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Skunk Works Reveals SR-71 Successor Plan
Ever since Lockheeds 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 Martins 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 Forces 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 Darpas 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).
>
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.
>
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, wed 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. Thats 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 proceedthe only thing holding us back is the perception that [hypersonics] is always expensive, large and exotic.
>
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 artists 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.
>
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-72s 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.
>
As for rumors of an existing high-speed ISR aircraft, Leland is dismissive. Its been almost 20 years since the SR-71 was retired. If there was a replacement, theyve been hiding it pretty well, he says.
Ever since Lockheeds 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 Martins 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 Forces 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 Darpas 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).
>
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.
>
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, wed 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. Thats 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 proceedthe only thing holding us back is the perception that [hypersonics] is always expensive, large and exotic.
>
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 artists 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.
>
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-72s 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.
>
As for rumors of an existing high-speed ISR aircraft, Leland is dismissive. Its been almost 20 years since the SR-71 was retired. If there was a replacement, theyve been hiding it pretty well, he says.
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