VASIMR plasma drive in GURPS...

[Anthony]

Originally Posted by Ulzgoroth

Er, I think your arithmetic is badly off.

My own estimate, which is about 40 W/kg ship mass (derived from the power output of high-TL chemical power plants), is good for about 0.005 mps*g.

Hm. Okay, 0.005 mps/tank is an exhaust velocity of 0.1 mps or 161 m/s. 1G is 9.8 N/kg. Power would thus be 9.8N/kg * 161m/s * 0.5 = 789W/kg.

If you want, we can break down further. For a 1 kg ship, 9.8N at 161m/s is a mass flow rate of 0.0609 kg/s, with a ke of 0.0609 * 161^2 * 0.5 = 789W.

I suspect your math error was by using exhaust velocity = mps/tank, not exhaust velocity = 20*mps/tank. Dividing by 20 does yield your numbers.

[Ulzgoroth]

Originally Posted by Anthony

I suspect your math error was by using exhaust velocity = mps/tank, not exhaust velocity = 20*mps/tank. Dividing by 20 does yield your numbers.

Yeah, looks like that's what I did.

I'm really starting to hate spaceships for doing things like this.

So, do we think a power point is somehow hundreds of watts/kg, or do we think that ion drives run on magic pixie dust?

[Agemegos]

Originally Posted by GoblynByte

I need help from folks who understand this stuff better than I do.

http://en.wikipedia.org/wiki/Variabl...oplasma_rocket

The drive presented here is supposedly capable of impulses up to 30,000 seconds and producing 300 km/s jet velocities.

Same thing. In the metric system specific impulse is exhaust velocity. Only in mediaeval units do you divide momentum by mass and get time rather than speed.

Basically, to represent a rocket system in GURPS Spaceships you need to know its thrust-to-mass ratio and its exhaust velocity or specific impulse in time units.

(1) Thrust to mass ratio is in units of acceleration. Divide this by 1 gee (32.17 ft/sec/sec, or 9.8 ms^-2) to convert it into gees. Then divide by twenty to get acceleration per engine system.

(2) If you are stuck with specific impulse in units of time, multiply it by Earth's surface gravity to convert it into natural units. That's 32.17 ft./sec./sec. or 9.8 ms^-2. Once you have specific impulse in natural units (or exhaust velocity, which is the same thing) first divide by 20 and then convert from ms^-1 or ft./sec. to miles per second (there are 1609 metres in a mile, and 5280 feet in a mile).