How high can planes fly?

Tough question.

The Next Geneartion Boing 737 is certified to fly up to 41,000 feet -- but in theory, it could go higher if they really wanted to. Military and experimental planes can fly much higher (85-96K). What is really weird is that it seems like the limiting factor at high altitudes is that the air becomes too thin to run the jet engines -- not too thin to continue to provide lift. Is that true?

You have to collect enough O2 molecules with a jet engine, unless it was a rocket. The rocket supplies its own fuel for engine combustion. RAM jets have a surface to collect oxygen and supplies the jet engine. Also, it is difficult at such altitudes because you need even higher power to make up for less lift.

The discontinued SR-71 spy plane flew high enough it technically left the atmosphere into near space. It used auxiliary rockets at higher altitudes as there was insufficient oxygen to run jet engines, provide lift, or for the control surfaces to function.

Passenger airliners (and most other jets) utilize turbofan engines, which can be thought of as a sort of hybrid between a turboprop (thrust is produced by a propeller) and turbojet (thrust is produced by exhaust) engines. Turbojets are much better engines at higher altitudes where the air is less dense. Since Newton taught us that Force = Mass x Acceleration, there are basically two ways to provide thrust. You can take a large mass of air, and accelerate it a little (which is what a propeller does), or you can take a smaller mass of air, and accelerate it a lot (which is what a turbojet does).

Since the air is thinner at higher altitudes (less mass), turbojet engines are more effective because they don’t require a large mass of air to begin with. Turbofan engines, however, lose thrust rapidly as air density decreases. The advantage is that they are cheaper, quieter and more fuel efficient at lower altitudes than turbojet engines, and also provide better thrust at lower altitudes, which means they don’t need as long of a runway.

The B-52 regularly cruises above 70,000 feet because it is powered by roughly a bajillion giant turbojet engines. But it also requires a runway roughly the size of Kansas.

What’s the temperature in a jet cargo bay at 35,000 ft,?