A proposed hypersonic reconnaissance and strike aircraft, the SR-72 would serve as a replacement for the famed SR-71 Blackbird, which was retired by the Air Force back in 1998. The SR-71 Blackbird could fly at 2200 mph (over 3 times the speed of sound).
Lockheed has said they are working on a combined-cycle engine. It uses both a turbine and a scramjet to achieve hypersonic speeds. Lockheed Martin is testing Aerojet Rocketdyne from 2013 to 2017. Two combined-cycle engines are planned to power the SR-72, which is designed to be about the same size of the SR-71 and could achieve first flight in the late 2020s.
An optionally piloted flight research vehicle (FRV) is also in the works to flight test elements of the SR-72 design. The FRV will be about the size of an F-22 and use a single combined-cycle engine for propulsion. Development of the FRV is expected to begin next year and first flights could occur as soon as 2020. Leading up to the FRV, Lockheed could be conducting ground and flight tests on even smaller demonstrators.