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RE: starship-design: Staged Fusion Power

Hello Steve,

>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.

In the past we calculated that scooping matter during travel in
interstellar space would likely get us too little mass. So I indeed assumed
that mass would be stored on board or that it would be launched in advance
along the track that the starship will follow.

>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.

For multiple flights that may be true, but for a single interstellar flight
optimizing the fuel may be worth the additional work of refining the wanted
isotopes for the fuel. Isolating isotopes may not be easy, but the
technology to do it is available.

>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 think another reason that makes isolating or synthesizing special
isotopes worth the expenses is that some of the reactions involving less
common isotopes may deliver more than 2 times the amount of energy per unit
of mass.

>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.

In my 8/17 letter I wrote:

   (II)  24 (2H) + 2 (3H)  -> 1 (54)Fe         + 401.4 MeV


   Reaction (II) is the short-circuit of the multi stage fusion.

I don't actually have/know all the steps in between, but it wasn't my
purpose to display what in reality was possible. I merely wanted to compare
a single-stage fusion reaction with the best possible way (energy-wise) to
get iron. (I could have chosen worse possibilities, that would have
strengthened my point that multistage fusion isn't worth the trouble.)

My goal was to show that some of the best single stage fusion reactions
would not be that much worse than complex and likely hard to realize multi
staged fusion reactions.