# What will happen to a modern high bypass engine at 80 000 feet with cruise speed of 0.8 Mach?

While keeping the typical cruise speed of 0.8 Mach, will the engine shut down due to "air starvation" at such high altitude? Is it possible to design a turbofan engine, which is able to provide thrust, similar to typical engines at lower altitude, with this given speed of 0.8 Mach?

• This is a very vague and broad question with no specific answer. Clearly though, we have air-breathing engines that function at ~80,000 ft, e.g the U-2. Oct 30, 2018 at 15:18
• @zymhan but it was not using a high-bypass engine as the question requires. Oct 30, 2018 at 15:26
• I see two questions here: "what will happen if we take a turbofan to 80,000 feet", and "is it possible to design a turbofan to function at 80,000 feet". If my understanding is wrong, you may want to edit to clarify. If you do want to ask two questions I'd suggest asking them separately, since they are fairly different. Oct 30, 2018 at 15:47
– M28
Oct 30, 2018 at 18:35
• At that altitude high bypass turbofan doesn't make much sense anymore. To make then engine run at all, you really want to use all the turbine's power to drive as big of a compressor as possible and don't have any power left to drive a fan to generate meaningful amount of thrust out of such thin air. Oct 31, 2018 at 1:39

In a conventional turbofan engine the oxygen pressure would be too low at 80,000 ft to make combustion possible. The engine would experience a flameout at much lower altitude (maybe 50,000 ft), making a climb to 80,000 ft impossible.

However, given the compression ratio of modern turbofans (1:30 to 1:50), combustion chamber pressure would be around ground pressure and with a longer combustion chamber it should still be possible to maintain combustion.

An engine shutting down, and engine providing any small amount of thrust, and an engine providing some necessary threshold of thrust(to weight or size or similar) are three different issues. A specific existing engine and a custom engineered engine are also totally separate issues.

Yes, you can make a high bypass turbofan that operates and produces some amount of thrust at fl800 while mounted on a plane traveling at mach 0.8

The other issues can't be answered based on how general the question is.