Philospher ponders lunar 3He mining

[james_davis_nicoll]

He's agin it.

Comments

[mmcirvin.livejournal.com]

After the Gerard O'Neill vision of an L5 colony/solar-power-satellite economy went out of fashion, fans of large-scale space expansion needed some kind of other justification for why people should do it. Mining helium-3 from the Moon emerged as one of the favorites, even though it doesn't make any sense on its own terms for a variety of reasons.

The space-fan party line is that this is a better and cleaner form of nuclear fusion than deuterium-helium, which is not clear in itself (and of course nobody's taken either type to the point of practical power generation), and that it makes more sense to get 3He from the Moon than to get it on Earth, which it doesn't.

[(Anonymous)]

It's cheaper to manufacture Helium-3 from tritium here on Earth; and if we had fusion power (which we DON'T) it would probably also be cheaper to scoop-mine it from the atmosphere of a gas giant.

It's another stupid marker.


Doug M.

[neowolf2.livejournal.com]

D-3He is the next easiest fusion fuel combination after D-T. By "next easiest" I mean "50 times less reactive". But it also produces a much lower fraction of its energy output in neutrons, especially if the tritium from DD side reactions can be removed before fusing, which helps solve one of the probable showstoppers for DT fusion (the first wall turning to shit due to every atom in it being displaced like a billiard ball 100 times a year.)

Manufacturing 3He on earth with fission reactors is a non-starter for enabling a "fusion economy". Each excess neutron from a fission reactor comes along with (optimistically) 100 MeV of fission energy output. This would enable the production of a single 3He atom, yielding just 18.6 MeV. So fusion would always be a minor adjunct to fission. In that case, why not just use fission?

3He breeding would be possible in DD reactors where tritium could be rapidly removed and allowed to decay. The levitated dipole is an example of this. Unfortunately, the US LDX experiment had its funding terminated in Nov. 2011 to focus on tokamaks. Not that it's anything to get too upset about, since even it would have been a long shot in the competition with more mundane energy sources.

Very long term, if they discover a moderate sized planet out beyond the Kuiper belt (say, Mars-to-Earth sized), it might be a good place to mine 3He, if the temperature at the planet's exobase was low enough to retain 3He in its atmosphere.

[mindstalk.livejournal.com]

100 Mev... heh. Oops.

But there could be other neutron sources. Spallation, say. Or fusors, though I'm doubting it makes energy sense to force D-T reactions to make neutrons to make He3 with, again unless one is specifically manufacturing rocket fuel.

[neowolf2.livejournal.com]

Spallation costs maybe 60 MeV of energy per neutron produced. And the best spallation sources are of fissionable materials, where most of the neutrons end up coming from secondary (subcritical) fission.

Spallation reactors might make sense for destroying higher transuranics (curium, etc.), which tend to have so few delayed neutrons when fissioned that you cannot operate a conventional reactor using them (far too much risk of going prompt supercritical -- BOOM). But if you're extracting and concentrating the transuranics for destruction, it's probably easier to just package them up and shoot them into space instead.

Fusors are fun toys, but cannot work as an energy source. Even as neutron sources, they really suck.

[mindstalk.livejournal.com]

What about Bussard Polywell?

Given existing rockets I have trouble seeing that as cheaper or safer than burnup.

Are you expert enough to comment on externally driven fusion rockets like Longshot? E.g. instead of trying to contain and extract power from fusion, simply using e.g. fission power to initiate fusion pulses that squirt out the way plasma wants to? I'd guess worst case it converts fission power to high impulse exhaust, best case you have net fusion energy dominating the exhaust and it's easy because you're not fighting the plasma. Mostly I'm wondering if it's as handwavy as everything else or if it's something we'd have a good chance of building if we wanted to.

[neowolf2.livejournal.com]

Polywell is bullshit. It's a sterling example of Asimov's corollary to Clarke's first law.

I don't know if Bussard was a con man in his old age, or just senile, but the entire affair was just deplorable. You will notice it has gone absolutely nowhere.

