Philospher ponders lunar 3He mining

[james_davis_nicoll]

He's agin it.

Comments

[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.