From: henry@spsystems.net   
      
   In article <106n2uikq2a333e@corp.supernews.com>,   
   Joseph S. Powell, III  wrote:   
   >So, let's say a very powerful fission reactor (interim until fusion can   
   >finally be developed) strapped on the ship - could the VASMIR engines   
   >develop enough thrust to, say, liftoff from the lunar surface...   
      
   Exceedingly unlikely.  VASIMR is basically a low-thrust technology, and   
   even quite powerful reactors are pretty damned heavy (especially after   
   you figure in all the power-generation incidentals).   
      
   >If the thrust isn't that powerful, what future possibilites for high-thrust   
   >engines can be developed that aren't chemical or of a NERVA type design?   
      
   Beamed power -- e.g., laser launch -- is about the only other high-thrust   
   option.   
      
   The fundamental problem is that if you want exhaust velocity better than   
   chemical rockets *and* high thrust, the power requirement is *enormous*.   
   Power is directly proportional to exhaust velocity times thrust.  Big   
   chemical rocket engines like the F-1 are already multi-gigawatt devices.   
      
   Two major problems immediately surface.   
      
   First, that power has to *come* from somewhere.  Chemical energy storage   
   is no good, because the waste chemicals count as part of the exhaust, so   
   no way does this give you better-than-chemical exhaust velocity.  There   
   basically isn't any lighter way of storing energy except nuclear.  The   
   alternative is to get the energy from outside, but natural sources are too   
   thinly spread, so beamed power is almost certainly the only alternative.   
      
   Second, that power has to be employed to expel exhaust, with a minimum of   
   machinery mass.  That's hard.  The only reason multi-gigawatt chemical   
   rockets can be so physically small -- multi-gigawatt power plants are   
   massive structures covering acres of land -- is that they manage to avoid   
   directly *handling* most of that power.  That nice property is just about   
   unique to thermal rockets.  In particular, the machinery mass for anything   
   electric is huge.   
      
   So if you want high thrust and are willing to operate near (for some   
   definition of "near" that depends on your beaming technology) a power   
   station, you can consider beamed power, probably with a thermal rocket   
   engine at the receiving end.   
      
   But if you want it to be self-contained, then some form of nuclear-thermal   
   engine is really your only choice.  Doesn't have to be NERVA, exactly:   
   Dumbo, gas-core, Orion, Medusa, pellet fusion, etc. are all options.  But   
   barring new discoveries, there is no real alternative to nuclear-thermal   
   for this.   
   --   
   MOST launched 30 June; science observations running     |   Henry Spencer   
   since Oct; first surprises seen; papers pending.        | henry@spsystems.net   
      
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