"Real Thrust"

  We have previously concluded, given one Maneuver Unit = 1000km and 1
Turn
= 15 minutes (900sec), that one Thrust = .25 gee (2.5m/sec^2).	 Assume
that ship mass remains constant (which it doesn't, but we'll also ignore
maneuver thrusters to make up for it).	Given these assumptions, what
kind
of drive might we have, and how much turns worth of fuel?
  Let's look at a 20 tonne ship (so 5% for the drive = 1 tonne) with a 1
thrust drive.  20,000kg * 2.5m/sec^2 * 900 sec = 45,000,000 kg-m/sec. 
I've
seen a prediction for specific impulse for a Nuclear Thermal engine of
Isp
3000.  (1 kg fuel with specific impulse 3000 will produce 1kg--or
Newton,
rather--thrust for 3000 seconds). Dividing this into our result above
yields 15 tonnes of fuel required for one turn's acceleration.	Not
good. 
If we bump it up to Isp 300,000 and assume "drive" is 25% engine and 75%
fuel, then we can use full thrust for 5 turns before we have dry tanks. 
We
are therefore pretty much forced to conclude that our Isp had better be
somewhere in the 3,000,000 range, and even so it gives us only 50 turns
of
acceleration at full power.  Therefore we will not see quick trips from,
say, Earth to Ganymede, boosting all the way.  We are also well out of
the
range of most drives--I think we're stuck with ion drives, perhaps
anti-matter powered jets, or something which extracts energy from vacuum
fluctuation.
  Or I could just be horribly, horribly mistaken (yes, it does happen). 
I'm actually hoping someone will tell me I've dropped a decimal place
somewhere, so please flame away.
laserlight@mci2000.com