From: dietz@dls.net   
      
   Jake wrote:   
      
   > One last thing: the heating isn't caused by "friction".  If that was the   
   > case, skydivers would burn.  The heating is caused by hypersonic objects   
   > supercompressing the air before it could move out of the way.  The   
   > supercompression heats up the air and the heat is transferred to the   
   > object via radiation.   
      
      
   It's caused by the gas going through a shock, not (just) by the gas being   
   compressed.  As the mach number increases, the density jump across   
   a shock does not increase unboundedly, but approaches some finite limit.   
   The *temperature* increases without limit, however.  (There is also some   
   heating from turbulence.)   
      
   Entropy increases across a shock; it's an non-isoentropic process, unlike   
   compression of gas in an apparatus where the motion of the gas is very   
   subsonic, like your typical air compressor.  In this sense a shock is   
   like friction, which is also an entropy-creating process.   
      
   	Paul   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)