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RE: starship-design: Staged Fusion Power

Timothy van der Linden writes:
 > Hello again Lee,
 > >Too bad we can't use the gamma rays produced in the D+D reaction though...
 > What would you use them for?

Raw energy, of course.  Gamma rays are high-energy photons.

My brief thoughts on the whole fusion power thing are:

Any fusion drive that depends on uncommon elements or isotopes
probably implies that the fuel has to be gathered ahead of time
and stored on board.  Deuterium, tritium, helium-3, and so on are
not easy to come by and take significant time and energy to
refine out of a planetary atmosphere or ocean or the interstellar
medium.  The main reason such isotopes are considered for fusion
power plants on Earth is that the lower cost of fusing them looks
attractive even compared to the cost of refining them out of
seawater.  For spacecraft fuel I really believe that becomes a
significant disadvantage.  I think the only mitigating factor is
that it might not be too expensive to synthesize the uncommon
isotopes from the far more abundant common isotopes in order to
be able to run the fusion reactor with lower temperatures.

I also don't know where you got the fusion reaction that produces
iron directly from a bunch of hydrogen and helium, unless it's
the abbreviation of a whole bunch of intermediate reactions.  If
it's possible, it's really darn unlikely to be able to do it all
in one step.

I could see running a fusion reactor that would allow for a
few stages (say fusing from hydrogen up to relatively light stuff 
like carbon and oxygen).  Isn't the most common fusion reaction
in the universe the carbon cycle that fuses hydrogen with carbon
to produce an unstable nitrogen isotope that decays back into
carbon again?