SR-72 Darkstar emerges?

• In November 2018, Lockheed Martin reported that a prototype of the SR-72 was scheduled to fly by 2025
• However, not all executives at Lockwood are cognisant of the programme.
• In June 2017, Lockheed Martin announced that the SR-72 would enter development by the early 2020s

Years of speculation have surrounded a new, hypersonic aircraft, as the U.S. Air Force is said to be close to test flights of the SR72, also said to have been influenced by the Darkstar aircraft of Top Gun fame.

Background

The SR-71 Blackbird was retired by the United States Air Force in 1998, eliminating a unique and valuable intelligence, surveillance, and reconnaissance (ISR) capability. Most fifth-generation jet fighters and planned drones intended for enemy airspace rely on anti-radar stealth technologies. Professor Justin Bronk, a senior research fellow in airpower and technology at the Royal United Services Institute (RUSI), argues that the rise of anti-access/area denial tactics and counter-stealth technologies renders speed more promising than stealth for penetrating protected airspace.

The first unconfirmed reports about the SR-72 emerged in 2007, when various sources reported that Lockheed Martin's Advanced Development Programmes (ADP) division, Skunk Works, was developing an aircraft capable of flying at six times the speed of sound, or Mach 6 (4,000 mph; 6,400 km/h; 3,500 kn), for the U.S. Air Force—about twice as fast as the SR-71.

Design and development

Since 2006, Lockheed Martin has been working to develop a suitable engine with Aerojet Rocketdyne. After the HTV-3X (DARPA FALCON Project) was cancelled in 2008, Aerojet Rocketdyne applied its scramjet (supersonic combustion ramjet) technology to the SR-72's engine design. This engine was to have an air-breathing propulsion system that could operate at subsonic, transonic, supersonic, and hypersonic speeds. Turbojet engines can function from zero speed and typically perform best up to Mach 2.2. Ramjets, which rely on aerodynamic compression with subsonic combustion, are most efficient around Mach 3, and can operate up to about Mach 6. (The SR-71's engines shifted to low-speed ramjets by redirecting airflow around the core and into the afterburner at speeds exceeding Mach 2.5.) Scramjets can cover the high-supersonic-to-hypersonic range. The SR-72 was to employ a turbine-based combined cycle (TBCC) system, with a turbine engine for low speeds and a scramjet for high speeds. The engines would share an inlet and nozzle but have separate airflow paths.

At speeds of Mach 5 and above, aerodynamic heating generates temperatures sufficient to melt conventional metallic airframes. Engineers then turned to making critical components from composites such as the high-performance carbon, ceramic, and metal mixes used in intercontinental ballistic missiles (ICBMs) and the retired Space Shuttle.

Plans for a prototype

In 2013, plans were made for an optionally piloted scaled demonstrator, about 18 m long, and powered by a single full-scale engine to achieve Mach 6 for several minutes. They projected it would be ready by 2018 for flight testing aligned with the High-Speed Strike Weapon timeline. The production version of the SR-72, company officials said, would resemble the SR-71 in size at over 30 m long, share its range, and enter service by 2030. 

The Air Force's thoughts

In 2013, Air Force Chief of Staff General Mark Welsh expressed the Air Force's interest in hypersonic flight, but he also clarified that the Air Force had not engaged with Lockheed Martin regarding the SR-72. Lockheed needed to raise the funds needed to continue with the programme, but in December 2013, the Air Force declined to fund the programme. 

In December 2014, NASA awarded Lockheed Martin a contract to study the feasibility of developing the SR-72's propulsion system using existing turbine-engine technologies. NASA had previously funded a Lockheed Martin study that determined speeds up to Mach 7 were achievable with a dual-mode engine combining turbine and ramjet technologies. The primary challenge in hypersonic propulsion has been bridging the gap between the top speed of a turbojet (around Mach 2.2) and the minimum operational speed of a scramjet (Mach 4), as typical turbine engines cannot accelerate sufficiently for a scramjet to take over. The NASA-Lockheed Martin study explored options such as a higher-speed turbine engine or a scramjet operable within a turbine's slower flight envelope; the DARPA HTV-3X had demonstrated a low-speed ramjet (DMRJ) functional below Mach 3. Existing turbofan engines from jet fighters and experimental designs were considered for modification. If successful, NASA planned to fund a demonstrator to test the DMRJ in a flight research vehicle.

On 14 December 2014, NASA's Glenn Research Centre awarded Aerojet Rocketdyne a multi-million-dollar contract to support mode-transition research. The two companies were collaborating on the TBCC propulsion system, aiming to begin development of the SR-72 hypersonic demonstrator in 2018, with an initial flight targeted for 2023.

In May 2015, the SR-72 was reported to be envisioned as an ISR and strike platform, though no specific payloads were identified—likely because existing payloads would be inadequate for an aircraft travelling at Mach 6 and up to eighty thousand feet (24 400 m) altitude, requiring hundreds of miles to turn. New sensors and weapons would need to be developed specifically for such speeds.

In March 2016, a breakthrough was announced that would enable the SR-72 to reach Mach 6 and permit a hypersonic demonstrator. 

In June 2017, Lockheed Martin announced that the SR-72 would enter development by the early 2020s, with a top speed exceeding Mach 6. Executive Vice President Rob Weiss remarked, "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."

However, not all executives at Lockwood are cognisant of the programme. Orlando Carvalho, executive vice president of aeronautics at Lockheed Martin, refuted reports of the SR-72's development, stating that no such aircraft had been produced. He added that hypersonic research was driving weapons development: "Eventually, as that technology matures, it could ultimately enable the development of a reusable vehicle. Before this, we may have referred to it as 'like an SR-72', but now the terminology of choice is 'reusable vehicle'"

In November 2018, Lockheed Martin reported that a prototype of the SR-72 was scheduled to fly by 2025 and would be equipped to launch hypersonic missiles. But 2026 is just around the corner, and no reports of the SR72 being flown have surfaced.

Only time will tell.