Ever since Lockheed’s unsurpassed SR-71 Blackbird was retired from U.S. Air Force service almost two decades ago, the perennial question has been: Will it ever be succeeded by a new-generation, higher-speed aircraft and, if so, when?

That is, until now. After years of silence on the subject, Lockheed Martin’s Skunk Works has revealed exclusively to AW&ST details of long-running plans for what it describes as an affordable hypersonic intelligence, surveillance and reconnaissance (ISR) and strike platform that could enter development in demonstrator form as soon as 2018. Dubbed the SR-72, the twin-engine aircraft is designed for a Mach 6 cruise, around twice the speed of its forebear, and will have the optional capability to strike targets.

Guided by the U.S. Air Force’s long-term hypersonic road map, the SR-72 is designed to fill what are perceived by defense planners as growing gaps in coverage of fast-reaction intelligence by the plethora of satellites, subsonic manned and unmanned platforms meant to replace the SR-71. Potentially dangerous and increasingly mobile threats are emerging in areas of denied or contested airspace, in countries with sophisticated air defenses and detailed knowledge of satellite movements.

A vehicle penetrating at high altitude and Mach 6, a speed viewed by Lockheed Martin as the “sweet spot” for practical air-breathing hypersonics, is expected to survive where even stealthy, advanced subsonic or supersonic aircraft and unmanned vehicles might not. Moreover, an armed ISR platform would also have the ability to strike targets before they could hide.

Although there has been evidence to suggest that work on various classified successors to the SR-71, or some of its roles, has been attempted, none of the tantalizing signs have materialized into anything substantial. Outside of the black world, this has always been relatively easy to explain. Though few question the compelling military imperative for high speed ISR capability, the astronomical development costs have made the notion a virtual nonstarter.

But now Lockheed Martin believes it has the answer. “The Skunk Works has been working with Aerojet Rocketdyne for the past seven years to develop a method to integrate an off-the-shelf turbine with a scramjet to power the aircraft from standstill to Mach 6 plus,” says Brad Leland, portfolio manager for air-breathing hypersonic technologies. “Our approach builds on HTV-3X, but this extends a lot beyond that and addresses the one key technical issue that remained on that program: the high-speed turbine engine,” he adds, referring to the U.S. Air Force/Defense Advanced Research Projects Agency (Darpa) reusable hypersonic demonstrator canceled in 2008.

The concept of a reusable hypersonic vehicle was an outgrowth of Darpa’s Falcon program, which included development of small launch vehicles, common aero vehicles (CAV) and a hypersonic cruise vehicle (HCV). As structural and aerodynamic technologies for both the CAV and HCV needed testing, Lockheed Martin was funded to develop a series of unpowered hypersonic test vehicles (HTV).

In the midst of these developments, as part of a refocus on space in 2004, NASA canceled almost all hypersonic research, including work on the X-43C combined-cycle propulsion demonstrator. The Darpa HTV effort was therefore extended to include a third HTV, the powered HTV-3X, which was to take off from a runway on turbojet power, accelerate to Mach 6 using a scramjet and return to land.

Despite never progressing to what Leland describes as a planned -HTV-3X follow-on demonstrator that “never was,” called the Blackswift, the conceptual design work led to “several key accomplishments which we didn’t advertise too much,” he notes. “It produced an aircraft configuration that could controllably take off, accelerate through subsonic, supersonic, transonic and hypersonic speeds. It was controllable and kept the pointy end forward,” adds Leland.

Fundamental lessons were learned, particularly about flight control systems that could maintain stability through the transonic speed regime. Lockheed Martin’s work proved the configuration could “take off without departing,” Leland notes. “We were able to drive down the takeoff speed and keep it stable and controllable. We proved all that in a whole series of wind-tunnel tests.”

Just as importantly, the Skunk Works design team developed a methodology for integrating a working, practical turbine-based combined cycle (TBCC) propulsion system. “Before that, it was all cartoons,” Leland says. “We actually developed a way of transforming it from a turbojet to a ramjet and back. We did a lot of tests to prove it out, including the first mode-transition demonstration.” The Skunk Works conducted subscale ground tests of the TBCC under the Facet program, which combined a small high-Mach turbojet with a dual-mode ramjet/scramjet, and the two sharing an axisymmetric inlet and nozzle.

