Engineers at Lockheed Martin’s Skunk Works® are working on the successor to the SR-71 Blackbird retired in 1998. With a top speed of Mach 6, or six times the speed of sound, the SR-72 will be twice as fast as its predecessor. An optionally piloted hypersonic aircraft, the SR-72 will have long range strike and reconnaissance capabilities. Lockheed Martin announced the project in 2013, and has been working on technologies for the plane for over a decade.

The key to the SR-72’s hypersonic speed is its turbine-based combined cycle (TBCC) propulsion system. In this system, a standard turbine engine is integrated with a supersonic combustion ramjet air breathing engine. Lockheed Martin has been collaborating with Aerojet Rocketdyne on the integration of the two propulsion technologies. In the TBCC system, the turbine engine provides thrust from takeoff to around Mach 3, at which point the ramjet accelerates the vehicle to hypersonic speeds, switching to scramjet mode at Mach 5. The two engines share a common inlet and nozzle, reducing spillage drag at the inlet and base drag at the nozzle.

The ramjet is crucial to the aircraft reaching hypersonic speeds. Air is collected at the inlet and compressed by the forward motion of the aircraft, slowing down to subsonic speeds as it travels through a diffuser toward the combustor, where it mixes with fuel and is ignited. The hot, expanding exhaust is accelerated out of the nozzle, producing thrust. In scramjet mode, the airflow remains supersonic throughout the engine.

DARPA’s Falcon Project

Artist’s concept of Hypersonic Technology Vehicle 2 (HTV-2) during atmospheric reentry.Artist’s concept of Hypersonic Technology Vehicle 2 (HTV-2) during atmospheric reentry.

The development origins of the SR-72 trace back to DARPA’s (Defense Advanced Research Projects Agency) Falcon project. The Falcon (Force Application and Launch from Continental United States) program sought to develop launch vehicles, common aero vehicles and a hypersonic cruise vehicle. As part of the Falcon project, Lockheed Martin worked with DARPA to develop the Hypersonic Technology Vehicle 2 (HTV-2). The project provided valuable data about the aerodynamics, aerothermodynamics and navigation, guidance and control of hypersonic vehicles. In flight testing, HTV-2 reached a top speed of Mach 20, or 13,000 mph, with a maximum surface temperature of 3,500° F.

A followup to the HTV-2, the HTV-3X Blackswift, was cancelled, but development work yielded a stable aircraft configuration and knowledge about flight control systems that maintain stability at transonic speeds. Work on HTV-3X also advanced the TBCC concept, resulting in the first mode-transition demonstration of a combined turbojet and ramjet propulsion system.

A fundamental design challenge for the TBCC engine was a speed gap between the turbine engine and the ramjet. The maximum speed of off-the-shelf turbine engines is around Mach 2.5, while ramjets do not function efficiently until speeds of around Mach 3. Lockheed has developed a method to bridge the gap, enabling an affordable TBCC Mach 5+ aircraft.

Hypersonic Speed

Traveling in excess of 4,000 miles per hour puts unique demands on an aircraft’s structure. At Mach 5, skin temperatures due to aerodynamic friction exceed 2,000° F. Conventional materials would fail in such circumstances. Composite carbon and ceramic materials and special metal alloys are needed to withstand the heat. In addition, all routes into the interior of the aircraft must be fully sealed to prevent superheated air from wreaking havoc on the aircraft’s interior.

Artist’s concept of the SR-72. Source: Lockheed MartinArtist’s concept of the SR-72. Source: Lockheed MartinThe airframe is also exposed to unique stresses in the hypersonic speed regime. The center of lift, at the back of the aircraft during supersonic flight, transitions toward the nose at hypersonic speeds due to drag on the leading edges. A gap between the center of lift and center of gravity is necessary for an airplane to maintain stability, so the SR-72 must be designed to accommodate the shifting center of lift and incredible stresses that hypersonic flight entails.

ISR + Strike

Like the SR-71, the SR-72 is being developed as an intelligence, surveillance and reconnaissance (ISR) platform. But unlike the SR-71, its successor will also be armed with the capability to strike targets. Traveling at hypersonic speeds, it could penetrate deep into enemy territory, gathering intelligence or engaging targets before the enemy could react. Advanced adversaries might have knowledge of satellite positions, allowing them to conceal mobile weapons platforms from satellite surveillance, but they would not have time to hide assets from the hypersonic spyplane. The SR-72’s speed is also intended to counter sophisticated air defense and stealth detection capabilities.

During its unveiling in 2013, Lockheed Martin’s Hypersonics program manager, Brad Leland, emphasized the importance of speed in military aviation. “Hypersonic aircraft, coupled with hypersonic missiles, could penetrate denied airspace and strike at nearly any location across a continent in less than an hour,” said Leland. “Speed is the next aviation advancement to counter emerging threats in the next several decades. The technology would be a game-changer in theater, similar to how stealth is changing the battlespace today.”

Component technologies for the SR-72 are thought to already be undergoing ground and flight tests. Development of a flight research vehicle (FRV) could begin in 2018, with flight tests starting in 2020. The FRV would be powered by a single full-scale TBCC engine. It would be around 60 feet in length, about the size of an F-22.

A full-scale demonstrator, around 100 feet in length, about the size of an SR-71, would begin flight tests in the late 2020s. A fully operational SR-72 could be ready by 2030.