From: henry@spsystems.net   
      
   In article ,   
   Gordon D. Pusch  wrote:   
   >> Basically shine the laser into a 45-degree mirror sitting under the   
   >> vehicle and heat up the air in the "combustion chamber"...   
   >   
   >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!   
      
   You cool it the same way jet-engine turbine blades are cooled:  with air.   
   (Turbine-inlet temperatures in modern jet engines are significantly above   
   the melting point of the blades.)  In principle, anyway...   
      
   >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...   
      
   Laser-supported-detonation thrusters have been demonstrated using air as   
   reaction mass.  There are a host of engineering problems associated with   
   making such a system work well in practice, but then, that's true of the   
   l-s-d thrusters in general.   
      
   >> 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...   
      
   Moreover, the competition for a laser launcher is not today's rockets, but   
   the same amount of money invested in new rockets.  Unfortunately, big   
   lasers are fairly expensive, and even if you're pessimistic about the   
   development cost of reusable launchers, they're in the same league.   
      
   >Also (and again like most CATS schemes), the system is most economical   
   >for small, light payloads that can tolerate high gees...   
      
   Small, light payloads, yes -- as with a lot of non-rocket schemes, the   
   hardware in a laser launcher is sized based on the size of an individual   
   payload, and is largely insensitive to the number launched.  Such systems   
   are conveyer belts, not semitrailers.   
      
   High gees, no.  Laser launchers are externally-powered rockets, not   
   catapults.  They accelerate over quite long distances and there's no   
   reason why their payloads should see high gees.   
      
   >...which means that it's not really usable for launching people...   
      
   Laser launchers indeed are not good for launching people, but for a   
   different reason:  people are too big.  The laser needed for a system that   
   can launch that big a lump at once is extremely expensive.   
   --   
   MOST launched 30 June; science observations running     |   Henry Spencer   
   since Oct; first surprises seen; papers pending.        | henry@spsystems.net   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)