Are military powers no longer interested in the race to build supersonic fighters like MiG-25, F-14 Tomcat, Lockheed SR-71 or is there another secret behind this trend?

Possessing outstanding stealth technology; surpass thousands of the most advanced radar systems; Perform the most complex maneuvers; those are new fighters such as Su-57, F-35 Lightning; superior to any predecessor. However, there is one feature that they lose to previous fighters: speed. No modern fighter jet exceeds Mach 2.2. Are military powers no longer interested in the race to build supersonic fighters like MiG-25, F-14 Tomcat, Lockheed SR-71 or is there another secret behind this trend?

The simplest answer is that insane top speed was intended for interception. Namely interception of high flying nuclear armed bombers. Now that bombers are obsolete for nuclear attacks, and ICBM travel way faster and much higher than a jet could ever hope to, top speeds in excess of mach 2 are unnecessary and the sacrifices required to reach it make the aircraft useless for anything else.

In most cases, fighters need range and endurance more than speed. In the early history of the jet fighter, the high altitude nuclear strategic bomber was a major threat. The fastest fighter jets were intended to be used defensively against these bombers. A variety of “interceptors” were designed for relatively short range, high speed, fast climb rate. They were not dogfighters, with poor maneuverability. They were simply designed to be take off, climb to 60,000 feet, get vectored to a bomber by ground control, and launch missiles at the bomber. These interceptors were the fastest operational fighter jets.

Missile technology rapidly improved, however. By the late 1960s it was clear that SAMs and long range air to air missiles would increasingly make the high altitude bomber vulnerable without requiring dedicated interceptor aircraft. Since then, ideas about strategic bombing have changed. Over time, weapons like cruise missiles have changed the game. The High-speed, high-altitude interceptors are no longer a priority.

Defense planners have realized that top speed is not that important. Fighter jets only flare up their afterburners when they are trying to escape a situation. An aircraft using afterburners consumes approximately 8x the fuel when compared to subsonic flight. As a result, when a fighter fires up its afterburners to escape a situation, intercepting fighters will almost never give chase, because they would likely run out of fuel if they tried. The vast majority of the time, fighter jets fly subsonic. Defense engineers have been designing fighters around other characteristics.

The maximum speed of fighter plane may have gone down, but the actual speed they use in operational flights may have gone up. What matters is how fast a plane is on the load it has, at altitude it is flying, and how quickly and fuel-efficiently it can accelerate to that speed.

Most of those old mach 2 planes consumed most of their fuel to accelerate and climb to altitude where they can fly at mach 2, and had almost non-existing range and remaining flight time after doing it. And they could not do those speeds while carrying any external fuel tanks, bombs or more than few missiles.

Because of the terrible fuel efficiency at high speed, most mach 2-capable planes spent over 90% of their flying time at subsonic speeds, and on most flights did never go supersonic.

Also, practically all Within Visual Range air combat happens at subsonic speeds; The maneuverability of any plane at high altitudes, which are required for high speeds, is quite bad because thin air does no give much lift to wings, and immediately when a plane starts to make a high-g turns at supersonic speeds, the induced drag from the turn slows down the plane quite a lot and very soon it’s subsonic.

Modern planes have much better fuel economy and some of them can actually cruise much faster than those older “faster” planes. And 5th generation jet fighters have internal weapons bay which means a typical weapons load adds practically no extra drag to the plane, they can reach high speeds even with considerable weapons load.

In the fifth generation, considerations of stealth and beyond-visual-range combat led to a situation where speed was less important, and sensors and missiles became more important. There have been remarkable improvements in missile technologies, with air-to-air missiles routinely being designed to go hypersonic in recent iterations, and for compatability with stealth aircraft. The F-22, F-35, J-20, and the Su-57 being the most pre-eminent. But speed is still something that is particularly appealing to military designers. The next or sixth generation of fighters or more accurately defence systems may have even more capabilities that may reinvigorate the need for high speed. In future, therefore, we may see very fast aircraft that are unmanned, and which have extremely high never exceed speeds, required to get to stratospheric altitudes, or to sub-orbital altitudes.

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