starship-design: Deceleration scheme

[Steve VanDevender <stevev@efn.org>]

The efficiency problem shows up in other areas.  Another blast from the
past I'll have to dig up is the derivation of fuel-to-payload ratios for
various possible fuel sources.  In summary, for a self-powered starship
to get to high relativistic speeds (I defined that as 0.8 c or greater)
you need 4-5 times as much matter+antimatter as payload just to boost up
to about 0.8 c (square to get the amount need to decelerate again;
square again for the amount needed for a round trip without refueling).
And it's much worse for anything else.  Fusion would probably require
lugging an ice moon around for fuel, at best.

Has anyone else thought of using a lightsail and beamed power to
accelerate, and a ramscoop to decelerate?  I think the concept has gone
by before, but it seems to me to be the best combination.  You don't
have to carry much fuel, except for maneuvering; you get free braking
_and_ more fuel from the ramscoop, and maybe you can even afford to
build another beamer in the target system for a return trip.  This also
turns the main disadvantage of a ramscoop (drag against the interstellar
medium) into an advantage.  There's almost no need to fuse the collected
hydrogen unless you want improved braking efficiency once you get slowed
down to low speed.