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It's well-known that the stated reason the Concorde was painted predominantly white was to mitigate heating problems.

However, given that the source of the Concorde's thermal woes wasn't excessive exposure to solar radiation, but, rather, direct conduction and convection of compression heat, I'm confused; in that case, shouldn't the Concorde have been painted black (or nearly black), like the SR-71, to better radiate heat away?

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    $\begingroup$ Even white paint can still be effectively "black" at the long infrared wavelengths that will comprise the dominant radiative wavelengths at the temperatures the hull will reach. I'm not sure if this is true of Concorde's paint, but plenty of materials "look" different in the infrared than they do in the visible. A silicon wafer, for example, looks opaque dark grey to us but just a tiptoe into the infrared it is rather more like a transparent window. $\endgroup$ – J... May 6 '18 at 22:00
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    $\begingroup$ The SR-71 was subject to the Rule of Cool. $\endgroup$ – Harper May 8 '18 at 5:11
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Concorde's average skin temperature was 92°C (365K). Calculating the black body radiation using the Stefan Boltzmann law we get 1006W/m². This the maximum heat flux possible with perfect radiation, and very similar to the heat flux of solar radiation, which is also about 1kW/m² at the earth's surface in the absence of clouds (and a bit higher at Concorde's typical cruising altitude.)

However, as others have pointed out, at these temperatures Concorde would radiate in the far infra red, and it is perfectly possible to have a selective paint that appears white in the visible region (reflecting much of the heat of sunlight) while also radiating in the infra red region.

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There is a discussion on it here that's worth reading but in short the requirements were just different. A few of the key points,

  • The black color on the SR-71 offered some night camouflage in addition to its heat dissipation
  • The Concorde had an Aluminium airframe while the SR-71 had a primarily titanium airframe which could lead to different coating types.
  • The Concorde was a commercial aircraft many of which are often painted white thermal benefits aside they may have simply been keeping with what they usually did.
  • The SR-71 flew substantially faster than the Concorde and had different thermal requirements.
  • The hottest point on the Concorde was the nose 127°C which was actually cooled by fuel being pumped through as a coolant. The hottest point on the SR-71 was the cockpit window which cooked in at 327°C again, very different requirements thermally.
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    $\begingroup$ I would not have expected the cockpit window to be the hottest part on the SR-71. Excellent references. $\endgroup$ – ke4ukz May 8 '18 at 0:54
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    $\begingroup$ The thermal conductivites of aluminium and titan make a big difference. Pure Titanium has 22 W/mK and Dural has 147 W/mK. This means that Concorde's surface could distribute the thermal load between hot and cold spots much easier than the SR-71's. $\endgroup$ – Ariser May 8 '18 at 6:34
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    $\begingroup$ @ke4ukz: I'm guessing it's partly because the SR-71's canopy sticks out into the airstream and causes a lot more air to get bunched up and compressed in front of it than the other parts of the SR-71. Also, it isn't, strictly-speaking, the hottest part on the SR-71 - that honour goes to the engines, the hottest part labelled in that diagram of which cooks in at 1050 F, and the nozzles of which are unlabelled, but, going by the colour-coding of the diagram, look like they might go up to 1200 F or even hotter. :-P $\endgroup$ – Sean May 11 '18 at 19:54
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The Blackbird was black so it could absorb radiation, not so it could emit radiation. Look at the Blackbird's predecessor, the A-12, it is fairly easy to find pictures of the A-12 with polished or partially polished finish. The Blackbird and A-12 moved to a radar absorbent black finish for reasons of observability, flying at the edge of space means a darker surrounding than lower in the atmosphere so the camouflage requirement is different than say a fighter jet. Any thermal emissions that do occur do so well outside the visible spectrum.

To reach the point at which it is beneficial to thermally dissipation for the aircraft to be painted black is to reach the point that the aircraft emits more energy as radiation than it absorbs in the visible spectrum. At that point the black aircraft would be brighter than a white aircraft.

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    $\begingroup$ The A-12 example you give is nice, also the Pepsi Concorde cruised at a slower speed (M1.5-1.7 depending on the source) because of the darker paint. $\endgroup$ – ymb1 May 6 '18 at 20:41
  • $\begingroup$ @XRF the color of the paint has very little to do with its radiation and in extension stealth characteristics. The black paint as in Dave's answer had to do almost exclusively with heat emission/absorption. And in terms of your observability point, the SR-71 and variants of cruised over enemy airspace so high that even with a bright coating of paint, it would be difficult and pointless to attempt to acquire it visually. Keep it mind that even seeing it on radar (which the Soviets were capable of with their newer radars) doesn't nearly mean being able to prove its existence NOR shoot it down. $\endgroup$ – Jihyun May 6 '18 at 22:03
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    $\begingroup$ @Jihyun I edited to increase clarity. Thermal emissions at these temperatures (and radar to be fair) occur outside the visible spectrum. The SR-71 evolved from the faster A-12 with the goal of reducing observability (An A-12 was hit by a missile). The paint was radar absorbent and covered the entire body because of that, thermal concerns were only important in a limited number of places. Observability is not limited to acquisition either, it is harder to get a good image of a low contrast subject. The U2 moved from grey to a similar paint scheme around the same time for the same reasons. $\endgroup$ – XRF May 7 '18 at 5:43
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    $\begingroup$ Yes I have heard of that close-call incident as well. You picked my curiosity. I found that the Vietnamese had similar hardware as the Soviets, but as a whole were much less capable at using it, so (in parallel with the official declassified report) I would assume that they could barely detect enough of a signature for a lock, instead extrapolating a lead and using a shotgun technique in the hopes of a hit, as the Soviets did with the earlier U-2 overflights. Also, just keep in mind that the U-2 flew at over 10,000 ft lower than the SR-71 and variants. $\endgroup$ – Jihyun May 7 '18 at 13:04
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Good question. There are two main reasons for the Concorde's specific coloring: Heat absorption and heat emission. The key to its color scheme is in the materials used in construction of each plane.

The Concorde was made of aluminum, which emits heat far more quickly and effectively than the SR-71's titanium skin. Contrary to popular belief, the plane will become ductile and lose structural integrity at far lower temperatures than the metal's melting point, and preventing the metal from reaching these temperatures is a key reason for the paint colors of each aircraft.

Aluminum becomes ductile and unacceptably weak at higher temperatures, but would rarely reach them at the Concorde's cruise speed, as the metal reflects/releases sufficient heat so that the paint can can do a more proactive job of keeping heat absorption at a minimum. If titanium had similarly good heat dissipation capabilities then I am confident that it too would have been white.

However, titanium at speeds around the Blackbird's cruise range would absorb far too much heat for it to release, so the black paint would actually be superior in terms of heat dissipation to compensate for titanium's natural characteristics. Short answer shorter, the aluminum on the Concorde didn't get hot enough for the aluminum to lose integrity, so white was the better option in terms of preventing the metal from reaching those critical temperatures.

The SR-71 however could not release heat as quickly as aluminum, so the paint was designed to aid it in that regard. Hope this helps, sorry about the formatting I am in a rush so if there was anything conceptual I left out then please comment.

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Concorde was a civilian commercial vehicle. It's mission is not 100% flying, but also embarking and disembarking of passengers, waiting for ATC permission, etc. This means, it spend considerable time, on the ground, on low power, not heated by the airflow - but heated by the sun. For this part of the mission, white paint is vastly superior.

SR-71 was a military aircraft and it was optimized for one thing only - the flight. Sunbathing at the airport was not an issue, it was kept in a hangar for as long as possible anyway.

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