XPost: sci.space.shuttle   
   From: mailer-daemon@bof.de   
      
   khatcat@hotmail.com writes:   
      
   >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?   
      
   Apart from the smug reply that the heat shield will probably already   
   be in place on launch... :) consider this:   
      
   Heating comes roughly from velocity times drag, with drag coming   
   roughly from density of the air, which decreases rapidly with   
   altitude. In this sense, it is not the same atmosphere, as the   
   upper layers (100km height and more) are almost a vacuum   
   compared to sea level.   
      
   On launch, energy from chemicals is used to accellerate from 0 to   
   mach 23 or something - orbital speed. You want to use the least   
   amount of chemicals to get at the required speed. Any heating   
   cum drag will WASTE chemicals, by putting their energy into   
   heating the air, instead of increasing the speed of your vehicle.   
   Thus, the engineers are told to avoid drag cum heating as good   
   as possible. Solution: go up almost vertically, at not-so-high   
   speed, until most of the air is below you. Then start to really   
   accellerate.   
      
   On reentry, it is the other way round. The vehicle must be   
   decellerated from mach 23 to 0 (or you won't survive landing...),   
   so all that energy must be shed. Now, it's a nice thing to have   
   this atmosphere cum drag - you don't need any more chemicals   
   to burn for decelleration! So, in this case, the engineers are   
   told to search for drag cum heating (within the limits of the   
   heat shielding, of course), because this drag is exactly the   
   thing you use as a brake.   
      
   I hope this layman's understanding was both correct and understandable.   
      
   best regards   
     Patrick   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)