From: henry@spsystems.net   
      
   In article ,   
   David Given   wrote:   
   >Is there enough energy from current RTG models to usefully drive a Stirling   
   >engine, which then drives a generator? Since the Stirling engine relies on   
   >macroscopic physics rather than the microscopic physics that semiconductors   
   >use, it should be far less susceptible to radiation damage. Given that the   
   >energy coming off the plutonium is pretty much free, efficiency isn't   
   >really an issue, provided the result is usable.   
      
   Unfortunately, no, efficiency *is* an issue, because the lower the   
   efficiency, the less electrical output you get out of a given mass of   
   Pu-238.  You can't just crank up the Pu-238 content arbitrarily, partly   
   because the stuff is very expensive, and partly because the mass of other   
   parts of the system -- cooling fins, physical protection against launch   
   accidents, etc. -- scales with the Pu-238 mass.  The power/mass numbers   
   for RTGs are already fairly bad by spacecraft-power-system standards.   
      
   There is considerable interest in conversion techniques other than the   
   current thermoelectric systems, but a large part of that is precisely   
   because alternatives promise higher efficiencies.   
      
   Stirling engines have been considered, if I recall correctly, although   
   they have a tendency to be heavy and spacecraft people mistrust systems   
   with moving parts.   
   --   
   "Think outside the box -- the box isn't our friend."    |   Henry Spencer   
                                   -- George Herbert       | henry@spsystems.net   
      
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