July 31, 1968

MEMORANDUM for Assistant Chief, Research Projects

Flight 1-78-138 was flown on July 16, 1968, by Major William J. Knight for the purpose of:

Satisfactory data were obtained from the Fixed-sphere flow sensor, Sky Brightness, and Fluidic probe experiments. Because of a malfunctioning pressure transducer, the number two hydraulic gage indicated a deterioration of hydraulic pressure, the pilot pushed over early and ground control elected not to extend the WTR experiment.

Flight Track and Profile

The radar track and profile for this flight are shown in figure 1.

The launch was at Railroad Valley #2 on a magnetic heading of 183°. The launch transients were normal. Engine light occurred 1.5 seconds after launch and 100 percent thrust occurred 1.3 seconds later.

The average angle of attack during rotation was between 10° and 11° with normal acceleration reaching 2.3 "g's." The planned pitch angle of 40° was reached 1 second later than planned at 32.6 seconds after light. At approximately 60 seconds, the pilot observed hydraulic pressure drops below 1000 pounds per square inch on the #2 hydraulic gage and elected to push over to an alternate profile. Pushover was initiated at approximately 62 seconds after launch and 0 "g" was reached at 67 seconds. The planned 0 "g" was held to shutdown at 4950 feet per second and 138,500 feet altitude. No pressure drops were observed by the pilot after shutdown.

The early pushover resulted in a peak altitude of 218,500 feet compared to the planned 250,000 feet. During reentry, the angle of attack peaked to 23° but averaged approximately 19°. Peak normal acceleration was 4.8 "g's" with an average of 4.4 "g's" during reentry and the dynamic pressure reached 900 pounds per square foot. After reentry the aircraft was to the left of the planned track. The pilot brought the aircraft back on track using a 3"g" right turn and speed brakes to reduce excessive energy. The approach and landing were completed without incident.

Stability and Control

The time history of the significant stability and control parameters telemetered during the flight is shown in figure 2.

A 4° nose-left thrust misalinement resulted in a three axes excursion at shutdown. At this time there was not enough aerodynamic force to right the aircraft and the pilot immediately went to ballistic controls. After bringing the aircraft attitudes to 0° the pilot engaged RAS. At approximately 2OO,O00 feet where the pilot began preparing for reentry, he reported experiencing a severe shaking in the aircraft and described the sensation as being similar to "moderate buffet." The shaking persisted for 20 to 30 thousand feet and subsided during "g" buildup. After reentry the pilot rolled out on heading and as the "g" level decreased the aircraft began to shake again and continued throughout the remainder of the flight. This problem is discussed further under operational discrepancies.

Air Force Western Test Range Launch Monitoring

Because of the early pushover ground control elected not to extend the experiment and no data were obtained. The equipment was returned to Northrop for re-calibration and is ready for flight.

Fixed-Sphere Flow Direction Sensor

Acceptable data were obtained and the experiment will be flown on the next flight.

Fluidic Probe

Acceptable data were obtained and the experiment will be flown on the next flight.

Sky Brightness

Acceptable data were obtained. The experiment has been re-calibrated and will be flown on the next flight.

Operational Discrepancies

Investigation of the indicated hydraulic pressure failure of the #2 system during flight disclosed a bad transducer and not a hydraulic system problem. The transducer has been replaced.

An investigation to determine the cause of the "buffet" reported by the pilot was conducted on the aircraft. Because the buffet began in the low dynamic pressure environment at 200,000 feet, this investigation was centered around the stability augmentation system (SAS). Engaging the roll-axis SAS initiated a high frequency "buzz" at the servo which excited the aircraft structural mode. With SAS disengaged, movement of the primary pitch-roll control system also excited the structural mode for approximately three cycles. The SAS and primary control induced vibrations were notable to an X-15 pilot through the control stick and the structure.

An investigation of the primary control system indicated the primary surface actuators were functioning normally, however, significant "slop" within the system was noted. The walking beam was changed because of worn bearings and the linkage has been tightened.

During the analysis of the SAS, the "buzz" stopped and further checks did not reveal the problem area. As a result of not being able to isolate the problem, the SAS case and pitch-roll servo's were replaced and will be analyzed in the laboratory.

Data System Discrepancies

Discrepancies noted on the flight were:

X-15 Research Project Office