From: henry@spsystems.net   
      
   In article <1107127000.539022.123860@z14g2000cwz.googlegroups.com>,   
   Allen Thomson  wrote:   
   >Supposing you wanted a reactive propulsion system -- say a   
   >nuclear rocket, electrical thruster, mass driver or other   
   >throw-mass-out-the-back widget that had continuous thrust >=   
   >1 kN, ISP >= 10,000 sec (>= 100 km/sec Ve) and a total run   
   >time >= 1 megasecond or greater...   
      
   Mmm...  Even at 100% efficiency, this requires 50MW of continuous power,   
   which will not be easily had.  (Jet power is 0.5*thrust*exhaustvelocity.)   
      
   That kind of exhaust velocity is almost certainly impractical with a mass   
   driver or anything similar.   
      
   Most electric thrusters are out too.  Ion thrusters can get up into that   
   range easily enough, although they are more usually optimized for lower   
   Isp; you will need a whole bunch of them to get 1kN.  The power source and   
   thruster hardware will be quite heavy.   
      
   There is no way that solid-core nuclear can do 100km/s.  Even gas-core   
   probably tops out around 50km/s.  100km/s or more should be feasible with   
   systems that don't try to separate fission fuel and propellant -- NSWR or   
   imploded-pellet fission, for example -- but operating costs will be high   
   and the exhaust generally rather dirty.   
      
   Fusion or antimatter rockets can deliver the performance, but fusion is   
   beyond what we can build now, and antimatter is beyond what we can fuel   
   now (the hardware technology is rather easier than fusion [!!], but   
   large-scale antimatter production would require massive infrastructure).   
      
   >Does current technology or anything that can be reasonably   
   >foreseen in the next 20 years support anything like that? If   
   >so, what might it be?   
      
   Current technology can stack up an array of ion thrusters, but powering   
   them is problematic.  That is much too big for current-technology solar;   
   the problem is not the solar cells but the structural dynamics of enormous   
   lightweight solar arrays.  100MW space reactors are not off-the-shelf   
   items either.  Solving either in the next 20 years is conceivable, but not   
   a small project, and probably will not happen without specific need.   
      
   100km/s dirty-fission or antimatter systems are probably buildable without   
   major new technology, but again, it would be a big project and is unlikely   
   to happen within 20 years unless someone makes a major effort to do it.   
   --   
   "Think outside the box -- the box isn't our friend."    |   Henry Spencer   
                                   -- George Herbert       | henry@spsystems.net   
      
   --- SoupGate-Win32 v1.05   
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