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.