Center for Strategic Communication

The Air Force’s X-51A Waverider hypersonic missile attached to the wing of a B-52 Stratofortress. Photo: USAF

Updated 2:39 p.m. ET

A crucial test for the Air Force’s experimental Mach 5 missile has ended in failure, according to the Air Force. Tuesday’s test is also the third time the hypersonic X-51A Waverider cruise missile has fallen short. But this time could be the last.

As first reported by Danger Room, the Waverider’s control fin failed, preventing the missile’s scramjet engine from starting. The Air Force later confirmed the result in a statement. That also corresponds to information provided by an insider familiar with the test, who said a problem with a missile’s fin caused a loss of control before the engine could kick in. The missile’s scramjet engine was supposed to power the missile at hypersonic speeds for 300 seconds. Instead, the Air Force identified a fault with the fin 16 seconds after launch. Fifteen seconds later, after the missile separated from its preliminary rocket booster — which builds up speed before the missile activates its scramjet – the Waverider broke apart.

The Air Force launched the missile over the Pacific sometime between 10 a.m. and 11 a.m. Tuesday from a B-52 Stratofortress based at Edwards Air Force Base in California. Traveling at 50,000 feet, the B-52 was to launch the missile, which would then scream across the Pacific at hypersonic speeds. The missile got away from the B-52, but then flopped uncontrollably into the ocean.

The stakes were high. So far the tests have cost the Pentagon some $300 million. True, the millions in research and testing have helped advance the field of hypersonics, but the shortage of tangible results have also helped spoil the confidence the Air Force once had in producing a working weapon. The failure also has serious implications for the military’s “prompt global strike” mission, which aims to use missiles with engines capable of hypersonic flight to wallop targets hundreds — even thousands — of miles away and do so within minutes.

The 26-foot-long X-51A was supposed to be a working candidate for prompt global strike, and a safer alternative to the Air Force’s former plan to stick conventional warheads onto intercontinental ballistic missiles, or ICBMs. Problem with using ICBMs, however, is that they look like nuclear missiles when launched, potentially provoking a devastating war. The X-51A is just a cruise missile — albeit one with an experimental scramjet engine.

It works in theory like this. After being launched from an aircraft, the X-51A fires its conventional rocket booster, which builds up subsonic speeds before the missile’s supersonic ramjet kicks in. Once the ramjet fires, the engine would begin to collect oxygen from the atmosphere and mix the air with jet fuel, burning both. The missile would next accelerate past Mach 5, staying in the air by using lift created from the missile’s shock waves.

But actually doing it is really hard. Hypersonic speed generates tremendous heat. And the missile also needs sophisticated guidance tools, sensors and navigation equipment to keep it in the air.

The X-51A’s test history is instructive. The missile’s first test flight lasted more than three minutes before a seal broke — a failure, but relatively better compared to what followed. A second test last summer lasted only a few seconds before the missile failed due to overpressure created by unexpected shockwaves. A possible solution was to tweak the timing for injecting fuel into the engine, which should also change the pattern of shockwaves. Though, it looks like Tuesday’s test didn’t last long enough for Air Force to find out if it would’ve worked.

The Air Force is currently building a fourth missile, but is short on funds for testing. And if tests keep getting worse, there may not be enough interest for another round. At the same time, the service is working on transitioning the X-51A into a serviceable missile called the “High Speed Strike Weapon.” The Pentagon’s researchers at Darpa have also been experimenting with hypersonic glider. The glider — and a potential weapons platform — is launched into near-space before zooming back to Earth at 20 times the speed of sound. Darpa wants to press ahead with another glider test after its last try-out crashed into the Pacific.

Kinda like what happened to the X-51A. Except for the Air Force’s cruise missile, there might not be another try, nor the budget and willpower.