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.”