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.
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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.
Armstrong had another role in the program: to assist in the development of the High Range, the flight route from Utah to Edwards along which all X-15 flights launched, at 45,000 feet and Mach .7, from a B-52 mothership. Radars and radio stations were placed on mountaintops, and miles-long runways were marked on a string of dry lakes so that an emergency landing site would always be available.
Bill Dana got to know the dry lakes well; the early part of his time in the program was spent setting out smoke flares at landing sites so that the X-15 pilot would know the wind direction. He didn’t become an X-15 pilot himself until 1965, but his first memory of the aircraft is much earlier: “I went to work October 1 of ’58, and they rolled the X-15 out at LAX [Los Angeles International Airport] on October 15. I got to see it the day after that, and I thought it was the ugliest airplane I’d ever seen. We’d spent our whole careers trying to reduce drag, and now they’d put a vertical tail that was square in the back. So I wasn’t too impressed with it until they put the big engine in it, and then it had to command your awe. It was a 33,000-pound airplane with 60,000 pounds of thrust, and it really left the scene immediately when you lit that engine.
“I got to see a lot of launches because I was launch chase, and it never failed to impress me. And I wanted in the worst way to fly the airplane, and eventually I got my chance. We went to ground school for six months. I knew the airplane pretty much backwards and forwards.”
Preparations for flying the X-15, once the pilots were out of ground school, consisted of long periods in the Iron Bird, a simulator in which pilots rehearsed flights over and over. Once every movement of the 10-minute adventure to come was second nature, the pilots ran through strings of unexpected emergencies, as space shuttle crews do today.
“The preparation was intense,” recalls Bob White. “We practiced the profile of the mission we were going to fly, and then we threw in failures of some of the rate dampers, the yaw, roll, or pitch damper, and the adaptive flight control system, and then when I was ready I would fly the profile again and they would throw things in unexpectedly I wasn’t prepared for.”
The part for which no amount of simulator time could prepare the pilots was the steep glide to a dead-stick, or engine-off, landing. The pilots accomplished it with a combination of guidance from NASA 1—a controller on the ground, usually another X-15 pilot—and every pilot’s ultimate tool, the eyeball. For every four and a half miles it covered over the ground, the X-15 lost a mile of altitude. Most airplanes were incapable of descending that steeply, but it was found that an F-104—whose general proportions were quite similar to those of the X-15—with its engine throttled back, flaps down, and landing gear and air brakes extended could match the X-15’s glide angle at 300 knots (345 mph). Actually, the F-104 could, in a pinch, descend even more steeply than the X-15. The late Joe Walker, asked whether it would be possible to land accurately out of such a steep approach, replied, “There’s no question of where you’re going to land, it’s how hard.” In fact, precise dead-stick landings in the X-15 were, in Bob White’s words, “a piece of cake.”
“We did a tremendous amount of practicing approaches in -104s to the uprange lake beds, and all of the lake beds,” says Joe Engle, “because each one of them was different and unique, and the approach was different and your cues were different, and they were different lengths. Some of them were [so short that it was] critical to touch down right at the end.”
Bob White worked harder on his landings, he acknowledges, than other pilots in the program: “Joe Walker, he made the first government flight, and Joe landed a couple of miles down from the intended touchdown point. Apparently he didn’t work the problem like I did. I took the -104, I would go to different lakes, and you know, engine back in the -104—Okay, here I am, I’m gonna dead-stick—and I set up, and I established all my cues around the landing pattern, and now I was going to make my first flight, and I remember Dick Day, one of the two engineers at Edwards, said, ‘Bob, how far from the landing spot do you think you’re going to be when you land?’ I said, ‘I’ll be within plus or minus 1,000 feet, no worse than that.’ And he said, ‘Oh, I’ll bet you a martini.’ And I said, ‘Make it two.’ And he bought me two martinis.”
The reentry and glide of the space shuttle resembled those of the X-15, and Joe Engle, who flew both, was the only pilot to hand-fly a shuttle at hypersonic speeds. He recalls the X-15 with evident warmth:
“I really look back on this airplane with fondness. I’m not at all shy or bashful to say that I enjoyed flying the airplane more than any other. If you have a favorite airplane it would have to be the X-15—because it was an absolutely awesome airplane. It was a very ingenious design for its time. It was really very, very advanced. It was a real pilot’s airplane; you weren’t separated from the airplane by a lot of computers and automatic control systems, and yet you got to fly a very, very impressive profile in both speed and altitude.”
