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

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

Lockheed said the aircraft should be operational by 2030.

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.