Forty Years Ago in the X-15 Flight Test Program, April 1962
No X-15 missions were launched between January 17 and the beginning of April 1962. The pace of the program picked up significantly in April. Sixteen X-15 missions were launched in the next four months. The X-15-3 was expanding its altitude envelope. The X-15-2 was slated to evaluate airframe heating, and the X-15-1 was expected to set another new altitude record.
After a series of aborted launch attempts, the X-15-3 was launched on its third flight on April 5. Major Fitzhugh Fulton and Captain John Campbell were at the controls of the NB-52A. Stan Butchart manned the launch panel operator's station.
Propellants stream from the prime drains of the X-15-3 immediately after launch. Photo courtesy AFFTC/HO.
The MH-96 flight control system passed the pre-flight analyzer tests with no problems. When Armstrong triggered the ignition of the XLR-99 engine over Hidden Hills Dry Lake, the igniter pressure fell to zero and the engine remained mute. He ran through the engine restart sequence, which seemed to him to take a long time. The X-15-3 had fallen 10,000 feet before he got it going uphill. As a result of the late engine start, Armstrong let the rocket burn for 9.2 seconds longer than the flight plan called for. The X-15-3 reached a peak altitude 10,000 feet higher than the planned 170,000 feet. Armstrong's evaluation of the MH-96 was largely positive, but his comments indicated that he felt that the operation of the reaction control thrusters needed some fine tuning.
Joe Walker was scheduled to make the 26th flight of the X-15-1 on April 17. The primary purpose of the mission was to evaluate the Alternate Stability Augmentation System (ASAS) of the X-15-1 at high angle of attack. The ASAS acted as the primary backup to the Stability Augmentation System (SAS). Walker would be launched over Mud Lake and use full throttle to climb at 30 degrees to an altitude of 58,000 feet. At that altitude he was to push over until the wings of the X-15 were generating no lift, resulting in a period of "zero-g" flight and minimizing drag. Although Walker would experience no upward or downward acceleration, he would be pressed into the pilot's seat with a force approaching 3 times the force of gravity from the thrust of the rocket engine. At 75,000 feet, he was to pull up until the X-15 pulled 1.5 gs. The propellants would be expended after about 83 seconds, when the X-15 was at an altitude of 100,000 feet.
After burn out, Walker was to pull the nose of the X-15-1 up to an angle of attack of 20 degrees and shut down the channels of the SAS, one at a time, to evaluate the response of the ASAS. The X-15-1 would reach its peak altitude of 153,000 feet after 76 seconds of coasting. Walker would pull the nose up to 20 degrees angle of attack twice again during the descent, landing on Rogers Dry Lake after a flight of about 11 minutes.
The reaction control system of the X-15-1 suffered from a hydrogen peroxide leak on the morning of April 17, so the flight was rescheduled for the next morning. Major Jack Allavie and RAF Squadron Leader Harry Archer took the NB-52A aloft with Walker in the cockpit of the X-15-1 under its right wing on April 18, but cloudy skies over Mud Lake prevented the launch that morning.
Jack Russel manned the launch panel operator's station as the twenty-sixth flight of the X-15-1 was launched over Mud Lake on the morning of April 19. The XLR-99 rocket engine fired up on the first try, and the X-15 was already popping and banging like a hot wood burning stove from thermal stresses as it climbed through 70,000 feet at full throttle. The rocket engine ran out of propellants after 83.4 seconds, as the X-15 was going mach 5.69. Walker noted some yaw instability as he increased the angle of attack to 20 degrees after engine burn out. The instrument that indicated the yaw angle was wired backward, so initially Walker's inputs to the reaction control thrusters increased the yaw angle. He reversed his responses to the sideslip needle, which produced the desired results. He was very pleased with the transition to the Alternate Stability Augmentation System when he shut down the roll channel of the Stability Augmentation System during the coast to 154,000 feet. The behavior of the X-15 hardly changed at all when the ASAS took over.
Another evaluation of the MH-96 adaptive flight control system of the X-15-3 by Neil Armstrong was next on the schedule. A stabilizer position indicator had been added to the instrument panel in the cockpit since the last flight. The flight plan called for a launch over Mud Lake, followed by a 30 degree climb at full throttle to an altitude of 60,000 feet. Armstrong was to pitch over to zero-g flight and then pull up to a 32 degree climb at 81,000 feet. He would shut down the engine at 120,000 feet after an 81-second burn. The X-15 should coast to a maximum altitude of 205,000 feet. During the descent, he was to evaluate the ability of the MH-96 to damp out oscillations about the pitch and roll axes while flying at an angle of attack of 20 degrees. He was expected to level out at 90,000 feet and decelerate before descending to land on Rogers Dry Lake eleven minutes after launch.
The fourth flight of the X-15-3 was postponed once. Armstrong's launch was rescheduled to April 20, just one day after Walker's flight in the X-15-1.
Major Jack Allavie and Major Russell Bement were at the controls of the NB-52B on the morning of April 20. Stanley Butchart manned the launch panel operator's station. There was severe turbulence at the launch altitude. The pitching and rolling of the NB-52B interfered with attempts to get a solid fix from the doppler radar input for the stable table, so the launch proceeded under the assumption that the inertial velocity supplied to the MH-96 aboard the X-15-3 would be slightly in error.
Armstrong got a good light of the XLR-99 engine on the first try. The flight followed the flight plan closely during the ascent. Armstrong made a series of pullups, evaluating the SAS at angles of attack up to 20 degrees. The X-15 was going mach 5.31 when he shut down the rocket engine eighty-one seconds into the flight. It reached a peak altitude of 207,500 feet.
