From: henry@spsystems.net   
      
   In article <1103633784.071266.280980@c13g2000cwb.googlegroups.com>,   
   Richard Hofer  wrote:   
   >...For the REALLY big missions,   
   >like ones to Mars, you would end up spending a lot of dollars on the   
   >propellant, but it actually turns out that it's such a small portion of   
   >the overall mission budget (millions among billions), that other   
   >mission drivers (availability, toxicity, thruster performance,   
   >storability, etc., etc.) end up driving your choice...   
      
   And indeed, the big snag is availability.  If you want a thousand tons of   
   xenon, you simply can't get it in any reasonable amount of time.  Even a   
   hundred tons would be problematic unless you order years ahead.  This is   
   essentially independent of the price you're willing to pay; even if you   
   outbid everyone else, there simply isn't that much production capacity.   
   --   
   "Think outside the box -- the box isn't our friend."    |   Henry Spencer   
                                   -- George Herbert       | henry@spsystems.net   
      
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