A high-speed mission analysis research process was used to define the technology challenges and, along with studies of new concepts and vision vehicles, drove the desired speeds higher. “All of the studies have been leading us in the direction of Mach 5 and above. We've started to lay out what are the technologies required to make that real,” Clay says.

The outline plan calls for demonstrations of a turbine-to-dual-mode-ramjet/scramjet transition system around 2020. In tests of the Air Force/Darpa Blackswift ramjet/scramjet engine model, “we ran into problems in the TBCC and high-temperature turbine engine. There was a lot of significant ramjet/scramjet work done and since then there's been a lot of progress in high-temperature turbines that hold out promise that a program like this can be done,” Clay notes.

Testing will be focused on a subscale airframe, though likely with a half- or full-scale flowpath. “It becomes a technology testbed for other systems we need to develop,” says Clay, referring to a wide array of secondary objectives, including ceramic-matrix composite structures, advanced power and thermal management, sensors and affordability trades and initiatives.

The Air Force science and technology research plan is focused on tackling the major challenges associated with the aircraft's propulsion system and its integration into the airframe. Other focus areas cover power and thermal management, guidance and control, structures and materials, configuration and aerodynamics, as well as sensors.

“Getting through mode transition is very important,” Clay says. “One thing to be looked at is what we can do with commercial off-the-shelf turbines. Can I extend the speed of those up a bit and perhaps extend the [dual-mode ramjet] down a bit?” The tests will evaluate scramjets scaled eight to 16 times larger than those flown to date. A recently added technology test target includes nozzle sealing. “We've seen on X-51A how simple things, like how we do seals, can lead to problems,” he says.

The X-51A experience also reminded the Air Force of another key lesson. “Flight testing is hard, but it has to be done, and you have got to have enough flight-test vehicles to get you where you need to go,” says Clay.