VASIMR plasma drive in GURPS...

[Ulzgoroth]

Originally Posted by Anthony

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).

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.

[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?

[Anthony]

Originally Posted by Ulzgoroth

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?

We think that electric drives (mass drivers have the same issue) should be errata'd.

[Fred Brackin]

Originally Posted by Ulzgoroth

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?

That's obviously where the bright white glow in Star Wars "ion drives" comes from. :)

To the OP, any proposed Real World drive that boasts of its' (potential) performance is comparing itself to earlier RW proposals that were eventually abandoned as impractical, not fictional drives.

[Agemegos]

Originally Posted by Ulzgoroth

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

I hate the nasty relationship between the units chosen for acceleration and the units chosen for velocity. Who wants a conversion factor in v = a*t ?

A 1-gee drive gets up to 1 mi./sec. in what? 164.2 seconds? What unit of time is that?

[Timolas]

Ooh - do I sense a cry to make GURPS metric...? ;-)

Come on SJG! You know it makes sense! The metric system is brilliant.

(...and the tool of the devil).

[Agemegos]

Originally Posted by Timolas

Ooh - do I sense a cry to make GURPS metric...?

It wouldn't be the first.

GURPS 0th edition (The Fantasy Trip) was metric. But SJGames has learned since that foreign buyers are better able to learn the complexities of the mediaeval English system than US buyers are to learn the simplicities of metric.

[GoblynByte]

Originally Posted by Fred Brackin

That's obviously where the bright white glow in Star Wars "ion drives" comes from. :)

To the OP, any proposed Real World drive that boasts of its' (potential) performance is comparing itself to earlier RW proposals that were eventually abandoned as impractical, not fictional drives.

Yeah. I honestly wasn't holding out great hopes or anything. I just saw it mentioned in a show the other day and, once again, I went through the emotional roller coaster of thinking maybe THIS one could do it. Okay, that's an exaggeration. It was more like and emotional 'ride on the cars with that track down the middle.' But you get the idea. ;)