It's not just that they need oxygen. Cold air is good for thermal efficiency. Even if the heat source were nuclear (as in https://en.wikipedia.org/wiki/Project_

It's not just that they need oxygen. Cold air is good for thermal efficiency. Even if the heat source were nuclear (as in https://en.wikipedia.org/wiki/Project_Pluto https://en.wikipedia.org/wiki/Project_Pluto engines) there would be a problem with the thermal efficiency. (see https://en.wikipedia.org/wiki/Thermodynamic_cycle https://en.wikipedia.org/wiki/Thermodynamic_cycle and https://en.wikipedia.org/wiki/Carnot_cycle https://en.wikipedia.org/wiki/Carnot_cycle for why) Air temperature rises from about 217 Kelvin to about 270 Kelvin as you climb up the stratosphere. The temperature goes back down again as you get up to around 50 km (80 miles) but that is really high altitude. There is an upside though. Subsonic aircraft can go faster as the temperature rises because the speed of sound increases. It goes from about 295 m/s to 329 m/s. The extra 11% is nice. Graph of the air by altitude: https://upload.wikimedia.org/wikipedia/commons/9/9d/Comparison_US_standard_atmosphere_1962.svg https://upload.wikimedia.org/wikipedia/commons/9/9d/Comparis...