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.