For the Pentagon's advanced research agency, blazing a trail in hypersonics has proved problematic. Now a decade-long program to demonstrate technology for prompt global strike is being wound down, with some hard lessons learned but no flight-test successes.

In its place, the U.S. Defense Advanced Research Projects Agency (Darpa) plans to switch its focus to shorter, tactical ranges and launch a hypersonics “initiative” to include flight demonstrations of an air-breathing cruise missile and unpowered boost-glide weapon. If approved, the demos could be conducted jointly with the U.S. Air Force, which is eager to follow the success of its X-51A scramjet demonstrator with a high-speed strike weapon program.

Darpa's original plan for its Integrated Hypersonics (IH) project was to begin with a third attempt to fly the Lockheed Martin Skunk Works-designed HTV-2 unmanned hypersonic glider, after the first two launches in 2010 and 2011 failed just minutes into their Mach 20 flights across the Pacific. This was to be followed by a more capable Hypersonic X-plane that would have pushed performance even further.

The original plan drew sharp criticism from Boeing executives, who viewed the proposed program as a thinly veiled excuse to fund a third flight of Lockheed's dart-like HTV-2, which they consider unflyable. In laying out its revised program plan, Darpa makes no mention of any political lobbying against the HTV-2, but acknowledges a third flight would not make best use of its resources for hypersonic research.

Instead, as the Pentagon refocuses on China as a threat, Darpa is looking to work with the Air Force to demonstrate hypersonic weapons able to penetrate integrated air defenses and survive to strike targets swiftly, from a safe distance. Air-breathing and boost-glide weapons present challenges different to each other and to HTV-2, but the agency believes the lessons learned so far will prove valuable.

Key take-aways from HTV-2, says Darpa program manager Peter Erbland, include that the U.S. “has got kind of lean” in hypersonics competency as investment has declined from the heady days of the X-30 National Aero-Space Plane, and that “we have to be careful assuming our existing design paradigms are adequate” when developing a new class of hypersonic vehicles.

The HTV-2 sprung some surprises on its two failed flights, first with aerodynamics then with hot structures. Working out what happened “required us to mine all the competency in hypersonics that we have,” he says, and took a team assembled from government, the services, NASA, the Missile Defense Agency, industry and academia.

Erbland says the decision not to fly a third HTV-2 was influenced by “the substantial knowledge gained from the first two flights in the areas of greatest technical risk: the first flight in aerodynamics and flight performance; the second in the high-temperature load-bearing aeroshell.” Another factor was the technical value of a third flight relative to its cost. A third was the value of investing resources in HTV-2 versus other hypersonic demonstrations. “We've learned a lot; what is the value of other flights?” he asks.

While the Air Force Research Laboratory had two successes in four flights of the Mach 5, scramjet-powered Boeing X-51A, Darpa's two HTV-2 flops followed three failures of the Mach 6, ramjet-powered Boeing HyFly missile demonstrator. But as is often the case in engineering, more is learned from failure than from success, and investigation of the HTV-2 incidents will result in more robust hypersonic design tools that increase the likelihood of future success, Erbland argues.

To ensure all lessons are absorbed, work on the HTV-2 will continue to early next summer “to capture technology lessons from the second flight, and improve design tools and methods for high-temperature composite aeroshells,” he says. Information from the post-flight investigation will be combined with additional ground testing to improve the models used to design load-bearing thermal structures—“how they heat up, the material properties, their uncertainties and variables, and how we use modeling and simulation to predict thermal stresses and responses.”

HTV-2 was intended to glide an extended distance at hypersonic speed—roughly 3,000 nm. in 20 min.—and required a slender vehicle with high lift-to-drag (L/D) ratio and a carbon-carbon structure to fly for a prolonged time at high temperatures. While Flight 1 in April 2010 failed when adverse yaw exceeded the vehicle's control power, Flight 2 in August 2011 failed when the aeroshell began to degrade, causing aerodynamic upsets that ultimately triggered the flight-termination system.

“From the first flight it was clear our extrapolation of aero design methods was not adequate to predict behavior in flight,” says Erbland. “From the first to the second flights we redid the ground testing, and rebaselined the aero using new tools. On the second flight, the changes were completely effective, even in very adverse flight conditions.” But the modifications set up the HTV-2 for failure on the second flight.

“Changes to the trajectory made it a more severe aero-thermal environment than the first flight,” he says. “We have been able to reconstruct how it failed from the limited instrumentation, and the most probable cause is degradation of the structure. Thermal stresses led to failure.” While the vehicle retained its structural integrity, temperature gradients over small areas led to local material failures that caused the upsets.

“From the second flight, we learned a lesson on how to design refractory composites, to improve our understanding of how to model hot structures under thermal load,” says Erbland. “We learned a critical lesson about variability and uncertainty in material properties. That is why we are taking time to fund the remediation of our models to account for material and aero-thermal variability.”

HTV-2 is all that remains of the once-ambitious Falcon program (for Force Application and Launch from the Continental U.S.), started in 2003 with the goal of demonstrating technology for prompt global strike. Falcon had two elements, a hypersonic cruise vehicle (HCV) and a small launch vehicle (SLV) needed to boost the cruiser into a hypersonic glide. The SLV effort helped fund Space Exploration Technologies' Falcon 1 booster, but the HCV went through several changes.

The original HTV-1 hypersonic test vehicle was abandoned in 2006 when the sharp-edged carbon-carbon aeroshell proved impossible to manufacture. Darpa and Lockheed proceeded with the easier-to-produce HTV-2, but then departed from the original unpowered HCV concept to propose an HTV-3X testbed, with turbojet/scramjet combined-cycle propulsion. Congress refused to fund the vehicle, dubbed Blackswift, and it was cancelled in 2008, leaving two HTV-2s as the remnants of Falcon.

Now Darpa is seeking to reinvent its hypersonics focus by moving away from the global- to the tactical-range mission. But while an air-breathing weapon can draw directly on the X-51, boost-glide over a 600-nm range is a different vehicle to the HTV-2. “To get the performance we need to look at high L/D with robust controllability. Thermal management is a different problem to HTV-2. We need robust energy management. And affordability.”

Boost-glide challenges include packaging a weapon for air and surface launch. “The mass and volume constraints are different. We had a very high fineness ratio for global strike; we will have to be very innovative to get high L/D without a high fineness ratio,” says Erbland. On the other hand, “trajectory insertion velocities are lower, and the booster problem could be more tractable. The problem with global range is that orbital launch systems with the energy needed are not designed to put a vehicle on an ideal start of glide, so we have to make them fly in ways they don't want to,” he says.

But Darpa believes its HTV-2 experience will prove useful. “It provided critical technical knowledge to enable us to design a future boost-glide vehicle capable of prompt global strike. We made huge progress in understanding what we need to do in ground-test and flight-test to design the aerodynamics and hot structure,” Erbland says. “These are lessons we would not have learned without flight test, because of the limitations with ground test. We know going forward how to use modeling and simulation and ground test to give us more confidence that we can design a successful system.”