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By higher I mean like 60k feet instead of 30k. Seems like the rate limiting step is engine performance at 60k feet. Are people working engines that can operate at higher altitudes? How close are we to good performance at high altitude (60k feet).

For example: Proteus high altitude aicraft

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    $\begingroup$ Possible duplicate: Q: Why are many jet aircraft designed to cruise around FL350-370? $\endgroup$
    – mins
    Jul 15, 2017 at 1:09
  • $\begingroup$ The Concorde cruised at 60,000 feet, so although you do need more powerful engines they are available. Engines are not the biggest problem. The limiting factors that make flight above 45,000 feet inefficient have more to do with Mach numbers, wing dimensions and structural requirements. These are addressed by the question mins linked above. $\endgroup$
    – TomMcW
    Jul 15, 2017 at 6:12

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There are several factors to take into account to explain your question, but it is a nice one.

In a cruise flight the aircraft force balance determines that the lift compensates the weight of the air plane and the engine thrust compensates the drag.

So, assuming the weight is given (which varies along the flight) the lift is determined to ensure the air plane is not falling. The lift influences the drag, the more the lift the more drag (actually a lineal variation with the variation of the square of the lift). And so, the engine compensates the drag.

So... engine thrust depends on the weight, essentially to the aerodynamic efficiency. So, my first answer, the more efficient the aerodynamic of the air plane, the higher it can fly. So, the engine is not the only factor.

However, you are highlighting an important point, the performance of gas turbine based propulsion. The problem of this propulsion is the need for an external oxidant. Essentially, the engine needs the oxygen of the atmosphere to be able to work. So that limitation requiring oxygen will make it impossible to reach much higher flight levels.

Another option is to change the propulsion concept and use an independent engine where not only fuel is carried but also oxidant is carried in the plane. The normal answer to that one is using a rocket engine. For example, the Bell-X (first manned A/C to flight faster than speed of sound) was having a ceiling of +90000 feet using rocket propulsion.

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  • $\begingroup$ Following your explanation, the U-2 could not have flown higher than maybe 40,000 ft, which it clearly did. This really doesn't answer the question. $\endgroup$
    – Peter Kämpf
    Jul 15, 2017 at 7:47
  • $\begingroup$ I am just highlighting the limitations not introducing numbers... $\endgroup$
    – Trebia Project.
    Jul 16, 2017 at 15:10
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The Armstrong Limit for human survival, even with supplimental oxygen, is around 60k-62k feet, where the fluids that keep one's lungs working will boil away. Even with supplimental O2, a human exposed to 60k altitude pressure will live maybe a couple of minutes, and has far less time than that to get back to a liveable pressure before permanent damage occurs.

So what happens if the pressurization system fails? I see that the Proteus boasts a 'shirt sleeve' environment, but I have to wonder what the procedure is on that aircraft to deal with loss of cabin pressure, and give the crew some chance of surviving. Most likely, the crew wears pressure suits, as are required for very high altitude aircraft like the U2 and SR71.

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    $\begingroup$ Well, this might answer why is not recommended more than why is not possible $\endgroup$
    – Trebia Project.
    Jul 14, 2017 at 20:51
  • $\begingroup$ The question did say airliner... It is possible to fly an aircraft at over 60,000 feet. An commercially viable airliner carrying paying passengers would have a substantial liability and publicity drawback, as the consequences of cabin pressure loss would likely be the death of everyone on board. $\endgroup$
    – tj1000
    Jul 14, 2017 at 23:06
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    $\begingroup$ The Concorde flew at 60,000 feet for years. This really doesn't answer the question. $\endgroup$
    – TomMcW
    Jul 15, 2017 at 6:02

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