- 2010 — Present
- 2000 — 2009
- 1990 — 1999
- 1985 — 1989
The Airplane That Flew Into Space
HOW THE X-15 BECAME THE MOST VALUABLE AND SUCCESSFUL RESEARCH PLANE EVER BUILT
Summer 2001 | Volume 17, Issue 1
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.