From: nospam@127.0.0.1   
      
   On 12/23/2020 6:20 PM, dumpster4@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   
      
   --- SoupGate-Win32 v1.05   
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