The Airplane That Flew Into Space

MAJ. WILLIAM (“PETE”) KNIGHT WAS in trouble, and he didn’t even know it. It was October 3,1967, and he had just set a world aircraft speed record of Mach 6.7 (6.7 times the speed of sound, or 4,520 mph) in a specially modified X-15 research plane flying at an altitude of 102,000 feet. As Knight approached the dry bed of Rogers Lake, at Edwards Flight Test Center in the Southern California desert, he found himself coming in fast and heavy. He was unable to jettison excess fuel, since the plane was carrying none, and he had no flaps to slow his touchdown speed. But X-15 pilots were used to tense situations. As Knight, today a California state senator, recalls, “I can probably count on one hand the number of flights we made where nothing happened in terms of an emergency, regardless of how big or small the emergency.”

But this time was different. During Knight’s highspeed run, shock waves had formed around a mockup of a “scramjet” test engine mounted on the X-15’s lower ventral fin. These shock waves had focused heat onto the undersurface of his airplane, raising its temperature to almost 3,000 degrees Fahrenheit. Despite the use of a special “ablative” paint designed to absorb heat and flake off, the heat had burned through the plane’s skin and spread up into the engine compartment, damaging vital components and leaving Knight seconds away from losing the hydraulic systems that operated his maneuvering controls. The aft end of the craft was severely weakened and in danger of disintegrating completely.

Knight managed to set the X-15 down on the lakebed and slid to a stop as a phalanx of recovery vehicles surrounded the aircraft. But he didn’t get the usual welcome home: “Normally everybody comes to the front of the airplane to congratulate me on the flight and help me get out of the airplane. This time that didn’t happen. Everybody went to the back of the airplane. And I said, there’s something wrong. So I got out, went around, and looked at the back end.… It was like you took a blowtorch to the lower ventral. Things had really been going to hell in a handbasket rather rapidly, which I hadn’t known at the time.”

A record set, new technologies tested, engineering theories proved and disproved, and a dash of hairraising adventure: just another day in the X-15 program. More than 40 years after its first flight, the X-15 remains the most successful experimental aircraft ever flown. It provided scientific data, technology, and techniques used by engineers from the Mercury program to the space shuttle, and it set records for aircraft speed and altitude that remain unbroken. The X-15 program had the misfortune to be inaugurated when men were about to orbit the earth and terminated when they were about to orbit the moon, so its accomplishments were always overshadowed. Still, aerospace engineers from the 1960s to the present have benefited enormously from the data and the lessons it yielded.

The origin of the X-15, like many other genesis stories, has official and unofficial versions. A. Scott Crossfield, the X-15’s first pilot, says the aircraft was born in 1951 during a fishing trip he took with Walt Williams, head of the High Speed Flight Station (later the Flight Research Center) at Edwards. At the time, Crossfield was a 30-year-old aircraft designer and test pilot with the National Advisory Committee on Aeronautics (NACA). In the latter role, in November 1953, flying a Douglas D-558-2 Skyrocket, he would become the first man to break Mach 2. Crossfield recalls: “We were coming home late at night, two of our friends asleep in the back seat. We heard on the radio that they’d just fired a 75,000-pound Viking engine. Walt and I immediately conjectured what we could do with a manned airplane with 75,000 pounds of thrust. He was driving, so I got a piece of paper out of his glove compartment. Literally, we calculated what it should do on the back of an envelope.”

Officially, though, the X-15 traces its birth to a 1954 NACA study on the possibility of a research airplane that would go beyond transonic (speeds in the vicinity of Mach 1) and supersonic (Mach l to Mach 3) flight to explore the region known as hypersonic. Although the X-15 would eventually fly into the lower reaches of outer space as well, no one was seriously thinking of such a thing yet. John V. Becker of Langley Aeronautical Laboratory, who devised the basic X-15 concept, remembers that “we were thinking more in terms of an airplane which would fly at very high speeds within the atmosphere. But we noticed right away that if you have enough propulsion energy to reach Mach 6 or 7 in the atmosphere, you have more than enough energy to fly out of the atmosphere into space for a short time and make a re-entry.”