Armstrong felt that the aircraft control and damping during re-entry were better than expected. The MH-96 system was using reaction control thrusters more than had been expected. That was depleting the hydrogen peroxide that fueled both the thrusters and the Auxiliary Power Units (APUs). The peroxide-low warning lamp came on as the X-15 descended through 160,000 feet. Armstrong swiched on the pump that transferred unused hydrogen peroxide from the engine turbopump to the tank that supplied the thrusters and APUs.
Although the flight plan had called for Armstrong to level out at 90,000 feet, he inadvertantly brought the nose up too high. The X-15-3 skipped back up to an altitude where its wings produced almost no lift or drag. Rather than decelerating, the X-15 continued at nearly unabated speed toward Edwards Air Force Base. Jack Allavie in the NB-52B could see that Armstrong was passing Rogers Dry Lake and called out, "Hard left turn, Neil", over the radio. Armstrong rolled the X-15 onto its left side and pulled back on the stick until the stabilizers were deflected as far as they could go, but the X-15 continued in a nearly straight line. He watched his landing site pass by as he was still traveling mach 3 and heading for Los Angeles.
Armstrong was many miles south of the base, flying over the southern San Gabriel Mountains, far from his planned flight course, when he got the X-15-3 turned around. The sonic boom from the X-15 swept across La Canada and the northern L. A. basin.
Armstrong had the impression that he was "in pretty bad shape for the south lakebed". For a moment he considered putting the X-15 down on the concrete runway at the Palmdale Airport. He pitched the X-15 to the angle of attack that generated the most lift for the least drag to stretch his glide as far as he could. He didn't bother with the usual spiraling descent, just brought it straight in to runway 35 on the south lakebed.
The X-15 pilots had not considered the prospect of landing on Rogers Dry Lake from the south. There were no pre-selected checkpoints to help guide Armstrong to his landing point. Late in the approach, he realized that he would be short of his initial aim point by about two miles. He was flying below the tops of the Joshua Trees before he reached the lakebed, and the X-15 landed on the hard, clay surface just a short distance from the sagebrush. The X-15 touched down 12 minutes and 28 seconds after launch, nearly a minute and a half longer than the original flight plan. This was the longest flight of the X-15 program.
Armstrong had missed his planned landing site by twelve miles. The convoy of recovery vehicles was at the far end of Rogers Dry Lake and would not arrive for a quarter hour.
The primary objective of the next planned X-15 flight was an evaluation of airframe heating and the ASAS of X-15-2 by Al White. The flight plan called for White to make a 20 degree climb at full throttle to an altitude of 54,000 feet, followed by a pushover to zero-g flight. White was to level off at 73,000 feet and throttle back to 30% thrust, adjusting the speed brakes to keep the speed of the X-15 at about 3,400 miles per hour. The engine would burn out after about 103 seconds. After burnout White would turn off the channels of the SAS, one at a time, to evaluate the response of the ASAS.
The sky was overcast on April 24, so the launch was postponed for a day. Major Allavie piloted the NB-52A with the X-15-2 under its wing on April 25. Colonel Charles "Chuck" Yeager made his only flight in the NB-52 mothership that day, serving as co-pilot. His one opportunity to observe an X-15 launch first hand was spoiled by another day of overcast skies.
Lieutenant Colonel Don Anderson replaced Chuck Yeager in the right seat of the NB-52A on April 26. Al White could not get the X-15 engine igniter lit, so the launch was aborted again. White's next X-15 flight was postponed until later in May. Major Robert Rushworth would make the heating test flight in the X-15-2 in early May.
Joe Walker was set to make an altitude build up flight in the X-15-1. Walker's flight plan called for him to make a 38 degree climb at full throttle, shutting down the engine after 81 seconds. Paradoxically, Walker was instructed to extend the speed brakes during the engine burn above 70,000 feet. This improved the directional stability of the X-15 in the rarified upper atmosphere. The X-15 would be going about mach 5.35 at engine burn out. It would coast to a peak altitude of 255,000 feet. Walker would begin his pullout at 180,000 feet and level out at 70,000 feet, landing at Rogers Dry Lake about 11 minutes after launching over Mud Lake.
Walker's flight had already been postponed once when Major Allavie and Squadron Leader Archer took the NB-52B aloft with the X-15-1 on April 27. Cloudy skies over the launch lake forced another scrub of the launch that morning. The launch was rescheduled to April 30. Major Russell Bement was in the co-pilot's seat of the NB-52B for the next launch attempt. Jack Russel sat behind the pilots of the NB-52B, monitoring the systems of the X-15.
The XLR-99 engine lit up right away and Walker established his climb. The flight progressed just as planned. Walker found that he had to strain to reach the throttle to shut off the engine under the force of nearly 3 gs of acceleration. The X-15 was traveling mach 4.73 (3,489 miles per hour) at engine shut down and continued to climb another 20 miles, reaching a peak altitude of 247,000 feet about 80 seconds later. As Walker pushed over at the peak altitude, he got a strong feeling that he was going to overshoot Edwards Air Force Base. The maximum mach number, mach 4.94, was achieved during the descent.
The staff of the NASA Flight Research Center headed over to the Elks club for a party that night to celebrate the new altitude record that Walker had set.
X-15 (The Nasa Mission Reports)
Angle of Attack : Harrison Storms and the Race to the Moon by Mike Gray. The biography of Harrison Storms, who was instrumental in the development and operation of the X-15.
At the Edge of Space : The X-15 Flight Program by Milton O. Thompson. The story of test flying the X-15 from the point of view of the pilot.
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