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advancedatheist comments on In memory of Leonard Nimoy, most famous for playing the (straw) rationalist Spock, what are your top 3 ST:TOS episodes with him? - Less Wrong
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Comments (40)
Some people, like Keith Henson, argue that we've blown the thermodynamic opportunity to get off planet because we've already squandered the best quality fossil fuels.
This seems very strange. First, most of our rockets use hydrogen and oxygen. One doesn't directly need to use fossil fuels. Second, there's still a massive amount of energy available in terms of fossil fuels, it just involves lower energy return on energy investment. Third of all, there are many other sources of energy, with nuclear power being one of the more obvious, but massive amounts of solar and wind also being available. Fourth of all, rockets are comparatively inefficient in general since one needs to move most of the fuel itself. Launch loops and space elevators are both obvious substitutes that are more energy efficient once they are off the ground. Do you know where Henson has made this argument and what his reasoning was in more detail?
I agree with all your points except the fourth. Once outside the atmosphere and up to speed, rockets are actually pretty efficient. The space shuttle main engines achieved 15% propulsive efficiency if I remember correctly; this is better than many land vehicles.
I've studied launch loops and they have massive power requirements, on the order of magnitude of heating up the atmosphere significantly (up to a degree or so). Space elevators can be efficient, if the power delivery system is efficient, and so far all workable proposals (laser power beaming, conductive cables) seem extremely inefficient.
Rockets are in many ways the ideal space launch system, the major disadvantage being enormous unit cost.
Yes, but getting out of the atmosphere is really inconvenient.
Do you have a citation or a back of the envelope for the statement about launch loops having such large power requirements? I don't think I've seen that before, and I'd be interested in seeing that.
I don't remember exactly where I read that, but here's a page with some power calculations: http://launchloop.com/LaunchLoopHeating
100 billion tons/year may sound like a lot but you really do need to launch that much cargo in order to recuperate construction and operation costs. The 0.1 degree heating cited on that page is a bit optimistic since it doesn't take into account secondary effects.
Ouch. Yeah, that's not reasonably viable.
What, exactly, is the claim?
Is the claim that launching a ton of payload by launch loop heats the earth more than launching a ton by rockets? That is very hard to believe. To a first approximation, the energy savings of the launch loop are heat savings.
Or is the claim that launching enough payload to make a launch loop worthwhile is too much heat, either way?
Good point. Hmm, now I'm confused.
I don't think Keith Henson refers to the energy expenditure of rockets but to the total cheap energy evailable to global society. Currently we are more concered with optimizing the last out of the reamining sources and keeping society running at all. Granted there are still significant technological improvements but these are mostly small scale. Rockets have not improved at all.
My second and third point directly responded to that interpretation of the claim.