From: spam@merlyn.demon.co.uk   
      
   JRS:  In article <402A2E6A.1050503@business.com>, seen in   
   news:sci.space.science, nafod40  posted at Wed, 11   
   Feb 2004 08:30:18 :-   
   >SeeBelow@SeeBelow.Nut.retro.com wrote:   
   >> I think the fuel danger is not excessive.  The fuel containers could be   
   >> kept at a considerable distance from the station, on a wire heading   
   >> toward the earth.  Tidal forces would keep the wire taught.  That might   
   >> be necessary if they were solid fuel, which could explode if hit by a   
   >> micro-meteorite.   
   >>   
   >> OTOH, if the fuel was two components, say oxygen and hydrogen, then   
   >> neither oxygen tanks not hydrogen tanks are explosive on their own.  To   
   >> keep them cold in space mainly requires sheilding from sunlight.   
   >   
   >We could send up water, then just let solar array-powered electrolysis   
   >slowly do its magic to make the fuel. Two years for a bag of fuel? No   
   >problem, no rush.   
   >   
   >That way no volatile components in the launch. You could freeze the   
   >water, and use it as part of the structure of the launch vehicle to   
   >reduce weight. Alternate launch techniques such as rail guns? The   
   >payload would certainly tolerate the G's.   
      
   To do a reasonably useful mission requires, give or take an order of   
   magnitude or so, enough fuel to accelerate a ton by 10 km/s, putting 0.5   
   * 1000 * 10000^2 = 5E10 joules into the payload; given the way a rocket   
   works, the energy in the fuel must be significantly greater - say   
   double, giving 1E11 joules.   
      
   The solar constant is about 1400 W/m^2; assume solar panel efficiency   
   about 35% which means 5E2 W/m^2 available, 5E2 J/s/m^2.  A year is 3E7   
   s, giving 1.5E10 J/yr/m^2.   
      
   On that basis, a modest 6 m^2 panel allows for a mission per year.   
      
   That's well-enough placed with respect to the ballpark to justify a   
   calculation using better figures.  The efficiencies above are   
   intentionally generous.   
      
   Load momentum is 1E7 units; to get that in a year needs a thrust of   
   about 0.3 N.  What Isp can an 0.3 N H-O rocket give?  I think of the   
   electrolysed gases being fed directly to an engine, so producing a   
   continuous-thrust mission running on sunlight and water.   
      
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