From: vince@offshore.ai (Vincent Cate) Newsgroups: sci.space.policy Subject: Re: Settle the moon first using tethers References: <9186edb5.0404210736.308d9697@posting.google.com> <9186edb5.0405061618.147d85b4@posting.google.com> <2g27hrF3g1vtU1@uni-berlin.de> <9186edb5.0405072126.dffecca@posting.google.com> NNTP-Posting-Host: 207.42.133.252 Message-ID: <9186edb5.0405100928.9801e8c@posting.google.com> Joe Strout wrote in message news:... > There's something I don't understand about momentum-transfer tethers. A > tether is not a rigid structure -- it's a flexible rope. So why, when I > attach a stationary load to the end of it as it comes by, does it lift > this load into orbit, instead of simply winding itself up like a yo-yo? You don't "attach a stationary load". The payload matches velocity with the tip of the tether before it attaches. Note that this can be a much lower velocity than the center of mass of the tether. If the payload is gaining momentum it will be attaching when the tip of the tether is going backwards relative to the orbital motion and then releasing when going the same direction as the orbital motion. For example, imagine a LEO tether where the center of mass is moving at 7.7 km/sec and the tip is rotating at 3.7 km/sec relative to the center of mass. A payload coming from a suborbital rocket would have to be going 4 km/sec to match velocity with the tip and then attach. When it releases it could be going 11.4 km/sec. It can gain 2 times the tip speed. Some moon rocks coming the other way could attach to the tether at 11.4 km/sec and release at 4 km/sec. To visualize what is going on I recommend that you run my Java applet at http://spacetethers.com/spacetethers.html Even in a Space Elevator you are not really stationary when you attach. A Space Elevator is really a rotating tether that rotates once per 24 hours, which is also once per orbit. When you get on at the equator you are really going about 1000 Mph, since the surface of the Earth spins about that fast at the equator. Just for the record, existing materials make such high delta-Vs not practical today, and maybe for a long time. For Spectra-2000 the mass ratios as a function of delta-V and safety factor looks like: http://spacetethers.com/tethermassratio.gif -- 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