Becker’s conceptual work was enough to convince NACA of the aircraft’s feasibility, and the committee set about planning a research program and finding a manufacturer to build it. The Air Force and Navy were NACA’s partners in the effort. The aircraft was dubbed the X-15, continuing series of experimental planes that had begun in 1946 with the X-1, in which Chuck Yeager became the first person to break the sound barrier. Under NACA’s plan, the X-15 would explore problems of aerodynamic heating, stability, control, and pilot physiology at speeds up to Mach 6 and altitudes as high as 250,000 feet.

In 1955 North American Aviation beat out Bell, Douglas, and Republic for a contract to build three X-15s. Reaction Motors, a division of Thiokol Chemical Corporation, was chosen to develop a millionhorsepower liquid-fueled rocket engine, dubbed the XLR-99. Its design would be based on the Viking engine that had inspired Crossfield and Williams in 1951. On October 17, 1958, North American delivered its first X-15 to Edwards. Since the XLR-99 was not ready yet, it was fitted with two smaller XLR11 rocket engines, of the type used in the X-1, so that the contractor demonstration phase could begin.

Even on the tarmac at Edwards, home of the world’s most advanced aircraft, the X-15 was a striking sight: a black, lowslung, 50-foot-long needle with thin, trapezoidal wings spanning 22 feet and an odd, chunky, wedge-shaped tail. Its skin was made of Inconel-X, a rugged nickel-chromium alloy capable of withstanding temperatures up to 1,200 degrees Fahrenheit. Most of the interior was taken up with tanks of liquid oxygen and anhydrous ammonia, which, when mixed and ignited at high pressures, release huge amounts of energy.

Milton O. Thompson, who flew the X-15 in the early and mid-1960s, described the craft as “a big propellant tank with a cockpit on the front end and an engine on the back end.” Its landing gear consisted of a nose wheel and a pair of skids in the aft fuselage, so the plane had to be wheeled on a special dolly while on the ground. But the X-15 was never meant to take off or lar.d like a conventional airplane. Instead, like the X-1, it was carried into the sky nestled under the wing of a mother ship—in this case, a specially modified Boeing B-52—and then released in the stratosphere. After its rocket engine burned out, which took about a minute and a half, the pilot had to set the plane down in a “dead-stick landing,” without power. As Thompson put it, “You knew you were going to be on the ground within 8 to 10 minutes one way or the other. You were either going to have made a successful landing, or come down in a parachute, or made a smoking hole.”

It was the brute force of the XLR-99 rocket engine that made the X-15 the fastest aircraft ever. The aircraft weighed about 33,000 pounds fully loaded with propellants but only about 15,000 pounds after its fuel was exhausted. The XLR-99’s 60,000 pounds of thrust gave the X-15 a thrust-to-weight ratio of four—more typical of a ballistic missile than an airplane. According to Thompson, “Our simulations and calculations indicated we could have gone to 400,000 or possibly 500,000 feet, but we could not safely re-enter from that altitude” because the X-15 would not have been able to withstand the heat of the longer re-entry period. Becker explains: “For short flights you can sort of cheat. You get into a high-temperature environment but it doesn’t heat up your structure all the way.”

In the spring of 1959, following nearly five months of ground tests, the X-15 was ready to fly. It took to the air for the first time on March 10, bound securely to its B-52 carrier. Then, after a few more captive flights, the X-15 was ready to be dropped from the B-52 and glide to a landing so its handling qualities, systems, and general flight characteristics could be checked. Scott Crossfield, who had left NACA in 1955 to join North American and shepherd the plane through design and development, was the first to try it.

The first attempt at a glide flight, on April 1, 1959, was aborted before release because of a radio failure. Several attempts over the next two months were also aborted, as frustration grew. Sputnik and other Soviet space successes had embarrassed the United States, and NASA (NACA’s successor, the National Aeronautics and Space Administration) was counting on the X-15 to help give America the lead in the space race—if it could ever take wing.

Finally, on June 8, Crossfield was released from the B-52 at 38,000 feet and Mach 0.79. For the first time, he was alone in the sky with the aircraft he’d helped conceive and build. “My checkout flight was 3 minutes and 58 seconds,” he recalls with amusement. “And therein was where I had to learn all I had to know to land the airplane for the first time.”

