Maximum Speed

All was ready on the morning of 3 October 1967 as Colonel Joseph P. Cotton and Lieutenant Colonel William G. Reschke, Jr., started the engines on the NB-52B. Pete Knight had already been in the cockpit of X-15A-2 for over an hour performing the preflight checklist with the ground crew led by Charlie Baker and Larry Barnett and a host of support personnel in the NASA control room. At 1331 hours the mated pair took off from Edwards and headed to Mud Lake. An hour later, Knight "reached up and hit the launch switch and immediately took my hand off to [go] back to the throttle and found that I had not gone anywhere. It did not launch." This was not a good start, but a second attempt 2 minutes later resulted in the smooth launch of flight 2-53-97.[341

The flight plan showed that X-15A-2 would weigh 52,117 pounds at separation, more than 50% heavier than originally conceived in 1954. As the X-15 fell away, Knight lit the engine and set up a 12-degree angle of attack resulting in about 1.5 g in longitudinal acceleration. As normal acceleration built to 2 g, Knight had to hold considerable right deflection on the side stick to keep X-15A-2 from rolling left due to the heavier liquid-oxygen tank. When the aircraft reached the 35-degree planned pitch angle, Knight began to fly a precise climb angle. The simulator had predicted a maximum dynamic pressure of 540 psf, remarkably close to the 560 psf measured during the rotation. Knight maintained the planned pitch angle within 1 degree.[342]

Knight jettisoned the external tanks 67.4 seconds after launch at Mach 2.4 and 72,300 feet. Tank separation was satisfactory, but Knight described it as "harder" than it had been on flight 2-50­89. The parachute system performed satisfactorily and the Air Force recovered the tanks in repairable condition. Free of the extra weight and drag of the external tanks, the airplane began to accelerate quickly, and Knight came level at 102,100 feet. As Knight later recalled, "We shut down at 6,500 [fps] and I took careful note to see what the final got to. It went to 6,600 maximum on the indicator."[343]

Seventy-one seconds after engine shutdown, Knight performed the first of a series of planned rudder pulses with the yaw damper off. The sideslip indicator did not rotate as expected, but post-flight analysis revealed that the aircraft achieved a satisfactory yaw rate and lateral acceleration. Since the maneuver occurred at approximately the same time the unprotected ball nose reached its maximum temperature, researchers theorized that differential expansion in the nose may have resulted in a false instrument reading. Almost amusingly, despite the significant heating experienced by the rest of the airplane, the aft viewing Millikan 16-mm camera installed in the center-of-gravity compartment froze because of a malfunction of the thermal switch that activated the camera heater.[344]

This is how the ventral stabilizer and ramjet installation looked on the morning of 3 October 1967 prior to Flight 2-53-97. The skid landing gear is extended in this photograph. (NASA)

As Knight decelerated through Mach 5.5, the HOT PEROXIDE light came on; unknown to anybody, the intense heat from shock waves impinging on the dummy ramjet were severely damaging the airplane. Unfortunately, the peroxide light distracted Knight from his planned maneuvers and his energy management. As worries mounted, NASA-1 directed Knight to jettison his peroxide and began vectoring him toward high key. The X-15A-2 came across the north edge of Rogers at 55,000 feet and Mach 2.2. When Knight went to jettison the remaining propellants so that the chase plane could find him, nothing came out. There would be no help from the chase. Knight was high on energy, unable to jettison his propellants, and unsure about the condition of his airplane. He turned through high key at 40,000 feet but was still supersonic. While on final approach, Knight tried to jettison the ramjet, but later indicated that "I did not feel it go at all."

The ground crew reported that they did not see anything drop. Something was obviously wrong, but things were happening too quickly to worry about it.[345]

Fortunately, things mellowed out after that and Knight made an uneventful landing. Once on the ground, Knight realized that something was not right when a majority of the ground crew rushed to the back of the airplane. After he finally egressed and walked toward the rear of the X-15, he understood: there were large holes in the side of the ventral with evidence of melting and skin

rollback.-1346!

Post-flight analysis showed that the airplane had managed to attain Mach 6.70, equivalent to 4,520 mph (6,629 fps), at 102,700 feet, an unofficial speed mark for winged-vehicles that would stand until the return of the Space Shuttle Columbia from its first orbital mission in April 1981. This was the only X-15 flight to exceed the original 6,600-fps design goal.!3471

Later analysis showed that the shock wave from the spike nose on the ramjet had intersected the ventral and caused severe heating. Flight planner Johnny Armstrong observed, "So now maybe we knew why the ramjet was not there." The telemetry indicated that the ramjet instrumentation ceased to function 25 seconds after the XLR99 shut down. Later that afternoon several people, including Armstrong, were reviewing the telemetry when they noted an abnormal decrease in the longitudinal acceleration trace that indicated a sudden decrease in drag. The conclusion was that this was when the ramjet had separated. When the flight profile was computed, it was determined that this happened at about the 180-degree point during the turn over the south area of Rogers Dry Lake at about Mach 1 and 32,000 feet. Armstrong began correlating the telemetry with recorded radar data: "I could say that I did a detailed calculation of the drag coefficient for a tumbling ramjet, then a 5th order curve fit of the potential trajectory, corrected for winds-but actually, I just made an engineering estimate." In other words, he guessed.!3481

Not everybody believed Armstrong, but Bill Albrecht, the NASA operations engineer for X-15A-2, and Joe Rief, the AFFTC airfield manager, thought the theory had merit. Albrecht and Armstrong checked out a radio-equipped carryall van, cleared it with the tower, and headed out onto the Edwards impact range. Armstrong had previously marked up a map with some landmarks near where the telemetry and radar indicated the ramjet had separated. As they drove, Armstrong indicated a place to stop. They got out and walked about 200 yards directly to the ramjet, which was lying in two major pieces. The pair gathered up the nose cone and pressure probes and then headed back to the van (the main body of the ramjet was too heavy for only two men to lift). The next day Albrecht and Armstrong directed a helicopter to retrieve the ramjet. Subsequent inspection showed that three of the four explosive bolts that held the ramjet on had fired, probably due to the excessive temperatures that had melted large portions of the ramjet and ventral.!349!