An unmanned experimental aircraft designed as a global bomber prototype, capable of gliding down from the upper atmosphere at 20 times the speed of sound, has lost contact with ground control on its second test flight, a Pentagon agency said.

The Falcon HTV-2 was launched from Vandenberg Air Force Base in California on a rocket and successfully separated from the launch vehicle, the Defense Advanced Research Projects Agency (DARPA) reported.

The arrowhead-shaped plane was expected to separate from the rocket near the peak of its ascent and glide back to earth, reaching hypersonic speed before rolling and plunging into the Pacific ocean, according to a test diagram posted online.

The hypersonic plane, which is supposed to travel at Mach 20, or 21,000 kph, could potentially provide the US military with a platform for striking targets anywhere on the planet within minutes using conventional weapons.

Philip Ewing, the editor of online journal DOD Buzz told Al Jazeera that "the Pentagon has, in past decade or so, wanted to have something it calls 'prompt global strike' with the ability to strike a target anywhere in the world within an hour.

"It is possible that the Pentagon has learned some valuable lessons from the crash, especially in dealing with something this fast at that kind of altitude," Ewing said, adding " the US would most likely say that mistakes like this must be when developing new forms of technology like this and that today's incident would inform other projects in the future."

Global strike potential

Such a weapon, still in development, is part of what the US Air Force has dubbed "prompt global strike" capability.

"The ultimate goal is a capability that can reach anywhere in the world in less than an hour," DARPA said on its website.

About 10 minutes after the flight began on Thursday, DARPA tweeted that the mission was "on track, entering glide phase."

But about 26 minutes later, DARPA tweeted that its monitoring stations had lost contact with the glider.

"Downrange assets did not reacquire tracking or telemetry," DARPA tweeted about an hour later. "HTV-2 has an autonomous flight termination capacity."

The loss of communications in the final stages of the test flight was a failure for the agency. During the initial flight test in April, researchers lost contact with the vehicle about nine minutes into the flight.