A spallation reactor burning a ton of transuranics a year would cost northward of a billion dollars. The current cost of launch a ton into orbit, on today's launchers, is maybe $5M (Falcon 9), and is projected to drop to about $2M on Falcon Heavy. Yes, shielding and a safety reentry package would be needed, as well as some means to boost beyond Earth orbit, but then these costs SHOULD continue to drop in the next few centuries.

I don't know anything about Longshot, but mixing heavy elements into a fusion plasma sounds like a tremendously bad idea, since they would cause fierce loss of energy to radiation (especially if they were only partially ionized, as they would be in a fusion plasma.)

[mindstalk.livejournal.com]

Why is it bullshit?
I keep hearing it needs $200 million for a proper experiment, so I don't know if "not going anywhere" means anything. One could conclude fusion in general is BS.

Boosting beyond Earth orbit is expensive, and I don't know why rocket costs should fall faster than reactor costs. Assuming between safety binding and GEO costs you get a factor of 3, that's $15 million per Falcon 9 ton. Granted, still compares decently to a billion dollar plant with even a 66 year lifespan. OTOH some of this stuff is really toxic and you really don't want it entering the atmosphere in a failure mode...

There's no mixing of heavy elements. A conventional fission reactor provides the electricity to cause fusion reactions to squirt out the rocket.

[neowolf2.livejournal.com]

The description of how Polywell was supposed to work never made any physical sense. It was never subject to scrutiny in the peer-reviewed literature (this should be a screaming red flag, btw), or if it was, it failed to pass muster for publication. The results they did report (in non peer-reviewed venues) were very inadequate to support any sort of optimistic assessment, or to bolster the case on how it was supposed to work. Conventional fusion, even if it is impractical, is subject to scrutiny to keep the players more or less honest.

Rocket costs should fall faster than reactor costs because we are building a lot more rockets than we are reactors. Generally, small things improve faster than large things. For the same reason, distributed energy systems (gas turbines, wind turbines, PV) are improving faster than large fixed baseload plants.

As for Longshot: so the fission part was just a distraction? Ok. That's basically "let's pretend we have a pulsed fusion reactor with mediocre Q". Not sure what that really buys you anyway, particularly if it's a DT reactor and 80% of the energy comes out as neutrons, which are pretty much useless for propulsion.

[mindstalk.livejournal.com]

Fission is where the driving energy is coming from but yes, the detail is unimportant. Like Daedalus, it envisioned D-He3.

"pretend we have a pulsed fusion reaction". Well, I guess that's what I'm asking about. I know that in a lab sense making fusion is easy; getting useful energy out sustainably has been the hard part. But what if you don't care about the fusion plasma being self-sustaining and happily let it escape? I envision it buying you exhaust at fusion fuel energy density and particle velocities, which is as good as we can hope for in an interstellar ship, short of antimatter or giant beams.

[neowolf2.livejournal.com]

This approach doesn't buy you much. The problem is the fission reactor will require large radiators to get rid of the waste heat, or will require a large mass flow to keep it cool. In the former case, the specfic thrust is bad, in the latter, the Isp will be bad (even if that mass flow is then heated by the fusion reactor output). So I don't really see what this is buying you, for the complexity/mass of having to add a fusion reactor.

[mindstalk.livejournal.com]

Well, given lack of progress in fusion power, any near-term interstellar ship would need fission for its primary power source anyway. So then the question becomes how to turn that into thrust: ion drive, heating up gas with lasers, lightbulb... Longshot sounded like a fusion afterburner as it were, compared to just hot gas.

The logic is that we're not choosing between fission-fusion and fusion, we're choosing between fission and fission-fusion.

[neowolf2.livejournal.com]

The system as described has, at high Isp, far too little thrust to be useful for interstellar travel. It's even of questionable value for the outer solar system.

Interstellar travel in reasonable time requires power/mass (in the vehicle) far beyond anything we can build today.