From: vince@offshore.ai (Vincent Cate) Newsgroups: sci.space.tech Subject: Re: Lunar Sample Return via Tether References: <9186edb5.0312061749.206011fa@posting.google.com> <9186edb5.0312101309.4509af66@posting.google.com> <3FD89AAE.5A716AE1@nospam.com> NNTP-Posting-Host: 207.42.133.230 Message-ID: <9186edb5.0312112210.6d51b0f8@posting.google.com> Andrew Nowicki wrote in message news:<3FD89AAE.5A716AE1@nospam.com>... > VC> You can make your tether shorter than 100 km, say 5 km, > VC> since the non-human payload can tolerate high Gs. > AN> True. > VC> This also reduces the chance of collision. > AN> False. The longer the lunavator, the safer it is. If you are going to say someone is wrong, it is nice to say why. So, why do you think that a longer tether is safer from collision? The odds of hitting a large object are not increased much by the thickness of the tether and they increase linearly with the length. Even on a simple swept area basis, a shorter cable is going to be safer. If you make a cable twice as strong and half as long, it is not twice as wide, only sqrt(2) as wide. > VC> The rotational momentum of even 1 Kg at the end of a > VC> 100,000 meter long tether is so huge that no momentum > VC> wheel will have any impact on it. > > Having a small momentum wheel is useful for fine adjustment > of the angular velocity of the lunavator. If you do the > adjustments with the winch only, the lunavator shakes > too much, and you have to use mini rocket thrusters. Do you have anything to backup either of these claims? > VC> The orbital momentum needs to be controlled by either > VC> leaving something on the surface of the moon of equal > VC> mass to what you are picking up, or using some kind of > VC> thruster. > > False! > > Pick up Moon dust, reel it in and drop it backwards. Ok, you can use the tether as a thruster with the regolith as your reaction mass. And this is fun, for sure. There are a couple issues, though it could be a reasonable thing to do. When you picked up the regolith it was going backwards relative to your center of mass about 1.6 km/sec. So you have to throw it backwards faster than this to get any net thrust. If you want to throw it back twice as fast then you are going to winch in like halfway first. So now you have twice the tip speed and half the length. Given that centrifugal force is v^2/r your tether now needs to be 8 times as strong (2^2/0.5). This is an issue, but it would not be as bad if you tossed slower. The second issue is that you can easily make all kinds of space junk that hits you or someone else on some future orbit. By picking your velocity to be something less than 3.2 km/sec backwards relative to the center of mass of the tether, then it will be less than 1.6 km/sec relative to the moon and fall to the moon. You might want to avoid hitting Apollo sites, or any man made objects on the moon. But this does not seem like killer problem either. Ya, I think it could work. Both issues get easier if you don't toss very fast. Since there is no shortage of mass, that is ok. Since you would not be limited by a fixed supply of xenon reaction mass, you could bring back more lunar regolith this way. It should take less in the way of solar power since the energy put into the reaction mass goes up with the velocity squared (1/2 m v^2). So you save mass on solar and on the xenon. So you can put more mass into making a bigger reentry capsule and bring back more regolith. I can imaging starting with 4,200 Kg in LEO from a Falcon-5 and bringing back a few times this mass in regolith. > There are many ways to loose > the extra angular momentum. Probably the cheapest one is > dragging an anchor on the dusty Moon surface. If you are rotating too fast, then you are going to be hitting the moon as you rotate (dragging sounds too gentle). It seems like it would be very hard on your anchor and tether. Winching in and out to use the gravity of the moon to increase or decrease you rotational momentum just has to make for a more reliable system. -- Vince