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How thin could at atmosphere be before it could not be flown in using a modern plane?
Does speed affect this to a reasonable degree?

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closed as off-topic by kim holder, Gwen, Brian Tompsett - 汤莱恩, TildalWave Oct 2 '15 at 21:53

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "This question is about other space sciences (physics, weather, astronomy, etc), and does not directly pertain to space exploration as outlined in the help center." – Gwen, Brian Tompsett - 汤莱恩, TildalWave
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There are two major constraints on aircraft operating in thin atmospheres:

  • Lift produced by aircraft (wing)
  • Thrust produced by the engine

The lift produced is directly proportional to the density; as altitude increases, density and hence lift reduces. This can be compensated to a certain extent by increasing the speed (or by increasing the lift coefficient). However, as the speed increases, the drag also increases and the required thrust goes up.

However,the engines need air to generate thrust (it doesn't matter if they are piston + propeller or gas turbine) and as the density falls, so does the thrust. As a result, the aircraft won't be able to generate enough speed to produce the required thrust to fly in a thin atmosphere. Usually this limitation is reached before the wings stop producing lift and the airplane would never reach theoretical considerations like the Kármán Line.

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    $\begingroup$ For anyone interested that doesn't know, the Kármán Line is 100km, the point at which traveling fast enough to produce lift would exceed orbital velocity. $\endgroup$ – Sarah Bailey Oct 2 '15 at 14:18
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Depends on the plane.

Airliners generally cruise at 10km altitude, where atmospheric pressure is about 1/4 of sea level.

Speed does indeed matter; lift is proportional to the square of airspeed, and higher-performance planes like the SR-71 can maintain some lift up to 25km.

As mentioned by TildalWave, the Kármán Line at 100km is the point at which you need to go at orbital speeds to develop any aerodynamic lift.

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