This spring, the Air Force was preparing for a groundbreaking test of the X-51 WaveRider, a hypersonic cruise missile that would reach speeds of up to Mach 6. But it looks like the WaveRider's debut flight will have to wait while some technical issues are addressed.

Boeing spokeswoman Christina Kelly confirmed to Danger Room that the test would have to be rescheduled. "We don’t have a firm date," she said. "It’s going to happen, but it’s just going move to the right."

The X-51 program is a collaboration between Boeing Phantom Works and engine maker Pratt & Whitney Rocketdyne to develop a new class of cruise missile that can reach targets much, much faster than current designs. As this Air Force video explains, the X-51 employs scramjet propulsion: It collects air from the atmosphere to mix and burn with its fuel, but unlike a jet, it doesn’t rely on a turbine to do the compression work.

Hypersonic flight was once considered too extreme for an air-breathing vehicle to handle in a controlled way. But as Danger Room's Noah Schachtman explained in Popular Mechanics a few years back, the X-51's unique shape uses the shockwaves created by hypersonic flight to create lift, and compress the air to mix with its fuel.

If the concept works, It fits neatly with the military's "prompt global strike" concept. The idea is to develop a new class of conventional weapon that can reach distant targets – say, a weapon of mass destruction, or an enemy command post – and hit it in a hurry. But a conventionally armed ICBM or submarine-launched ballistic missile might not be the ideal solution: You don't want your global strike mission to be mistaken for a nuclear attack.

Testing the X-51 is a fairly complex thing. First, a B-52 has to carry the WaveRider up to 50,000 feet, and then drop it away. A solid rocket booster will then accelerate the aircraft to about Mach 4.5. After the booster drops, the scramjet is supposed to ignite, taking the WaveRider up to Mach 6.

Lt. Col. Todd Venema, director of the Hypersonic Combined Test Force, said in this Air Force news item that the flight test would push the altitude limit for the B-52. And tracking the flight will also require some orchestration. "Telemetry has to be relayed to the Naval Air Station at Pt. Magu to a control room with about 35 people, all watching the various telemetry," he said. "So there will be a lot of team work aspects to the whole project."