Other than vehicles with a detached
shock wave, all aircraft heat up more the faster they move. From the NASA book Facing the Heat Barrier: A History of Hypersonics [1]:
At Mach 3 and higher, there was the Lockheed SR-71 that cruised at 85,000 feet. The atmosphere at such altitudes, three times higher than Mount Everest, has a pressure only one-fiftieth of that at sea level. Even so, this airplane experienced aerodynamic heating that brought temperatures above [260C] over most of its surface.
The SR-71 Blackbird gets to 260C
over most of its surface. Aluminium
anneals (softens) at 177C (alloys can be higher)
making it unsuitable as a material for use on the outside of the
Blackbird. The Blackbird used titanium instead.
This temperature of 260C is in a vehicle moving
at Mach 3.
The US Air Force's experimental
rocket X-15 moved at Mach
5 and was designed to withstand temperatures of 1240F (671C) in some
places. If Mach 5 takes the temperature beyond the aluminium melting
point (660C)*, how did the Apollo
reentry capsule hold together at Mach 36?
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