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> NNTP-Posting-Host: 207.42.133.230 Message-ID: <9186edb5.0305201423.6ad24a0e@posting.google.com> "Joe D." wrote in message news:<4%7ya.623$Zk.33146@news.uswest.net>... >Sorry for my ignorance of Physics - correct me if Im wrong, but an >orbital vehicle is travelling at 25000kmh and must loose this speed >to drop out of orbit. Decelerating through the atmosphere generates >a great deal of heat from friction. Why is it not possible to come >in at a shallower angle so that loss of speed is more gradual and >friction is less? The problem is that as you reduce your speed you fall out of orbit. Without the centripetal/centrifugal "force" holding you away from the Earth("up"), gravity starts pulling you toward the surface of the Earth, messing with your planned "shallower angle". To fight gravity you can have a vehicle that has lift. But for any given vehicle and speed there is a minimum air density to hold the vehicle up. Sadly, this same speed and density still makes your vehicle rather hot. And the other sad part is that wings add to the weight you need to lift into orbit. A little bit of lift (say L/D of 0.5) is a good idea if you have humans onboard because it reduces the maximum Gs experienced on reentry. But even a capsule can get a little bit of lift. At some point (maybe L/D of 0.5 even) you are better off designing for the heat than trying to get more lift. Another theoretical possibility is to use a rocket to slow you down. But it takes the same fuel to payload ratio to accelerate with rocket fuel as it does to decelerate with rocket fuel. And you would first need to lift the rocket fuel to orbit that you were going to use on the way down. If it takes you 30 lbs of fuel to get 1 lb to orbit, and you need 30 lbs of fuel to slow 1 lb down on the way back, you need like 900+ lbs of fuel for every lb that goes up and down. There are better solutions to the reentry problem than this. Reentry from orbit is a hard problem. Designing a practical (not too heavy) human reentry vehicle that is totally reusable (just fill up tanks and launch again) is a problem so hard that we have yet to demonstrate a single solution. -- 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