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Re: starship-design: Staged Fusion Power

On Sun, 16 Aug 1998, Steve VanDevender wrote:

> Zach Johnson writes:
>  >    N. Lindberg wrote:
>  > 
>  > >         I was reading some of the numbers for the various engines that
>  > > could be used to power a starship, and I noticed that every fusion
>  > > reaction shown only used its fuel once.  If a closed powerplant (not
>  > > rocket) could be run hot enough, there's no reason I can see not to run
>  > > the fuel up to heavier elements instead of just throwing it away after it
>  > > turns to helium. Note:  I didn't do any math for this one, it might be
>  > > impractical.  Although I realize that a scheme like this would require
>  > > reactors far superior to a today's can't-quite-ignite tokamaks, it might
>  > > be doable in fifty years. The power from this type of reactor could be
>  > > used to power a laser or ion drive, the latter prehaps adding the
>  > > reactor exhaust to the Xenon reaction mass.
>  > >         The upshot is, exhaust recycling could reduce the amount of fuel
>  > > required by which is one of the major hurdles of starflight.
>  > > Best Regards,
>  > > Nels Lindberg
>  > 
>  >     Could you combine fusion and fision to produce a continous reaction?
> Well, conservation of energy says that there has to be some limit 
> to that.  You should also remember that both fission and fusion
> conserve nucleons (although beta decay and inverse beta decay may 
> change a neutron into a proton and electron and neutrino, or vice 
> versa).  So that puts an even more stringent limit on the amount
> of energy you could get; no matter what happens, the energy tied
> up in the nucleon's masses is never released, only the binding
> energy holding them together.
> One problem with a multi-element fusion reactor is that it takes
> progressively higher temperatures and pressures to induce fusion
> in heavier nuclei.  So the cost of fusing the heavier elements
> may not be well offset by the additional energy provided.

Actually combining fusion and fission is a total waste...

For all elements ligther than Iron (Fe) Fusion produces power and fision
costs power... Vice Versa for elements Heavier than Iron.

As for using some kinda multi-stage Fusion device I personnaly think that
it will both be MUCH simpler (from an engineering POV) and more efficient
to just use the fusion products (probably Helium) as reaction mass.

Just my 2c