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Comment by tomswartz07 | Hacker News Reader

Realistically, any form of power generation could be used in space. It's really only a question of whether it's financially and practically feasible. I've been interested to see if Liquid Flouride Thorium Reactors (LFTRs) ever become a thing.

From what I've read about LFTRs, they can produce a whole lot of power- the most efficient system produces 1000-MW(e) from 700kg of fuel. That lasts approximately 12.5 years, give or take.

Obviously, thats on an Earth-based system and most spacecraft probably don't need anywhere close to 1GW.

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Pu-238 has the highest energy density which is why its so useful (both as a weapon and an energy source). Its the cheapest to use because it adds the least amount of weight to the launch vehicle.

238Pu doesn't have anything to do with nuclear weapons. It is a highly unwanted contaminant of weapons-grade Plutonium.

Yes totally correct, its Pu 239 that's weapons grade. I got mixed up.

> "Realistically, any form of power generation could be used in space. It's really only a question of whether it's financially and practically feasible."

Are you referring only to nuclear power sources? Because a literal reading of your comment ("any form of power generation") is obviously silly (e.g. steam power). And questions of financial and practical feasibility are two of the most important considerations for nuclear power systems, so one shouldn't dismiss those concerns so easily.

Yes, good point. That's what I meant.

Closed loop steam power is actually not that unreasonable in space.

Concentrated solar cells have been considered for a range of space operations. They generate a lot of waste heat, which opens the way for a range of heat engines. Currently, non concentrated solar is a clear winner but serious effort went into studying combined systems.

Long term several space mining concepts involve using concentrated sunlight to heat and refine asteroids. This would tend to make quite a bit of waste heat and steam power is one method to utilize that waste heat.

Well, "any" is a very broad definition

As long as it doesn't have any (constantly) moving parts, doesn't have requirements relating to pressure or radiation exposure, works in a variety of temperatures, it could be used.

Solar panels and RTGs fit the bill today. Maybe the ASRGs have moving parts, given their name?

Solar panels don't fit the bill for missions far from the sun.

Thorium reactors may be useful on earth. But the hype is misplaced for space probes. Either they need maintenance (extracting fission products from the fuel) or closed cycle processing which makes them quite bulky (by space mission standards, not by nuclear reactor standards).

RTGs and stirling engines are fairly lightweight and compact. reactors provide more power per weight, but they have a much higher minimum weight.

And molten salts aren't exactly the most hassle-free technology there is. they are highly corrosive, so you have to compromise on the materials you use and it also strongly limits their lifetimes. RTGs on the other hand often operate long beyond their intended lifetime, albeit at reduced capacity.

I think the issue with Stirling Engines is the fact that they have moving parts that could wear and break down.

Most (all?) RTGs are for all intents and purposes, solid state.

Yes, but I was comparing to molten salt reactors. Stirling engines are still much simpler than reactors, even if they are slightly more complicated than RTGs. AIUI they already have research models that are capable of running for decades without maintenance.

I worked for about a year and a half on the ASRG project, specifically doing reliability work for the Stirling engines. They had a bit of a ways to go to prove out the system (its a tricky problem trying to "prove" that your device will last 17 years), but they had made a whole lot of progress. The biggest problem for the project though was that the ASRG wasnt being designed for any one mission, so its performance requirements were a bit of a moving target.

>I think the issue with Stirling Engines is the fact that they have moving parts that could wear and break down.

In NASA's ASRG design there is a stirling generator with only has one moving part: a free piston moving inside a coil, cushioned by the working gas. Wear and tear should be minimal, even for a multi-decade mission.

https://solarsystem.nasa.gov/rps/docs/ASRGfacts2_10rev3_21.p... (page 2)

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