From: vince@offshore.ai (Vincent Cate) Newsgroups: sci.space.tech Subject: Re: Reentry without ablation or ceramics? References: <9186edb5.0305121517.3f7bfc4f@posting.google.com> <9186edb5.0305152010.d27c97@posting.google.com> <9186edb5.0305192235.3607d942@posting.google.com> <9186edb5.0305201423.6ad24a0e@posting.google.com> NNTP-Posting-Host: 207.42.133.230 Message-ID: <9186edb5.0305211726.5e689c2e@posting.google.com> soup@penrij.uucp.jtan.com (John R. Campbell) wrote in message news:... >Dumb question: Assuming you have enough altitude and a long >enough tether, couldn't a tether be used to effectively lower >an entry vehicle's velocity to below hypersonic speeds? Good question. The thing that keeps you from just making longer and longer tethers is that they have to support their own weight, not just the payload weight. With the real materials available today you have to design with a taper where it is thicker near the top and gets thinner near the bottom. Most of the thick part is really just supporting the weight of the tether below it. For either a rotating or hanging tether what really matters is the tip speed relative to the center of mass (note that a "hanging" tether rotates every orbit). So for a 2.6 km/sec tip speed and a safety factor of 2, using Spectra-2000 as your material, you end up needing a tether that is like 50 times as heavy as your payload. This mass ratio is an exponential function of the tether tip speed. So if you go to 3.2 km/sec you need a tether 128 times as heavy as your payload. For 3.7 km/sec I get 404 times. As your tether gets heavier and rotates faster the suborbital rocket needs to do less and less to catch the end of it. At some point it seems easier to just make a reusable rocket than to push the tether any more. I think this is for a tether that is around 50 times payload. If you could lift 15 payloads per day (once every orbit) and had a market, then you could justify setting up something that is much heavier than your payloads. If you can start with a small tether and grow cheaply using that tether to setup a big tether, things could even be really good. My hope is that tourism is the market. If you actually had dream materials, you could have really long tethers. With dream materials you could make a tether going from the surface of the Earth past GEO to a ballast so it was fixes relative to the surface of the Earth. Then you could just go up and down without any rocket. There are those, like www.highliftsystems.com that think this could happen in the next 30 years. Maybe, but I am personally much more interested in tethers that we could build today. -- 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