Re: starship-design: Plasma Engine

[Steve VanDevender <stevev@efn.org>]

KellySt@aol.com writes:
 > In a message dated 8/30/98 11:25:13 AM, lparker@cacaphony.net wrote:
 > >Correction, AIMSTAR is claiming delta v in excess of 900 km/sec, making it
 > >almost four times better than the nearest plasma engine.
 > >
 > >For interstellar use, we would need to boost this to at least 90,000
 > >km/sec....
 > >
 > >Lee
 > 
 > The fusion systems I used for the Explorer and Fuel/sail designs had spec imp
 > of over a million.  Even with that the fuel ratios for the ships were
 > stagering.
 > 
 > Kelly

The analysis I did a long time ago, which was in terms of
mass-energy conversion and fuel-to-payload ratios, implied that
fusion had pretty a exorbitant fuel-to-payload ratio for getting
to high relativistic speeds (0.8 - 0.9 c), on the order of 10^6:1
or worse.  To get a fuel-to-payload ratio of less than 10:1 we
pretty much have to have an antimatter photon rocket.

Timothy did indicate that you can get pretty good thrust with
using lots of low-velocity reaction mass, but unfortunately if
you make the reasonable definition that "fuel" is really "energy
source plus reaction mass", this implies an even worse
fuel-to-payload ratio, especially if you're trying to go for long
continuous acceleration.  Sure, you can get theoretically good
results if you make a reaction that pushes a lot of mass at a low
speed in one direction and a little mass to very high speed in
the other, but that only works if the reaction is essentially
instantaneous.

The ultimate result is that even for really fast interplanetary
transportation you don't need to have any major amount of
mass-energy conversion, but for relativistic interstellar travel
you do; you have to convert a mass larger than the payload to
energy to get a payload to speeds of 0.8c or higher, and this
requires either an incredible amount of low-efficiency fuel or
even a pretty amazing amount of antimatter.