Share this story On October 3, 1967, test pilot William J. “Pete” Knight climbed into a black, dart-shaped aircraft bolted to the wing of a B-52 bomber. At 45,000 feet over Mud Lake, Nevada, the B-52 released him. Knight lit the rocket engine. In the next 140.7 seconds of engine burn, he accelerated through Mach 1, Mach 2, Mach 3, Mach 4, Mach 5, Mach 6, and kept going. He hit Mach 6.7 — 4,520 miles per hour — at an altitude of 102,100 feet. That speed record for a piloted, powered aircraft still stands today. Nearly sixty years later, no human being has flown faster in an airplane. The North American X-15 was not a fighter. It was not a bomber. It was a research vehicle — a flying laboratory designed to answer questions that no wind tunnel or computer simulation of the era could resolve. What happens to an airframe at hypersonic speeds? How does a pilot control an aircraft at the edge of space? What are the thermal, structural, and physiological limits of manned flight? The answers the X-15 programme produced shaped everything that came after: the Space Shuttle, the scramjet programmes, and the very concept of reusable spaceflight.

Born from a B-52’s Wing

The X-15 could not take off under its own power. It was too small to carry enough fuel for a ground takeoff, too fast to use a conventional runway, and too specialised to waste any of its precious propellant on the mundane act of getting airborne. Instead, it was carried aloft by a modified B-52 Stratofortress — nicknamed “Balls 8” — slung beneath the bomber’s right wing like a black dagger. At approximately 45,000 feet, the X-15 dropped free. The pilot had a few seconds of unpowered glide before igniting the Reaction Motors XLR99 rocket engine, which produced 57,000 pounds of thrust by burning anhydrous ammonia and liquid oxygen. The burn lasted roughly 80 to 120 seconds. In that brief window, the X-15 accelerated to speeds and altitudes that no other piloted aircraft could reach. Once the fuel was gone, the X-15 became a glider — a very fast, very heavy, very unforgiving glider that needed to bleed off energy and land on the dry lakebeds of Rogers Dry Lake at Edwards Air Force Base. There were no go-arounds. There was no second chance. The pilot had to get it right the first time, every time.

Where Air Ends and Space Begins

The X-15 did not just fly fast. It flew high — so high that conventional aerodynamic controls stopped working. At extreme altitudes, the air is too thin for ailerons, elevators, and rudders to bite. The X-15 solved this with small hydrogen peroxide reaction control thrusters in the nose and wingtips — the same type of attitude control system that would later be used on the Space Shuttle and Apollo spacecraft. On August 22, 1963, NASA test pilot Joseph Walker reached 354,200 feet — 67 miles above the earth. He was above 99.99 per cent of the atmosphere. He was, by any reasonable definition, in space. The curvature of the earth was visible through his canopy. The sky was black. Eight X-15 pilots earned astronaut wings by exceeding 50 miles altitude, the U.S. Air Force’s definition of the boundary of space. Only one of them went to space again: Joe Engle, who commanded the Space Shuttle missions STS-2 in 1981 and 51-I in 1985. Neil Armstrong, the most famous X-15 pilot of all, never took it above 50 miles; his highest flight reached 207,500 feet.

The Heat Problem

At Mach 6, the leading edges of an aircraft reach temperatures that would melt aluminium. The X-15’s skin was made from Inconel X, a nickel-chromium alloy that could withstand temperatures exceeding 1,200 degrees Fahrenheit. The aircraft was painted black not for aesthetics but because dark surfaces radiate heat more efficiently. Even with Inconel X, the thermal challenge was brutal. The windshield was made of fused silica glass with a retractable outer pane. The landing gear had to be kept retracted until the final approach because the speed would have ripped conventional gear doors off the airframe. Internal components required careful thermal management to prevent instruments, electronics, and even the pilot’s pressure suit from overheating.

The thermal data the X-15 collected was priceless. Engineers learned how heat distributes across a hypersonic airframe, where the hottest spots form, and how different materials and coatings respond. This knowledge fed into later hypersonic research and into the thermal protection of the Space Shuttle.

The Pilots

Twelve men flew the X-15. They included some of the most accomplished test pilots in history: Neil Armstrong, who made his last X-15 flight on 26 July 1962, seven years before he walked on the Moon. Scott Crossfield, who made the first powered X-15 flight. Joe Walker, who set the altitude record. Pete Knight, who set the speed record. And Michael Adams, who died on November 15, 1967, when his X-15 entered a hypersonic spin during re-entry and broke apart. Adams’s death was the programme’s only fatality in 199 flights — a safety record that is remarkable given the extreme conditions. The X-15 operated routinely at the absolute edge of what was physically possible, in an era before fly-by-wire, before digital flight computers, before any of the safety systems that modern test pilots take for granted. The margins were razor-thin. The pilots knew it.

Flight 188: The Day the X-15 Burned

The record flight on 3 October 1967 was not flown in an ordinary X-15. Knight was flying the X-15A-2, serial 56-6671, a rebuilt and stretched version with two external propellant tanks. For its fastest flights NASA covered it in an ablative coating, a pink material called MA-25S topped with a white sealant, designed to char and erode away so the metal underneath stayed cooler. Under the ventral fin hung a dummy ramjet, a full-size shape meant to test how a future air-breathing engine would sit on the aircraft at hypersonic speed. It had no engine inside it.

