The Lockheed Martin Skunk Works revealed a hypersonic aircraft design that can take off and land conventionally using turbine-based combined-cycle engine technology. The company said it has been working with rocket propulsion specialists Aerojet for several years on the project, using company funds. Although the design could lead to a Mach 6 unmanned strike aircraft, Lockheed Martin has dubbed it the SR-72, after the company’s SR-71 Blackbird manned strategic reconnaissance aircraft that reached Mach 3 but was retired in 1997.
“Hypersonic aircraft, coupled with hypersonic missiles, could penetrate denied airspace and strike at nearly any location across a continent in less than an hour,” asserted Brad Leland, Lockheed Martin program manager, hypersonics.
The engine design modifies a standard military turbine such as an F100 or F110 and couples it to a supersonic combustion ramjet (scramjet), using a common inlet and nozzle. Aerojet has been publicizing its Trijet combined cycle concept for some time already, noting that it would bridge the so-called “thrust gap” between turbines that reach Mach 2.5 and scramjets that work above Mach 3.5. Aerojet was merged with the former Pratt & Whitney Rocketdyne business to form Aerojet Rocketdyne earlier this year.
Lockheed Martin said that the design is “affordable” and could be operational by 2030. Leland told Aviation Week that a series of wind-tunnel tests have proved that the design is controllable from takeoff through hypersonic speeds. Subscale ground tests of the combined cycle engine, intakes and nozzle have taken place, as have scaled tests of other components. Details of how the thrust of the turbine is augmented and how the scramjet can work at lower speeds than hitherto remain proprietary, Leland added. He said that work on a single-engine, F-22-size demonstrator could start in 2018, with a first flight in 2023. The full-scale SR-72 is 100 feet long.
The Skunk Works–also known as Lockheed Martin Advanced Development Projects (ADP)–has previously been associated with both “black” and “white” supersonic and hypersonic programs. In the 1990s there was much speculation that a secret line (unintentionally revealed) in the U.S. Air Force budget named “Aurora” contained funds for an SR-71 replacement. In fact, the line concealed funding for the then-secret B-2 bomber. More recently, the Skunk Works developed the rocket-launched Hypersonic Technology Vehicle 2 (HTV-2) using funding from the Defense Advanced Research Projects Agency (Darpa) Falcon program. Lockheed Martin said that the SR-72 design incorporates lessons learned from the HTV-2, which flew at Mach 20.
Leland did not fully explain why ADP is revealing the project now. He linked the move to the U.S. Air Force high-speed strike weapon (HSSW) missile project. The service’s hypersonics roadmap envisions the HSSW as a stepping stone in 2020, to an SR-72-type vehicle by 2030. But Leland acknowledged that hypersonics are perceived as expensive and exotic, despite the recent success of the Boeing X-51A Waverider project.
Meanwhile, the USAF has let risk-reduction contracts for a supersonic long-range strike bomber. Lockheed Martin has teamed with Boeing for that program.