Built to test aerodynamics at hypersonic speed, the X-15 had more in common with a rocket than a conventional aircraft, making it extremely difficult to fly and land.

The X-15 remains one of the most remarkable experimental aircraft ever built, nearly 70 years after its first flight. Completing 199 flights between 1959 and 1968, the X-15 was a monumental success—reaching Mach 6.70 (roughly 4,520 miles per hour) and 354,200 feet altitude, to establish the program’s speed and altitude records.

At those extremes, the X-15 encountered problems conventional aircraft obviously did not: things like hypersonic heating, rocket-engine hazards, underpowered landings, and the fact that aerodynamic controls became useless in near-space. Yet, despite operating at the edges of human engineering performance, the program suffered only one fatal flight, though there were a handful of close calls.

About the X-15 Rocket Plane

• Year Introduced: 1959
• Number Built: 3
• Length: 49 ft 2 in (15 m)
• Wingspan: 22 ft 4 in (6.81 m)
• Weight (Gross): 33,500 lb (15,195 kg)
• Propulsion: One Reaction Motors XLR99-RM-2 liquid-fueled rocket engine; 70,400 lb thrust
• Top Speed: ~4,520 mph (7,270 km/h); approx. Mach 6.7’
• Range: ~280 mi (450 km)
• Service Ceiling: ~354,330 ft (108,000 m)
• Loadout: None
• Aircrew: 1

A consortium of US government agencies—NASA, the US Navy, and the US Air Force—launched the X-15 program in the late 1950s in order to determine flight characteristics at extremely high speeds and altitudes, effectively helping to bridge the gap between aircraft and spacecraft. The program was fundamentally about research, not military achievement, and the X-15 had virtually no value as a military aircraft. However, it could reach truly astonishing speeds; the fastest recorded X-15 flight took place on October 3, 1967, when test pilot William Knight flew at a speed of Mach 6.7, a speed record that remains in place as the fastest speed ever recorded by a manned and powered aircraft. Like the earlier X-1—the first aircraft to ever break the sound barrier—the X-15 had no launch capabilities of its own; it was carried under a B-52 Stratofortress “mothership” and dropped at altitude, saving it valuable fuel and allowing it to start its flights from a high speed and altitude.

Three X-15s were built in total, and flew from 1959 until the program’s closure in 1968, amid budget cuts and the sense that it had achieved its scientific goals. In total, the three aircraft completed 199 successful test flights during that nine-year period. Two of the aircraft survive to the present day; one is housed at the National Air and Space Museum in Washington, DC, the other at the National Museum of the Air Force in Ohio.

The X-15’s Major Problem: It’s Hard to Land a Rocket

On November 5, 1959, test pilot Scott Crossfield suffered an engine-related emergency shortly after his drop from a B-52 near Edwards Air Force Base in California, forcing him to make an emergency landing with the X-15 still loaded with fuel and consequently still heavy. Landing on nearby Rosamond Dry Lake, the X-15 suffered severe structural damage, breaking its back behind the cockpit. Crossfield was fortunate enough to walk away, and the aircraft was later repaired and returned to operational status. 

The Crossfield incident highlighted how sketchy the X-15’s landings were, even under the best of circumstances. The basic problem was that the X-15 had more in common with a rocket than an aircraft; it was powered by rocket fuel that burned for only 80 to 120 seconds, depending on the mission profile. After that, there was no more propulsion available. That meant that pilots couldn’t miss their approach and fly a go-around, as an aircraft could. Once they were on a path to land, they were going to land, for better or worse.

As a result of these conditions, pilots had to meticulously manage their energy while gliding towards a dry lakebed. (Fortunately, the aircraft was tested in the arid Mojave Desert in California and Nevada, where there were many such lakebeds available.) Touchdown occurred at roughly 200 miles per hour. And before landing, the X-15’s enormous ventral fin had to be jettisoned because the thing extended below the rear skids.

Aerodynamic heating was another dangerous condition of the program’s routine operations. Mach 5–6 flight generates tremendous aerodynamic heating. The aluminum construction used on conventional aircraft wouldn’t have been able to handle such heat. Engineers therefore relied on “Inconel X,” a superalloy that could maintain strength at high temperatures. But the realities of high-heat operations remained; the X-15 was forced to confront material expansion, structural dimension changes, seal and joint stress, and temperature variance across the airframe—all potentially dangerous conditions.

An X-15 Killed Its Pilot in 1967

The program’s only fatality took place on November 15, 1967, with Air Force Major Michael Adams. After climbing from the initial drop altitude of around 30,000 feet to 266,000 feet, Adams experienced electrical and control difficulties, leading to pilot disorientation that resulted in the aircraft deviating from its intended heading. The X-15 entered a hypersonic spin during descent. Remarkably, Adams managed to recover from the spin—but the aircraft then entered severe oscillations while descending at high speeds. At Mach 3.9, the aerodynamic loads exceeded the X-15’s structural limit, breaking the airframe apart roughly 60,000 feet in the air and killing Adams.

Danger was inherent to the X-15 program. Put simply, the program existed because engineers didn’t understand hypersonic flight. The X-15 was built to test those boundaries, to generate knowledge of hypersonic aerodynamics, thermal protection, reaction controls, high-altitude physiology, reentry, and flight-control systems. The unknowns were vast, and the margins were thin. But the knowledge gained fed directly into American spacecraft and high-speed aircraft development.

About the Author: Harrison Kass

Harrison Kass is a writer and attorney focused on national security, technology, and political culture. His work has appeared in Tablet, City Journal, The Hill, The Spectator, and The Cipher Brief. He holds a JD from the University of Oregon and a master’s in Global & Joint Program Studies from NYU. More at harrisonkass.com.

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