Reverse engineering Jet Engines - how do velCurve and atmCurve work?

[antbin]

340m/s = mach

That's what the code says? Why would they go to the trouble of working in Mach numbers if the speed of sound doesn't change with density/altitude? Just for cross-planet conversion?

I'd like to start plotting some of these for typical aircraft.

Edited April 28, 2015 by antbin

[Radam]

That's what the code says? Why would they go to the trouble of working in Mach numbers if the speed of sound doesn't change with density/altitude? Just for cross-planet conversion?

I'd like to start plotting some of these for typical aircraft.

Apparently it does go to 295 at 10km or so then back to 340. Details...

The stall curves? Maybe the people behind FAR will make a plugin for stock aero.

[NathanKell]

The math for temperature appears based on the new temperature curves in CelestialBody. From aeroGUI I can tell you that the daytime temp at the equator is about 40C, night is more like 30C. Gets colder the further away from the equator you go. Temperature varies with altitude, and sun appears to have varying influence on the "offset" temperature at various altitudes.

Mach is indeed temperature (and thus altitude) related.

[antbin]

Probably not stall curves, more interested in maximum speed (thrust or thermal limited) and ceiling, in terms of throttle %.

For this thread though I'm interested in the intersection of thrust vs. airspeed vs. altitude curves with drag vs. airspeed vs. altitude curves. Both should be computable for level flight and 'typical' aircraft shapes / drag. Gives a ballpark answer to the questions:

* How fast / high will it go?

* What if I strap a takeoff booster to it to get the RAPIER above Mach 2?

* How fast will it overheat and explode?

[Radam]

Just need to streamline your craft...

Edited April 28, 2015 by Radam

[antbin]

Nice graphs!

http://imgur.com/yciepgi,Cp3Hk6P#0

http://imgur.com/Ngris7N

[BlueCanary]

So from this, is there a way to work out the most efficient (as in miles per gallon type efficiency) speed/altitude for engines? Because that'd be really useful for designing efficient long range basic/turbojet cargo planes and airliners (sorry if I should have worked that out already, I've got a bunch of maths exams coming up at the moment so I'm too maths-d out to even try .

[antbin]

Good question. As far as I know, the ISP for air-breathing engines is constant now, and fuel consumption scales with thrust (not throttle or altitude). So for long flights the main thing affecting fuel economy is how little thrust you need to overcome drag.

Full throttle at the highest altitude feasible seems (to me) to be the best for fuel economy.

After exams, why don't you enter my fuel efficiency challenge and see whose designs work out best?

Edited May 18, 2015 by antbin

[BlueCanary]

Good question. As far as I know, the ISP for air-breathing engines is constant now, and fuel consumption scales with thrust (not throttle or altitude). So for long flights the main thing affecting fuel economy is how little thrust you need to overcome drag.

Full throttle at the highest altitude feasible seems (to me) to be the best for fuel economy.

After exams, why don't you enter my fuel efficiency challenge and see whose designs work out best?

Yeah, I was thinking that might be it but what I've found from my (very limited) experimentation is that the difference in speed from 2/3 throttle to full throttle is relatively small and some high TWR planes can get close to top speed even with 1/3 throttle (I have a single basic jet fighter, about 4.5 tonnes, that in 1.0 could cruise at about 260m/s on 1/3 throttle at 0.02 fuel units a second) so it might be that staying below top speed helps. I guess that would make sense if drag increased exponentially up to top speed, but I'm not sure how much that is true. Then again, this was all 5000-10000m and basic jets, so at high altitudes and speeds might be completely different.

The challenge looks fun, I might try it at the weekend as I have a week off from exams then. What I was thinking for the craft is that if I make a plane with peak Lift/Drag ratio at the engines best speed (the turbojet now has a thing saying max thrust at mach 3.4, so if my plane flies best at mach 3.4 it should be able to attain highest altitude and so best efficiency there I think, if that makes any sense at all) it should be really efficient. Then again, I'd have to figure out the Alt-F12 Lift/Drag chart thing to do that.

