re: Fuel Chat

>On Fri, 27 Nov 1998, Sean Bayan Schoonmaker wrote:

>executive summary: fusion fuel is not an issue. reaction mass is
probably 
>an issue.
>
[snip]
>the question is, how are you going to turn this energy into thrust? if
>you have grav drives or photon drives, then it is complicated but just
>requires some machinery. 

Right.	So if you're using a 'reactionless' drive, you need very little
fuel with either fission or fusion reactors to power the drive.

>if you are going to use a reaction drive (ie a
>nuclear rocket), you will need reaction mass. you may well need a lot
of
>reaction mass, depending on how fast you can accelerate it. if you go
for
>boiling water with the fusion energy and venting it into space, you
will
>need an immense amount, and will need to replenish frequently - icy
>comets, asteroids, planetary ring bodies, etc, are possible sources. if
>you are using a super-high-velocity ion drive, then your reaction mass
>needs will be more modest.

ummm, from what I've read, a fusion rocket produces thrust in a two part
process:  first the frozen fuel (H or an He-Deuterium mix depending on
the type of reaction you're using) pellet drops into a magnetic
confinement chamber.  Second, your fusion reactor powers up a laser or
particle beam which zaps the fuel pellet and produces a fusion explosion
that the confinement chamber channels out the back end of the ship as
thrust.  This kind of fusion pulse drive was used in Pournelle and
Niven's _Footfall_ by the aliens.  Again, depending on the reaction you
use, you can achieve ISPs from 5 to 40 times more efficient than the
best theoretical designs for ion drives.  Fuel use is moderate, and
gives credence to the gas giant/ice moon/comet refueling stations.  If
you want to use ion drives, all your refueling stations have to be on
planets or asteroids (or maybe out of the deep atmosphere of a gas
giant) where you can pick up the heavier ionizates-  Argon or whatever.

Sources:  Advanced Propulsion Study, Robert Forward (incidentally
inventor of the Forward Mass Detector), Hughes Research Labs 1987.

The Future of Flight, L. Myrabo and D. Ing, 1985.

and probably some others.

>Probably the easiest way to create reaction "mass" is to use a 
>very powerful lamp with a parabolic reflector. If you could find 
>a way to get a high efficiency from fusion reaction to photon 
>production, such a drive could actually be quite powerful. Of 
>course heat is always a problem, so you would need both an 
>efficient lamp and an efficient reflector at the frequencies 
>you would want to deal with.

What the heck is a 'photon' drive?  How does this produce thrust?

Curious

Sid Jones