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Re: starship-design: Staged Fusion Power

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.