From: g_d_pusch_remove_underscores@xnet.com   
      
   Frd.Gunther_NOSPAM@aol.com writes:   
      
   > There have been proposals in the past for ground-based high-energy lasers   
   > to provide the initial boost for a medium- to heavy-lift vehicle.   
   > Basically shine the laser into a 45-degree mirror sitting under the   
   > vehicle and heat up the air in the "combustion chamber".  Pulse the laser   
   > to let the hot air escape and cold are come in and then be heated in   
   > return.  The escaping hot air is what pushes the vehicle up -- need to   
   > use on-board gas after reaching a high altitiude.   
      
   I think you've been reading too many old Jerry Pournelle stories. The   
   system you describe requires "combustion chambers" made of Unobtainium ---   
   _NOTHING_ is sufficiently reflective to not melt without some form of   
   transpirational or ablative cooling at those power levels!   
      
   Modern "laser launcher" vehicle designs using approaches such as   
   "laser-heated thermal rockets" or "laser-assisted detonation waves"   
   require a goodly supply of on-board ablative or transpiration mass;   
   so much mass, that it winds up representing the lion's share of the   
   reaction mass in the "jet."   
      
      
   > Those who have worked the numbers say that this will work quite well.   
      
   ...However, like most such "Cheap Access To Space" (CATS) schemes,   
   the investment required in capital equipment is so high that it is not   
   economical unless the launch-rate is more than an order of magnitude higher   
   than the current rate, leaving one with a "chicken and egg" problem...   
      
   Also (and again like most CATS schemes), the system is most economical   
   for small, light payloads that can tolerate high gees --- which means   
   that it's not really usable for launching people, unless you want them   
   to arrive as "Soylent Green(tm)"...   
      
      
   -- Gordon D. Pusch   
      
   perl -e '$_ = "gdpusch\@NO.xnet.SPAM.com\n"; s/NO\.//; s/SPAM\.//; print;'   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)