Meanwhile, the U.S. Air Force Research Laboratory’s parallel HiSTED (High-Speed Turbine Engine Demonstration) program essentially failed to produce a small turbojet capable of speeds up to Mach 4 in a TBCC. “The high-speed turbine engine was the one technical issue remaining. Frankly, they just weren’t ready,” recalls Leland. This left the Skunk Work designers with a familiar problem: how to bridge the gap between the Mach 2.5 maximum speed of current-production turbine engines and the Mach 3-3.5 takeover speed of the ramjet/scramjet. “We call it the thrust chasm around Mach 3,” he adds.

Although further studies were conducted after the demise of the HTV-3X under the follow-on Darpa Mode-Transition program, that fell by the wayside, too, after completion of a TBCC engine model in 2009-10. So, Lockheed Martin and Aerojet Rocketdyne “sat down as two companies and asked ourselves, ‘Can we make it work? What are we still missing?’” says Leland. “A Mach 4 turbine is what gets you there, and we’ve been working with Rocketdyne on this problem for the last seven years.”

Finally, he says, the two achieved a design breakthrough that will enable the development of a viable hypersonic SR-71 replacement. “We have developed a way to work with an off-the-shelf fighter-class engine like the F100/F110,” notes Leland. The work, which includes modifying the ramjet to adapt to a lower takeover speed, is “the key enabler to make this airplane practical, and to making it both near-term and affordable,” he explains. “Even if the HiSTED engines were successful, and even if Blackswift flew, we’d have had to scale up those tiny turbines, and that would have cost billions.”

Lockheed will not disclose its chosen method of bridging the thrust chasm. The company funded research and development, and “our approach is proprietary,” says Leland, adding that he cannot go into details. Several concepts are known, however, to be ripe for larger-scale testing, including various pre-cooler methods that mass-inject cooler flow into the compressor to boost performance. Other concepts that augment the engine power include the “hyperburner,” an augmentor that starts as an afterburner and transitions to a ramjet as Mach number increases. Aerojet, which acquired Rocketdyne earlier this year, has also floated the option of a rocket-augmented ejector ramjet as another means of providing seamless propulsion to Mach 6.

Although details of the proposed thrust-augmentation concept remain under wraps, Leland says a large part of a successfully integrated mode-transition design is the inlet. “That’s because you have to keep two compressor systems [ramjet and turbine] working stably. Both will run in parallel,” he adds.

Lockheed has run scaled tests on components. “The next step would be to put it through a series of tests or critical demonstrations,” Leland says. “We are ready for those critical demonstrations, and we could be ready to do such a demonstration aircraft in 2018. That would be the beginning of building and running complete critical demonstrations. 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.”

The 2018 time line is determined by the potential schedule for the high-speed strike weapon (HSSW), a U.S. hypersonic missile program taking shape under the Air Force and Darpa (see page 36). “We can do critical demonstrations between now and then, but we don’t believe it will be until HSSW flies and puts to bed any questions about this technology, and whether we can we truly make these, that the confidence will be there.” In spite of the recent success of demonstration efforts, such as the X-51A Waverider, Leland observes that “hypersonics still has a bit of a giggle factor.”

The timing also dovetails with the Air Force hypersonic road map, which calls for efforts to support development of a hypersonic strike weapon by 2020 and a penetrating, regional ISR aircraft by 2030 (AW&ST Nov. 26, 2012, p. 40). Key requirements for the high-speed ISR/strike aircraft is the ability to survive a “day without space”—communication and navigation satellites—and to be able to penetrate denied areas. With a TBCC propulsion system, the Air Force has pushed for increasingly greater speeds since defining Mach 4 at initial planning meetings in December 2010. The latest requirements are thought to be at least a Mach 5-plus cruise speed and operation from a conventional runway.

