Re: starship-design: Deceleration scheme

[Steve VanDevender <stevev@efn.org>]

Timothy van der Linden 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
 > 
 > Using just as much antimatter as matter is a waste of energy for most
 > velocities!

I meant that the fuel mass (which is matter and antimatter in equal
parts) is 4-5 times the mass of the payload.

In a ship that carries its own fuel you get the lowest fuel-payload
ratio by having the highest-velocity exhaust.  Photons are optimal; the
mixture of photons and high-velocity particles you get from
matter/antimatter reactions is about the best you can do.  If you react
a quantity of antimatter with a larger quantity of matter then you get
slower exhaust velocity and a higher fuel-payload ratio, and things get
more or less exponentially worse with decreasing exhaust velocity.