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,?

This is really more a question of comfort and safety. For a passenger jet, the highest most will every fly is 45,000 feet. Higher is possible but at such high altitudes pressure in the cabin required to keep passengers comfortable begins to strain on the cabin walls. At higher altitudes stronger cabin walls are required to hold the higher pressure in the cabin. The extra weight of reinforced cabin walls aren’t worth the efficiency achieved by flying higher.

In aircraft like the SR-71 and U-2 these aircraft were not fully pressurized and the pilot wore a pressurized space suit to operate the aircraft in order to save on weight by not pressurizing the cabin. These aircraft could fly up to 85,000 feet. Although this is very high, it is nowhere near the edge of the atmosphere. In fact, this is well within the stratosphere.

The X-15 was able to reach altitudes of 350,000 feet, but these were ballistic zoom climbs and not really flight per se. The runner up for highest flying aircraft is the X-43 Hyper-X (featured in my avatar) which sustained level flight using air breathing SCRAMJET engines at 110,000 feet. If you want to add dirigibles into the equation, these could theoretically reach higher heights than conventional aircraft could due to added buoyancy, but since the question is specific to planes, I’d have to say that 100k feet is probably close to the upper limit for air breathing aerodynamic aircraft.

@Poser Also, the B-52 has a service ceiling of 50,000 feet. This means the aircraft cannot maintain steady level flight above this altitude. 70,000 feet is far above this and would certainly cause extreme fatigue on the airframe (if not cabin pressure failure) due to the pressure differential across the cabin walls.

@Shuttle128—That’s what I get for not checking my sources.

I flew a B-52F at over 52,500 feet, the F model is a turbojet. We were doing a procedure that as the plane burns off fuel it is allowed to climb. The original B-52s were going to be used as spy planes and the crew would have worn pressure suits, and in fact the oxygen control panel has a control built in for the Face Plate defroster on the suit. I have no idea why they didn’t use them that way, probably because the U-2 could go higher and was much less of a target. Also, the much better performing H model with turbofan engines, could not climb as high. So, that published figure of 50,000 feet was just a wag at what the plane could do. It is all based on gross weight, and an empty B-52 weighs about 185,000 lbs and can carry a load of fuel and whatever else of around 300,000 lbs on the ground. The limiting factor is the weight the gear can support on the ground, and they call it the Maximum Takeoff Weight. That total weight in the G and H models is 488,000 lbs. They have actually refueled a B-52 in flight to a gross weight over 560,000. I can remember as a young co-pilot in some of my first refuelings (the copilot runs the fuel panel and is in charge of the weight and balance) keeping all of the fuel tanks valves open as they had devices that would shut each tank off as it became full and an little light would come on. The fuel panel looked like a pinball machine. B-52s are amazing machines, and the biggest reason they are so sucessful is that with 8 engines they had massive ability to correct for things that failed. It was actually possible to land with 4 engines out on one side, or even make a landing with one side of the landing gear inoperative.