# VASIMR - Propellant usage data?

So I am using the VASIMR data of approximately 5N of force per 250kw of power draw. As far as I can see, this scales fairly linearly, so 500kw would result in 10N of force. (Might require two engines, I'm not sure how ion drives actually scale up, but that's a separate question).

However I can't seem to find the propellant (xenon, I think is usually used) usage for that 5N of force.

I do suspect there's an obvious way to work it out from the specific impulse, which I know is Ns/kg.. but I'm not quite sure how I use that information. Does it mean that an engine with a specific impulse of 12000Ns/kg, can put out 1N of force for 12000 seconds, using 1kg of propellant?

(and a second minor question, how do engines like the VASIMR scale up? If you want 50N of force, do you build 10* 5N engines, or can you build a single much bigger engine (I don't know if the technology can be increased in size like that), or can you just pump 200MW into a single normal engine (if you can keep it from exploding). Would be interested in the solution - I suspect the answer is to build 10 engines)

Thanks :)

I do suspect there's an obvious way to work it out from the specific impulse, which I know is Ns/kg.. but I'm not quite sure how I use that information. Does it mean that an engine with a specific impulse of 12000Ns/kg, can put out 1N of force for 12000 seconds, using 1kg of propellant?

Exactly so. "Specific impulse" is short for "mass-specific impulse", meaning "impulse produced per mass of propellant". Impulse is force times time (e.g. newton-seconds), and is dimensionally equivalent to momentum (e.g. kg•m/s) -- application of impulse yields change in momentum.

So for 5N at 12000 N•s/kg, you're spending 5/12000 = 0.000417 kg of propellant every second.

Flipping it around, mass flow rate (kg/s) times specific impulse (N•s/kg) equals thrust (N).

(and a second minor question, how do engines like the VASIMR scale up? If you want 50N of force, do you build 10* 5N engines, or can you build a single much bigger engine (I don't know if the technology can be increased in size like that), or can you just pump 200MW into a single normal engine (if you can keep it from exploding). Would be interested in the solution - I suspect the answer is to build 10 engines)

I would guess you need bigger grids as you run more power, but there are probably some economies of scale to be had. One hopes that stated engine stats are at the best-performance power level. For back-of-the-envelope spacecraft design it's probably best to assume that engine mass and power requirements scale linearly with thrust, as if you were just stacking additional engines alongside the first.

• Thankyou! I would upvote your answer, but stack doesn't let me do that yet :( appreciate your help though! Nov 5, 2019 at 1:03