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.”