From: schillin@spock.usc.edu   
      
   Sander Vesik  writes:   
      
   >John Schilling  wrote:   
      
   >> >cells giving you 600W/kg exist - they use aluminium foil as substrate -   
   >> >and designs using say kapon as the substrate could get up to at   
   >> >least 2KW/kg. Sure, this will degrade over time in space and is distance   
   >> >sensitive.   
      
   >> Be careful; solar *cells* are not solar *arrays*.  To get power on a   
      
   >right, my bad.   
      
   >> spacecraft, you need solar cells, interconnects, wiring harness, cover   
   >> glass, a structure to tie it all together, slew mechanism to point it   
   >> at the sun, and probably a power regulator.   
      
   >But are all of these - and all of these being heavy - really necessary?   
      
   Pretty much, yes.   
      
      
   >The glass covers of the solar panels are going to last much longer than   
   >the mission time for most missions. Same applies to the large rigid   
   >structure they are built in. In most cases, building solar panel structures   
   >that are going to last for centuries is utter waste - anything beyond 2x   
   >mission time really is.   
      
   Believe me, nobody is building solar arrays to last for centuries.   
      
   The issue with solar array structure is not some gradual weakening or   
   erosion of the structure, but dynamic failure.  Either you've got enough   
   rigidity and internal damping to keep the thing from shaking apart in   
   its first few hours, or you don't.  If you do, you've got something   
   pretty close to the present state of the art in solar array design.   
      
   And the cover glass, the issue there isn't how long the glass itself   
   will last but whether it is thick enough to stop the deluge of energetic   
   electrons and protons that would kill the solar cells.  If you've got   
   a cover glass that is half as thick as it needs to be to stop most of   
   the radiation, yes, your cover glass will still last decades.  But   
   the solar cells will be dead in weeks to months depending on the   
   environment.   
      
   There is, at the margin, a mass vs. lifetime trade.  But most of what   
   can be eked out of that trade already has been, and it's going to take   
   Extreme Cleverness to get any more substantial reductions in the mass   
   of the support systems without killing the arrays in very short order.   
      
   And people are working the Extreme Cleverness angle from several directions,   
   but that's not the sort of thing you can count on paying off.   
      
      
   --   
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