How do i calculate how much electricity and fuel an ion engine needs?

[Rabada]

The stats for one of the stock ion engines says:

Engine max power: .5

Propellants

- ElectricCharge (12)

- XenonGas (.1)

What do these numbers actually mean?

Edited December 18, 2013 by Rabada

[Rhomphaia]

Max power is full throttle thrust in kN

The values for propellants is how many units are consumed per second at full throttle

[Vlk]

I think that its better if you test it. Fortunately, you dont have to launch it to space, you can check it on the ground as well: just open the panels, fire up the ion engines, and see if the electricity runs out or not.

But: the farther from the sun you are, the less electricity the panels provide... if you visit Jool, you will probably need twice as much panels if you want to use the engines at max capacity.

[Rabada]

Max power is full throttle thrust in kN

The values for propellants is how many units are consumed per second at full throttle

This doesn't seem right, many of the ion engines in mods have a lower value for Electric charge than 12, yet those parts definitely need a lot more juice.

I think that its better if you test it. Fortunately, you dont have to launch it to space, you can check it on the ground as well: just open the panels, fire up the ion engines, and see if the electricity runs out or not.

But: the farther from the sun you are, the less electricity the panels provide... if you visit Jool, you will probably need twice as much panels if you want to use the engines at max capacity.

The main reason I am asking is because I like to mod the values of parts, and I can't figure out how the values for ion engines work. If I'm just playing I test it like you suggested.

[Soda Popinski]

The stats for one of the stock ion engines says:

Engine max power: .5

Propellants

- ElectricCharge (12)

- XenonGas (.1)

What do these numbers actually mean?

I believe the rule of thumb is one Gigantor PV array per ion engine, at least near Kerbin orbit. Farther out, you need more panels, closer in, less. I'm not sure if the game really follows the inverse square law, but it does in real life.

[Rabada]

I believe the rule of thumb is one Gigantor PV array per ion engine, at least near Kerbin orbit. Farther out, you need more panels, closer in, less. I'm not sure if the game really follows the inverse square law, but it does in real life.

Thankyou for the rule of thumb, but unfortunately it doesn't answer my question because I want to know how the formulas work for some person modding I do.

[Soda Popinski]

This doesn't seem right, many of the ion engines in mods have a lower value for Electric charge than 12, yet those parts definitely need a lot more juice.

That's probably because the mod ion engines are overpowered. Even the stock ion engine is greatly overpowered. We don't know the actual electricity units in game, but the Gigantor XL (looks like the ISS solar array) produces 18 E / second. The ISS solar arrays can generate at peak 30 kW (or 30 joules / second). The real world highest thrust ion engine uses 250 kW (~8 ISS solar arrays / Gigantors). For all of that power, it only produces 2.5 N of thrust. Compare that to the stock KSP ion engine of 500 N of thrust for one 2/3 of the power of one Gigantor XL solar array. On the other hand, the real world ion engine has a specific impulse of 19,300 s.

[Guest]

Actually, this calculation is incorrect. KSP only cares about the ratios, not actualy numbers. So, 120 units of EC per 1 unit of Xenon. But the fuel consumption needs to be calculated from Isp formula. In that case, you'd only use Xenon, because EC is massless. Then, you'd calculate EC from the ratio.

[Vlk]

Hmm, maybe i still don't understand the problem well, but as i checked, the mechanics look clear for me:

Engine max power: .5 is .5 kN or 500N - its the force it generates to push the ship.

- ElectricCharge (12) - it takes 12 E per second if you run it on max power

- XenonGas (.1) - it takes .1xenongas per second if you run it on max power

(in the game i see 14.54, and .1212 Xenon - maybe its just the new version. and if i fire up two ion engines i see an energy loss of 29 per sec - so it is correct)

Edited December 18, 2013 by Vlk

[Rhomphaia]

This doesn't seem right, many of the ion engines in mods have a lower value for Electric charge than 12, yet those parts definitely need a lot more juice.

Yep, got that wrong. it is the ratio. 12 units electricity for every 0.1 unit of xenon.

0.23 displays the full throttle consumption of the engine instead.

[Rabada]

Hmm, maybe i still don't understand the problem well, but as i checked, the mechanics look clear for me:

Engine max power: .5 is .5 kN or 500N - its the force it generates to push the ship.

- ElectricCharge (12) - it takes 12 E per second if you run it on max power

- XenonGas (.1) - it takes .1xenongas per second if you run it on max power

(in the game i see 14.54, and .1212 Xenon - maybe its just the new version. and if i fire up two ion engines i see an energy loss of 29 per sec - so it is correct)

I'm sure this is not the case. I tested it with 2 different engines with the following stats:

Engine A

Power: .5

Electric Charge (12)

Xenon Gas (.1)

Engine B

Power: 2.5

Electric Charge (12)

Xenon Gas (.1)

I put each engine on a 1000 unit battery and ran it at full throttle. By your logic, they should both drain the battery at the same rate. However this did not happed, Engine B drained the battery much quicker.

Edit: Same ISP for both engines too

Edit 2: Perhaps the 12 means it uses 12 units of electricity per second per unit of thrust

Edited December 18, 2013 by Rabada

[Rhomphaia]

I'm sure this is not the case. I tested it with 2 different engines with the following stats:

Engine A

Power: .5

Electric Charge (12)

Xenon Gas (.1)

Engine B

Power: 2.5

Electric Charge (12)

Xenon Gas (.1)

I put each engine on a 1000 unit battery and ran it at full throttle. By your logic, they should both drain the battery at the same rate. However this did not happed, Engine B drained the battery much quicker.

Edit: Same ISP for both engines too

Edit 2: Perhaps the 12 means it uses 12 units of electricity per second per unit of thrust

12 units of electricity per 0.1 units of xenon.

Units per second are calculated based on ISP and thrust of engine so in your example engine B would drain electricity 5 times as fast.

[RadHazard]

To directly answer your question, you need to calculate the mass flow rate of the ion engine. From wikipedia:

We start off with this equation:

V[SUB]e[/SUB] (effective exhaust velocity) = g[SUB]0[/SUB] (surface gravity) * I[SUB]sp[/SUB]
g[SUB]0[/SUB] is actually 9.81 m/s^s, but I believe KSP uses 9.82 m/s^s for mass flow calculation.
For the ion engine, the effective exhaust velocity is 9.82 m/s * 4200 s = 41,244 m/s

To find mass flow rate using V_e, we use this equation:

Thrust = V[SUB]e[/SUB] * m (mass flow rate)
We rearrange it to get m = 0.5 kN / 41,244 m/s = 0.0000121 kN*s/m, or 0.0121 kg/s

Xenon in KSP weighs 0.1 kg per unit and electricity is weightless, so we get a final flow rate of 0.121 units of Xenon per second. Since the ratio listed on the stats determines how much electricity it uses, we get 14.52 units of electric charge per second. Stock Gigantors produce 18 EC/s in Kerbin orbit, so 1 Gigantor per ion engine is a decent rule of thumb. Unfortunately KSP doesn't have a proper inverse square relationship with solar power, so it's more difficult to calculate how many you'd need in, say, Jool orbit.

[Rabada]

Thankyou RadHazard, that was exactly what I was looking for!