From: henry@spsystems.net   
      
   In article <41DE1E92.BA2B3402@Hovnanian.com>,   
   Paul Hovnanian P.E.  wrote:   
   >As far as introducing some sort of coolant into the skin, you'd have to   
   >determine the amount of heat generated. A good estimate is that all of   
   >the shuttle's kinetic energy is converted into heat at the skin   
   >interface...   
      
   No, that's a lousy estimate.  There is more than enough kinetic energy in   
   a vehicle reentering from orbit to vaporize the entire vehicle, no matter   
   *what* it's made of.  The only reason reentry is practical at all is that   
   with careful design (notably, a very blunt leading surface) only a *tiny*   
   fraction of the heat actually reaches the skin.   
      
   >Then, given the specific   
   >heat of various coolants, calculate how many tons of coolant you'd have   
   >to haul up at launch and throughout the mission in order to cool the   
   >skin.   
      
   The fast answer will be:  too much.  The shuttle makes a prolonged reentry   
   with quite high total heat loads, which would require an awful lot of   
   expendable coolant.  Expendable coolants work much better with Apollo-style   
   lifting-capsule reentries, which are short and sharp, with higher peak   
   heating rates but much lower total heat loads.  That's what an ablative   
   heatshield is:  solid expendable coolant.   
      
   >The shuttle is cooled by transferring the heat of friction to the   
   >surrounding atmosphere which carries it away.   
      
   No, the air is in general hotter than the surface.  Almost all of the heat   
   is spread into the air and never reaches the surface.  What does reach the   
   surface is to some small extent soaked up (heat continues to soak through   
   the tiles for quite a while after reentry -- the cargo-bay temperature   
   actually peaks *after* landing), but mostly radiated.  Conveniently, even   
   fairly hot air is essentially transparent to radiated heat.  But that does   
   require a very hot surface -- radiated heat flux scales with the fourth   
   power of temperature -- with very good insulation behind it.   
   --   
   "Think outside the box -- the box isn't our friend."    |   Henry Spencer   
                                   -- George Herbert       | henry@spsystems.net   
      
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