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As a follow-up question to What is the typical temperature of an airliner's hull during flight? I wonder how very fast airplanes, such as the SR-71 mentioned, avoid overheating with a total air temperature outside of more than 400 degrees Celsius.

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    $\begingroup$ The 400 degrees mentioned is only present in the stagnation point (the only place where the speed is zero), as soon as you move away from this (imaginary) point, the temperature drop will be considerate. $\endgroup$ – ROIMaison Dec 18 '15 at 12:10
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To avoid overheating, the usual trick is to select the right material:

  • The Concorde used a special aluminium alloy, called Hiduminium, which had higher strength at elevated temperatures and allowed the Concorde to cruise at Mach 2.02. Aluminium melts at 660°C.
  • The MiG-25 used stainless steel instead of aluminium to make its top speed of Mach 3.1 possible (top speed without engine damage was Mach 2.83). Titanium would had been even better, but was expensive and difficult to work with at that time. Stainless steel melts between 1400°C and 1450°C.
  • The XB-70 used titanium in addition to stainless steel. Its cruise speed was also Mach 3.0. Titanium melts at 1668°C.
  • The SR-71 used titanium for 85% of its structure to allow a cruising speed of Mach 3.2.
  • The X-15 was built from Inconel-X, a nickel-chromium alloy which has excellent strength at high temperature (it starts to melt at 1393°C) and allowed it to go up to Mach 6.5.
  • The Space Shuttle used a ceramic insulation on top of the load-carrying aluminium structure which kept the heat away from the structure during descent. Note that the Shuttle had to be connected to a heat sink to cool the on-board systems after landing.

The next trick is to fly higher up where air is less dense. Lower density does not reduce the air temperature, but it reduces the heat flow so the airframe will settle at a lower temperature. Remember that the eventual heat is the combination of convection, radiation and heat conductivity. Flying high up allows especially the top surface to radiate heat freely into the black of space above.

The hottest aircraft ever was the North American X-15 A-2. For a speed record attempt, the whole aircraft was covered with pink, ablative paint so the process of sublimation would carry some more heat away. To protect the paint from liquid oxygen, a white topcoat was applied also.

X-15 A-2 shortly after being dropped from the B-52 carrier

X-15 A-2 shortly after being dropped from the B-52 carrier (picture source). Note the small barrel under the lower fin: This was a ramjet which was test-flown on this occasion.

Damage to the ventral stabilizer

Damage to the ventral stabilizer. The ramjet separated prematurely due to frictional heating (picture source)

A third trick is to fly fast only for a short time. A heat-seeking air to air missile will easily reach Mach 3, but only for less than a minute. To keep the sensor cooled, a pressurized gas (argon or nitrogen) would be expanded (the AIM-9X even uses a stirling cryo-cooler). By heating the structure or using an internal heat sink, the limited heat load can be tolerated, albeit only for a very limited time.

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    $\begingroup$ The shuttle heat sink that you are talking about was not for mitigating aeroheating of the structure. It was for cooling internal mechanical and avionics components. There was no active cooling of the shuttle's heat shield after landing. $\endgroup$ – Tristan Dec 18 '15 at 15:02
  • $\begingroup$ @Tristan: No, the heat shield did not need cooling, but the internals did due to the heat creeping through the ceramic foam and heating the airframe from the outside. $\endgroup$ – Peter Kämpf Dec 18 '15 at 16:30
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    $\begingroup$ The SR-71 also had body panels designed to expand from the heat so they were very loose on the ground. This means it leaked large amounts of fuel at take-off. I recall reading somewhere that it would take off with a full load of fuel, go supersonic to heat the skin to close up the gaps, then need refueling because it used up almost all the fuel between leaks and expanding the skin. Then it would go about its mission. $\endgroup$ – tpg2114 Dec 18 '15 at 17:07
  • $\begingroup$ @PeterKämpf The heat rejection for the internals was first and foremost due to their self-generated heat -- they are actively cooled from first powerup well before liftoff to powerdown well after landing. The shuttle's tiles were such poor heat conductors that postlanding structural heating wasn't a major concern. $\endgroup$ – Tristan Dec 18 '15 at 17:23
  • $\begingroup$ @Tristan Internal, self-generated heating required the Shuttle to open the cargo bay doors soon after reaching orbit, and if the doors would't open the mission had to be aborted. After landing, the doors stayed shut. So you say the external cooling just did what the radiators on the inside of the cargo bay doors did? What I learned is that the Shuttle descent was highly instationary, absorbing much more heat than it could handle continuously, and without removing the heat stored in the tiles, the structure would be permanently damaged. $\endgroup$ – Peter Kämpf Dec 18 '15 at 19:46
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I'd like to answer with a focus on the SR-71, since I happen to have a book that gives details on its design.

