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

 

Like its predecessor¹, 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.

 

 

The SR-72 is being developed at Lockheed Martin's Skunk² 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 relatively³ 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 adversary⁴ would have no time to react or hide".

 

For the SR-72, Lockheed Martin is drawing on work done on the Falcon⁵ 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 advancement⁶ 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（秘密，鬼祟） is changing the battlespace today."

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