Mike Van Pelt  wrote:
> "James A. Robbins"  wrote:
>
> > 1) A power source capable of accelerating you to high velocities
>
> Having a power source on-board your ship to get you up to very close
> to light-speed is problematic -- at .87 c or thereabouts, the kinetic
> energy of your ship is equal to it's rest mass-energy, and this gets
> very much worse as you go faster.

That sure puts limits on how fast a fusion torch can get
while carrying its own fuel.  I envisioned a fusion torch as
the primary power plant of a colony ship with its exhaust
tuned to be not particularly efficient at generating power
so it also gives thrust.  It wouldn't be much but plenty
for manueuvering needs.  Initial acceleration and final
decellaration would be from separate reserves.  One
comet as fuel to accellerate, another as a radiation shield
and fuel to deccellerate needing less and less shilding as
the speed decreases.  I think Star Trek already covered
this in a TOS episode.

> One workaround is to propel the ship by some sort of beamed power.
> Lightsails are used in a lot of stories. G. David Nordley has used
> particle beams in a number of his stories, and I think others have,
> too. =A0This requires massive infrastructure which can be arbitrarily
> powerful at each star system to propel the spacecraft back and forth.
> That infrastructure has the advantage of not having to go anywhere.

The idea of the Starwisp.  Have a network of thin cables
close enough together to act as a microwave dish.  At the
joints have some chips that control the wires and at as
the intelligence of the ship.  At launch use the output of
a solar power array (the type designed for geosync power
stations) for a year to push it to near 0.1 C.  At the end
of the flight the same power station floods the distant system
with its beam and that's enough to power the Starwisp but
not decellerate it.  The Starwisp goes through at 0.1 C or
maybe crashes into the sun.  No way to decellerating.

Switch this into a two way system and it works to conduct
ships among existing colonized star systems but not to
uninhabitted ones.  Commerce among systems would be
mostly by radio chatter on a very slow network.
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