The SR-71 had a moving engine inlet spike, so that would be an operating limit based on temperature and speed. Fun fact: the SR-71 flew its entire mission in r

The SR-71 had a moving engine inlet spike, so that would be an operating limit based on temperature and speed. 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.