Lockheed Martin SR-72. The Lockheed Martin SR-72, colloquially referred to as "Son of Blackbird", is an American hypersonic UAV concept intended for intelligence, surveillance and reconnaissance (ISR) proposed privately in 2013 by Lockheed Martin as a successor to the retired Lockheed SR-71 Blackbird. In 2018, company executives said an SR-72 test vehicle could fly by 2025 and enter service in the 2030s.

Design and development

Background and early work
The SR-71 Blackbird was retired by the United States Air Force in 1998, eliminating a unique and useful intelligence, surveillance and reconnaissance (ISR) capability. Though most fifth-generation jet fighters and projected drones intended to operate in enemy airspace incorporated anti-radar stealth technologies, some came to believe that the growth of anti-access/area denial tactics and counter-stealth technologies meant that speed, not stealth, was the most promising approach to penetrating protected airspace.

The first unconfirmed reports about the SR-72 appeared in 2007, when various sources said Lockheed Martin was developing an airplane able to fly six times the speed of sound or  for the Air Force—about twice as fast as the SR-71.

Lockheed Martin had been collaborating with Aerojet Rocketdyne since 2006 to develop an appropriate engine. After the 2008 cancellation of the HTV-3X, the company used its scramjet technology for the SR-72 engine. The SR-72 was envisioned to have an air-breathing propulsion system that could handle subsonic, transonic, supersonic, and hypersonic speeds. Turbojet engines can work at zero speed and usually perform best up to Mach 2.2. Ramjets, using aerodynamic compression with subsonic combustion, perform poorly below Mach 0.5, are most efficient around Mach 3, and can go up to around Mach 6. The SR-71's 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 (supersonic combustion ramjets) cover the range of high supersonic to hypersonic speeds. The SR-72 was to use a turbine-based combined cycle (TBCC) system to use a turbine engine at low speeds and a scramjet engine at high speeds. The turbine and ramjet engines shared common inlet and nozzle, but with separate airflow paths.

At speeds of Mach 5 and above, aerodynamic heating creates temperatures hot enough to melt conventional metallic airframes, so engineers considered making critical components from composites such as high-performance carbon, ceramic, and metal mixes, which have been used in intercontinental ballistic missiles and the retired Space Shuttle.

On 1 November 2013, an article on Lockheed Martin Skunk Works' SR-72 work was published by Aviation Week & Space Technology. Public attention to the news was large enough to overwhelm the Aviation Week servers. Company officials announced plans to build an optionally piloted scaled demonstrator that would be  long, about the size of a Lockheed Martin F-22 Raptor, and powered by one full-scale engine to fly for several minutes at Mach 6; it would be ready in 2018 for flight testing that would follow the timeline for the hypersonic High Speed Strike Weapon. The production SR-72 would be similar in size to the SR-71 at over  long, have the same range, and enter service by 2030, company officials said. It would follow the U.S. Air Force's hypersonic road map for developing a hypersonic strike weapon by 2020, and a penetrating ISR aircraft by 2030. Lockheed officials said they had talked with government officials, but had not secured funding for the demonstrator or engine.

On 13 November 2013, Air Force Chief of Staff General Mark Welsh said the service was interested in hypersonic flight, which would reduce the time an adversary would have to react to an operation. He said it was among the capabilities that could help the Air Force against the advanced air defenses of the mid-2020s. Welsh said the service was pursuing hypersonic technology, but lacked the material ability to construct a full-size plane like the uncrewed SR-72. He also said the Air Force had not talked to Lockheed about the SR-72.

By December 2013, the Air Force declined to fund the SR-72 program. Pressed by budget cuts, the service chose instead to develop the Northrop Grumman RQ-180 stealth UAV—expected to be cheaper and less complex to design and manufacture—to conduct ISR missions in contested airspace.

2014 NASA contracts
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 $892,292 (~$ in ) contract funded 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 scramjet at Mach 4. Typical turbine engines cannot achieve high enough speeds for a scramjet to take over and continue accelerating. The NASA-Lockheed Martin study is looking at the possibility of a higher-speed turbine engine or a scramjet that can function in a turbine engine's slower flight envelope; the DARPA HTV-3X had demonstrated a low-speed ramjet (Dual Mode 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. Aerojet Rocketdyne was awarded a $1,099,916 contract by NASA's Glenn Research Center on 15 December 2014 during mode transition. The two firms were reported to be collaborating on turbine-based combined cycle (TBCC) propulsion system, with plans to begin developing the SR-72 hypersonic demonstrator in 2018, with the first flight expected in 2023.

In May 2015, it was reported that the SR-72 was envisioned as an ISR and strike platform, but no payloads were specified, likely because current payloads would be insufficient on an aircraft flying at Mach 6 up to  high requiring hundreds of miles to turn. New sensors and weapons would likely have to be created specifically to operate at such speeds.

In March 2016, Lockheed Martin CEO Hewson said that the company was on the verge of a technological breakthrough that would allow its SR-72 to reach Mach 6 and allow a hypersonic demonstrator aircraft the size of an F-22 stealth fighter to be built for less than $1 billion.

2017 to present
In June 2017, Lockheed Martin announced that the SR-72 would be in development by the early 2020s, with top speed past Mach 6. Executive Vice President Rob Weiss said, "We've been saying hypersonics [are] two years away for the last 20 years, but all I can say is the technology is mature and we, along with DARPA and the services, are working hard to get that capability into the hands of our warfighters as soon as possible."

In January 2018, Lockheed Vice President Jack O'Banion gave a presentation that credited the advancements in additive manufacturing and computer modeling, stating that it would not have been possible to make the airplane five years ago and that 3D printing allowed a cooling system to be embedded in the engine.

In February 2018, Orlando Carvalho, executive vice president of aeronautics at Lockheed Martin, pushed back on reports of the SR-72's development, saying that no SR-72 had been produced. He also said that hypersonic research is fueling weapons development. "Eventually as that technology is matured, it could ultimately enable the development of a reusable vehicle. Prior to this we may have referred to it as a 'like an SR-72', but now the terminology of choice is 'reusable vehicle'", he said.

In November 2018, Lockheed Martin said that a prototype of the SR-72 was scheduled to fly by 2025. In addition the company stated that the aircraft is to be equipped to fire hypersonic missiles. The SR-72 could enter service in the 2030s.

See also
 Aurora (aircraft)
 Boeing X-51
 DARPA Falcon Project
 MD-22
 Prompt Global Strike
 Reaction Engines Scimitar
 Tupolev Tu-360

References

Aircraft with retractable tricycle landing gear
Hypersonic aircraft
SR-072
Low-wing aircraft
Proposed military aircraft of the United States
Scramjet-powered aircraft
Strategic reconnaissance aircraft