VASIMR plasma drive in GURPS...
 

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. I'm just too dense to figure out the math of it all and how it would translate into GURPS starship construction.

Here is my question: assuming a 'mature' version of this type of engine, how does it compare to 'super science' drives of sci-fi movies? Would this be capable of producing low weight ships with high thrust or does it still run into the problem of massive fuel needs?

In other words, would a fully realized VASIMR drive be be used to create a sci-fi style space fighter?

Re: VASIMR plasma drive in GURPS...
 

If only I was still in College Station, I might be able to help you. I had 3 friends working on this project...

Re: VASIMR plasma drive in GURPS...
 

Probably not; using the numbers at Atomic Rocket and the formula from the Designer's Notes, I calculate only 0.002 G per system. And that's using the "high-thrust" gearing.

Re: VASIMR plasma drive in GURPS...
 

A jet velocity of 300,000 meters per second, in a 1g field, can be equated to a specific impulse of a shade over 30,000 seconds. That means that to support a weight of 30,000 lbs. you need to expend 1 lb./second of reaction mass. Double that if you want a net acceleration of 1 g.

So say you make your ship with 10% of its mass in reaction mass, or 3,000 lbs. out of 30,000 lbs. That's a low enough fraction so you can treat it as linear rather than logarithmic and not have a huge error. At 2 lbs./second you're looking at 1500 seconds of thrust. It takes about 1,150 seconds to reach escape velocity, so you can do that and have some left over for maneuvering, but you won't have enough left for a powered lander; you'd better have glider wings.

On the other hand, if you're in circular orbit to start with, you can maneuver pretty freely. You might be able to make interplanetary flights, if you were satisfied with circular orbit at the other end. I suspect it would be a one-way trip, though.

Bill Stoddard

Re: VASIMR plasma drive in GURPS...
 

Ah. That's an important limitation. Should have followed the link.

Bill Stoddard

Re: VASIMR plasma drive in GURPS...
 

300 km/s exhaust, if I'm doing my math right, won't even get you up to 10 mps per tank. So you need lots of reaction mass, though it might be substantially better than the Spaceships stats for the ion drive.

Achieving high thrust is going to demand you take the power level up a lot. At the 1.5 kg/kw, assuming that's power of the exhaust jet, it looks like it would take a single engine system almost 8 days to go through a tank of fuel. At the 30km/s exhaust. High impulse mode is much worse.

Re: VASIMR plasma drive in GURPS...
 

No joy there. It's just an Ion Engine that can switch between between normal and high thrust modes, and even the high thrust mode ain't much.

Re: VASIMR plasma drive in GURPS...
 

All drives are subject to a basic limit: (thrust)*(exhaust velocity) cannot exceed 2 * (drive power). Electric drives are generally limited by the power density of electric power sources, which will generally give an overall power density not exceeding 1 kW/kg and probably substantially lower (1 EP in spaceships appears to be around 50W/kg).

Using that figure, ISp per fuel tank of a 1 EP drive will not exceed 0.00031mps/(acceleration per drive). That limits an ion drive (1 EP, 0.0005G) to 0.62 mps/tank, probably indicating that thrust for electric drives should be errata'd down by a factor of 10 (non-electric thrusters can have orders of magnitude higher power).

Re: VASIMR plasma drive in GURPS...
 

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).

Re: VASIMR plasma drive in GURPS...
 

My maths agrees with yours. I get 9.32 mi./sec./tank, without allowing for the actual mass of the tank.