A scramjet generates thrust by sucking in air while traveling at supersonic speeds—meaning that a separate engine has to push the airplane to those speeds before the scramjet can engage. The Combined Cycle engine makes the dual-engine approach viable by having the turban and scramjet share the same inlets and exhaust nozzles.
Hypersonic weapons—those capable of flying over five times the speed of sound—are the hot new buzz word of defense industrial complexes across the globe. China, Russia and the United States have all vigorously and relatively openly pursued a diverse array of hypersonic weapons programs, adding fuel to the fire of a growing arms race.
While long-range ballistic missiles could already attain hypersonic speeds, they travel in predictable arcs and can be detected well in advance, giving military and political leaders time to react. Furthermore, an increasing number of air defense systems may be at least partially capable of intercepting ballistic missiles.
(This first appeared earlier in 2019.)
However, back in 2013 Lockheed executive Robert Weiss caused a stir when he told Aviation Week the aerospace titan was well into developing a hypersonic aircraft—and invoked the legendary SR-71 Blackbird spy plane by dubbing it the SR-72.
No manned aircraft in operational service has matched the remarkable long-distance Mach 3 cruises of the Blackbird. Until recently, SR-71s simply outran missiles fired at them on photo-reconnaissance missions over North Korea and the Middle East. Now the latest surface-to-air missiles render Mach 3 speeds inadequate to assure survival, but a hypersonic aircraft might again outpace the threats arrayed against it.
The SR-72 depicted in Lockheed’s concept art was described as capable of cruising at six times the speed of sound. The challenge, however, lay not so much in designing an aircraft that could attain hypersonic speeds as ensuring that it could also take off and land at slower speeds. The rocket-powered X-15 testbed, which in 1967 recorded the fastest flight by a manned, powered aircraft ever at Mach 6.7, had to be carried aloft and released mid-air by a B-52 bomber!
Weiss told journalist Guy Norris “…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... I can’t give you any timelines or any specifics on the capabilities. It is all very sensitive... We can acknowledge the general capability that’s out there, but any program specifics are off limits.”
Reportedly, Lockheed and the firm Aerojet Rocketdyne made a breakthrough by developing a Combined Cycle engine involving both a turbine for speeds below Mach 3 with a scramjet engaged for hypersonic cruising. A scramjet generates thrust by sucking in air while traveling at supersonic speeds—meaning that a separate engine has to push the airplane to those speeds before the scramjet can engage. The Combined Cycle engine makes the dual-engine approach viable by having the turban and scramjet share the same inlets and exhaust nozzles.
Weiss made clear he hoped Lockheed would receive funding to build an optionally-manned sixty-foot long (jet-fighter-sized) single-engine test-bed aircraft that would cost “only” $1 billion. This would then lead to the development of an operational twin-engine one-hundred-foot-plus SR-72.
In the six years following Weiss’ comments, Lockheed officials continued drawing atypical levels of attention to a supposedly secret program too sensitive to reveal to the public, teasing statements that kinda-sort-of implied they had already built an SR-72 testbed.
For instance, at a science convention in 2018, Lockheed vice president Jack O’Banion stated “Without the digital transformation [of three-dimension design technology], the aircraft you see there could not have been made. In fact, five years ago, it could not have been made.” However, Executive Vice President Orlando Carvalho subsequently said to Flight Global “I can tell you unequivocally that it [the SR-72] has not been built”, claiming O’Banions quotes had been taken “out of context.”
Lockheed’s hyping of a hypersonic aircraft which may-or-may-not already exist seems explicitly intended to build support for additional funding. This may be because it’s pursuing the project with the Defense Advanced Research Programs Agency (DARPA), which focuses on innovative development of cutting-edge technologies often well ahead of capabilities in operational service, rather than fulfilling an Air Force requirement.
While the U.S. Air Force is interested in deploying hypersonic aircraft in the long term, it already knows what it wants in the near future: lots of F-35 stealth fighters (also built by Lockheed) and forthcoming B-21 Raiders flying-wing stealth bombers. As the air warfare branch already can’t procure all the aircraft it wants, carving out funding for a highly-expensive avant-garde concept won’t be easy.
The Blackbird’s unique “SR” designation stood for “Strategic Reconnaissance,” reflecting that its job was to penetrate defended airspace on short notice and snap up photos of what was going on below before anyone could move or cover it up. However, the appellation SR-72 is arguably misleading for a number of reasons.
A hypersonic SR-72 would almost certainly be an unmanned aerial vehicle (UAV)—in other words, a drone normally receiving a “Q” designation. To what extent it would rely on man-in-the-loop (which might be susceptible to disruption) or pre-programmed control versus its own autonomous algorithms, remains an interesting question.
Furthermore, while an SR-72 would have an Intelligence, Surveillance and Reconnaissance (ISR) role, it would also surely be intended to strike targets with little advancing warning—in other words, it would be a bomber. Traveling around 4,000 miles per hour, a hypersonic bomber could theoretically depart from a base in the continental U.S. to hit targets across the Pacific or Atlantic in just 90 minutes. Unlike the various hypersonic missiles under development, it could then return to base and load up for further sorties.
Weiss stated from the beginning the SR-72 “had strike-capability in mind.” The SR-72 project, in fact, is reportedly an outgrowth of the rocket-powered Falcon HTV-3 hypersonic test-bed, which was associated with America’s Prompt Global Strike program.
However, the cost-efficiency of a hypersonic bomber/spy plane is debatable. It would surely lack stealth characteristics, as the heat generated by travel at such high speeds would make them highly visible to sensors and burn away radar-absorbent materials. Thus adversaries would probably see it coming, even if they had relatively little time to react.
While it might exceed the capabilities of contemporary air defense missiles, the SR-72’s existence would surely further spur the development of surface-to-air missiles capable of engaging hypersonic targets. An SR-72 bomber would also require expensive development of munitions designed for launch at such high speeds.
The Blackbird was retired and not replaced because its ISR capabilities had become niche due to improving spy satellites and because of slow but stealthy long-endurance drones like the RQ-170. Sure, Blackbirds could rapidly penetrate defended airspace, but a stealth drone could do that more slowly but also more discretely, and sustainably orbit an area of interest, delivering real-time video feeds for hours. In fact, the Pentagon’s decision to contract Grumman to build ultra-stealthy long-endurance RQ-180 drones may be perceived as coming at the SR-72’s expense.
The SR-72’s promoters argue that “speed is the new stealth,” reflecting a growing belief in some quarters that improved networked sensors will eventually diminish the survivability of stealth aircraft, making speed once again more prominent as means of defense. Given the Pentagon’s blossoming interest in all kinds of hypersonic weapons, it’s possible Lockheed’s Schrodinger's cat of a hypersonic UAV may attract additional funding. However, that may place it at odds with the stealth-oriented paradigm the Air Force is currently committed to.
Sébastien Roblin holds a master’s degree in conflict resolution from Georgetown University and served as a university instructor for the Peace Corps in China. He has also worked in education, editing, and refugee resettlement in France and the United States. He currently writes on security and military history for War Is Boring.
Image: Creative Commons.