Had an idea but I need some help

[jab136]

I am trying to write a matlab script to create optimal ascent profiles for launch vehicles and I was wondering if anyone knew if there was a way to get thrust curves for every engine as a function of pressure (for rocket engines)  and the total thrust curves for jet engines.  I also need a method of exporting Cd and Cl curves from the game for FAR, and as a note, I am using AJE which affects the thrust curves for the jet engines.

Edited May 20, 2016 by jab136

[Gaarst]

From Wikipedia:

F thrust, Isp specific impulse in vacuum (s), g_o standard gravitational acceleration, m mass flow rate, A_e cross sectional area of nozzle exit and p_o external ambient pressure (Pa)

You are not given Ae but you can work it out fairly easily using given asl stats.

I'm not sure KSP calculates atmospheric thrusts like this though, you should probably check that this works.

 

For jet engines, you have to account for speed as well. I don't know the exact equations, but several thrust curves with speed have been made. You can find them searching a bit on the forums.

Edited May 20, 2016 by Gaarst

[jab136]

1 minute ago, Gaarst said:

From Wikipedia:

F thrust, Isp specific impulse in vacuum (s), g_o standard gravitational acceleration, m mass flow rate, A_e cross sectional area of nozzle exit and p_o external ambient pressure (Pa)

You are not given Ae but you can work it out fairly easily using given asl stats.

I'm not sure KSP calculates atmospheric thrusts like this though, you should probably check that this works.

 

For jet engines, you have to account for speed as well. I don't know the exact equations, but several thrust curves with speed have been made. You can find them searching a bit on the forums.

I know the real relations (I am currently working on my graduate AE degree and it looks like my thesis will be on propulsion), I was just trying to figure out if anyone knew how squad programmed it, since it may not actually follow that curve very well.

[Gaarst]

The best answer to that is to do the experiment.

Calculate predicted thrusts using the formula. Measure the thrust of an engine at different pressures/altitudes in KSP. Compare.

If the two are the same, then great. If not, well, at least you have experimental measurements, try to work out the formula using them.

Edited May 20, 2016 by Gaarst

[jab136]

I pulled up the config file for the skipper and found a bit that is 

atmosphereCurve
{
	key=0 320
	key=1 280
	key=6 .001
]

I assume this is the thrust curve, but I wasn't sure how they interpolate between the points, I also assume the first number is the number of atmospheres where this is true, and the second number is the Isp at that pressure

8 minutes ago, Gaarst said:

The best answer to that is to do the experiment.

Calculate predicted thrusts using the formula. Measure the thrust of an engine at different pressures/altitudes in KSP. Compare.

yah, I will probably end up doing that, I was just kind of hoping I could find someone who knew how it was actually coded

Edited May 20, 2016 by jab136

[Gaarst]

1 minute ago, jab136 said:

yah, I will probably end up doing that, I was just kind of hoping I could find someone who knew how it was actually coded

Or you can wait a bit, I'm not the only one here on the forums, and there are probably even users that know how this works

[jab136]

yah, I am currently breaking in my new laptop anyway (applying moderate OCs to most of the components) so I wasn't going to boot KSP for at least a day anyway.

[Gaarst]

Just did a quick test with a Skipper. The formula seems to fit the measurements pretty well (only a difference of 2kN at high pressures, but it is probably due to rounding errors).

[jab136]

yah, I did the test with the "swivel"  so I guess they didn't model it completely properly since technically the exit conditions are dependent on the outside conditions (v_e and P_e aren't actually necessarily constant since you can have issues if P_e and P_a have a large difference).  Oh well, that makes it easier for me.