November 01, 2013
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.
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).
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.
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.
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. Some concepts that could make this possible include a “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.”
