The limit to the blackbird’s speed

[Marshal Mo Hare, SASS #45984]

The Blackbird, contrary to what most people believe, did not have its speed limited by its fuselage not being able to withstand higher temperatures or its engines overheating.

The problem with the SR-71, besides the much shorter flight time at higher speeds due to the higher fuel consumption, was the "glazed" parts of the two canopies and spy cameras, which would not withstand speeds above Mach 3.3 for long periods. The "glazed" parts of the SR-71 Blackbird were made of double layers of solid quartz, 1.25" thick, ultrasonically fused to the titanium structures. This material was chosen for its resistance to high temperatures and optical properties. In addition, they were coated with a thin layer of gold, which helped to dissipate the heat generated by solar radiation and air friction. Gold is an excellent reflector of infrared radiation, helping to keep the interior of the cabin or the electro-optical equipment bays more comfortable and protected.

Aircraft like the X-15 were already flying at Mach 6.7, so the problem is not the structure or the external coating of the aircraft, but rather its "glazed" part. So much so that the X-15, for tests at hypersonic speeds, took off with one of the "glasses" covered by a titanium plate, and when it slowed down, the exposed "glass" was already completely burned, and they lowered the plate of the other to so that the pilot could see the outside.