"Tim Attwood" <tim### [at] comcastnet> wrote:
> I'm of the opinion that a large enough power source, such as
> a nuclear reactor, could produce enough electric power to fuel a large
> ion thuster. If a thrust to power ratio of around 50 mN/kW is assumed, and
> a total ship weight of 10 tons then we're looking for a power source of
> about 2 gigawatts. This would let a ship accelerate a little over 1G for
> extended periods. Such a ship could make a round trip to Alpha Centari A in
> about 9 years. If such a vehicle was unmanned then little of the radiation
> shielding would be needed, which is what turns a reactor into a big building
> with 6ft thick cement walls.

Does this estimate take relativistics effects into account?  (I've long
forgotten how to do the math.)  A vehicle that gets to Alpha cen in 4.5
years has to go pretty close to the speed of light.  The relativistic mass
gain would surely affect the thruster's ability to maintain acceleration.

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