From: joe@strout.net   
      
   In article <45MMe.388$x21.293@read3.inet.fi>,   
    Mikko  wrote:   
      
   > > But 30,000ft and, say, 600mph isn't much of a boost. It's a long, long,   
   > > long way from orbit. The difficulty of getting to orbit is better   
   > > described by "17500 miles per hour" than any figure of altitude.   
   >   
   > Why is that speed needed?   
      
   Because that's orbital velocity in low-Earth orbit.  Any slower than   
   that, and you're falling to the ground.   
      
   > Only thing I can think is, that since earths gravity effects the craft   
   > whole time, the longer it takes, the more gravity will "drag back" the   
   > craft?   
      
   Hmm, I suppose that's one way of looking at it.  Here's another way:   
   gravity is pulling you toward the ground at (roughly) 9.8 m/sec.  But   
   the Earth is also round; if you are moving forward fast enough, then by   
   the time you fall to the Earth, it's no longer there -- it's curved away   
   and is now behind you a bit.  At 17500 mph or so, you're moving so fast,   
   that the Earth is *always* behind you; you're going around it as fast as   
   you are falling towards it.   
      
   Hmm, I think that was a lousy explanation.  Surf the web a bit and I bet   
   you'll find better ones.   
      
   In pithy form: orbiting is the trick of falling towards the ground and   
   missing.   
      
   > But also gravity gets smaller when you get more away from the earth?   
      
   Yes, but not for a LONG way.  You can pretty safely ignore that effect.   
      
   > How high does one have to go to have only half of gravity?   
      
   Well, gravity (like pretty much anything else, due to basic geometry)   
   falls off with the square of the distance.  So you can write g2/g1 =   
   (r1/r2)^2.  The radius of the earth is 6400 km; call that r1, and you   
   want the r2 where g2/g1 is 0.5.  0.5 = (6400/r2)^2, do the algebra, r2   
   comes to about 9100 km, or about 2700 km altitude.   
      
   So this decrease of gravity with altitude isn't much help in reaching   
   orbit.   
      
   > What if someone built a 30,000 ft high tube, similar to magnetic trains   
   >    - electrical magnets around it. Then you could just put metallic   
   > cargo inside - without any engine or fuel, and shoot it up. The tube   
   > would have to be high enough that there is no air where the cargo comes   
   > out, and maybe part of the tube would have to be a vacuum.   
      
   Yes, this has been explored before (it's generally called a mass driver   
   -- try a google search).  It would be a massive engineering project,   
   probably more so than you realize, if you want to be able to launch   
   anything other than bulk materials like water.  You'd need a barrel   
   length of over 50 km to keep the acceleration tolerable for humans (say,   
   2 Gs or so).   
      
   > Propably not something to do today, but still lot shorter than the   
   > "space-lift", and it would give near 100 % payload.   
      
   By "space-lift" I assume you mean a space elevator.  Yes, it's   
   dramatically shorter than that, but has a number of additional   
   operational complications, like keeping the thing up in the sky and   
   getting your payloads to and from it.  It also doesn't help much with   
   getting stuff back down, which is itself a rather hard problem.   
      
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