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We’ve all been flying high ever since the development of the jet engine in the mid-1950s. But why were jets were designed to fly at those high altitudes in the first place?

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Sources:
http://www.airliners.net/aviation-forums/tech_ops/read.main/56218/
http://www.decodedscience.com/why-do-jet-airplanes-fly-at-high-altitudes/9815
https://www.grc.nasa.gov/www/k-12/BGP/PAT/Fuel_and_Air2_act.htm
https://www.grc.nasa.gov/www/K-12/airplane/lift1.html

You frequent flier types are probably used to it, but the fact is, when we travel by air, we are way up there. The average cruising altitude of a commercial jet is 7 and a half to 11 kilometers. In horizontal terms, that is at least as far as the average distance as between you and the nearest Starbucks at any given moment. In vertical terms, well that's a long way down. The good news is that [if] the plane starts to fall, it has a long time to figure out how to stop falling.

But we've been flying that high ever since the development of the jet engine in the mid 1950's. And jets were designed to fly at these high altitudes because there's less air up there, and that's what engineers, passengers, and airlines all prefer. Basically, there are far fewer air molecules at say 30 thousand feet, or 9 kilometers, than at sea level, so the plane is literally running into fewer molecules. This means it doesn't need to produce as much thrust in order to maintain the speed necessary to fly. So it can travel more efficiently, which is what the airlines want.

What the passengers want is to not feel like they're flying in the air at all, and flying at higher altitudes means being able to fly over at least some of the weather patterns and air currents that older, less powerful propeller planes often had to fly through. So flying higher usually means a more comfortable flight.

But there are some trade offs for this efficiency and comfort, in order to stay in the air, an airplane needs to maintain lift, the force that counteracts its weight. At lower altitudes, having lots of air around helps a plane get lift, but the higher it goes, the harder it is to maintain. So engineers had to find ways to generate more lift in other ways, like making planes with larger wings.

Jets can't fly too high though. In order to, like, continue working, jet engines need to burn fuel. That is an important part of the process, and to burn stuff, you need oxygen. So planes have to stay at altitudes where there is still enough oxygen to mix with the jet fuel and allow combustion to happen. To get any higher, your aircraft would have to pack canisters of air to mix with the fuel, and once you do that, you're not an airplane anymore, you're basically a rocket. So engineers have done the math and found the optimal height for efficient travel, and designed planes to operate best at that height. Yay engineers!

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