The path to the SR-72 would begin with an optionally piloted flight research vehicle (FRV), measuring around 60 ft. long and powered by a single, but full-scale, propulsion flowpath. “The demonstrator is about the size of the F-22, single-engined and could fly for several minutes at Mach 6,” says Leland. The outline plan for the operational vehicle, the SR-72, is a twin-engine unmanned aircraft over 100 ft. long (see artist’s concept on page 20). “It will be about the size of the SR-71 and have the same range, but have twice the speed,” he adds. The FRV would start in 2018 and fly in 2023. “We would be ready to launch the SR-72 shortly after and could be in service by 2030,” Leland says.

According to Al Romig, Skunk Works engineering and advanced systems vice president, “speed is the new stealth.” This is perhaps just as well, given the inherent challenges involved in reducing the signature of hypersonic vehicles. With large engine inlets and aerodynamic requirements overriding most considerations, the SR-72 concept shows little in the way of stealthy planform alignment. Although the surfaces could be coated with radar-absorbing material, the requirement for thermal protection along sharp leading edges is likely to be a complicating factor. Like the HTV-3X, the vehicle may also feature hot metallic leading edges and a “hot/warm” metallic primary structure designed to handle the high thermal flux loads.

The deep nacelles, mounted close inboard, indicate the “over-under combined cycle” engine configuration outlined for the HTV-3X, as well as integrated inward-turning turbo-ramjet inlets. “One of the differences with this demonstrator compared to the HTV‑3X is that with that, we were limited to small turbines with a low-drag design,” Leland says. “With fighter engines, we accelerate much more briskly. It’s a significant improvement in adding margins. It is also very important [that] you have a common inlet and nozzle because of the significant amount of spillage drag in the inlet and the base drag in the nozzle.”

Aerodynamically, the forebody appears to be shaped for inlet compression at high speed, but without the characteristic stepped “wave-rider” configuration of the X-51A. “We are not advocates of wave riders,” Leland says. “We found that, in order for a wave rider to pay off, you have to be at cruise and be burning most of your fuel at cruise. But these designs burn most fuel as they accelerate, so you want an efficient vehicle that gets you to cruise. You end up with a vehicle that is hard to take off and land, has little fuel volume and high transonic drag.”

The planform is characterized by chines that blend into a sharply swept delta extending back roughly halfway along the hump-backed fuselage. The chine and delta are likely designed to provide increased directional stability as well as a larger amount of lift at high cruise speeds. Outboard of the engine inlets, the leading-edge angle abruptly aligns with the fuselage before the wing extends into a trapezoid. The angle of the cranked wing would provide vortex lift to assist with low-speed flight.

The SR-72 is being designed with strike capability in mind. “We would envision a role with over-flight ISR, as well as missiles,” Leland says. Being launched from a Mach 6 platform, the weapons would not require a booster, significantly reducing weight. The higher speed of the SR-72 would also give it the ability to detect and strike more agile targets. “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 ISR,” he adds. Lockheed envisages that once the FRV has completed its baseline demonstrator role, it could become a testbed for developing high-speed ISR technologies and supporting tests of the SR-72’s weapons set, avionics and downlink systems.

“It is time to acknowledge the existence of the SR-72 because of the HSSW going forward,” says Leland. Together with the strategic “pivot to the Pacific,” the concept of high-speed ISR is “starting to gain traction,” he notes. “According to the hypersonic road map, the path to the aircraft is through the missile, so now it is time to get the critical demonstration going.” These would test individual elements of the propulsion system, which would then be integrated for the full-scale FRV evaluation.

“We have been continuing to invest company funds, and we are kind of at a point where the next steps would require large-scale testing, which would significantly increase the level of investment we’ve had to make to-date. Between Darpa and the Air Force, it would be highly likely they’d have to fund the next steps,” Leland says. The FRV will also give the Skunk Works a better idea on overall development costs, he adds.

As for rumors of an existing high-speed ISR aircraft, Leland is dismissive. “It’s been almost 20 years since the SR-71 was retired. If there was a replacement, they’ve been hiding it pretty well,” he says.

See a detailed comparison of the SR-71 and SR-72 engine cycles.