The X-51A was originally aimed at testing in excess of Mach 6, but none of them exceeded Mach 5.1. Brink says early in the flight-test program, “we had a real long discussion of flight time versus the Mach 6 number, and the consensus was it was more important to show we could control the engine acceleration and fly it on out, run it dry and control it at hypersonic speed. I'm not saying we shot for 5.1. We thought it might be in the mid-Mach 5 range, but we found changes to inlet geometry [which was changed to a stiffer columbium-based alloy] made the wedge much thicker. Plus there were changes to cowling to incorporate the fourth engine. So we knew we had added drag counts.”
Following engine shutdown, the unpowered vehicle was commanded to perform various “parameter identification” maneuvers to characterize its aerodynamic handling and controllability. Three sets of data were collected at decreasing Mach numbers as part of evaluations which will help pave the way for future hypersonic testing and see “what it takes to contain and safely contain these vehicles,” says Jorris. This testing takes place during the unpowered phase of the flight because “we understood the engine parameters. Now we have to understand the vehicle itself, so with the engine off we can isolate the pure aerodynamic phenomenon.” The vehicle's response to specific pitch, roll and yaw inputs will be compared to pre-test predictions made by NASA Langley.
Testing was monitored until telemetry was lost at 20,000 ft., which was caused by loss-of-signal dropout between radar-tracking and telemetry-monitoring sites at Point Mugu and Vandenberg AFB. However a U.S. Navy NP-3D relay aircraft positioned down range was still receiving signals from the vehicle. “We believe we'll get more data all the way down to splashdown, but we still need to reduce that data,” says Brink. “We got all the data we wanted—we were ecstatic with the results.”