Re: starship-design: Deceleration scheme

[jimaclem@juno.com]

On Tue, 29 Jul 1997 16:17:32 -0700 Steve VanDevender <stevev@efn.org>
writes:
>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.
>
Sigh, I'm beginning not to like the sail concept as much.  It is simple
and elegant, but between the deceleration problems, plus the difficulties
of such mega-structure operation, and the power beam generation problems,
I think the use of matter-antimatter and/or fusion ideas may be the
ultimate best way to go.

Jim