Unbalanced Jet Engine Force at High Atmosphere

[Frank_Black]

Hello everyone, I am a fresh man in aircraft building. Here I got a problem.

Right now I built a simple aircraft with twin engine, but once I reached about 23km, one engine started to lose trust power, but another engine continued to increase power, which caused the whole jet to spin like a gyroscope!

The next time I tried to fly higher, when I reached 20km, I lowered the nose down, decreased the climb rate to around 20m/s, but the same thing happend, this time I see very clearly that engine 2 trust level power dropped directly around 10kN, then it began to lose power while engine no. 1 continued to increase power. I had to cut the trust level of both engine to idle, then easily increase both trust. After a while, engine No.2 stopped again and once more I did the same thing to regain control of the aircraft...

Although I finally managed to reach 31km high, maintained stable altitude for several minutes, I still do not know why engine No.2 just keep losing power. None of them were flamed out, they were working the whole time.

Edited April 28, 2014 by Frank_Black

[FREEFALL1984]

This is because there is insufficient intake air available, all the intake air comes from a common pool, when there is plenty of air available both engines will take their maximum required air from this pool. when there is no enough air available, the air will be taken for one engine first, then the second, even thought the engine is still running, the slight lack of required air causes the second engine to slightly lose power, this causes the craft to turn its intakes away from the angle of attack and amplify the issue. All I can recommend is more intake spam to boost up the total pool which will help the issue, but I normally have a single jet in the middle, then when the intake air gets too low I just kill my second and third engine and keep my middle engine running and since it now has the whole intake air pool to itself it can run for much longer.

[Frank_Black]

Oh, I see.. Thank you~~

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Oh I see.. Thank you !~~

[MisterFister]

For the record, the term for what you've experienced is "asymmetric high-altitude jet flameout." It's a thing, and must be somehow accounted for in the design stage. One way, mentioned above, is to leave one jet engine along the thrust vector, and shutdown all other outboard jet engines, usually using action groups. (For fancy-pants multi-engine designs, you can shut off engines in pairs from outboard inward toward the center -- and like the suggesting party said above, with every engine shutdown [or flameout, as you've discovered] the same "pool" of IntakeAir becomes re-available to the still-running engines.)

Another way to address this is with onboard staging -- you can assign engines to various stages, and there even exists a "stage" action group. (Just because it's a stage-activation doesn't mean you need a decoupler to decouple.) At high altitude, you can switch from jets to [other type of jets, perhaps ramjet, or to] rockets, for an SSTO ascent profile.

To better understand the engine behavior, I recommend right clicking one or more of them at low altitude, and pay attention to all the complex-seeming numbers. None of those numbers is "eye candy," they ALL have an extremely specific purpose for being displayed. After a few test runs -- and one of the greatest things about .23.5, in my view, is that you can quicksave in atmosphere, allowing you to replay from the same point repeatedly -- you should be able to anticipate when flameout becomes inevitable just before it actually occurs. Once you can identify it with the readouts, identifying it without the readouts will begin to become easier as well. Eventually, you'll be able to feel it happening just by the engine sound, throttle setting, and navball / altimeter readings. It is once you can do THAT, that "designing around it" becomes the easiest.

God I love this game.

[Temeter]

You could also use Mechjeb. There is an 'prevent jet flameout'-option, which automatically lowers thrust to avoid flameouts.

[Random Tank]

Yeah, I like to use the odd engine number technique for my SSTO's; that means you can still have power at super high altitudes and when the single engine does flameout, it doesn't spin the plane. You can detect flame outs because you see the nose start twitching, and then start to pull left or right; when this happens just cut or lower the thrust.

Apparently the engine that is decided to flame out first is dependent on placement in the SPH; when placed with symmetry, the one that is actually clicked onto the attachment node is the one that will flame out second; the other, copied one will be first.

[Claw]

You could also use Mechjeb. There is an 'prevent jet flameout'-option, which automatically lowers thrust to avoid flameouts.

Yeah, I like to use the odd engine number technique for my SSTO's; that means you can still have power at super high altitudes and when the single engine does flameout, it doesn't spin the plane. You can detect flame outs because you see the nose start twitching, and then start to pull left or right; when this happens just cut or lower the thrust.

Apparently the engine that is decided to flame out first is dependent on placement in the SPH; when placed with symmetry, the one that is actually clicked onto the attachment node is the one that will flame out second; the other, copied one will be first.

Random Tank is on the right track. Whichever jet engine is placed last during building will be the first to flame out. When placing via symmetry, the mirrored part is considered "last" and will flame out first. His tips about watching the aircraft nose are also good. In addition to watching the plane, you can also watch the trim indicators in the bottom left corner. When you see the yaw trim start to push out to one side, then it's time to throttle back some.

MechJeb does have an auto throttle. You won't get maximum performance out of your engines if you use it, but it will prevent asymmetric flameout.