From: nospam@127.0.0.1   
      
   Alain Fournier  writes:   
      
   > On Nov/5/2020 at 02:22, JF Mezei wrote :   
   >> On 2020-11-03 18:47, David Spain wrote:   
   >>> Will NASA use it?   
   >>>   
   >>> No. But a private company might.   
   >>   
   >> If you're carrying humans (or Martians, apparently they have bee voting   
   >> illegally in USA :-), I have to assume that shielding requirements will   
   >> be very hefty. Does all that extra mass make this type of propulsion   
   >> worthwile?   
   >   
   > You just put the reactor far from the crew and put a little shielding near   
   the   
   > reactor. You also have to put shielding around the crew, which you would have   
   > to do even if your propulsion method was chemical. There are all kinds of   
   > radiations in space, you need shielding.   
   >   
   Water makes for great shielding. So most designs I've seen put large   
   fuel tanks of just plain (pureified) water between the crew and the reactor(s).   
      
   >> And out of curiosity, how does nuclear propulsion work?   
   >>   
   >> I am aware of nuclear batteries that generate electricity.   
   >> Am aware of nuclear reactors that heat water into steam and drive   
   >> turbines.  But how is nuclear fission used as propulsion?   
   >   
   > There are several different methods. But basically, if you heat a gas to high   
   > temperature, whether you heat it by chemical reactions, nuclear energy or   
   some   
   > other method, the gas wants to expand. You just have to make sure that the   
   > expansion happens in the opposite direction to where you want the rocket to   
   > go.   
   >   
   > Nuclear has some important advantages over combustion. Low mass molecules   
   move   
   > faster at the same temperature than higher mass molecules. So if you are   
   > heating hydrogen to 3000 °C, the molecules of hydrogen will move faster than   
   > water at the same temperature. Therefore heating hydrogen with a nuclear   
   > reaction can give more push than burning it and exhausting water.   
      
   Yeah but hydrogen is a PITA to store. I suspect early Nuclear (fission)   
   Thermal Propulsion systems NTP in the parlance would just shoot   
   pureified water right through the moderator flash it over and out a   
   vacuum optimized nozzle.   
      
   Another optimization that I haven't seen discussed anywhere is the   
   possibility of a multi-stage NTP design. Where first water is pumped   
   into the reactor and allowed to dissocate thermally into its elemental   
   constituents of oxygen and hydrogen which is then heated further in a   
   main chamber before being exhausted. Could be a good way to get that higher   
   ISP w/o the storage headaches of cyrogenics.   
      
   Nuclear electric ion-drive was the proposed method of propulsion for   
   Discovery from 2001 A Space Odyssey. But they dispensed with the reactor   
   radiators in the movie version design because they thought that having a   
   space vehicle with what appeared to be wings would be a hard sell to the   
   near (almost) scientific literati according to what I read by A.C. Clark   
   in his book on the making of the movie.   
      
   Dave   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)