Lockheed Martin SR-72

SR-72
Lockheed Martin SR-72 rendering
Role	Hypersonic strategic reconnaissance UAV
Manufacturer	Lockheed Martin
Status	Design proposal
Primary user	United States Air Force (intended)

The Lockheed Martin SR-72 is a conceptualized hypersonic UAV intended for intelligence, surveillance and reconnaissance proposed by Lockheed Martin to succeed the retired Lockheed SR-71 Blackbird.^[1]

Design and development

The SR-72, the proposed successor to the SR-71 Blackbird retired in 1998,^[2] is expected to fill what is considered a coverage gap between surveillance satellites, manned aircraft, and unmanned aerial vehicles for intelligence, surveillance and reconnaissance (ISR) and strike missions. With the growth of anti-satellite weapons, anti-access/area denial tactics, and counter-stealth technologies, a high-speed aircraft could penetrate protected airspace and observe or strike a target before enemies could detect or intercept it. The proposed reliance on extremely high speed to penetrate defended airspace is considered a significant conceptual departure from the emphasis on stealth in fifth-generation jet fighter programs and projected drone developments.^[3] There were unconfirmed reports about the SR-72 dating back to 2007, when various sources disclosed that Lockheed Martin was developing a Mach 6 (4,567 mph; 7,350 km/h; 1 mi/s) plane for the United States Air Force.^[4][5] Skunk Works' development work on the SR-72 was first published by Aviation Week & Space Technology on 1 November 2013.^[1][2] Public attention to the news was large enough to overwhelm the Aviation Week servers.^[6]

To attain such speeds, Lockheed Martin has been collaborating with Aerojet Rocketdyne since 2006 on an appropriate engine. The company is developing the system from the scramjet-powered HTV-3X, which was canceled in 2008. The SR-72 is envisioned with an air-breathing hypersonic propulsion system that has the ability to accelerate from standstill to Mach 6.0 using the same engine, making it about twice as fast as the SR-71.^[1] The challenge is to design an engine to encompass the flight regimes of subsonic, supersonic and hypersonic speeds. Using turbine compression, turbojet engines can work at zero speed and usually perform best up to Mach 2.2.^[7] Ramjets, using aerodynamic compression with subsonic combustion, perform poorly under Mach 0.5 and are most efficient around Mach 3, being able to go up to around Mach 6. The SR-71's specially designed engines converted to low-speed ramjets by redirecting the airflow around the core and into the afterburner for speeds greater than Mach 2.5. Finally, scramjets with supersonic combustion cover the range of high supersonic to hypersonic speeds. The SR-72 is to use a turbine-based combined cycle (TBCC) system to use a turbine engine at low speeds and a scramjet engine at high speeds.^[2] The turbine and ramjet engines share common inlet and nozzle, with different airflow paths in between.^[8]

When flying at and above Mach 5, aerodynamic friction becomes hot enough to melt conventional steel airframes, so engineers are looking at composites to make it out of including high-performance carbon, ceramic, and metal mixes, such as the types used for the noses of intercontinental ballistic missiles and space shuttles. Although the SR-72 is envisioned as an ISR and strike platform, no payloads have been specified, likely because current payloads will be insufficient on an aircraft flying at Mach 6 up to 80,000 ft (24,000 m) high requiring hundreds of miles to turn. New sensors and weapons will likely have to be created specifically to operate at such speeds.^[9]

Construction of an optionally-piloted scaled demonstrator is planned to start in 2018. The demonstrator will be about 60 ft (18 m) long, about the size of an Lockheed Martin F-22 Raptor, and powered by one full-scale engine to fly for several minutes at Mach 6.^[1][2] Flights of the demonstrator are to be conducted starting in 2023. The SR-72 flight testing follows the planned timeline for the hypersonic High Speed Strike Weapon. The SR-72 is to be similar in size to the SR-71 at over 100 ft (30 m) long and have the same range, with entry into service by 2030. The SR-72 follows the US Air Force's hypersonic road map for developing a hypersonic strike weapon by 2020, and a penetrating ISR aircraft by 2030. At the time of the concept's unveiling, Lockheed Martin had engaged in talks with government officials, but has not secured funding for the demonstrator or engine.^[1][2]

On 13 November 2013, Air Force Chief of Staff General Mark Welsh revealed that the service was interested in the SR-72's hypersonic capabilities, but had not spoken with Lockheed about the aircraft. Its high speed appeals to the service to reduce the time an adversary would have to react to an operation. They are pursuing hypersonic technology, but don't yet have the material ability to construct a full-size plane like the unmanned SR-72. The SR-72 was unveiled in the midst of sequestration budget cuts that have forced the Air Force to prioritize acquisition projects and sacrifice mission readiness. By the mid-2020s, it is believed that foreign countries will produce and export advanced aerial technologies that could end up in battlespaces against the United States. This drives the Air Force to further develop new systems, including hypersonic, to replace legacy systems that would be outclassed in those situations.^[10]

The SR-72 may face significant challenges to being accepted by the Air Force, as they are opting to develop the Northrop Grumman RQ-180 stealth UAV to perform the task of conducting ISR missions in contested airspace. Compared to the SR-72, the RQ-180 is less complex to design and manufacture, less prone to problems with acquisition, and can enter service as soon as 2015.^[11]

In December 2014 NASA awarded Lockheed Martin a contract to study the feasibility of building the SR-72's propulsion system using existing turbine engine technologies. The $892,292 contract funds a design study to determine the viability of a TBCC propulsion system by combining one of several current turbine engines, with a very low Mach ignition Dual Mode Ramjet (DMRJ). NASA previously funded a Lockheed Martin study that found speeds up to Mach 7 could be achieved with a dual-mode engine combining turbine and ramjet technologies. The problem with hypersonic propulsion has always been the gap between the highest speed capabilities of a turbojet, from around Mach 2.2 to the lowest speed of a ramjet at Mach 4. Typical turbine engines cannot achieve high enough speeds for a ramjet to take over and continue accelerating. The NASA-Lockheed study is looking at the possibility of a higher-speed turbine engine or a ramjet that can function in a turbine engine's slower flight envelope; the DARPA HTV-3X had demonstrated a low-speed ramjet that could operate below Mach 3. Existing turbofan engines powering jet fighters and other experimental designs are being considered for modification. If the study is successful, NASA will fund a demonstrator to test the DMRJ in a flight research vehicle.^[12]

See also

• DARPA Falcon Project
• Boeing X-51
• Prompt Global Strike
• 2037 Bomber
• Aurora (aircraft)
• Reaction Engines Scimitar

References