Here’s a first look at the SR-71 successor, Lockheed Martin’s SR-72 Blackbird. Unlike its predecessor, this one will use a new hypersonic engine design that combines turbines and ramjets, making the unmanned SR-72 twice as fast as its predecessor with a cruising speed of Mach 6.
According to Leland, a Mach 6 platform would not only leave very little time for an enemy to respond, but it also be a very effective way to launch hypersonic missiles.
Since these wouldn’t need a booster rocket when launched at six times the speed of sound, they can be of much lighter and simpler construction. The key to the SR-72 is what Lockheed calls Turbine-Based Combined Cycle Propulsion, which incorporates Lockheed’s experience in building the HTV-2 hypersonic demonstrator that flew at Mach 20 (15,224 mph, 24,501 km/h) in tests.
In this new system, the twin engines of the SR-72 are actually two engines in one.
Official Source: http://www.gizmag.com/lockeheed-martin-sr-72-blackbird/29634/