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I've got a question which is nearly answered by the following thread however not fully to my satisfaction.

Why do jet engines get better fuel efficiency at high altitudes?

The question ultimately comes down to:

Why do jet airplanes fly around tropopause and not higher ?

In the following I will relate what I believe to know and try to explain the reasoning in my mind. Please correct me in all cases where my reasoning is not entirely correct.

Here is what I get. -Fuel efficiency becomes better with altitude

This is because even though air gets less dense in altitude (amount of oxygen per volume decreases) the ram effect compensates so that the same amount of air enters the compressor. So in comparison with low altitude, in high altitude the same amount of air is entering the compressor so the same amount of air (number of oxygen particles) enters the combustion chamber. If the same amount of air enters the combustion chamber the same amount of fuel will be used to combust. so the same pressure and temperature is created inside the combustion chamber. What we need for higher propulsion of the gas is the highest possible pressure difference in comparison to the ambient pressure. Higher pressure in the combustion chamber is limited by the maximum temperature the turbine blades are able to resist, so the only way to get more propulsion is a lower ambient pressure wich is the case in higher altitude.

On top of that, thermal efficiency increases with lower temperatures. meaning more of all the energy inside the fuel is transferred into heat and pressure when combusted. Only this is no more than approximately 5% in higher altitude.

These two lead to my conclusion that in altitude you will need high speed (to create ram) this will make your engine give more propulsion/thrust for the same amount of fuel burned when flying at the same speed at lower altitude. So you are able to reduce fuel burned to create the same thrust in high altitude and thus increasing fuel efficiency.

What I also know is that airplanes try to fly around the tropopause. Does it have anything to do with the characteristics of the tropopause ? because why don't they fly higher ? is this because temperature doesn't decrease anymore ? (this is the original story I recall from my classes a few years ago but I can't find the reasoning behind it any more) I find it now hard to believe since thermal efficiency is almost negligible. Perhaps this is because the structure of the airplane is not able to withstand the pressure difference due to the fact the cabin pressure needs to remain below 10.000 feet ? In this it would still be true that an airplane flying higher and higher than the tropopause would get more and more fuel efficient. But also this I find hard to believe because with newer and modern composites an aeroplane should be able to withstand higher pressure differentials without adding weight and so be able to increase fuel efficiency by going higher. Cheaper and easier than developing more efficient engines I believe.

So where am I wrong ?

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    $\begingroup$ @Federico this question seems more centered on "why isn't getting above the tropopause worth the fuel efficiency gains?" $\endgroup$ – BSteinhurst May 23 '15 at 15:47
  • $\begingroup$ @BSteinhurst to me the answer is contained in the answers to that question. we'll see also what the OP thinks and the rest of the community. $\endgroup$ – Federico May 23 '15 at 15:57

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