The Concorde is painted white for thermal reasons. If I understand correctly, it is done to reflect as much radiation as possible in order to lower tempereture. But when the skin is really hot (SR-71, reentry vehicle such as the Space shuttle,...), it is better to have a black/dark skin so that it radiate internal heat.

In short: when flying supersonic the aircraft should be painted white to reflect incoming radiation, and when flying hypersonic the aircraft should be painted black to radiate internal heat.

My question is: what is the speed threshold below which painting white results in lower temperature and above which painting black results in lower temperature?

  • $\begingroup$ the sr-71 was not hypersonic $\endgroup$ – Abdullah May 16 at 16:21
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    $\begingroup$ @ABJX I agree. The SR-71 flew fast enough for black skin to be thermally interesting. Flying hypersonic (i.e. faster) would be fast enough for black skin to be interesting. $\endgroup$ – Manu H May 16 at 19:16
  • $\begingroup$ wait what? the sr 71 did not glow. if it did, then that would make sense. but do aircraft already glow at mach 5 (hypersonic)? $\endgroup$ – Abdullah May 17 at 6:40
  • $\begingroup$ what is interesting is being black in IR, not visible.......aviation.stackexchange.com/questions/51268/… $\endgroup$ – Abdullah May 17 at 6:41

This might be more of a question of altitude and the ability to remove heat as a function of air density. The SR-71 has a cruising altitude of ~26km and the Concorde has a crusing altitude of ~18km. Of course a black coating makes it absorb more solar irradiance, but the designers would have taken this into account and ensured that energy balance favors a desirable temperature. The SR-71 had at most two passengers, so comfort cooling can be focused on the cockpit compared to a passenger plane. The Concorde on the other hand had a much larger passenger cabin which would lead to much higher parasitic cooling losses with a black skin.

This chart shows the decrease in density, but whether it's enough to justify a different approach to passive thermal control is hard to say:

Air Properties vs. Altitude

The implication is that at high altitudes, heat generated inside the vehicle needs to be removed by a combination of convection and radiation. This is especially important for the Space Shuttle as it needs to maintain temperatures in the lack of atmosphere although the black tiles did not serve as the orbiter's primary radiators. They only needed to insulate the orbiter from aerodynamic forces and the high temperatures. While reentering the atmosphere, the heat comes from air compression in the atmosphere and the black tiles would have helped to re-radiate some of the heat away from the orbiter.

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