Re: fuel chat

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.  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. 

	You're correct, though Mr. Jones isn't all wrong.

	If :

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

	Then:

Isp = Ve / g
Ve  = g * Isp

	The factor that every one is interested in is "delta-V"
	or "change in velocity".  In EFSB, this corresponds to
	how far one can move the endpoint counter when the
	ship does a burn.

	If:

Mbs   = mass of ship at start of burn (kilograms)
Mbe   = mass of ship at end of burn (i.e., Mbs - mass of propellant
used)
1n(x) = take the natural logarithm of x

	Then:

deltaV = Ve * 1n( Mbs/Mbe)

	Isps I have on hand:

CHEMICAL ROCKET
Propellant				Isp
===========================================
Hydrogen-Fluorine (F2/H2) ideal 	528
Hydrogen-Oxygen (O2/H2)  space shuttle	460
Hydrogen-Oxygen (O2/H2)  ideal		528	
(O3/H2)  ideal				607
(F2/Li-H2)				703
(O2/Be-H2)				705
Free Radicals (H+H)->H2 		2,130
Metastable Atoms (e.g. Helium)		3,150

SATURN V FIRST STAGE		Isp = 430 seconds
SPACE SHUTTLE			Isp = 455 seconds
NERVA/DUMBO "Atomic Rocket"	Isp = 850 to 2950 seconds
ORION "old Bang-bang"		Isp = 1000 to 5000 seconds
ARCJET (Electrothermal) 	Isp = 800 to 1200 seconds
MPD (Electromagnetic)		Isp = 2000 to 5000 seconds
ION (Electrostatic)		Isp = 5e3 to 4e5 seconds
LIQUID CORE NUCLEAR REACTOR	Isp = 1300 to 1600 seconds
GASEOUS CORE NUCLEAR REACTOR	Isp = 3570 to 7000 seconds
GASEOUS CORE COAXIAL FLOW REACTORIsp = 1800 seconds
DAEDALUS (Fusion Microexplosions)Isp = 1e6
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

> 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.
	In chemical rockets, the reaction mass and the power fuel
	is the same thing.
	In an ion rocket, they are different.  For instance,
	the cesium reaction mass will be accelerated by
	power from a nuclear reactor fueled with plutonium.

-- 
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