He had no problems until just before touchdown. One of Crossfield’s tasks was to test a newly developed “sidearm” control. This was a stick on the right armrest that allowed the pilot to fly the plane solely with hand and wrist motions during high-speed flight, when he would be pinned to his seat by G forces and unable to use the center stick. But as Crossfield used the sidearm control to line up for his landing, the aircraft began to “porpoise,” its nose pitching up and down wildly. Crossfield couldn’t damp the motion, and he was plummeting toward the ground at 200 miles per hour. If he failed to time his touchdown just right, so that the X-15’s landing skids touched the ground at the bottom of an oscillation, the craft would smash apart on impact.

Crossfield’s piloting skill saved the X-15—and his life—on that first landing. He hit the lakebed hard, but he and the X-15 survived. Crossfield worried that a serious design flaw had been overlooked, but the cause was soon traced to a faulty adjustment in the sidearm control system, which was easily repaired. Crossfield later received an award from the Southern California Soaring Society for setting a record: the shortest time from 38,000 feet to the ground in a glider. The trophy consisted of a streamlined brick mounted on a plaque.

The next milestone was the first powered flight. After several abortive attempts, Crossfield took X-15 number 2 to just over Mach 2 on September 17, 1959. Except for a small engine fire, it was a textbook mission, as was the next flight a month later. The third powered flight, in November, proved more dramatic.

After the X-15 was dropped from the mother ship and Crossfield started the rocket engines, a chamber in the lower engine exploded, blowing out the back end of the plane and sparking a fire in the engine compartment. Crossfield prepared for an emergency landing, but the plane was nose-heavy with a full propellant load. The overladen X-15 touched down smoothly on its rear landing skids, then broke in two as the nose wheel hit the lakebed. The plane slid to a stop, badly damaged and leaking fuel, but Crossfield was uninjured. “There was a design oversight in the nose gear, and that’s what caused it to break in two,” he says. “In fact, it probably should have broken on the first landing, because the deficiency was there.”

With the initial kinks worked out, Joseph A. Walker began NASA’s X-15 flights in March 1960, alternating missions with Maj. Robert M. White of the Air Force. They soon found out it wasn’t easy to keep the X-15 reined in. In May 1960, White took the X-15 above 100,000 feet for the first time and Walker made the world’s first “official” flight above Mach 3 (though Crossfield had briefly exceeded the mark in January, to little notice, and back in September 1956 Milburn Apt had reached Mach 3.2 in a Bell X-2 only to die in a crash while attempting to land). Impressive as these feats were, however, it wouldn’t be possible to realize anything like the aircraft’s full potential without the big XLR-99 engine.

In March 1960 the first XLR-99 was finally delivered to Edwards and installed in X-15 number 3 for testing. Like the plane itself, the XLR-99 needed some adjustments in its shakedown phase. During a ground test-run with Crossfield at the controls, the engine exploded, blowing the cockpit 30 feet forward and subjecting Crossfield to a momentary acceleration of 50 Gs. A million-horsepower engine makes a very big explosion. Crossfield recollects: “It was like being in the sun, bright orange all around me. I only had a light shirt and tie on; I was in a business suit. But I was better off than anybody. I was in a steel container that was designed to accept very high temperatures.” Fortunately for the program, the cause of the explosion lay not in the design of the XLR-99 or the X-15 but in a flawed pressure regulator.

Testing and modifications continued, and in December Crossfield made his final flight in the X-15. Once the plane had passed all its tests, North American had fulfilled its contract, and now it belonged to NASA. Crossfield, as a North American employee, had no further role as a pilot in the research program. Now it would be up to Walker, White, and a new group of NASA and military pilots to take the X-15 to its design limits. Before long, it became clear that the X-15’s designers had been thinking much too conservatively.

In 1961, when all three X-15s had the big XLR-99 engine, the X-15 program hit full stride. The core pilot group consisted of Walker, Jack McKay, and Neil Armstrong (who, of course, would go on to even greater things before the decade was out) for NASA; White and Maj. Robert Rushworth for the Air Force; and Comdr. Forrest Petersen for the Navy. The group of six pilots began the envelope-expansion phase, pushing the X-15 ever faster and ever higher in carefully measured, methodical steps.

