Re: starship-design: Genuine STR question

[Steve VanDevender <stevev@efn.org>]

To respond to this in short. the ability of a self-powered
spacecraft to accelerate to high relativistic velocities (roughly
meaning > 70% of c) depends on the efficiency of mass-to-energy
conversion of its fuel as well as the fuel-to-payload ratio.
Efficiency of mass-to-energy conversion happens to correlate well
with exhaust velocity.  A fuel that reacts with low
mass-to-energy conversion, like hydrogen fusion, requires
tremendous fuel-to-payload ratios on the order of 10^6:1 to
accelerate the payload to high relativistic speeds, while the
ideal matter-antimatter reaction can achieve something like 
80-90% mass-to-energy conversion and requires only a 4:1
fuel-to-payload ratio to get the payload to the same speed.

When I get time I'll dig up the URL to the archived item where I
derived these results.  It's true you can reduce the total
reaction mass you have to carry by accelerating it to high
exhaust velocities, but if you use an inefficient fuel you end up
having to carry an amount of fuel that more than makes up for the
reduction in reaction mass.  And the only way to reduce the fuel
mass is to increase the efficiency of mass-to-energy conversion
of the fuel.