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The Lockheed SR-71 and the (old) Lockheed U-2 were engineered to maintain cabin altitudes in excess of 25000 feet AMSL at an altitude of more than 70000 feet AMSL.

However, in both airplanes, pilots are required to wear space suits which maintain the safe pressurisation and oxygen content anyway. If the space suit develops a pressurisation leak, I think it's unlikely that the very high cabin altitude in these airplanes would buy the pilots any more time to descent to a safe altitude safely as the time of useful consciousness at 25000 feet AMSL is the order of a few minutes. Furthermore, it seems strange that aerospace engineers would install a pressurisation system on the Lockheed U2, an airplane already struggling to be as light as possible.

So why is this pressurisation maintained at such a high altitude in the airplanes?

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Even the unmanned RQ-4 Global Hawk has a pressurized compartment. The low air density at altitude would not be sufficient to cool the electronics, so a pressurized electronics compartment had to be added.

I am not sure, but I expect the same reason required pressurization in the U-2 and the SR-71. The electronics were, besides the pilots, also an occupant of their pressurized cockpits.

At the pressure of 25.000 ft a human will not stay conscious, but will be able to survive. When the pressure drops below that of 30.000 ft, the breathing reflex will stop. If the pilot initiates a descent when his pressure suit starts to leak, he has a good chance of waking up at lower altitude before the aircraft hits the ground if the cabin can prevent the air pressure from falling to that of ambient air.

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    $\begingroup$ Wikipedia suggests that some pressurization was needed so that an aircon system could cool the cockpit. $\endgroup$
    – David Richerby
    Oct 5, 2014 at 9:05
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    $\begingroup$ @DavidRicherby: So pressurization was needed to cool all cockpit occupants, humans and electronics. $\endgroup$
    – Peter Kämpf
    Oct 5, 2014 at 10:09
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    $\begingroup$ Isn't it very cold anyway at FL700+? $\endgroup$
    – shortstheory
    Oct 6, 2014 at 2:59
  • $\begingroup$ ^at least in the U2's case I mean. $\endgroup$
    – shortstheory
    Oct 6, 2014 at 4:02
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    $\begingroup$ Regardless of the temperature, with insufficient air pressure convective heat transfer just doesn't work. It can be mighty cold in space (if in the shade), but it's non-trivial to get rid of that heat (you have to resort to radiative cooling, far harder). $\endgroup$
    – Nick T
    Oct 6, 2014 at 4:40

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