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starship-design: Antimatter Annihilation Products

DotarSojat@aol.com writes:
 > [And after we convert antimatter energy to usable energy, we
 > still have to convert the usable energy to exhaust kinetic
 > energy (via some kind of engine/electrical generator powering an
 > accelerator), so the "antimatter conversion efficiency" which is
 > the overall efficiency used in the calculation of the starship
 > properties reported in my 2/4/97 note will be even lower than
 > 0.8.]

If the result of a proton-antiproton or proton-antineutron annihilation
is a lot of high-velocity particles, there's no point to trying to
capture those and turn them into electrical energy to accelerate
reaction mass.  Either throw the reaction products directly out the back
or play them into your reaction mass to heat it, then throw the hot
reaction mass out the back.

I have never understood Timothy's claim that lower exhaust velocities
are more efficient.  My own conclusion is that, physics permitting,
converting mass directly into photons provides the highest payload
velocity for a given amount of fuel mass.  Lower exhaust velocities mean
higher fuel-and-reaction-mass-to-payload ratios and expending more
energy to accelerate a given payload plus the additional reaction mass.