FLYING THE HIGHEST-PERFORMANCE aircraft ever built required intense practice, and still there were surprises. The research program moved at a fast pace, and pilots had few opportunities to share impressions that were not related to the research goals of their flights. Milt Thompson was startled by the violence with which the X-15 detached itself from the B-52 mothership, and at finding, on his first flight, that when acceleration from the huge engine pinned him against his seat, he could no longer scan the instruments in the way he had developed while slouched comfortably in the simulator. Other pilots must have experienced the same thing, but no one had warned him.
“There’s some professional pride there,” Bill Dana comments. “You don’t want to help the other guy do too good a job on his program. I never worried about that—I was never in it for the reputation. But a lot of people did.”
In the chase for records during the X-plane era, some test pilots clearly focused on their own achievements. But Joe Engle remembers the experience differently. “The real thing I’m grateful for is to have gotten to be part of the X-15 program,” he says. “The X-15 program had—I don’t want to sound gooey about it—almost a family attitude about it. Everybody there was family, you weren’t holding back from anybody.... Among the pilots there wasn’t any competition, everybody had the ultimate design limits in mind, and to be part of that climbing-the-mountain process made everybody part of the same team. I would love to do it again.”
One thing most pilots remembered in the same way: the hard physical work of flying the airplane.
“You’re talking about the change in the acceleration as you continue to accelerate faster,” says Bob White. “Going from Mach 2 to Mach 3 took so many seconds, 3 to 4 took less, 4 to 5, you’re cutting down, and the pressure on your chest, you get up to the point where you’ve got 4 Gs and it’s difficult to breathe. And so as Milt said it was the only airplane he ever flew where he was glad when the engine quit.”
As fuel was consumed and the airplane grew lighter, acceleration increased, and all the X-15 pilots experienced a peculiar illusion: the sensation that although they were holding a steady pitch attitude—a 30- to 40-degree climb—the airplane was actually continuing to climb until it was rotating over onto its back. White himself once failed to make his planned altitude because the illusion of over-rotating was so compelling that he had to push the nose down momentarily in order to glimpse the horizon.
Flights were extremely short—usually 10 or 11 minutes from B-52 to lake bed.
“The time went by like a flash,” says Joe Engle. “I remember counting down the last minute of countdown. There are certain things you do and check-list and hitting the release button [to detach from the B-52] and then from then on, right after the flight I would have been hard-pressed to go into a lot of detail, between that and the time when you finally slid to a stop and cracked the canopy.
“The other thing I do recall is that the cockpit and the suit were pressurized with liquid nitrogen that could be released through a valve. Cooling was the same way: You just opened up the valve to cool it down. Some of it I’m sure is because the skin would heat up in flight, but I recall turning it up, because everybody said turn it up all the way before you launch, and being almost cold, you know, and then again, this flight going by like the snap of a finger, and sliding out on the lake bed and cracking the canopy, wanting to get it open because I was just drenched in sweat.”
Ships 1 and 2 had conventional controls, plus a three-axis stability augmentation system, which would weakly counteract any unintended motions in pitch, yaw, or roll. Ship 3 had the fly-by-wire adaptive flight control system that Armstrong helped design. The system had two purposes: to make the airplane handle similarly in all flight regimes, and to seamlessly integrate the thrusters, used during the weightless coast above the atmosphere, with the aerodynamic controls. It was thought that the airplane might not be controllable during reentry without artificial stability augmentation until Pete Knight experienced a total electrical failure.
“Pete Knight was the best test pilot that I’ve seen,” says Bill Dana. “He had a flight that launched over Smith’s Ranch and headed for Edwards. At 100,000 feet and Mach 4, both his generators went offline. All the lights came on for a few seconds, and then they all went out. And he never had another electron, that he could see, in the whole flight. He flew the climb using ballistic controls to keep the wings level. He didn’t have an artificial horizon, but he could apparently see out. He kept the wings level over the top and then he wanted to get back to Mud Lake to land there, because it’s a long runway. So he made a 180-degree turn to the left and he used more back stick when he wasn’t developing wing rock or lateral-directional instability, and when the airplane was flying too squirrelly he backed off on the G and he came around, and now he had aerodynamic controls—he had reentered, in other words—and so he flew a dead-stick landing into Mud Lake. To me, that’s the greatest single feat of airmanship that I know of.”
