But that has not put them off the idea of hypersonic flight. The Pentagon also recently started work on the High Speed Strike Weapon, a hypersonic missile that will be launched from an aircraft. “It’s the next step,” says Mark Lewis, a former Air Force chief scientist, who was involved in the X-51 effort. “It’s looking at making hypersonics more operational and practical.”
Another programme is also underway: Darpa, which has also funded the X-51, held an open meeting this week for those interested in bidding on a new hypersonic program, which according to the agency announcement, is expected to lead to a hypersonic “X-plane” in 2016.
The programme, however, is based on the design of the Falcon Hypersonic Test Vehicle-2, or HTV-2, an unmanned glider that can travel at speeds of up to Mach 20, but suffered two previous catastrophic failures. The HTV-2 was designed to be launched off a rocket, and then glide, unpowered. But in its first flight, the vehicle began to spin like a football eventually crashing; the second flight also failed.
HTV-2 was once considered as a candidate for a mission known as “Prompt Global Strike,” a weapon that could travel anywhere in the world within an hour.
For critics of the HTV-2, Darpa’s decision to continue with a vehicle that already failed twice in flight for apparent design failures is a repeat of mistakes made in previous hypersonic programmes. “It’s a bad design. It flew twice, it was lost twice,” says Hallion. “No matter how much lipstick you put on this pig, it’s still a pig.”
Some advocates of hypersonics, however, believe that the real problem with the government and military programmes is that they try to do much, and then simply cut off support when things go wrong. “You cannot achieve this until you commit yourself to doing it,” says Preston Carter, who previously managed hypersonics programs at Nasa, Darpa, and the Lawrence Livermore Laboratory in California,
Carter is now in the private sector trying to find funding for a commercial aircraft designed to travel at speeds of around Mach 5, which would make the trip from New York to London in about half the time that it took the Concorde.
Hypersonics is challenging, Carter acknowledges, but it’s not impossible. He points to previous aircraft, like the SR-71 Blackbird, a supersonic spy plane, and the civilian Concorde that, though not hypersonic, pushed the envelope of what was technically possible. Those aircraft also had seemingly monumental engineering challenges at the start, but they eventually succeeded, Carter says.
The same approach needs to be taken to hypersonic aircraft. “The reason why they were able to do it because they started,” he says. “The reason we won’t do it is because we haven’t started.”
This story was updated at 2010GMT on 15 August to state that a faulty control fin caused the failure of the X-51.