Super-fast successor to legendary SR-71 "Blackbird" stealth plane unveiled

Lockheed Martin, the company behind the iconic SR-71 "Blackbird" spy jet, has announced plans to build a successor in the form of a super-fast unmanned plane capable of reconnaissance and combat missions.

The SR-72 is being developed by the 'Skunk Works' division of Lockheed - the sub-unit responsible for developing the F117 Stealth Fighter as well as the USA's current generation of highly advanced fighter jets.

In keeping with the SR-71's legacy as the fastest plane of its time, the SR-72 will be capable of 'hypersonic' speeds - reaching a cruising speed of Mach 6, or 4,567mph. At that speed, the new jet could cover the distance from London to New York in just 45 minutes.

The SR-71 was used for espionage missions over the Soviet Union during the Cold War. Famously, it was never shot down, relying on its extreme speed to outrun incoming missiles. The SR-72 would have the same defence mechanism, but there are early indications that it would not be limited to intelligence-gathering.

In an interview with Aviation Week, Brad Leland, a portfolio manager for air-breathing technologies at Lockheed, revealed that  “We would envision a role with over-flight ISR [Intelligence, Surveillance and Reconnaissance], as well as missiles.”

Leland also emphasised the new project's focus on speed. "Speed is the new stealth," he said, adding that “Even with the SR-71, at Mach 3, there was still time to notify that the plane was coming, but at Mach 6, there is no reaction time to hide a mobile target. It is unavoidable.”

To achieve such speeds, the SR-72 will use what's known as a 'scramjet' engine. Whereas a normal jet engine uses turbines to compress air in which to burn fuel, a scramjet relies on the aircraft travelling fast enough that the air entering the engine is already highly compressed. To do this, the plane must reach supersonic speeds (using conventional means) before the scramjet kicks in and takes it on to its cruising speed.

Exactly how Lockheed has bridged this gap, they won't say - safely transferring from supersonic (Mach 2.5 - Mach 3) to hypersonic speeds (Mach 4 and above) has been the stumbling block for previous projects. Development of an engine capable of this process while "keeping the pointy end forward" took seven years, according to Leland. He adds "We have developed a way to work with an off-the-shelf fighter-class engine".

He added that the SR-72 could fly in prototype form in 2018. "The next step would be to put it through a series of tests or critical demonstrations.We are ready for those critical demonstrations, and we could be ready to do such a demonstration aircraft in 2018. As of now, there are no technologies to be invented. We are ready to proceed—the only thing holding us back is the perception that [hypersonics] is always expensive, large and exotic."

Initially, the SR-72 will be built as a 60ft long, single-engined research vehicle - about the size of a current F-22 fighter jet. This could be operational by 2023, Leland said, with a view to an airworthy full-size SR-71 replacement launching in 2030. This is what is currently shown in the artist's impression - a 100ft, twin-engined jet with strike capability.

Travelling at hypersonic speeds raises huge engineering challenges. The SR-71 was known to leak fuel when grounded, as its body parts were only designed to fit together at high speed, where the heat generated would cause the metal to expand. At landing, the exterior temperature was more than 300 degrees Celsius. Any plane travelling at twice the speed will be subject to even greater heat pressures.

The SR-71 was a product of, and came to define, the Cold War espionage programmes of the 1970s and 80s. With the US military talking of a "strategic pivot to the Pacific", Leland notes that the SR-72 programme is "starting to gain traction". Lockheed's Skunk Works isn't known for playing its hand early, so there's a good chance this jet will be over Chinese skies in years to come.