Malfunctions as severe as Knight’s were rare, but if the X-15’s pilots felt anxiety, it probably would have been over that sort of thing—being left helpless out at the edge of the world to be burned up or torn apart by an airplane that had turned savage. Says Bob White: “If you didn’t have a little fear when you stepped into this thing, there was something wrong with you, believe me.”
Milt Thompson was the only pilot to write a book about flying the X-15 (At the Edge of Space, Smithsonian Institution Press, 1992), and because writing about it forced him to turn it over and over in his mind and examine, as writers will, all the feelings that it brought forth, he clothed his memories in metaphor. He described the airplane as a “black bull,” and even imagined a scene in which a young warrior is sent out to explore a mysterious land, clad in armor and mounted on just such a beast. “Beware of the bull,” the tribal elders tell the young man as he is about to set out. “He is awesome in battle. However, if you lose control of him or fall off, he will kill you as quickly as he would kill your enemy.”
Though Bill Dana and Milt Thompson were close friends, “I didn’t understand that part,” Dana says. “I don’t know what he was trying to get across. I didn’t think it was dangerous until right toward the end of the program.”
That was when the long lucky streak ended.
X-15s had never been reliable airplanes. They were complex and novel, and on most flights one system or another would act up. But they had always brought their pilots back alive, no one had ever had to eject, and only one pilot, Jack McKay, had been seriously injured. Even McKay—who was measurably shorter after suffering several crushed vertebrae when a landing skid failed and his airplane flipped over—eventually returned to fly the X-15 again. But Mike Adams didn’t return.
It happened in the Number 3 airplane, the one with the adaptive flight control system, which constantly adjusted the authority or “gain” of the controls in order to make the airplane feel the same regardless of speed and altitude. It was the latter phase of the program, by which point the original research goals had all been met and the X-15s were being used as mules to carry scientific experiments to extreme speeds or altitudes. An electric motor that was part of an experiment carried on the wingtip created a disturbance that interfered with the flight control system as the airplane shot out of the atmosphere. Adams, whose known susceptibility to vertigo had been ignored when he was assigned to the X-15 program, apparently became disoriented. An additional trap lay in wait: A needle on his primary attitude indicator could be selected to display either roll or yaw. Adams got mixed up and tried to correct with yaw for what was actually a roll cue. Controllers on the ground could not tell what was happening, but the airplane was rotating about its vertical axis until, when it reentered the atmosphere, it was flying sideways. A violent and dizzying ride followed. Adams reported that he was in a spin—a situation that had never before been encountered in hypersonic flight, and for which no recovery procedure was known. At first, however, the black bull corrected itself, its rotation slowing as it weathercocked back into alignment with its flight path. For a brief period it was inverted but stable, with sufficient altitude for a recovery.
But then the adaptive flight control system began pitching the aircraft up and down with increasing violence until, somewhere beyond 8 Gs, the airplane broke apart. A switch on the panel could have shut the runaway system off, but no one thought of it until too late.
“I have always associated the end of the program with Mike’s accident,” says Bill Dana. “We were going along with three airplanes, getting lots of data, and had lots of plans. And when Mike was killed, it kind of took the heart out of the program. And I think there were a lot of people that would have liked to quit the program right there. I think the program quit itself. You can imagine the emotion involved there, when Mike got killed.”
But it did not end immediately.
“Paul Bikle wasn’t for canceling programs when they got tough,” Dana continues, “and so he said we’re going to fly one more year, one more calendar year, and that was 1968, and that was what we flew—we flew eight flights in 1968, and that was the end of the road.”
Bill Dana was the pilot on the last of the X-15’s 199 flights. Freakish weather, including a snowstorm, frustrated several attempts to make it a round 200 before the program ended.
Today the first X-15 hangs in the Smithsonian’s National Air and Space Museum; the second X-15, the fastest airplane ever flown, is at the National Museum of the U.S. Air Force in Dayton, Ohio.
It would be difficult to overstate the X-15’s importance in the history of flight. It is the keystone of the bridge between Earth and space. It was, in a way, the ultimate airplane: No other airplane has ever approached its performance, or needed to. The X-15—a simple, direct, straightforward machine in the classical tradition of aeronautical engineering—had leaned on the door to hypersonic flight, and the door had swung open. After the X-15 came spacecraft and computers—and a new and alien era in flight.