I understand the SR-71 had to deal with a lot of issues in order to keep its speed (special fuel which leaked on the runway, titanium fuselage and probably other stuff). But wouldn’t the same type of engine be able to power a relatively slower fighter jet capable of easily cruise at match 2-2.5, so it doesn’t have to deal with so much friction as the SR-71 at match 3.
But while the engines exist since the 1960s, relatively few fighters go faster than match 2 and it took all the way to the 21st century to have fighters capable of supercruise (and still below match 2). So I guess there has to be a reason for that.
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There have been many experimental aircraft whose sole purpose was going fast and some of them flew faster than the SR 71. They were going fast more or less for the sake of it, since all they were made for was to study aircraft behavior in such high speeds.
But when an aircraft has to be produced in large numbers for civilian or military use, it has to serve a purpose other than just going fast. The SR 71 was not a fighter plane, it was a reconnaissance aircraft. It was an incredibly expensive and complex aircraft for what was essentially a flying camera. At the time it was developed that was the best and safest option for reliable spying, but now it’s obsolete since satellites and UAVs are much cheaper and more effective at the same task. When it was new it could fly so high and so fast that no interceptor aircraft or even ground to air missile could catch it. However it didn’t take long for such aircraft and missile systems to be developed for that exact purpose.
The military shifted its focus from having a wide variety of highly specialised single role aircraft to having more versatile multi purpose fighter jets. Such an aircraft can’t minmax and sacrifice everything for speed, especially since missile technology has advanced to a point where there’s no way to make a plane fly faster and higher than a missile. So the simple answer is that there’s simply no reason to make other aircraft that fly so fast. No role a modern fighter jet undertakes requires it to fly so fast, and it’s much better to have an aircraft that does a lot of things well than one that does one thing extremely well and is bad at everything else.
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