SCOTT CROSSFIELD WAS THE FIRST to fly the X-15, and he probably knew the airplane better than anyone else. He had left his job at the National Advisory Committee for Aeronautics in 1955 and gone to North American Aviation, which had just won the X-15 contract, to bring a pilot’s perspective to the design. Crossfield was an extraordinary test pilot, but at the end of the first flight, a seemingly simple power-off glide on June 8, 1959, the airplane tested him.
As he approached the dry lake bed at Edwards Air Force Base in California, he pulled the nose up to slow his descent. The nose came up too far, and he had to push it back down—and now he knew, and watchers on the ground knew, that the airplane had entered a divergent oscillation, galloping along a sine wave that increased in amplitude as Crossfield descended. Another X-15 pilot, Milt Thompson, later wrote that it was “a terrifying sight.” Crossfield couldn’t stop it, but he managed to get the landing skid on the ground at the bottom of a cycle, saving the airplane and possibly his own life. The problem turned out to be due to a poorly adjusted pitch damper; it was easily corrected.
By the time of that test, Crossfield already had 80 rocket flights under his belt, many in the Bell X-1 and Douglas Skyrocket, precursors of the X-15 that had been investigating supersonic flight since 1947. NACA, after spending eight years working up to the neighborhood of Mach 3 and an altitude of 100,000 feet in a series of barely adequate aircraft, now wanted the new research airplane to achieve Mach 6.6 and an altitude of 50 miles in a single leap. The X-15 would go where no airplane had ever gone before: into the void beyond the edge of the “sensible atmosphere,” where aerodynamics no longer exists, and to speeds at which the heat generated by the friction and compression of the air would melt the customary materials of aircraft structures.
LITTLE WAS KNOWN about flight in the hypersonic range—above Mach 5, or about 3,300 mph. Scanty data had been gleaned in wind tunnels by firing tiny models from guns into fast-moving streams of air. Two things had been learned from earlier rocketplane experience: First, stability—the quality that enables an airplane to be controlled by a pilot—decreased steadily with increasing speed; and second, aerodynamic heating would weaken and distort an airplane’s structure in flight. The aerodynamic design of the X-15, and particularly of the all-important tail surfaces on which it depended for both stability and control, was largely a matter of inspired guesswork. Its structural design, on the other hand, involved an immense amount of imaginative and skillful engineering together with novel methods of working with its recalcitrant structural materials: titanium and the heat-resistant hard nickel alloy called Inconel X.
Because he was a pilot, Crossfield’s contributions to the X-15’s design are often overlooked. Trained as an aeronautical engineer, he injected keen engineering intuition, a grasp of aerodynamics and human factors, and a powerful and decisive personality into a process usually entrusted to non-flying engineers. The result was one of the most successful research aircraft ever built.
North American trucked the first two airplanes—there were three in all—to Edwards late in 1958. For more than a year, teething problems—including an explosion that broke the second airframe in half and a hard landing, which broke it in half again—bedeviled the X-15. Aborted missions far outnumbered completed ones. But in 1960 things took a turn for the better. When, after a series of shakedown flights, Crossfield first turned the airplanes over to the government, they were still temporarily powered by a pair of the Reaction Motors four-chamber engines that had driven Chuck Yeager’s X-1 past Mach 1 more than a decade earlier. The total thrust from the smaller engines was just under 12,000 pounds. Crossfield came back that year to test-fly the new 60,000-pound-thrust XLR-99 engine, and then his role in the program ended.
Eleven other pilots flew the X-15. Three opened up the flight envelope: Air Force Major Robert White and NASA’s Neil Armstrong and Joe Walker. White was the first pilot to fly Mach 4, 5, and 6, and to surpass 200,000 and 300,000 feet—milestones that the X-15 effortlessly swept aside in rapid succession. Air Force Lieutenant Colonel Bob Rushworth flew the most X-15 flights—34; Lieutenant Commander Forrest Petersen was the only Navy pilot to fly the rocketplane. The other pilots were more or less equally divided between the Air Force and NASA. NASA’s Jack McKay, ex-Navy, happy-go-lucky, was the group’s “best stick-and-rudder man,” according to a number of pilots and program staff. Air Force Captain Joe Engle, who would go on to pilot the space shuttle, startled program director Paul Bikle by rolling the X-15 on his first flight, an unauthorized maneuver. NASA pilots Milt Thompson and Bill Dana both subsequently served as chief engineer at the space agency’s Dryden Flight Research Center, Dana retiring in 1998. Major William Knight, known as Pete, set a speed record for airplanes, 4,520 mph, that has never been surpassed. And Air Force Major Michael Adams was the program’s only casualty.
Of the 12 pilots, four are still living: White, Armstrong, Engle, and Dana. The most famous, as it turned out, would be Armstrong, whose trip to the moon eclipsed all of his previous accomplishments. He made seven flights in the X-15, going above 200,000 feet and nearly 4,000 mph before transferring to the space program in 1962.
Armstrong was the most talented engineer in the group, and he occasionally let his intellectual curiosity get the better of his piloting instincts. “He would let things go a little bit farther than, say, Jack McKay might have,” says NASA flight planner and stability specialist Bob Hoey. Armstrong made a famous mistake in the program, accidentally bouncing back out of the atmosphere during reentry while focused on a technical question about the behavior of the flight control system. He later told James Hansen, author of the Armstrong biography First Man, that he “felt the obligation to demonstrate” every aspect of the control system; he had consulted on its design, and he flew the missions to test it.
He coasted all the way to the edge of the Los Angeles basin before managing to turn the airplane around and land it at Rosamond Dry Lake, miles short of the originally planned landing site. It was jokingly said that on his final approach he cleared the cactus at the edge of the lake bed by a good margin—but only horizontally. It was the longest-duration flight in the X-15 program: 12.4 minutes.