From: schillin@spock.usc.edu   
      
   alexterrell@yahoo.com (Alex Terrell) writes:   
      
   >schillin@spock.usc.edu (John Schilling) wrote in message news:<   
   4vfa8$djg$1@spock.usc.edu>...   
      
   >> >Out as far as Mars, solar power is ligher than nuclear power for a   
   >> >given power level.   
      
   >> If the given power level is ten kilowatts or so, yes.   
      
   [...]   
      
   >> You seem to be assuming that nuclear and solar power scale linearly,   
   >> that a one-megawatt solar array or nuclear reactor will have a hundred   
   >> times the mass of a ten-kilowatt solar array or nuclear reactor.  This   
   >> is not the case - solar power starts to have problems above ~100 kW,   
   >> and nuclear at *less* than ~100 kW.   
      
   >Probably a false assumption on my part, but why does solar have   
   >problems above 10KW? SSPS was envisioned at 5GW, and there seems no   
   >fundamental reason why a (collection of) low thrust electric engine(s)   
   >couldn't pull one of those around.   
      
   I said 100 kW was the point where solar starts to have problems.   
      
   The reason for this is mostly the structural rigidity, or lack thereof,   
   of the solar arrays.  The fact that there is no gravity does not mean   
   that there are no loads and one can make the structure arbitrarily weak.   
   There are always *some* loads, external ones such as light pressure and   
   internal ones such as the torque of the motors and gimbals that keep   
   the array pointed at the sun.   
      
   If these were steady-state loads, there would be little problem, but   
   some of them have transient components.  And, reducing structural   
   mass reduces structural rigidity and damping in proportion.  So when   
   your array-pointing motor twitches, a ripple bounces back and forth   
   across that gossamer structure and is loathe to just die away.  If   
   it's still bouncing back and forth the next time the pointing motor   
   twitches, things start to get complicated...   
      
      
   Simple expansion of existing solar array designs to >100 kW levels,   
   gives you something that is in danger of tearing itself apart in   
   ordinary operation.  Solving that problem, requires either the sort   
   of structural reinforcement that puts you on the wrong side of a   
   cube-square law, or Extreme Cleverness in the area of structural   
   dynamics.  And spacecraft designers take a dim view of anyone who   
   proposes Extreme Cleverness for mission-critical systems.   
      
      
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