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re: fuel chat

From: Sid Jones <sjones@n...>
Date: Fri, 27 Nov 1998 20:15:07 -0500
Subject: re: fuel chat

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, since in space the
faster you throw something away from you the faster you're pushing
yourself forward.  The less efficient your engine is, the more fuel you
have to carry around (can anyone say Space Shuttle?).  With many of
these advanced engines (especially the ion engine) the mass flow rate,
or the amount of propellent you can dump overboard to increase your
thrust, is also very important.  The space shuttle couldn't get you to
Mars, but in a drag race between the shuttle and a high-tech ion engine
rocket, the shuttle's going to win every time because it can blow a lot
of LOX all at once.

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

standing corrected 


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