Re: starship-design: Staged Fusion Power

[Bjorn Nilsson <f96bni@student.tdb.uu.se>]

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

Bjorn...