XPost: sci.space.shuttle   
   From: joe@strout.net   
      
   In article <1122575549.091205.19130@f14g2000cwb.googlegroups.com>,   
    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?   
      
   Yes, but much more slowly.  Most of the acceleration to orbital velocity   
   happens outside the atmosphere.  Coming back down, however, we no longer   
   have enough fuel to do that much deceleration outside the atmosphere --   
   not by a long shot.  The only way we have to slow down is by plowing   
   into the air (converting kinetic energy to large amounts of heat).   
      
   Best,   
   - Joe   
      
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