EDIT: Looking again at the graphs you posted it looks like the one for basic jets seems related to what I found testing planes with them, that 7000m seemed to be the best cruise altitude as far as speed on low throttle was concerned. Don't really know what that means though, just thought it seemed relevant. The main thing I found though was that (at least on the planes I have on 1.0.2 at the moment, mainly subsonic basic jet planes such as this one http://forum.kerbalspaceprogram.com/threads/120305-1-0-MP-2-Seahorse-Multipurpose-Aircraft-%28UAJ%29 ), reducing throttle by 10% didn't mean a 10% reduction in speed, suggesting full throttle might not always work best, but again I'm not entirely sure what that means and also it might not translate so well to high altitudes and speeds. It would be interesting to see how your challenge entry (which is really impressive, especially considering most of my recent designs even with basic jets consume 89 units before they're even over the KSC boundary) would have done differently cruising at say 2/3 throttle at the same altitude, if you put a craft file up I might try.

Edited May 18, 2015 by BlueCanary

[Insanitic]

So here's my question. How does the thrust factors in atmCurve relate to the thrust factors in velCurve? Are they added up in game?

[NathanKell]

Multiplied.

[Insanitic]

Oh, so just to clarify, if useVelCurve and useAtmCurve are both set to true, and each atmcurve and velcurve have their own thrust multipliers and therefore thrust values, the two thrust values are multiplied for the total thrust?

Sorry if my question sounds wordy, I just want to make sure that you know that I'm not questioning about how for example, velCurve modifies thrust, but how the both atmCurve and velCurve gets "summated" to give the total thrust when both are enabled.

[Zaddy23]

I hope this isn't too dead, but I need help, I'm trying to createa scramjet-type engine, and I'd like to modify these curves so it produces no thrust below mach 3 and 10km, dropping off at mach 6 and 30km respectively. How would I do this? My current values:

        // Jet params        
        atmChangeFlow = True
        useVelCurve = True
        useAtmCurve = True
        machLimit = 8
        machHeatMult = 6.0
        velCurve
        {
            key = 0 0 0 0
            key = 0.2 0 0 0
            key = 0.72 0 0 0
            key = 1.36 0 0 0
            key = 2.15 0.1 0 0
            key = 3 3 0 0
            key = 4.5 5 0 0
            key = 5.5 0 0 0
        }
        atmCurve
        {
            key = 0 0 0 0
            key = 0.045 0.166 4.304647 4.304647
            key = 0.16 0.5 0.5779132 0.5779132
            key = 0.5 0.6 0.4809403 0.4809403
            key = 1 1 1.013946 0
        }

I'm pretty happy with the thrust, but I have no idea what I'm doing with the atmosphere curve.

[antbin]

Hi Zaddy. If you want the engine to produce no thrust below 10km, and no thrust above 30km, then the second (thrust multiplier) column in your atmCurve would need to begin at 0 and end at 0 too.

The first column (percent pressure) in your atmCurve would need breakpoints at 0,10,20(?), and 30km at least, maybe more. You can convert between altitude and atmospheric pressure with roughly Altitude = -5000 ln(percent Pressure) or percent Pressure = e^(-Altitude/5000). So 10000 is about 0.135, 20000m is 0.018, and 30000m is 0.0025.

Maybe 0 0 ; 0.0025 0 ; 0.018 1 ; 0.135 0 ?

The third and fourth columns are tangents - without a visual editor it's hard to see the implications of changing them. Doesn't Unity have an AnimationCurve editor?

[Zaddy23]

Maybe 0 0 ; 0.0025 0 ; 0.018 1 ; 0.135 0 ?

Okay, Thanks for replying! I will try these values when I next get a chance to play KSP.

Love the Tintin reference btw, that book in particular was awesome.

EDIT: okay, I have plugged in the values, and as with the velocity graph I made the tangents 0, for simplicity's sake. It seems to work fairly well, as it gives no thrust until about mach 2.8 and is above 8.4km, which is acceptable to me. Although looking back on it I did have fun cruising around at sea level at speeds to impress the SR-71, so now I'm wondering if I should change it so that that is still possible.

Edited August 5, 2015 by Zaddy23
Added experimental results