The B-52H crew were trying to thread a needle through an invisible point in the sky to hit the correct launch conditions for the test flight of the X-51A Waverider hypersonic demonstrator.

Everything had to be just right, so the pressure was intense. It was May 1 and the fourth and final opportunity for the X-51A test team to prove that a hydrocarbon-fueled, air-breathing vehicle could accelerate and fly hypersonically for a sustained period until its fuel ran out. But the actual flight of the small, missile-like vehicle was only one part of the story. The weather had to be good at Vandenberg AFB, Calif.; the main diversion airfield, the Point Mugu test range to the west of Los Angeles, had to be “green” and clear; the NASA chase aircraft had to be in position; and the X-51A needed to be primed to go at just the right time.

Central to this carefully rehearsed and choreographed performance was the venerable bomber—operating at the very upper edge of its flight envelope—entrusted with transporting the test article under its left wing to meet a finely defined launch window. The B-52H had to arrive on time, flying to within +/- Mach 0.04 of the precise Mach 0.8 target speed and within +/- 500 ft. of the 49,500-ft. release altitude in a small 1-nm-sq. launch “container.” Just as critical, the crew, facing rough northerly winds and the classic Dutch roll of the B-52H at these high altitudes, were coaxing the bomber to maintain a precise true heading tolerance of +/- 3 deg. If they strayed 1 deg. outside of this cone, they knew range safety controllers would have to terminate the vehicle.

The challenges, detailed for the first time by the U.S. Air Force at the recent Society of Experimental Test Pilots meeting in Anaheim, Calif., once again underline the difficulties researchers face in delivering air-breathing hypersonic experiments to the correct launch conditions. Capt. Thomas Meagher, B-52 test pilot at the 419th Flight Test Sqdn., and Lt. Col. Timothy Jorris, director of the Hypersonic Combined Test Force at Edwards AFB, Calif., say that to avoid overheating the X-51A avionics, the climb had to be minimized to get the aircraft on range, as well as through systems checks and a single “dry” pass, to ensure the launch. The weight and drag of the X-51 at launch also meant a minimal fuel load, while an asymmetric fuel loading compensated for the store loading under the left wing. The pilots manually transferred fuel from 10 separate tanks to the eight engines in five non-standard sequences during the mission to maintain an optimal center of gravity.

“Key to flight-test success was planning and rehearsal,” says Meagher. “But the big thing was we had to be so light. [That is why] we carried less than one-quarter of a normal fuel load. To maintain the target indicated airspeed until we got to the launch condition, we kept the throttles open the entire time. We had to maintain engines at full power because if you lost it once, you'd never get it back in time.”

The crucial timing involved meant that instead of the usual mode—preparing the avionics and systems all the way to the range—the telemetry and flight-termination checks, navigation solution, vehicle status and preparations for the single dry pass all had to be completed once in the range itself. “If the aircraft was not on conditions, the X-51A had problems, or the range became unavailable, there was not enough fuel to attempt the X-51A launch,” says Jorris.

Although NASA F/A-18s had been available as chase planes for the previous X-51 tests, the only aircraft available for the fourth test with sufficient performance was a single agency F-15B, with no back-up option. On the day of the test, issues at takeoff with both the B-52H and F-15B contributed to the tension. One of the aft landing gears on the B-52 failed to retract, a situation that was rectified in time by the crew. The F-15 then aborted takeoff and the ability of the chase pilot to expeditiously resolve the problem and take off safely “literally saved the mission,” Jorris adds.

To get the X-51A steady on its course for launch, the crew also applied drift to maintain a constant true heading in the critical minutes prior to release. Based on high-altitude wind data, the pilots flew to a calculated offset midpoint on the run into the launch “container” and then allowed the B-52 to drift down into the release point while maintaining a constant true heading. “Due to the decreased roll precision and small rudder area, preventing the Dutch roll from taking the heading outside the tight tolerances provided an additional challenge,” the test team notes.