From: henry@spsystems.net   
      
   In article ,   
   Daniel Walker   wrote:   
   >...why have   
   >spacecraft tended not to use engine-based deceleration to reduce the   
   >thermal effect of re-entry?   
      
   Because making even a modest difference requires vast amounts of fuel, far   
   more than can reasonably be made available.   
      
   >...so at what point (what altitude) does the   
   >weight of thermal protection tiles beat the extra fuel needed for an   
   >orbital craft?   
      
   The fast answer is that there is no crossover point:  aerodynamic braking   
   is *always* a lot cheaper in mass than doing the same braking with rocket   
   fuel.  That's not quite 100% true when you start examining specialized   
   situations, but for normal reentry from orbit you can take it as given.   
   --   
   "Think outside the box -- the box isn't our friend."    |   Henry Spencer   
                                   -- George Herbert       | henry@spsystems.net   
      
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