Why Do Modern Fighter Jets Have Lower Top Speeds?

There's a peculiarity that emerges when looking at the evolution of fighter jets. The F-100 Super Sabre — the first supersonic fighter jet in the world – debuted in the early '50s, and could exceed 900 mph, cruising at 590 mph. Shortly thereafter, engineers developed the Lockheed F-104 Starfighter which could travel at Mach 2 — twice the speed of sound. By 1989, the fastest fighter jet in America's arsenal, the F-15E Strike Eagle, was capable of speeds as fast as 1,875 mph — Mach 2.5. So, with technology advancing all the time, what speed can fighter jets go today?

Analysis shows that this trend stopped. The F-35 Lightning II, America's newest fifth-generation fighter jet, only gets up to around 1,200 mph — Mach 1.6. Technology may have improved but the United States military re-evaluated its priorities with fighter jets. Rather than optimizing airframes to sustain exceptionally high speeds, the focus instead turned to improving the thrust-to-weight ratio for maneuverability.

One contributing factor to this change was the Persian Gulf War. It was the largest air campaign since Vietnam and a conflict that saw multiple jets in the air capable of Mach 2. However, in no engagements did U.S. aircraft ever exceed Mach 1.03, even when enemy combatants flew faster.

Modern fighter jets favor stealth over speed, but that's not the only technology the military values. In fact, speed doesn't necessarily detract from a fifth-generation's ability to avoid detection. That's a myth that stemmed from the nascent stealth technology used on the F-117 Nighthawk. Today, maneuverability, situational awareness, and advanced weaponry also prove hugely beneficial against air defenses, and are high on the list of priorities. Speed is good, but it's not going to help against an interceptor's guns.

It's especially not going to offer an advantage against a missile exceeding Mach 3, which has become the primary weapon used in air-to-air engagements since the Vietnam War. Therefore, optimizing an aircraft for acceleration and maneuverability proves wise. Fighter jets being slower than their predecessors in no way means they're less powerful, though. There's an equation to determine how much force an aircraft can withstand for its weight — the thrust-to-weight ratio. The history of fighter jets has shown results ranging from 0.1 to 1.2, and when any aircraft exceeds 1.0, the aircraft can accelerate vertically.

Many of the fighters with the highest thrust-to-weight ratio fall in the fourth and fifth-generation category. An F-22's thrust-to-weight ratio, for example, is 1.08. Not much is known yet about America's F-47 sixth-generation fighter jet, but there's a good chance its max speed will be similar to the F-35.

If any generation of fighters were to be considered the fastest on average, it would be the third-generation jets, built throughout the '50s and '60s. The F-4C Phantom came about near the end of the third-generation in 1963 and reached 1,400 mph. Meanwhile, the F-104A Starfighter set a world speed record of 1,404 mph in 1958, and its fastest version, the F-104G, could reach 1,528 mph – Mach 2.0. Average speeds started gradually declining in the following generations.

When the first Persian Gulf War took place in the '90s, no U.S. aircraft exceeded Mach 1.03, but they were clearly capable of doing so. There were 48 F-15E jets flying operations over the gulf, which had no issue pushing all 29,000 pounds of thrust out of each of its twin Pratt & Whitney engines to exceed Mach 2.0. 

It's not just American fighters slowing down. China's J-20 fifth-gen fighter is only suspected of reaching 1,533 mph. Russia's Su-57 Felon is also slower, as it's believed to reach 1,326 mph.