Kerbal Space Program

When an engine flames out because of the lack of air (for example when closing the intakes) and the player throttles up, the engine will (when the air intakes are open again) already be at that speed without any delay. That is weird. Its a great way of saving runway though :P

Reproduction¶

1. Launch the attached craft (Airbreather Jig).
2. Open the action menu for the engine (right click) to monitor engine thrust.
3. Stage to activate the engine (it will remain static on the pad)
4. Note engine note rising and thrust increasing - wait until it levels out.
5. Press action group 1 to toggle air intake. (alternatively, use action menu on intake.
6. Note the engine flames out and quickly spools down.
7. Press action group 1 again to open intake.
8. Note engine note and thrust level
9. Apply full throttle (z) and wait for stabilisation (74.2kN)
10. Repeat steps 5-7 at any throttle setting.

Expected¶

On flameout due to air starvation, the engine should spool down. When air is available again, the engine should spool up at an equivalent or similar rate to step 4.

Observed¶

The engine will spool up gradually when first staged, but on closing the intake and starving it of air, the engine quickly spools down.(expected. see notes)
On opening the intake again, the engine will instantly return to the level of thrust it was at prior to flaming out.
Adjusting the throttle up or down during air starvation will make the engine initially return to the previous thrust value and then raise or lower its thrust accordingly.
Only the fuel required to raise to a higher throttle setting will be used. Not the fuel required to spool up from a standstill. (this can be seen by waiting for the appropriate amount of time for a normal spool down.

Video demonstration

Notes¶

Opening and closing the throttle produces a gradual increase or decrease in thrust that lags behind the throttle setting as expected from a gas turbine. Intake air velocity may increase the rotation and speed spooling up, but this should not happen instantly.
Flameout produces a quicker than expected spool down, and does not follow the same behaviour as shutting an engine down using (x).
The engine appears to return to a thrust value that may have been stored on the flameout event.