How does KSP calculate ISP for engines?

[-ctn-]

Okay, so I've been learning some of these rocket equations in my spare time. And I've come up on a brick wall.

Let's say I have an engine I want to make for the game, and I know the thrust and ISP I want it to have. But in the config files, you can input the thrust and fuel consumption rate values.

The equation for fuel consumption is Thrust (in newtons) / (gravity * ISP ). Otherwise known as Thrust / exhaust velocity.

So if I use this equation on one of KSP's engines, let's say the O10 monopropellant engine, the number I get does not match the stats in game.

Thrust = 20 kN

Isp = 250 s

and fuel rate = 2.04/s.

If I use the real world equation, I get: 8.15.

I've tried it with several engines but it never matches up...

[UmbralRaptor]

There's an extra conversion -- units of LF/Oxidizer/Mono are "volumetric", rather than mass.

20 kN /(250 s *9.80665m/s²) == 0.0081577 tonnes per second

Note that the "density" of monoprop is 0.004 tonnes/l

0.0081577/0.004 == 2.0394324 units/s

[-ctn-]

Ahhhhhh thank you so much!

[UmbralRaptor]

Uh, also to expand a bit, mind the ratios -- you can do the same with LF/Oxidizer engines (0.005 tonnes/l), but keep in mind that they run 55% oxidizer, 45% liquid fuel.

[-ctn-]

Err, still having problems with this. For example - I have an engine I made for a mod. In the config file, I put 0.93 for engine fuel ratio. In game however, the stats say "5.09" for fuel consumption and ISP of 250 s.

Thats exactly what ISP I want, however for future reference, what is the extra calculation there? None of the config files seem to match up to what the fuel ratio is in-game.

[UmbralRaptor]

Could you post the entire CFG file (say, on pastebin), and what the engine is doing?

(And if there are any mods, this should probably be moved to gameplay questions)

[-ctn-]

You can see it on the stock monopropellant engine.

http://wiki.kerbalspaceprogram.com/wiki/Parts/Engine/OMSEngine/omsEngine.cfg

ratio says "0.9" and in game the stats are "2.04".

[UmbralRaptor]

The ratio isn't an actual ratio, but a weighting factor. As best I can tell, KSP adds them all up, then uses the ratio value over the sum. (A notable example is the ion engine)

I was also asking, because I was wondering about the details of your modded engine (eg: is it the O-10 just with that number changed, or...?).

[-ctn-]

Over the sum of what? Thrust / atmosphere curve?

My engine was just the o-10 with a different FX and changed values. Ratio was changed to 0.93 and thrust to 50. In game, it says 50kN, isp of 250 s and fuel rate of 5.09 / s

[UmbralRaptor]

"Ratio" of that propellant supply over the sum of all other "ratios." Things should only get messy if you have multiple propellants of varying densities.

Since the engine still only burns monopropellant, that's 0.93/0.93 == 1

mass flow == thrust/(g0*Isp) == 50/(9.80665*250) == 0.0020394 tonnes/s

"volumetric flow" == mass flow/density * proportion == 0.0020394/0.004*1 == 5.0986 units/second

[-ctn-]

Well, that's helpful to know... But sorry, I may be missing something. In the configuration files, you can only specify RATIO and THRUST. So how does KSP calculate fuel flow and ISP from only these two values?

[UmbralRaptor]

Isp *is* specified (at 0 atm, 1 atm, and some higher amount where it hits 0.001 s) in the CFG files. eg:

atmosphereCurve
		{
			key = 0 250
			key = 1 120
			key = 4 0.001
		}

250 s at 0 atm, 120 s at 1 atm, 0.001 s at 4 atm. Interpolation is messy.

[-ctn-]

Isp *is* specified (at 0 atm, 1 atm, and some higher amount where it hits 0.001 s) in the CFG files. eg:

atmosphereCurve
        {
            key = 0 250
            key = 1 120
            key = 4 0.001
        }

250 s at 0 atm, 120 s at 1 atm, 0.001 s at 4 atm. Interpolation is messy.

Haha, you've got to be kidding me.

Thank you so much for your help and patience!

[Armchair Rocket Scientist]

There's an extra conversion -- units of LF/Oxidizer/Mono are "volumetric", rather than mass.

20 kN /(250 s *9.80665m/s²) == 0.0081577 tonnes per second

Note that the "density" of monoprop is 0.004 tonnes/l

0.0081577/0.004 == 2.0394324 units/s

AFAIK the units are actually mass (one unit of LF or oxidizer is 5 kg, but units of monoprop and xenon have different values. Can't remember how solid fuel is handled.

However, RealFuels does have volumetric units (liters for everything).

[K^2]

Interpolation is messy.

A bit, but not too complicated. It's a standard cubic spline, keyed from values and slopes. The slopes are computed by taking average from the two neighbors. Just in case anyone is curious.

[mhoram]

And for the calculation, gravity is not 9.81 m/s² but 9.82 m/s².

Have a look here: http://wiki.kerbalspaceprogram.com/wiki/Specific_impulse#Conversion_factor

[UmbralRaptor]

And for the calculation, gravity is not 9.81 m/s² but 9.82 m/s².

Have a look here: http://wiki.kerbalspaceprogram.com/wiki/Specific_impulse#Conversion_factor

Was 9.82, but as of 1.0 changed to 9.80665.

(The page will probably require an extensive rewrite, given the way engines also changed.)

[mhoram]

Was 9.82, but as of 1.0 changed to 9.80665.

Thanks for the info. Have not known about this change.