Re: fuel chat

On Tue, 1 Dec 1998, Nyrath the nearly wise wrote:
> Thomas Anderson wrote:
> > On Fri, 27 Nov 1998, Sid Jones wrote:
> > > For the curious, the Isp (specific impulse) of a rocket engine is
a
> > > measure of how fast the propellent is expelled from a rocket.
> > erm, i'm not quite sure that's right.
>	You're correct, though Mr. Jones isn't all wrong.

fair enough.

> Isp = Specific Impulse (Seconds)
> Ve  = Exhaust Velocity (meters / second)
> g   = acceleration due to gravity ( 9.81 meters / second^2 )
> Isp = Ve / g
> Ve  = g * Isp

aha! Isp is just a slightly more convenient way of stating exhaust
velocity. i get it.

force is rate of change of momentum, which is exhaust velocity times
rate
of flow of mass. acceleration of a ship is given by the exhaust velocity
times the flow rate divided by the ship's mass. thus, if you multiply
specific impulse by flow rate and divide by mass, you get the ship's
acceleration in multiples of g. clever.

sid and my two definitions of specific impulse were in fact equivalent
(although they were both wrong, as we both left out the g factor).

> CHEMICAL ROCKET
> Propellant				Isp
> ===========================================
> Metastable Atoms (e.g. Helium)		3,150

eh? (a) how is helium metastable? (b) how does a helium-fuelled rocket
work?

> NERVA/DUMBO "Atomic Rocket"	Isp = 850 to 2950 seconds
> GASEOUS CORE NUCLEAR REACTOR	Isp = 3570 to 7000 seconds
> ORION "old Bang-bang" 	Isp = 1000 to 5000 seconds
> DAEDALUS (Fusion Microexplosions)Isp = 1e6

these are the options we have run over, i think. i have read of GCNRs. i
thought they were of similar efficiency to scnr (nerva type), but
apparently they are much, much better. better, in fact, than orion.
still,
the daedalus-type drive is best by far. the orion type is used my the
human ship Michael in footfall; it has been said that the alien ship
used
a daedalus type, but i don't recall that.

> ARCJET (Electrothermal)		Isp = 800 to 1200 seconds
> MPD (Electromagnetic) 	Isp = 2000 to 5000 seconds
> ION (Electrostatic)		Isp = 5e3 to 4e5 seconds

so the ion drive has a higher Isp than Orion; i suppose it would have
to.
the thing is that it is very hard to have a high mass-flow rate (the
other
factor in determining thrust) with an ion drive, so an orion will give
you
a higher, less efficient thrust.

> BORON FUSION			Isp = 1e6
> HYPOTHETICAL FUSION TORCH	Isp = 5e4 to 1e6 seconds
> ANTIMATTER			Isp about 3.06e7 seconds
> PHOTON DRIVE			Isp about 3.06e7 seconds

hmm, now this is getting a bit blue-sky. ultraviolet-sky? i've never
heard
of boron fusion before; sounds like a bit of a daft idea to me.

excellent stuff! added to my collection of reference works cribbed from
Nyrath ... shall i webbify this (and star-list?) or are you on the case?

> > good point. the photon drive is well established, needs no fuel, and
is
> > still a really daft idea.
>	Well, the photon drive needs no *reaction mass*, but
>	it needs *power*, which will come from fuel.

sorry, sorry. still, if it is fusion-powered it will not need much fuel.

feeling much wiser,
Tom