The Falcon Hypersonic Technology Vehicle 2 attached to a rocket.

The Falcon Hypersonic Technology Vehicle 2 attached to a rocket. Photo: DARPA

Hopes of fast-tracking an experimental aircraft that could fly from Sydney to London in 49 minutes have taken a nosedive with US military scientists today losing the plane during testing.

They launched the hypersonic aircraft but lost contact with the experimental plane as it flew over the Pacific Ocean in its second test flight.

The Defence Advanced Research Projects Agency (DARPA) says the unmanned Falcon Hypersonic Technology Vehicle 2 (HTV-2), designed as a global bomber prototype capable of a mind-boggling 20 times the speed of sound, was launched successfully from California aboard a Minotaur IV rocket.

The Falcon Hypersonic Technology Vehicle 2.

The Falcon Hypersonic Technology Vehicle 2. Photo: DARPA

But after the plane separated from the rocket in the upper reaches of the atmosphere for its "glide" phase, contact was lost, DARPA said.

"Range assets have lost telemetry with HTV2," DARPA wrote in a Twitter post after the launch.

The agency provided no other details about the flight nor how long it had been separated from the rocket.

Last year, scientists lost contact with the HTV-2 after nine minutes in its inaugural flight.

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

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.

In theory, the Falcon could travel between New York City and Los Angeles in less than 12 minutes.

Unlike a ballistic missile, a hypersonic vehicle could manoeuvre and avoid flying along a predictable path. Moreover, analysts say it would not be mistaken for a nuclear missile, avoiding the possibility of triggering a nuclear confrontation.

But Loren Thompson, an analyst at Lexington Institute with links to the defence industry, said there was still much work to be done before the hypersonic bomber became a reality.

"The military has a long way to go before hypersonic vehicles are ready for deployment," Thompson said.

After separating from the rocket on Thursday, the Falcon undertook some manoeuvres before contact was lost, DARPA said.

The test flight plan called for the Falcon to roll and dive into the Pacific Ocean.