What is the highest an air-breathing engine can operate?

[Privvet]

What's the highest they can operate? Can you get them to operate higher, like around 50km by using more intakes?

[KerikBalm]

No, you can't. You could before 1.0.

Each jet engine has a thrust curve that depends on altitude and speed, and a flameout percent... when the thrust gets below a certain amount, the engine flames out.

The engine that can operate the highest is the rapier (followed closely by the whiplash). Its altitude curve is here:

and its mach curve is here:

Now at high-ish mach numbers, you can get the thrust multiplier high enough to operate the Rapier around 28-30km. I no longer play on the stock system, so I'm a bit fuzzy on the exact numbers, but I think its about 30km for the rapier, and 27km for the whiplash.

 

Again, Intake Spam does not help. It did in pre-1.0 versions.

 

[Fearless Son]

The exact height they can operate at for an extended period before your plane begins to lose altitude will depend a lot on it's mass.  As they approach their flight ceiling, air-breathing engines gradually drop thrust, even if you have plenty of intakes, and a more massive plane will begin to lose altitude more quickly than a less massive plane due to that decrease in thrust.  

[Adenosine Triphospate]

In terms of practial use, for a normal sized aircraft using turbofan engines (Juno, Wheesley, Goliath) it will struggle to get above 15,000 metres, because the thrust isn't enough to sustain level flight. For the other engines (Whiplash, RAPIER), a high speed can be achieved, which allows an aircraft to glide to about 35,000, however, power from these engines will drop at about 25,000 to almost nothing.

[TheTripleAce3]

28,500m on rapiers and 22,200 on whiplashes for me.