The flight log records the launch over Mud Lake, Nevada, a 140.7-second engine burn and a peak of Mach 6.70, 4,520 mph, at 102,100 feet. (The Air Force quotes the same flight as Mach 6.72 and 4,534 mph; NASA’s official figures are the lower ones.)

The speed was a success. The heating was not. Shock waves coming off the dummy ramjet hit the ventral fin and the lower fuselage, and NASA says temperatures there exceeded 2,800°F, far beyond what the ablator had been sized for. The heat burned through the coating and into the Inconel X structure underneath, opening holes in the ventral area. Decelerating through Mach 5.5, Knight saw a HOT PEROXIDE warning light come on. The ramjet, its mountings weakened by the heat, separated from the aircraft at around Mach 1 and 32,000 feet over the Edwards range, where it was later found. Knight had tried to jettison it by switch, and had felt nothing: “I did not feel it go at all.”

He brought the heavy, still-fuelled aircraft down safely on Rogers Dry Lake. The engineers who inspected it afterwards were alarmed by what they found. The X-15A-2 was repaired but never flew again, and it now hangs in the National Museum of the United States Air Force in Dayton. On 3 December 1968, President Lyndon B. Johnson presented Knight with the Harmon International Trophy for 1967. Knight later served as mayor of Palmdale and as a California state assemblyman and senator, and he died in 2004.

A Record That Still Stands

Pete Knight’s Mach 6.7 speed record from October 1967 remains unbroken by any piloted, powered, winged aircraft. The SR-71 Blackbird’s official top speed of Mach 3.3 is less than half of what the X-15 achieved. No modern fighter, no experimental aircraft, no hypersonic demonstrator with a human pilot has gone faster. The record has survived because the X-15 occupied a unique niche: fast enough to be a spacecraft, low enough to be an airplane, and crewed by pilots who were willing to ride a rocket into the unknown with nothing but their skill and a pressure suit between them and the void. The three X-15 airframes are now museum pieces. One hangs in the Smithsonian’s National Air and Space Museum in Washington. The second, Pete Knight’s record-setting X-15A-2, is at the National Museum of the United States Air Force in Dayton, Ohio. The third was destroyed in Mike Adams’s fatal flight. But the data they collected flies on — in every spacecraft, every hypersonic vehicle, every piece of thermal protection designed to keep humans alive at impossible speeds. The X-15 did not just set records. It wrote the textbook that made the next fifty years of aerospace possible.

Sources: Dennis R. Jenkins, X-15: Extending the Frontiers of Flight (NASA SP-2007-562), including the program flight log; NASA Armstrong Flight Research Center; Smithsonian National Air and Space Museum; National Museum of the United States Air Force; President Lyndon B. Johnson, remarks presenting the Harmon Trophy (3 December 1968); collectSPACE

Share this story

Frequently Asked Questions

What was the North American X-15?

The North American X-15 was a rocket-powered research aircraft built to explore hypersonic flight at the edge of space. Three were built, and between 1959 and 1968 they flew 199 missions for NASA, the US Air Force and the US Navy. It still holds the speed record for a piloted, powered aircraft.

How fast did the X-15 fly?

The X-15 reached Mach 6.70, or 4,520 mph, flown by Air Force pilot William J. “Pete” Knight in the modified X-15A-2 on 3 October 1967. NASA uses those figures; the Air Force quotes Mach 6.72 and 4,534 mph for the same flight. No piloted, powered aircraft has flown faster since.

What happened to the X-15A-2 on its record flight?

Shock waves from a dummy ramjet mounted under the X-15A-2 concentrated heating on its lower fuselage and ventral fin, where NASA says temperatures exceeded 2,800°F. The heat burned through the ablative coating and holed the Inconel X structure, and the dummy ramjet fell away. Pete Knight landed safely, but the X-15A-2 never flew again.

What altitude did the X-15 reach?

The X-15 reached a maximum altitude of 354,200 feet, about 67 miles, flown by NASA pilot Joe Walker on 22 August 1963. Eight X-15 pilots flew above 50 miles, the US definition of the edge of space, and qualified for astronaut wings.

Did X-15 pilots become astronauts?

Eight X-15 pilots qualified as astronauts by flying above 50 miles: Robert White, Robert Rushworth, Joe Engle, Pete Knight and Michael Adams of the Air Force, and Joe Walker, John McKay and Bill Dana of NASA, whose wings were awarded in 2005. Only Joe Engle flew in space again, commanding the Space Shuttle missions STS-2 and 51-I. Neil Armstrong flew the X-15 but never above 50 miles.

How was the X-15 launched?

The X-15 could not take off on its own. It was carried beneath the wing of a modified B-52 Stratofortress and released at about 45,000 feet, after which the pilot lit its rocket engine. After the engine burned out, the X-15 glided to a landing on a dry lakebed, usually Rogers Dry Lake at Edwards Air Force Base.

What engine powered the X-15?

Most X-15 flights used a single Reaction Motors XLR99 rocket engine producing about 57,000 pounds of thrust, burning anhydrous ammonia and liquid oxygen. Pete Knight’s record flight in 1967 used a burn of 140.7 seconds.

Where are the X-15s today?

X-15 number one, 56-6670, is displayed at the Smithsonian National Air and Space Museum in Washington. The X-15A-2, 56-6671, in which Pete Knight set the speed record, is at the National Museum of the United States Air Force in Dayton, Ohio. The third aircraft was destroyed on 15 November 1967, killing pilot Michael Adams.