Only one flight in 1961 was slower than Mach 2. In March, Bob White became the first man to fly beyond Mach 4. Joe Walker broke White’s speed record in May, only to lose it a month later, when White made the first flight to Mach 5. Before the end of the year, White became the first to exceed Mach 6 as well. In October he set an altitude record by taking the X-15 to 217,000 feet, more than 41 miles above the earth. It didn’t quite qualify as a space flight—the Air Force defined space as beginning at 50 miles, while NASA pegged it at 100 kilometers, or 62 miles—but it was close. By the end of its first year of full-scale operation, the X-15 had repeatedly broken every speed and altitude record for winged aircraft.

But the public’s attention was focused elsewhere. The USSR’s Yuri Gagarin had become the first human in space in April, and Alan Shepard made America’s first manned space flight in May. Amid all the rocket spectaculars of the Cold War, experimental airplanes just weren’t as interesting anymore. Still, the people at Edwards could live with anonymity, because they were less interested in records and public acclaim than in research. All X-15 flights were monitored by chase planes and ground stations, which recorded scientific data about hypersonic aeronautics and aerodynamics. Edwards researchers knew that their work was making possible the successes of the more visible space program.

THE MERCURY ASTRONAUTS, FOR EXAMPLE, TRAINED ON flight simulators derived from one developed for the X-15. The X-15’s telemetry and tracking range, spread out across Nevada, became a model for NASA’s orbital space-flight monitoring network. The A/P22S-2 pressure suit, which was inflated to maintain circulation and allow breathing in thin air or none at all, was another technology developed for the X-15 and adapted for Mercury. This unit introduced such features as helmet defogging and the aluminized outer layer, which reflected heat and ultraviolet light and protected a pilot in case of fire.

Most important, the bounty of X-15 data on aerodynamic heating, maneuvering in space, and controlled re-entry techniques found application not only in the Mercury program but in the development of Gemini and Apollo spacecraft as well. In 1968 NASA’s John Becker observed of the X-15 that “the spaceoriented results have been of greater value than the hypersonic aeronautics contributions. This is the reverse of what was expected in the beginning.”

The pace of the X-15 program required more pilots, since each flight typically required 10 to 14 days of preparation beforehand as well as ample time for recovery and debriefing afterward. Over the next several years, Milt Thompson and William H. Dana of NASA and Pete Knight, Capt. Joseph H. Engle, and Maj. Michael J. Adams of the Air Force were tapped to fly the craft. They found themselves immersed in a heady and demanding environment unlike anything they had experienced before.

Training was brutally intense. No two-seat X-15 trainer existed for practice or checkout flights, so pilots spent countless hours in a simulator preparing for 10 minutes of actual flight time. They trained for the X-15’s dead-stick landings in F-104 fighters, which had similar aerodynamic qualities. Pilots had to be familiar with designated emergency landing sites in the area between Edwards and Wendover, Utah, thoroughly familiarize themselves with all the X-15’s systems, and memorize the location of every instrument in the cockpit.

There were two basic flight profiles: speed runs and altitude flights. In a speed run, the pilot stayed inside the atmosphere, generally below 100,000 feet, and accelerated to whatever Mach number was required for the particular flight plan, sometimes performing other maneuvers or experiments along the way. During these missions, the X-15 essentially flew like a normal airplane, except much faster, and could be controlled with its aerodynamic surfaces.

The altitude flights, by contrast, were a ballistic roller-coaster ride into space and back. Shortly after launch, the pilot lit the engine and pitched the aircraft up to a steep angle. The X-15 streaked away from earth, continuing out of the atmosphere even after the engine burned out. Once the plane was in space, with wings and flaps useless, reaction controls came into play. These were small jets that created thrust by breaking down hydrogen peroxide into oxygen and steam. These thrusters could be used to correct yaw, pitch, and roll (respectively, rotation of the nose to the left or right, of the nose up or down, and of the wings up on one side and down on the other).

As the X-15 coasted to the peak of its ballistic arc, the pilot experienced a few minutes of weightlessness while he enjoyed the view at the top of the world. He could see the blackness of space above and the curvature of the earth ahead. After a few minutes, the craft began its descent into the atmosphere. This was the most dangerous and tricky part of the flight; as Joe Walker put it, “You have to do this maneuver right, or buddy, you’ve bought the farm.” The pilot brought the nose up and made the X-15 plunge earthward belly first, presenting a blunt surface to the air beneath. This slowed the plane more efficiently and, contrary to intuition, reduced frictional heating. The nose and the leading edges of the X-15’s wings and tail glowed dull red as the craft rapidly entered the dense lower atmosphere, while the pilot experienced rapidly building G forces. Finally, back inside the atmosphere, the pilot leveled out the aircraft, maneuvered to expend his excess speed (a process called “energy management”), and brought the X-15 down for a landing.

