Re: starship-design: Genuine STR question

[Steve VanDevender <stevev@efn.org>]

Johnny Thunderbird writes:
 > right --- you didn't speak to the issue of decoupling the drive system
 > from the energy production system. I went to some length to stick a
 > battery in the ship with the potential to power the entire journey,
 > just to point up you can make an interstellar journey on stored energy
 > alone. No fusion reaction, no antimatter reaction, no coal, just passive
 > reaction mass which is not fuel, pushed out by stored energy. I don't
 > like a drive without a reaction, but it makes the point a feline can be
 > flayed more than one way, to widen perspective on the way things
 > have to be.

Just how would you store all that energy?  The best way to store
energy using the least excess mass that I know of is to store it
as equal parts of matter and antimatter.  Energy in a battery
_is_ mass.

And remember, to get something up to something like 80-90% of c
with an acceleration that passengers and other normal matter can
stand requires approximately _4 times_ as much fuel mass as
payload in the ideal case of using antimatter fuel; it goes
rapidly downhill from there.  That's a tremendous amount of
energy.  If you didn't have to worry about gradual acceleration
then you'd need only the mass-equivalent of excess kinetic
energy.

This does change significantly if you can send energy to the
payload for propulsion rather than have it carry its own fuel.
However, if you can beam power to the ship then you may as well
just deploy a lightsail to reflect that beam during the boost
phase, as that turns out to be the most efficient way to use
those photons to accelerate the ship.  Then you don't even have
to carry excess reaction mass (and any collector that you use to
pick up the energy would likely weigh as much as the sail).  Then
you only have to carry fuel/reaction mass for the deceleration
phase, although I'm of the opinion that finding a way to harness
the drag of the interstellar medium would probably save you a lot
of that work.