From: Laserlight <laserlight@q...>
Date: Wed, 15 Jul 1998 23:03:36 -0400
Subject: "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.