XPost: sci.space.shuttle   
   From: bondage@frontiernet.net   
      
   khatcat@hotmail.com wrote:   
   >   
   > I think I understand why objects heat up when going through the   
   > atmosphere and therefore why re-entry vehicles need heat shielding.  My   
   > question is why we don't need heat shielding when launching.  Aren't we   
   > going through the same atmosphere?   
   >   
   > BigKhat   
      
      
      Yes. But there's a difference between going up, when speed is   
   increasing, as air density is decreasing, and coming down, where air   
   density is INcreasing, and you're tryibg to use that atmosphere to   
   decelerate (rather than rockets, which would be impractical in this   
   case), and that kinetic energy can only be converted to heat.   
      
      This is one of the problems with the idea of flying hypersonic   
   airbreathers to orbit. By definition, it has to stay in the meaningful   
   atmosphere to keep picking up oxidizer at increasing speed. As close to   
   orbital speed as your materials will allow you, with a small rocket kick   
   the rest of the way to orbit.   
      
     Pure rockets can take the fastest path into less dense air. Not just   
   for termodynamic reasons, but because rocket engines are more efficent   
   in decreasing atmospheric density.   
      
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