From: vince@offshore.ai (Vincent Cate) Newsgroups: sci.space.tech Subject: Re: Lunar Sample Return via Tether References: <9186edb5.0312061749.206011fa@posting.google.com> <9186edb5.0312080237.2e387357@posting.google.com> NNTP-Posting-Host: 207.42.133.230 Message-ID: <9186edb5.0312081938.42c2e0e4@posting.google.com> henry@spsystems.net (Henry Spencer) wrote in message news:... > In article <9186edb5.0312080237.2e387357@posting.google.com>, > Vincent Cate wrote: > >...with the momentum > >from 1 Kg of xenon we can pick up 98 kps / 1.6 kps or about 61 Kg > >of regolith. If you simply deposited equal mass on the lunar surface > >you would only pick up 1 Kg for every 1 Kg you put down. So this way > >is cheaper for a probe on a sample return type mission. > > Yes and no and maybe. It uses less fuel, but that does not necessarily > equate to "cheaper". The issue is not fuel cost -- xenon is one of the > few fuels whose cost actually *is* comparable to LEO launch costs, but > even so, that's a relatively minor issue here -- but hardware costs and > complexity and reliability. I should not have said "cheaper", I really meant "a better business case". If you did not use a high ISP thruster, xenon, or lots of solar cells, then you could clearly make something that was cheaper. However, without a high ISP thruster the deltaV from LEO to LLO (low lunar orbit) and the deltaV from LLO back to Earth each reduce your mass by some factor, maybe a factor of 10 together. So on top of the factor of 61 above we have reduced our return by another factor of 10 for a total reduction factor of 610. So if we had 6,100 Kg of returned regolith in the high ISP method, now we have 10 Kg of regolith to sell for profit. It may not be exactly this bad because you save something on solar Kg too. But whatever the exact number, but I don't think it would be better business. If we have a high ISP thruster for getting to the moon and back, then we might as well use it to reboost momentum after we do a tether pickup. It may well be that a Hall Thruster makes more sense than an Ion Drive because it is cheaper, needs less solar power, (and lasts longer?). I can can not say what the ideal ISP or type of thruster is just yet. But I am convinced that some kind of high ISP thruster makes sense for this type of mission. -- Vince