How do you know when to turn off your air breathing engines

[GalaxyGryphon]

I am making a spaceplane, but i want to get the most out of the jets before i switch to rockets, it seems to die around 0.41 but its kinda random.

Edited June 21, 2013 by GalaxyGryphon

[Spaceisbeautifulul]

Well, I usually cut mine at about 6 for a normal rocket. But if I feel like I'm not going fast enough, I let em go a bit longer (or vice-versa).

[UmbralRaptor]

When they're suffering from flameouts at 1/15 or less of max-throttle. This assumes a design that can safely fly east until it's in an unstable orbit, though. (If you do this right, you need only ~30 m/s on the rockets to pull your Pe out of the atmosphere)

The exact amount of intake air at flameout seems... wacky. It's higher when you have more intakes.

[numerobis]

The IntakeAir resource printout is not reliable; what it means depends on the order in which parts are processed by the physics.

If you right-click on an intake, you can see how many units of air per second it produces (it produces less at low altitude; there's a max of 0.2 per timestep, but that doesn't matter at high altitude). Multiply by the number of intakes to get a good approximation of the air you have available. Multiply your liquid fuel usage per second by 16 to see how much air you need. Throttle back to keep the required air less than the available air. You can keep going up indefinitely if you have enough intakes.

Easier is to calculate ahead of time a table: X airflow in one intake means you get at most Y LiquidFuel flow in the resources panel. Then you can right-click the intake and throttle appropriately by looking up in the table.

(Or you can use the mechjeb2 throttle control, which does basically this but more accurately.)