot how to do it.

A few more details here

https://phys.org/news/2023-02-extremely-lightweight-fission-...

(although still pretty vague)

You might like <https://www.frontiersin.org/articles/10.3389/frspt.2023.1197...>. Same thruster, but scaled down for a trip to Mars and back. Still not very detailed, but not nearly as vague.

The heading with 6 nouns in a row is amusing)

Skeptical of the superconducting magnet component. Main issue I see (when I used to think about fission fragment designs) is that some statistical fraction of the fragments are going to hit your superconducting wires, introducing defects in the material that may kill the superconductivity, introducing heat that needs to be dissipated to or it will also kill superconductivity.

The aerogel approach is novel though so excited to see what they come up with.

There’s a layer of material between the fission fuel and the superconductors, same as the inside of a nuclear reactor. It has to withstand not just fission fragments, but also the infrared from the rather hot fuel pellets in the core. An earlier paper (<https://www.frontiersin.org/articles/10.3389/frspt.2023.1197...>) estimates that for a ship designed to go to Mars and back it would have to withstand 20MW/m² just from that alone.

A fusion mockup with molten fissile core ejecting the bottom? Genius.

But why Aerogel? Is it easily compressible?

From the poster that gene-h linked:

> Why Aerogel?

> - Individual fuel particles must be micron sized to permit fission fragment escape

> - Accumulation of critical mass of micron dust particles presents a challenge (levitation not feasible)

> - Embedding in solid material could solve this, but need very low density to allow FF escape

> - High temperature, low density, IR-transparent aerogel is an ideal matrix for fission fragment engine core

Thanks!

So basically the fragments are escaping through the aerogel.

Yep. Then, because they are electrically charged and inside a magnetic field, they are accelerated out of the core and become propellant. If the magnetic field is strong enough, it seems they can reach quite a high speed. 5% of the speed of light even. It would be pretty cool to go to Mars and back on 20kg of fuel.

We need a new High Frontier thruster card.

We would need actual specs of the engine to be sure, but mechanically a 6⋅⅛ thruster could do the job easily. A 6⋅¼ might squeak in under the wet mass limit. Probably better to split it between thruster and reactor cards though; it’s a little bit like the Vortex Confined thruster + the VCR Light Bulb reactor, but rather more efficient since that only gives you 6⋅1.

In fact, the 100,000sec ISP figure they quote is insane; it’s an exhaust velocity of around 0.3% of the speed of light. That really makes it a 6⋅0 thruster, in game terms (or a 6⋅1⁄25 if you want to be precise, but none of the other cards are that precise). That puts it in the middle of the pack for the end–game TW thrusters, which is very impressive considering it doesn’t use fusion; the game’s GW and TW thrusters are almost all based on fusion. And apparently it doesn’t even need to be manufactured in space.

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