As chief engineering
test pilot for North American, Crossfield played a major role in the design and development of the
North American X-15 and its systems. Once it was ready to fly, it was his job to demonstrate its airworthiness at speeds ranging up to Mach 3 (2,290 mph). Because the X-15 and its systems were unproven, these tests were considered extremely hazardous. Crossfield flew 14 of the 199 total X-15 flight tests with most of these tests establishing and validating initial key parameters. Crossfield was not only involved with the design of X-15 from the beginning, but introduced many innovations, including putting engine controls of the rocket plane into the cockpit. Previously, all engine adjustments resulted from technicians making adjustments on the ground based upon results of flight profiles. It was during this time that Crossfield was part of the
U.S. Air Force's
Man In Space Soonest project. On June 8, 1959, he completed the airplane's first flight, an unpowered glide from 37,550 feet. The flight was troubled as the flight controls had not been set up properly. As Crossfield attempted to land the unfueled X-15, it went into what Crossfield described as "a classic PIO" or pilot induced oscillation. He managed to set down the X-15 on the desert runway at the bottom of one of the severe oscillations saving himself and the airframe. On September 17, 1959, he completed the first powered flight. Because of delays in the development of the X-15's mammoth 57,000 pounds force (254 kN) thrust
XLR-99 engine, the early flights were completed with a pair of interim
XLR-11 rocket engines. Shortly after launch on his third flight, one of these engines exploded. Unable to jettison his propellants, Crossfield was forced to make an emergency landing during which the excessive load on the aircraft broke its back just behind the cockpit. He was uninjured and the airplane was repaired. On June 8, 1960, he had another close call during ground tests with the XLR-99 engine. He was seated in the cockpit of the No. 3 X-15 when a malfunctioning valve caused a catastrophic explosion. Once again he was uninjured as Dr.
Toby Freedman, NAA Medical Director, pried open the cockpit to save him and despite being subjected to a later calculated acceleration force of near 50 Gs (although Crossfield stated in the Discovery Channel's series
Frontiers of Flight that he began to have debilitating issues with his night vision after the accident) and the airplane was completely rebuilt. On November 15 of the same year, he completed the X-15's first powered flight with the XLR-99 engine. Two flights later, on December 6, he brought North American's demonstration program to a successful conclusion as he completed his final flight in the X-15. Although it had been his hope to eventually pilot one of the craft into space, the USAF would not allow it, and gave strict orders which basically amounted to "stay in the sky, stay out of space." Altogether, he completed 16 captive flights (mated to the
B-52 launch aircraft), one glide flight and 13 powered flights in the X-15. The retirement of the X-15 (due to funding cutbacks) after its record-setting Mach 6.70 (4,520 mph) flight prompted pilot
Pete Knight to remark that he would have pushed it to even faster speeds if he knew it was the last flight. In his remarks to a number of aviation groups, Crossfield cited the X-15 as one of few aircraft that caused grown men to cry upon its retirement. He remained at North American as systems director of test and quality assurance in the company's Space and Information Systems Division where he oversaw quality, reliability engineering and systems test activities for such programs as the
Apollo command and service modules and the Saturn S-II booster. In 1966, he became the division's technical director for research engineering and test. == Civilian career ==