Re: starship-design: HOTOL SeaDragon idea.

[Steve VanDevender <stevev@darkwing.uoregon.edu>]

ben franchuk writes:
 > KellySt@aol.com wrote:
 > > In space its the specific impulse that is more critical.  And its not clear 
 > > that a anti-mater system would have a better thrust to weight ration.  You 
 > > forget the extream weight ratin of the antimater storage and handeling gear.
 > > 
 > > Also, fusino propulsion - or power - is generally considered doiable in a 
 > > short term dev program if there is focused work on it.
 > 
 > I have been waiting for them to hit break even with fusion since the
 > 1970's. The same could be said of re-usable space transport. Anti-matter
 > designs barring getting anti-matter seem to be practical and in the 
 > reach of small industry so development could proceed faster. I don't 
 > think the anti-matter tank would be much bigger than a bell-jar.

If your antimatter tank is the size of a bell jar, then it won't hold
enough antimatter to get anything bigger than about the size of a bell
jar to even low relativistic speeds.

Essentially, boosting a ship to a decent relativistic speed
(which I'll call > 0.5 c for purposes of this discussion) requires
additional energy input of approximately the initial rest energy of the
payload.  That means that a relativistic ship needs at least as much
antimatter + reaction mass as payload mass to reach relativistic speeds.

The situation with fusion power is worse, but in some ways not too much
worse; there you need something like a 1,000,000 :: 1 ratio of fusion
fuel :: payload.  That's a lot of fuel, but it's much easier to store
that much hydrogen than it is to store the much smaller amount of
antimatter in the case above.

Unfortunately ramscoops don't help the fusion situation much since
scooping up interstellar gas for fuel induces drag proportional to the
speed of the ship through the interstellar medium, which quickly matches
the thrust you get from fusing the fuel.  The expected top speed for a
traditional ramscoop ends up being something on the order of 0.1 c,
maybe a bit better if you do some other clever things.

I keep intending to put together a sort of breezy summary of the various
interstellar travel scenarios that have been considered and their
various tradeoffs, partly to get some newer list members up to date, but
mainly in the hopes it will revive active discussion on the list.