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| SR-72 Concept. Photo Credit: Lockheed Martin |
Lockheed recently announced plans to develop the SR-72, an unmanned aircraft capable of cruising at Mach 6. The SR-72 is being projected as a successor to the SR-71 manned spy plane and is referred to as Son of the Blackbird by the latest issue of AW&ST. Conceptually, the two aircraft are similar but modern technology would probably make the SR-72 far more lethal than its predecessor.
The SR-71 Blackbird was developed in the 1960s by Lockheed's Skunk Works division and served with the U.S. Air Force from 1964 to 1998. A total of 32 aircraft were built of which 12 were lost in accidents. No SR-71 was downed by enemy action as the aircraft would easily outrun any attacking enemy missile.
The SR-71 cruised at Mach 3.2 using it's unique Pratt & Whitney J58-P4 engine, a turbojet ramjet hybrid. The engine encapsulated a conventional turbojet within a ramjet, with the turbojet producing most of the power at speeds upto Mach 2.8 and the ramjet doing most of the thrusting thereafter.
The engine featured a two stage compressor, with some of the air from the first stage compressor being directed to the afterburner and the rest being routed to the afterburner through the a second stage compressor and turbine.
At low speeds most of the first stage compressor air would be directed to flow through the second stage compressor and turbine and the J58-P4 would function largely like a conventional jet. At high speeds the shock cone of the engine and first stage would compress and heat the airflow to an extent where routing it through the turbojet would be unsafe, as it would result in excessive heat generation and melting of the turbine blades. Therefore, most of the first stage compressor air would be routed directly to the afterburner effectively turning the J58-P4 into a ramjet; only a small amount of air would flow through the turbojet portion.
The J58-P4 engine could produce a static thrust of 32,500 lbf (145 kN) and cruise efficiently at speeds around Mach 3.2.
The maximum speed of the SR-71 was restricted to Mach 3.2 because the aircraft's compressor inlet temperature couldn't exceed 800 °F (427 °C).
Engineers at Lockheed Martin's Skunk Works lab in Palmdale, California, claim to have solved the problem, but haven't revealed details of their solution.
Following the SR-71's retirement in 1998 technology has advanced and studies have shown that inlets speeds of Mach 6 should now be possible.
Ramjet Gives Way to Scramjet in SR-72
Ramjets support higher cruise speeds because they have no moving parts - there is no fear of turbine blades melting! However, ramjets are limited to speeds a little above Mach 3 since the airflow within them is subsonic. In order to facilitate cruise at Mach 6 a ramjet would need to support supersonic flow within its combustion chamber. Such engines are referred to as scramjets.
Lockheed's Skunk Works has been working with Aerojet Rocketdyne for several years to develop a method to integrate an off-the-shelf turbine with a supersonic combustion ramjet air breathing jet engine to power the SR-72 from standstill to Mach 6.
Skunk Works aircraft earlier partnered with the Defense Advanced Research Projects Agency (DARPA) to develop the the rocket-launched Falcon Hypersonic Technology Vehicle 2 (HTV-2). The HTV-2 achieved flight speeds of Mach 20, or 13,000 mph, with a surface temperature of 3500°F and the project helped collect data on three technical challenges of hypersonic flight: aerodynamics; aerothermal effects; and guidance, navigation and control. The SR-72’s design incorporates lessons learned from the HTV-2.
The SR-72 would be capable of reaching any point on the globe within an hour and penetrating all conceivable enemy defenses. The ability would prove game changing, perhaps more so than stealth.
