From: jimdavis2@earthlink.net   
      
   John Schilling wrote:   
      
   > Among other things, the ISS 890 kg photovoltaic blanket is not   
   > the latest and greatest word in photovoltaic cells.  For that   
   > matter, I'm pretty sure its mostly not solar cells at all, but   
   > support structure and wiring harness and protective coating and   
   > whatnot.   
      
   I was using only the figure for the photovoltaic blanket. Larsen   
   and Pranke break down the solar array as follows:   
      
   Photovoltaic blanket    	890 kg   
   Mast    	    	    	    	330 kg   
   Gimbal    	    	    	    	540 kg   
   Electrical equiptment    	610 kg   
   Thermal control    	    	730 kg   
   Misc. integration    	    	610 kg   
   Total    	    	    	     3710 kg   
      
   > There are credible solar power system designs in advanced   
   > development with specific power levels of ~200 W/kg.  If such a   
   > system could be tightly integrated with the airship envelope,   
      
   Very, very tightly integrated. The solar cells are not only going   
   to convert sunlight to electricity but be load carrying structures   
   as well.   
      
   > those numbers would actually add up pretty well, with your 92.6   
   > kW/kg for the overall vehicle a not entirely unreasonable   
   > figure.   
      
   No, it does not make sense even then. Grant them the 200 W/kg power   
   level. Assume the airship is made up of nothing but these cells   
   when it gets to orbit. This means it the it has to convert 46.3% of   
   the electric power to useful work to acheive orbit in 4 days. An   
   ion thruster operating at an Isp of 5000 s over a delta V of 8000   
   m/s only converts a maximum of 15% of the electric power to useful   
   work with about 15% of the initial mass as propellant. You can   
   improve the situation by lowering Isp but then your propellant   
   loads increase. At 1250 s you can theoretically get the required   
   46.3% but now 48% of the initial mass is propellant with the rest   
   of the airship made up of nothing but solar cells. It just doesn't   
   add up even ignoring the impossibly high L/D ratios required.   
      
   > Those are actually two fairly common educated-amateur level   
   > mistakes in electric propulsion and rarefied gas dynamics,   
   > respectively, and if you grant those two mistakes I can almost   
   > make the numbers work for the rest of the system.   
      
   I would be interested in seeing what you can come up with those   
   stipulations.   
      
   Jim Davis   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)