From: eaglesondouglas@gmail.com   
      
   On Monday, December 28, 2020 at 10:11:01 AM UTC-5, David Spain wrote:   
   > On 12/23/2020 6:20 PM, dump...@hotmail.com wrote:    
   > > “Nuclear propulsion would be advantageous if you want to    
   > > go to Mars and back in under two years,” says Jeff Sheehy, chief   
   engineer in    
   > > NASA’s Space Technology Mission Directorate. To enable that mission    
   > > capability, he says, “a key technology that needs to be advanced is the   
   fuel.”"    
   > >   
   > I think the key thing to keep in mind here is that by fuel they mean    
   > what and how constructed is the fissionable material in the reactor. For    
   > now we can forget fusion unless you are using fission-fusion-fission    
   > bombs for propellant ala the nuclear Orion type spacecraft, which I    
   > don't think is under serious re-think at least now with all the arms    
   > treaties in place. So what exactly makes up the best FISSIONABLE    
   > material, how is it constructed and clad are all research items. There    
   > are pre-existing examples of how to build a Nuclear Thermal Rocket. In    
   > the 1960's the NERVA project (long since defunct) developed a working    
   > prototype rocket engine that was actually tested in the desert decades ago.    
   >    
   > My main point is that it is important when talking about a nuclear    
   > rocket to distinguish between FUEL (reaction-able (i.e. fissile)    
   > material) vs. PROPELLANT. One of the easiest propellants to feed a    
   > nuclear thermal rocket is water. Water is so handy in space it's hard to    
   > overemphasize its importance. In fact if water is super flash heated in    
   > a reactor it might also be possible to dissociate it and then combust    
   > the hydrogen and oxygen gases for additional propulsion, although    
   > chemically you are still not going to get the ISP you would from just    
   > flashing the water into extremely high pressure steam and jetting it out    
   > the nozzle. The other nice characteristics of water in space is that it    
   > provides a very good radiation barrier for that cone-of-safety between    
   > the crew compartment and the reactor, doesn't require cryogenics to    
   > store (although it might require some heating to maintain a liquid state    
   > at *reasonable* pressures), and can be electrolytically converted into    
   > hydrogen and oxygen for combustion for other purposes.    
   >    
   > The other means of nuclear propulsion is nuclear electric. Where the    
   > reactor provides enough electricity to power an ion propulsion system    
   > that ultimately probably also uses water as the the source propellant,    
   > but a far far smaller quantity. Nuclear ion is low thrust but can be    
   > sustained for a much much longer period of time, allowing to the    
   > spacecraft to reach tremendous velocities given enough time. Sort of the    
   > high mileage version of nuclear propulsion vs the drag racer that is    
   > nuclear thermal. I view nuclear electric as a much much harder task,    
   > certainly in my view more mechanically complex. Maybe a second    
   > generation of nuclear propulsion?    
   >    
   > Dave   
      
      
   Thank you for clarifying the meaning of fuel in a reactor engine.   
      
   The nuclear engine powered aircraft designs used air as fuel.   
      
   In space a duel use fuel is as you said.  Chemical fuel,   
   reactor thermal and/or electric boosted.   
      
   I vote for simple hydrazine plasma boosted. The reactor   
   design needs plasma cooling though.  Radiant heating   
   lowers the allowed power density.   
      
   Putting some kind of radiators on the reactor is allowed.   
   The shield using fuel is maybe an option, but what happens   
   on approaching the Earth with low tanks.   
      
   Think big and have  a thousand  foot long ship and use   
   water to shield. This is human supply water. Coming back   
   with empty water tanks will have a lower ship mass to   
   return with.  Lowering the engine power demands. And   
   the crew radiation dose.  Added hydrazine shielding   
   is a bonus.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)