Is the SR-71 constrained by its engine or airframe? I know I’ve read that if the plane went any faster something would start melting and the plane would be lost but I don’t recall if it was the airframe or the engine.
(Doing this from memory so I may get something wrong here), the SR-71 usually cruised at mach 3.2, it could accelerate to mach 3.3 when fired upon, and could go to mach 3.4 if the pilot felt it was critical to preserve the crew/airframe however the plane needed to return to base after getting out of harms way and needed to be inspected before returning to the flight line. A lockheed test pilot apparently got the SR-71 to mach 3.5 (or maybe Mach 3.6?) but said it started to get scary. If this is accurate, it's probably the case that no one knows what fails first on the SR-71.
Fun fact: the SR-71 flew its entire mission in radio silence, including multiple aerial refuelings, to avoid pesky Russian trawlers, etc. tracking it.
Generally aluminum is good to Mach 2.2, then you go to steel or titanium for either leading edges or the entire fuselage.
Which makes the F-5 all that more impressive, as its fuselage was designed for Mach 2.0, near the limit of aluminum, even theough the engines limited it to Mach 1.3 to Mach 1.6. (The F-5 was so small that it was optically stealthy - adversaries couldn't see it outside 5 miles.)
Titanium was used as a last resort in the F14 to reduce weight in the central fuselage box, which meant building the world's largest titanium casting plant at the time. Your tax dollars at work.
Funnily enough, having read the Sled Driver[2], I don't remember any clear explainer whether the engine or the airframe was thermally limited. Perhaps it was there and I forgot by now?
[edit] Wikipedia to the rescue, "Maximum flight speed was limited by the temperature of the air entering the engine compressor, which was not certified for temperatures above 800 °F (430 °C)." [3] The inlet air was necessarily heated as side effect of being slowed down to subsonic speeds, necessitated by the nature of the turbine engine.
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[1] by circulating the JP-7 fuel through critical sections, before sending it off to the engines
[2] https://www.goodreads.com/book/show/1349004.Sled_Driver
[3] https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird#cite_...
And all that was done without a computer.