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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 | 
