Lockheed Martin has begun work on a successor to the supersonic Blackbird SR-71 spy plane.

The unmanned SR-72 will use an engine that combines a turbine and a ramjet to reach its top speed of Mach 6 - about 3,600mph (5,800km/h).

Like its predecessor, the SR-72 will be designed for high-altitude surveillance but might also be fitted with weapons to strike targets.

Lockheed said the aircraft should be operational by 2030.

Jet engines

The SR-72 is being developed at Lockheed Martin's Skunk Works R&D centre in California that designed and built the original Blackbird.

That aircraft first flew in 1964 and was a mainstay of US Air Force spying and surveillance work until 1998. It typically flew at altitudes of 24,000m (80,000ft) and could reach speeds of Mach 3.

In a blogpost about the SR-72, Lockheed Martin said the aircraft would operate at similar altitudes but would fly far faster. At Mach 6 the plane could travel the 3,500 miles (5,500km) from New York to London in less than an hour.

While spy satellites can photograph enemy territory, the relatively long time it takes for them to be moved to a new orbit so they pass over a target can limit their usefulness.

By contrast, wrote Lockheed Martin, the SR-72 "would be so fast, an adversary would have no time to react or hide".

For the SR-72, Lockheed Martin is drawing on work done on the Falcon HTV-2 hypersonic technology vehicle. This is a test-bed for the futuristic technologies needed to support safe hypersonic flight and cope with the extreme conditions encountered by any object flying at such a speed.

For instance, on one test flight of the HTV-2, the aircraft hit a top speed of Mach 20 and its flight surfaces reached 1,927C (3,500F).

To reach Mach 6, the SR-72 will use an engine that acts like a normal jet turbine until speeds of Mach 3 are reached but which then operates like a ramjet to accelerate beyond that.

"Speed is the next aviation advancement to counter emerging threats in the next several decades," said Brad Leland, Lockheed Martin's hypersonic programme manager on the blog. "The technology would be a game-changer in theatre, similar to how stealth is changing the battlespace today."