The US Air Force’s Mach 5 missile program suffered a major blow this week as its third flight test of the X–51A WaveRider ended in failure. The test was meant to be a 300-second scramjet-powered flight over the Pacific Ocean, but just 15 seconds after it detached from the B–52 Stratofortress carrying it to 50,000 feet, a faulty cruiser fin left the shark-shaped cruise missile uncontrollable, sending it careening into the sea. The failure was first reported by Danger Room.

Ramjets rely on the air pressure generated from forward motion

The X–51A uses two types of engines to provide thrust: a modified solid rocket booster to get it up to speed, and a Pratt & Whitney Rocketdyne SJY61 scramjet (supersonic combustion ramjet) engine. Ramjets use the air compression from forward motion to produce thrust so they can’t move an aircraft from a standstill. To get around this problem, the preliminary booster first gets the WaveRider up to speed (Mach 4.5 or so), then a connecting interstage drops away and the scramjet gets lit, burning a mix of oxygen in the air and JP–7 jet fuel. The scramjet kicks things into hypersonic speed, and lift is improved by exploiting the shock waves the missile produces (called compression lift), giving rise to the WaveRider name. This is what happens when everything goes according to plan, at least — neither of the last two tests have gotten that far.

In a statement to the press posted by Aviation Week, the Air Force Research Laboratory’s X–51A program manager Charlie Brink said, “it is unfortunate that a problem with this subsystem caused a termination before we could light the Scramjet engine. All our data showed we had created the right conditions for engine ignition and we were very hopeful to meet our test objectives.” The Air Force originally created four X–51As, and of the three flights so far, only the first was successful, logging 200 seconds of flight time. This most recent failure casts doubts both on the fate of the fourth missile (the Air Force hasn’t determined when or even if it will fly), as well as the feasibility of a free-flying scramjet-powered vehicle.