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RE: starship-design: Plasma Engine


> The analysis I did a long time ago, which was in terms of
> mass-energy conversion and fuel-to-payload ratios, implied that
> fusion had pretty a exorbitant fuel-to-payload ratio for getting
> to high relativistic speeds (0.8 - 0.9 c), on the order of 10^6:1
> or worse.  To get a fuel-to-payload ratio of less than 10:1 we
> pretty much have to have an antimatter photon rocket.
> Timothy did indicate that you can get pretty good thrust with
> using lots of low-velocity reaction mass, but unfortunately if
> you make the reasonable definition that "fuel" is really "energy
> source plus reaction mass", this implies an even worse
> fuel-to-payload ratio, especially if you're trying to go for long
> continuous acceleration.  Sure, you can get theoretically good
> results if you make a reaction that pushes a lot of mass at a low
> speed in one direction and a little mass to very high speed in
> the other, but that only works if the reaction is essentially
> instantaneous.
> The ultimate result is that even for really fast interplanetary
> transportation you don't need to have any major amount of
> mass-energy conversion, but for relativistic interstellar travel
> you do; you have to convert a mass larger than the payload to
> energy to get a payload to speeds of 0.8c or higher, and this
> requires either an incredible amount of low-efficiency fuel or
> even a pretty amazing amount of antimatter.

All true. To put it simplistically, an interstellar drive needs to expel
LOTS of reaction mass at HIGH velocity. Nothing we currently have can do
both at the same time. These newer plasma engine concepts come close and the
various concepts for "hybrid" antimatter catalyzed fusion may do even
better, but they all are still far short of what is really needed for a true
interstellar drive.

Being a science fiction author is SO much easier...