WHICHEVER PROFILE A PILOT WAS CALLED ON TO fly, it was never a routine day at the office. Milt Thompson observed: “It’s happening so fast. The whole flight depends on how well you perform in the first 82 seconds, because that’s how long the engine burns. The success of the mission is established right there, and you’re just trying like hell to keep up.” He added, “It was the only airplane I ever flew where I was glad when the engine quit.”

Bill Dana, now a contract engineer at NASA’s Dryden Flight Research Center, cites his highest flight, to more than 306,000 feet in 1966, as the one he remembers best. But he enjoyed the speed flights more, he says, “because you were in a region that you could control the airplane continually. They were the biggest challenge, because the pilot was expected to get from the start to the end point and stay on profile the whole way, whereas on an altitude mission, once the engine shut down, you were basically ballistic.” Other pilots don’t make such distinctions, though. Asked which flight was his most memorable, Joe Engle, now an Air Force general, answers without hesitation: “All of ‘em!”

The X-15 made its first true space flight in July 1962, when Bob White took it to 314,750 feet. By breaking the Air Force’s space boundary of 50 miles, he became the first pilot to be awarded astronaut wings. His flight is still recognized as an official world record for altitude in a winged aircraft, although Joe Walker set the altitude record for the X-15 a little more than a year later in his last flight of the program, soaring to 354,200 feet, or more than 67 miles. (Walker’s record is not officially recognized because it did not meet all the criteria of the Fédération Aéronautique Internationale, the body that certifies aviation records.) Although Walker, Dana, and the other high-flying civilian pilots were not eligible for the coveted Air Force astronaut wings, Walker’s buddies took him out to dinner after his record flight and pinned homemade cardboard astronaut wings on his chest.

Despite some tense moments—such as Knight’s fiery 1967 flight and a November 1962 crash that seriously injured Jack McKay and tore apart his plane—the program enjoyed an excellent safety record, with no pilot lost and no plane damaged beyond repair. That changed on November 15, 1967, when Mike Adams took X-15 number 3 up for a high-altitude mission. Adams reached his maximum altitude of 266,000 feet, barely above the 50-mile mark, then found himself in a spin. The ground crew and chase pilots were shocked. A hypersonic spin was unheard of, and no one knew how to recover from one. Somehow the X-15 snapped out of the spin as it dived to lower altitude, but a malfunction of the control system caused the aircraft to oscillate and pitch wildly. A disoriented Adams was unable to regain control. X-15 number 3 broke up in mid-air, and Adams was killed. He was posthumously awarded astronaut wings for the flight.

The death of Adams cast a shadow over the program that never entirely lifted. However, X-15 number 1 was still flying, and number 2, which had been damaged in McKay’s 1962 crash, was rebuilt and modified for high-speed scramjet experiments, culminating in Knight’s Mach 6.7 flight. The scramjet, or supersonic combustion ramjet, is basically a tube that takes in air at one end, slows it down to increase the pressure, and then uses it to burn fuel. The combustion products escape from the rear of the tube at very high velocity, propelling the aircraft. The scramjet-modified X-15 had external propellant tanks, which were jettisoned (with parachutes) when empty and retrieved for reuse.

Unlike a rocket engine, a scramjet needs air, so it cannot be used in space. Moreover, it must be moving very fast—Mach 3 at least—to take in enough air to burn its fuel, so the only planes it is useful for are extreme high-end models like the X-15. Before a scramjet can start to function, it has to be brought into the supersonic region by another engine. The technology was originally developed for use in long-range missiles, which can be boosted to the necessary speed with rockets.

The scramjet was intended to bring the X-15 to blistering speeds of Mach 7 or even 8, leading to a new generation of hypersonic aircraft and winged orbital spacecraft. But the near-loss of Knight’s flight showed that protecting the plane under the stress of high-altitude and high-speed flights would be very difficult. In the wake of Adams’s death just a month later, further plans for the scramjet, as well as a delta-winged X-15, were canceled. Funding was drying up, and interest was shifting elsewhere. “I think management felt they’d had a pretty good run and only killed one person, and maybe they ought to fold their tent,” Bill Dana says.

