Resource Usage of Air-Breathing Engines

[KerikBalm]

"This implies that even at "InfinityU" output, the air intake has a hard limit of how much it can generate per second. It's just very high."

Well, each intake has a certain capacity. I suspect that when the output is infinity, each "tick" or "frame" that intake gets its resource bar filled.

For example, a radial intake has an intake air capacity of 1 (you can close the intake, and tranfer intake air into it, to get "bottled air" to use the jets briefly in space).

My guess is the sequence goes like this -> fill intake capacity, regardless of what it is -> subtract required intake air from that pool.

If the intake air required per tick is more than the total amount that the intakes can store, you have a problem.

I suspect if you modded the .cfg file to have each air intake hold 1000 units of intake air, the jets wouldn't suck it dry any more.

"All other values respond as mathematically expected, but none of them affect flameout altitude from what I can see."

As I already noted, there seem to be other things affecting the equlibrium air amount. I suspect that 10km is not an adequate test altitude.

You aren't getting more than 2.8 units/sec consumption at the 0.005 density

Try going higher, or setting the Isp even lower... drop it another order of magnitude (80, 250, 120 Isp curve) , and then use 150:1 instead of 15:1

And just to be sure, drop it another order of magnitude (8,25,12 Isp curve), and set the intake air to 1500:1

Ppen Intakes seem to reach an equilibirum fullness at less than 100% even when no airbreathing engines are active.

I don't know how the intake air works, but it may be the consumptionis too low to affect the equilibirum at that altitude.

Or the answer may be related to why the "infinite" flow still had flameouts.

Maybe its a matter of you simply get .16 intake air per "tick", and if your engines need more than that, you get a flame out.

The big flaw here is that you didn't test any of these to the flameout point.

Scale the Isp and intake air ratios to have the same LF consumption, but orders of magnitude different intake air consumptions.

If the difference in consumption is ~54x, then given the scale height of 5km, and that e^4 is 54, you should have a difference in flame out altitue of 20 km.

Its 2 flights to try, first the low air consumption one, hold a 30 degree climb, full throttle, until it flames out, do the same with the high intake air consuming one.

I suspect it will be very different (I don't have time to test right now, GF is coming over soon)

[Streetwind]

Or the answer may be related to why the "infinite" flow still had flameouts.

Maybe its a matter of you simply get .16 intake air per "tick", and if your engines need more than that, you get a flame out.

Maybe. I'm not sure. But it appears that it is some form of relative intake performance indicator. And as I just found out, in most cases it seems entirely independent of the engine consumption. This kind of surprises me, because then I have no idea how the turbojet managed to suck the intake dry (although it didn't flame out!) in the previous test. Right now I had a test with a basic jet flaming out on me even with 0.06 still displaying in the resource bar. If the resource bar was in any way affected by engine air consumption, then it should have read zero for several seconds at least before the flameout.

The bar being unaffected by engine air draw also explains why I saw the same result at the same altitude for all of the various ratio and Isp tests.

At 1:15, 2000-1800-1000, the basic jet flames out at 27km.

At 1:150, 200-180-100, the basic jet flames out at 15.5km.

In both cases, I still see 0.16 in the resource bar at 10km height.

But then what the hell happened with the turbojet sucking the bar dry on the runway? That makes no sense whatsoever...

I'm honestly considering to test under FAR instead, because FAR has a flight readout that's a million times more useful. It actually shows you your current intake performance as a percentage of engine air draw. It starts out at several thousand percent when you're low down and getting full intake performance, then dropping and dropping as you raise into thinner regions of the atmosphere (and also as your Isp is dropping and the engine gets more hungry). Eventually it drops to the point where there is only 100% left. And then, as it drops below 100%, your engine starts to spool down due to air deprivation. And finally, when it drops to 10%, the engine flames out. Which is exactly what the basic jet and turbojet show in the VAB: "Flameout under: 10%". The RAPIER meanwhile flames out under 33%, so it has a lower flameout ceiling than the turbojet despite sharing almost all other stats.

But I don't know what other things FAR might do that change up the results =/

[KerikBalm]

Well, there you have it, the intake air resource bar is not a good proxy for flameout altitude, and the jets that use less intake air flame out later.

It seems since we can't make intake air work if its massless, to get realistic jet Isps requires a lot of playing around with intake, fuel:air ratios, and Isps...

darn...

[GoSlash27]

Well, there you have it, the intake air resource bar is not a good proxy for flameout altitude, and the jets that use less intake air flame out later.

It seems since we can't make intake air work if its massless, to get realistic jet Isps requires a lot of playing around with intake, fuel:air ratios, and Isps...

darn...

KerikBalm,

It's not actually as difficult as you think.

The intake air resource merely shows how much air is left in the intakes after the engine has used it's allotment of air in the previous timestep.

As the engine operates at higher altitudes, Isp decreases (the curve I mentioned earlier). Likewise, the intakes are drawing air in proportion to AVpcos(a).

As the speed increases, thrust is reduced in accordance with the other curve.

If the engine finds itself with insufficient air, it will throttle back even more.

Getting an engine to operate at higher altitudes with the same intakes is simply a matter of increasing the high altitude Isp (allowing it to consume less air) and/or high speed thrust (allowing it to go faster to collect more air).

Best,

-Slashy

[Streetwind]

KerikBalm,

It's not actually as difficult as you think.

The intake air resource merely shows how much air is left in the intakes after the engine has used it's allotment of air in the previous timestep.

But that's precisely not what I saw happening in my tests. I've had an engine drawing 0.14 units/sec of Intake air show the exact same resource bar as one drawing 2.86 units/sec. That's twenty times as much, and there was no difference in the resource bar. None whatsoever. You can see it all a bit further up in the test results table that I posted.

The flameout altitude is different, but the resource bar just shows the same thing every time.

[GoSlash27]

But that's precisely not what I saw happening in my tests. I've had an engine drawing 0.14 units/sec of Intake air show the exact same resource bar as one drawing 2.86 units/sec. That's twenty times as much, and there was no difference in the resource bar. None whatsoever. You can see it all a bit further up in the test results table that I posted.

The flameout altitude is different, but the resource bar just shows the same thing every time.

You would expect it to; that's a function of the initial capacity number in the intake config file.

example from the XM-G50:

MODULE

{

name = ModuleResourceIntake

resourceName = IntakeAir

checkForOxygen = true

area = 0.006

intakeSpeed = 10

intakeTransformName = Intake

}

RESOURCE

{

name = IntakeAir

amount = 0.5

maxAmount = 1.0

The intake starts with an initial amount of .5 sitting still at sea level regardless of any other considerations.

But more importantly, it has absolutely no bearing on flameout altitude/airspeed. All that matters there is the intake area, airspeed (ground),angle of attack, and atmospheric pressure. If the air collected is >= the air consumed at the throttle setting/ Isp, the engine will stay lit. Note that I did not say "thrust produced"; that is completely independent of these factors and is set by a different atmospheric curve.

This is all from the KSP script here

Best,

-Slashy

Edited January 11, 2015 by GoSlash27

[numerobis]

The resource bar number depends on build order. It's mostly a useless number.