re: fuel chat

On Fri, 27 Nov 1998, Sid Jones wrote:
> in a really garbled message earlier, I said:
> 
> >The problem is that the internal-style combustion and NERVA
> >rockets son't produce the thrust needed to get anything anywhere.  
> >Their Isps are down around 10-20 thousand km/sec.  At that low an 
> >efficiency, like 80% of the rocket has to be fuel just to get from 
> >Earth to Mars- not exactly the ship design described in the FT rules.
 
> >Ion engines put out (note-  I'm still going by the sources mentioned 
> >last time) around 160 km/sec, while an He(3)-D fusion pulse engine
puts 
> >out on the order of 8,000 km/sec...
> 
> Those Isps are actually:
> - NERVA and fusion (steam exhaust):  10-20 km/sec.
> - ion engines:  160 km/sec.
> - fusion:  8000 km/sec.
> 
> For the curious, the Isp (specific impulse) of a rocket engine is a
> measure of how fast the propellent is expelled from a rocket.  This
> turns out to be a good measure of rocket efficiency

erm, i'm not quite sure that's right. i think specific impulse is the
amount of velocity change you can get from 1 kg of drive (or 1 kg of
fuel,
i'm not sure) with a 1 kg ship. so, with a nerva which took up one fifth
of your ship's mass, you could burn at 1 gravity for 200 seconds (3
minutes 20 seconds). this ain't much good for a combat starship, so, yes
i
think exernal fusion is the order of the day. i may have this all wrong,

though.

anyway, i can well believe that the daedalus/orion type drives are far
more powerful than thermal-expansion drives. however, the prospect of
setting off a nuke half a mile behind me every few seconds (a la orion)
is
not too attractive. on the other hand, the daedalus - which uses many
small explosions, and in a very sophisticated form might almost use a
continuous stream - is quite neat.

> A photon engine would be 100% efficient (no mass, all acceleration,
Isp
> approaching infinity)

well, photoons do have mass, that's the point. energy is mass and vice
versa (if you heat something up, it gets heaver by a tiny amount). a lot
of the energy from a photon drive is going into making photons; this
potential energy stays locked up until the photon hits something. i'm
not
sure a photon drive is 100% efficient.

> but I still think if I had a laser source that
> powerful I'd just sit in the backyard here in Houston and burn those
> pesky Martians from here without bothering to build a spacefleet.

good point. the photon drive is well established, needs no fuel, and is
still a really daft idea.

Tom