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​The X-51A Waverider prepares to launch its fourth and final flight. The cruiser achieved Mach 5.1 traveling 230 nautical miles in just over six minutes, making this test the longest air-breathing hypersonic flight ever. Air Force photo by Bobbi Zapka.

​The Air Force’s follow-on hypersonic test projects fail to leverage the progress made with the X-51 Waverider and potentially jeopardize development of a key, strategic capability, former Air Force Chief Scientist Mark Lewis said. “We had a moment in time with X-51 that we didn’t adequately capitalize on,” Lewis said during AWS16. Air Force Research Laboratory is breaking X-51’s booster and air breathing scramjet concepts into two separate test vehicles that fail to build on the lessons learned with X-51, repeating some “technical ground that X-51 already established” and in some cases making “technical mistakes” that set research back, he said. Worse still, having two separate test vehicles is “daring some bean counter” to cut one of the programs, Lewis stressed. “At some point, someone is going to say, you’ve got an air breathing missile, you’ve got a boost flight missile, we can’t afford both, … and that would be a really bad answer,” he noted.  Although the X-51 wasn’t a missile, its characteristics are similar enough to make it a useful development platform and “sometimes, frankly, there’s tremendous value in continuing to do what you’re doing,” he stressed. The Pentagon has currently set aside roughly $600 million for hypersonic test programs, which lack a unifying “theme and overall mission,” said Lewis. With additional X-51 test flights costing between $11 million and $20 million each, the Air Force could have seriously advanced the state-of-the-art at a reasonable cost. “Why don’t you take that configuration and keep building on it and fly a lot more and learn about it, put it through its paces … that’s something that we didn’t do,” he said.