From: dw229@hermes.cam.ac.uk   
      
   Reading these postings about SS1's re-entry and from what we've heard   
   recently regarding the space shuttle's TPS and its problems, why have   
   spacecraft tended not to use engine-based deceleration to reduce the   
   thermal effect of re-entry? If the orbital velocity is a major factor in   
   re-entry speed, why not take this off and just drop in? Dropping in from   
   400km would give you a nominal speed of ~2.8km/s if you reached the   
   Earth's surface (assuming I've done my sums right!).   
      
   I imagine the answers will involve the excess fuel needed, manoeuvring,   
   restartable engines, etc., so at what point (what altitude) does the   
   weight of thermal protection tiles beat the extra fuel needed for an   
   orbital craft? I know there's lots of variables involved, but wondered if   
   anyone had considered this?   
      
   Dan   
      
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