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The fastest air breathing aircraft, the Blackbird SR71 flew at speeds above Mach 3. Its engines could achieve much higher speeds but were limited by the aircraft's skin which would melt due to friction with the surrounding air should it fly faster.

Is it possible to use refrigeration technology to cool down the aircraft's skin so it can reach much higher speeds?

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    $\begingroup$ Wasn't the fuel being circulated in the wings to help cool them down? If fridge technology was used, what are your ideas for cooling the liquid after being heated in the wings? $\endgroup$ – CrossRoads Feb 27 at 15:13
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    $\begingroup$ SR71 burs fuel so fast at that speed that the fuel flow cooling is sufficient. If it goes even faster or doesn't burn enough fuel, then I guess eventually it will go with some kind of consumable refrigeration e.g. evaporative cooling, e.g. liquid nitrogen. $\endgroup$ – user3528438 Feb 27 at 16:11
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    $\begingroup$ As Niels pointed out the limiting factor for SR-71 speed was not fuselage temperature. At mach 3.3 the wings were not hot enough to even bake a pizza properly as they were only about 500 F... Titanium can take temperatures over 1000 F. $\endgroup$ – Jpe61 Feb 27 at 20:53
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Is it possible to use refrigeration technology to cool down the aircraft's skin so it can reach much higher speeds?

Fundamentally: no.

Refrigeration technology moves heat energy around, it doesn't get rid of it. Your fridge moves heat from inside to the outside.

To get rid of it, you have two means, broadly speaking:

  • Heat up some mass and throw it away
  • Radiate it away as light/IR

Air cooling fundamentally heats up the surrounding air, and moves the now hot air away. I believe the SR-71 used it's fuel as heat sink as well, heating the fuel before combusting it, and ultimately throwing it away.

Radiating it is inefficient at low temperatures. And it's directly dependent on surface area, which is at a premium on a plane.

So in essence you'd have nowhere to put the heat energy you removed from the skin of the aircraft...

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    $\begingroup$ I'd argue that circulating cold fuel under the hot skin to move the heat is a form of refrigeration, it's just not a closed-loop system. It's not the same as an AC compressor/heat pump, but the overall effect is almost the same. $\endgroup$ – zymhan Feb 28 at 16:21
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    $\begingroup$ I do not disagree with that. It fits with wikipedias definition of refrigeration. $\endgroup$ – vidarlo Feb 28 at 16:30
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The SR-71 speed was limited by the maximum allowable temperature of the air entering the first stage compressor at the front of the engine (~800F). The airframe heat load at mach 3 would far exceed the heat transfer capacity of any refrigeration system that could possibly fit inside the plane, and its weight would reduce its useful payload to zero.

A better solution might have been to coat the hottest parts of the plane with insulating tiles like those used on the space shuttle, but that technology was unavailable in the late 1950's and early '60's when the design of the SR-71 was developed.

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As already pointed out in another answer, the SR-71's speed was limited by engine intake heating, its wings were fine. One can add that the main undercarriage tires in particular needed cooling by the fuel on its way to the engine, due to their close proximity to the more heat-resistant wing.

Yes it is thought practicable to refrigerate the engine intake air, though it has yet to be demonstrated in flight. Reaction Engines are currently developing the SABRE air-breathing rocket engine, which features a helium-refrigerated "pre-cooler" heat exchanger. This chills the incoming air and is expected to enable speeds up to and beyond Mach 5. The waste heat is used to drive the engine's thermodynamic cycle and pre-heat the hydrogen rocket propellant. It has attracted major funding from several international aerospace bigshots. At the time of writing (early 2020) the pre-cooler has successfully completed high-temperature testing in a purpose-built facility, and a new funding round for a prototype flight demonstrator is under way.

Note that at sufficiently high speeds, airframe heating does become a problem. Whether it might prove practicable, and to what extent, to refrigerate other key hotspots such as the fuselage nose or the wing leading edges must remain an open question.

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