Ben Rich was the lead of the propulsion and thermodynamics group for the SR-71, and Kelly Johnson's successor as head of the Skunkworks for later programs. In the "Faster Than a Speeding Bullet" chapter of his memoir Skunkworks (pg 203 in the first paperback edition, 1994), he writes:

... I volunteered some unsolicited advice about how we could use a softer titanium that began to lose its strength at 550 degrees. My idea was to paint the airplane black. From my college days I remembered that a good heat absorber was also a good heat emitter and would actually radiate away more heat than it would absorb through friction. I calculated that black paint would lower the wing temperatures 35 degrees by radiation. But Kelly [Johnson, head of Skunkworks and the then-A12 project] snorted impatiently and shook his head... Overnight, however, he apparently had second thoughts..."On the black paint," he said, "you were right about the advantages and I was wrong." He handed me a quarter. It was a rare win. So Kelly approved my idea of painting the airplane black, and by the time our first prototype rolled out the airplane became known as the Blackbird.

The chapter goes on into greater detail on various unique material selections:

  • stainless steel hydraulic lines

  • Hastelloy X ejector flaps

  • Elgiloy control cables

  • Gold-plated plumbing lines

  • Titanium screws and rivets

  • Special rubber for the landing gear wheels, which were then inflated with nitrogen

  • Jet fuel with a higher flash point (JP-7)

Pg 205:

The fuel acted as an internal coolant. All the heat built up inside the aircraft was transferred to the fuel by heat exchangers. We designed a smart valve -- a special valve that could sense temperature changes -- to supply only the hottest fuel to the engines and keep the cooler fuel to cool the retracted landing gear and avionics.

Pg 207:

We designed the cockpit air-conditioning to bleed air off the engine compressor and dump it through a fuel air cooler, then through an expansion turbine, into the cabin at a frigid minus 40 degrees F, which lowered the ovenlike 200-degree cockpit to a balmy Southern California beach day.

So I guess there's three generic design principles embodied here:

  • selection of materials to withstand high temperatures
  • rejection of as much heat as possible--here by radiation from the aircraft's surface and convection out the engine exhaust via the fuel
  • sequestration of heat away from critical areas, as much as possible
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  • $\begingroup$ I'd add a note about using fuel, and structure, as heat sinks, as well. Before ablative techniques, atomic and nuclear warheads on missiles had copper heat sinks $\endgroup$ – Bill IV Nov 16 '16 at 10:47
  • $\begingroup$ I already have a note on the fuel being used as a heat sink. If you have something about the structure being used as a heat store, feel free to add your own answer (to this older question, I just noted a typo in my answer and changed it). I'd be surprised by the structure being used as a sink, since it seems like that's what you'd try to pull heat away from to reduce thermal stresses. $\endgroup$ – Erin Anne Nov 16 '16 at 23:36
  • $\begingroup$ Sorry, I was too terse. The "three generic design principles" ought to be "four generic design principles" and specifically mention sinks. It is possible to infer sinking (and exporting) heat from the second item, "rejection of as much heat as possible". But stating it clearly seems simpler, and is amply supported in the text above. Yes, in 2016 it is surprising to have structure intended as a heat sink, but heavy lumps of copper were part of early re-entry vehicle design. That's why they should be mentioned. $\endgroup$ – Bill IV Nov 17 '16 at 18:27
  • $\begingroup$ Hmm. Sinking is really only part of the process, though. You sink the heat to the fuel because you're about to dump it overboard. Hence the hottest-fuel smart valve. The rejection does encompass both radiation and convection...maybe I'll expand that. I'm not going to go into the structural heavy lumps of copper thing. I limited the scope of my answer to SR-71s because it's a question about fast-flying aircraft and I had a good SR-71 resource. I don't know anything about using structure as a heat sink in fast-flying aircraft. $\endgroup$ – Erin Anne Nov 19 '16 at 1:01
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There are multiple ways to avoid overheating.

  • The simplest method is to use a high temperature material- like Titanium in SR-71 or steel alloy in Mig-25; in Concorde, a special aluminum alloy (AU2GN) was used. In case of other aircraft, steel or Ti is used in stagnation points (like wing leading edges), while the other exposed surfaces are made out of other materials.

  • In SR-71, the fuel was used as a heat sink to dissipate the heat generated in the airframe.

  • In X-15, an ablative insulation was used to overcome heating problems:

The airplane was covered with ablative insulation designed to permit flights to Mach 7.4. A silicone elastomeric ablator was sprayed on in variable thickness appropriate to the local heat loads. Leading edges were protected by a related erosion-resistant material applied in preformed sections

  • Aerothermodynamic design is important for high speed aircraft. For example, when NASA tested a dummy ramjet in X-15, the mounting pylon nearly failed due to interference heating, with leading-edge heating rates of the order of seven times compared to those without interference being estimated.

X-15

Failure of pylon due to interference heating from dummy ramjet, image from history.nasa.gov

  • To help reflect and radiate the high amount of heat produced during supersonic flight, the Concorde had a high-reflectivity white paint that was about twice as reflective as the white paint on other jets.
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