On Saturday, July 21, 2018 at 12:31:12 PM UTC-5, David Ellis wrote:   
   > These are fair points, though I have been thinking of using normal protium   
   rather than, say, water or helium or anything else, because of hydrogen's   
   superior performance as a reaction mass purely in terms of exhaust velocity,   
   even compared to deuterium.   
        
   >    
   > Deuterium is also something I've tried to avoid partly because, while I   
   would, indeed, be making use of helium-3/deuterium fusion for these rockets, I   
   would be trying to run them deuterium-lean, thus minimizing neutronic D-D   
   side-reactions as much as    
   possible.  I wonder if using deuterium as a propellant that is allowed to mix   
   with the still extremely hot, high pressure plasma being vented from the   
   reactor's torus would run the risk of allowing additional D-D reactions to   
   occur.  While this would, I    
   suppose, further increase the thermal energy being used to propel my reaction   
   mass, it would also force me to cope with an even larger amount of neutron   
   emissions.     
   >    
   > Let's put the choice of cryogenic hydrogen aside for the moment, though.    
   I'm most interested in trying to sort out what secondary thrusters make the   
   most sense.     
   >    
   > I was thinking of arcjets, as mentioned above, since the selection of   
   electrothermal rocket types seems to be a fairly short list and using an   
   electrical arc not limited by the melting point of a solid material, as a   
   resistojet would be, seems like an    
   obvious advantage.     
   >    
   > Granted, periodic replacements of electrodes would create some maintenance   
   demands, but I suppose a machine can't exist that doesn't require maintenance   
   at some point.     
   >    
   > Atomic Rockets, however, also has the microwave electrothermal thruster   
   listed under electrothermal rockets, and it seems to be something very similar   
   to an electrodeless version of the arcjet, in the sense that electricity is   
   supplied to a system that    
   makes physics happen and then hot plasma gets exhausted.     
   >    
   > The AR section on microwave thrusters does list water as the propellant, and   
   I don't know for sure if something other than water can be used, so maybe the   
   microwave rocket isn't automatically the best option.     
   >    
   > That said, the idea of heating a material using electromagnetic waves to   
   excite the reaction mass into a plasma before exhausting it through a magnetic   
   nozzle sounds an awful lot like VASIMR, which is also, from what I can tell,   
   very much an    
   electrodeless electrothermal/plasma rocket.  So, perhaps I should just go   
   right ahead and settle on a VASIMR style of reaction control thruster, since I   
   DO know that VASIMR seems to be able to use hydrogen or other materials   
   instead of water.   
      
   The neutron problem does not come from the D-D reaction but from the Tritium   
   byproduct of 50% of the D-D reaction chain reacting with new deuterium to   
   create the D-T reaction which is 80% neutronic. If you run at higher   
   temperatures, then the He3    
   byproduct of the other 50% of D-D reactions will interact with either new   
   Deuterium or the produced Tritium. Either way, the neutronicity of the   
   reaction falls. If you can separate the Tritium from the D-D plasma and   
   prevent it from reacting, then you    
   will also run neutron lean.   
      
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