From: vince@offshore.ai (Vincent Cate) Newsgroups: sci.space.history,sci.space.policy Subject: Re: $5M Moon Rock Stolen From Malta Museum References: <373a0bc9.0405211458.bde154a@posting.google.com> <9186edb5.0405260624.b0faebd@posting.google.com> <4d6b07f3.0405261021.5663547f@posting.google.com> <9186edb5.0405270445.7b321552@posting.google.com> <1085688231.56339@haldjas.folklore.ee> <9186edb5.0405272142.48605589@posting.google.com> <1085752590.703258@haldjas.folklore.ee> <9186edb5.0405282143.7ee3dc3@posting.google.com> NNTP-Posting-Host: 207.42.133.252 Message-ID: <9186edb5.0405291235.5fd3169@posting.google.com> Doug... wrote in message news:... >Vince: >>Sander: >>> Vince: >>>> As I said about costs above, "... it is clearly hard to say". But >>>> a tether to lift 5 Kg from the moon only needs to be like 15 to 30 Kg. >>>> So the system does not need to be really huge. >>> >>> Maybe you should start doing some numbers then? Like say how much a 100 km >>> spool would weight? >> >>It would probably only be 10 or 20 km long. I am happy that a few people >>understand the idea and agree it seems interesting. Trying to get people >>to agree to what a spool weighs is not something I care to do. >[...] >I'm very, very surprised that you say your simulator takes all of these >things into account, and yet you can't come up with any kind of baseline >for the weight of the tether. I thought Sander was asking the weight of an empty spool. The "15 to 30 Kg" was the mass of the tether. This is using "Spectra-2000" as the tether material (a commercial product). I don't know the weight for the winch or other parts. And depending on the mass of the scoop the tether might need to be heavier. But if things get out of hand you could always design for 3 Kg or even 1 Kg of regolith at a time instead of 5 Kg. I have a nice graph showing the required ratio of tether mass to payload mass for different tip speeds at: http://spacetethers.com/tethermassratio.gif The mass of the tether depends on the tip speed and safety factor, but to a good approximation the length does not matter. >Also -- you say you think you only need a tether that's 10 to 20 km >long? That means that the center of mass of the orbiting tether must >have a perilune of between 5 and 10 km, right? No. I am thinking the probe could be 1,000 to 2,000 Kg and we are picking up between 1 Kg and 5 Kg at a time. So the center of mass is far closer to the probe. So for a 20 km tether the closest the probe would get to the moon is 20 km and the center of mass is only slightly closer than this. > Now, even a satellite >orbiting the Moon at 100 km will see its orbit deteriorate (such that it >will impact the surface) after only a few weeks, due to the rather >extreme uneven nature of the lunar gravitational field. (See what >happened to the Apollo 16 particles & fields subsatellite, if you don't >believe me.) Just how do you propose to continually re-shape the >tether's orbit to keep it in orbit? You could just use a hall-thruster/ion-drive to keep fixing things up. Probably would use only the electric thruster for the first sample tossed back, just to keep things simpler. But more fun is to pickup extra regolith, winch in partway to spin faster, and throw it backwards like rocket exhaust, to regain momentum. Since we will probably toss back multiple samples, this could be tried on the same mission. But even with a "regolith-thruster" you would want some kind of electric thruster to let you fully control things. >Won't that limit the trajectories on which you >can place your collected lunar materials, possibly in such a way to make >it impossible to send your lunar materials where you want them? You are limited in the trajectories you can throw to. But you can set things up so that you can throw things back to Earth. The regolith pickup/toss is balanced by more regolith pickups with tosses in the other direction. The fine control comes from your thruster. -- Vince ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Vincent Cate Space Tether Enthusiast vince@offshore.ai http://spacetethers.com/ Anguilla, East Caribbean http://offshore.ai/vince ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ You have to take life as it happens, but you should try to make it happen the way you want to take it. - German Proverb