Darpa plans to switch its hypersonic 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.
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.”
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.”
SOURCE - Aviation Week
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