“They canceled development of the scramjet engine. Therefore, there was no reason to push the envelope of the X-15 any faster,” says Knight. “And the total program was becoming expensive as well. We knew it was going to end sometime.” Still, he thinks the cancellation of the X-15 scramjet program was a big mistake. “If you look at the projects today, they’re talking scramjets for the X-43 [the latest research aircraft]. If we had continued with the scramjet engine, we could have had an operational engine in a couple of years with the X-15.”

BY 1968 THE X-15 PROGRAM HAD SURPASSED ALL ITS original goals. Dana and Knight were the only pilots still flying. Dana made the last flight, number 199, on October 24, 1968, although he didn’t know it was the last one at the time. Knight was scheduled to make flight 200 before the program officially ended at the close of 1968, but a combination of technical problems and bad weather kept him grounded. The aircraft he would have flown, X-15 number 1, now hangs in the National Air and Space Museum, on permanent display next to Chuck Yeager’s X-1, the Spirit of St. Louis , and the Wright Flyer . X-15 number 2 is at the U.S. Air Force Museum, at Wright-Patterson Air Force Base, in Dayton, Ohio.

Referring to the historic Mach 6.7 flight, Knight says, “I set a record in 1967. That’s 30-some years ago. It’s unheard of for that kind of record to stand that long with no foreseeable possibility of exceeding that record. We have stopped our advancement in the areas of aviation, aerospace, aerodynamics, hypersonic flight.” As Crossfield points out, “every airplane that significantly exceeded Mach 2 is now in a museum.” Yet many who flew the X-15 believe that turning away from winged hypersonic aircraft and the “air-breathing” orbital spacecraft that would have followed was a tremendous mistake.

A single-stage-to-orbit vehicle to replace the expensive, aged shuttle is the Holy Grail for aerospace engineers and poses staggering technical problems that have yet to be solved. Budgetary concerns and engineering problems have caused the cancelation of the X-33 and Venturestar spacecraft, and no clear successor to the shuttle is yet on the horizon. Some argue that if research in winged hypersonic flight had been continued beyond the X-15, the United States would currently have a better and much cheaper way to orbit.

Crossfield argues that “to get to orbit and back is an aerodynamic problem. I think there’s a lot better way to power things into orbit than by rocket. One, it’s very expensive, two, it’s very inefficient, and three, it doesn’t have much flexibility. With all the oxygen provided for you by the atmosphere, you can get very efficient engines. The shuttle would weigh, I’d guess, 20 percent of what it does now if it were an air breather. It would carry a bigger load for much cheaper.”

Whatever promises it may have left unrealized, no one questions the X-15’s enormous success and influence. Over the years, almost 800 scientific and engineering papers have been written based on X-15 data. The program gave NASA scientists invaluable insights into spacecraft design, heatresistant materials, flight techniques, tracking and communications technologies, and the biomédical aspects of space flight. The first practical throttleable rocket engine, the fullpressure space suit, reaction controls, space-flight simulation technology, and many other innovations trace their origins directly to the X-15.

But perhaps the most visible and lasting legacy of the X-15 is the space shuttle. Engle, the only person to fly both vehicles, remarks: “The flight-control system developed on the shuttle to transition from exoatmospheric flight to atmospheric flight really was developed and tuned in the X-15. It was sitting there on the platter waiting to be installed.” On his first landing in the shuttle, Engle discovered that at the subsonic speeds used for landing, “the X-15 and shuttle are surprisingly similar. The lift-to-drag ratio of both vehicles is pretty much the same.” Shuttle pilots today use the same techniques to dissipate their re-entry speed for approach and landing that were developed by X-15 pilots 40 years ago. And the research on high-speed aerodynamic heating and angle of attack during re-entry conducted with the X-15 provided important clues to protecting the shuttle from the searing temperatures that develop during its return to Earth.

More than a technological marvel, however, the X-15 is a symbol of a past age of daring, the willingness to take chances and push back frontiers for the sake of knowledge. “In those days we astonished ourselves every month with what we could do,” Crossfield remembers. It’s an attitude he finds lacking in the current play-it-safe climate: “Our biggest risk for the future is to not risk for the future.”