AIR & SPACE MAGAZINE

X-15 Walkaround

A short guide to the fastest airplane ever.

November 2007

Still the fastest airplane ever flown, the North American X-15 earned its title 40 years ago, when on October 3, 1967 Air Force Major William “Pete” Knight flew the rocket-powered aircraft to 4,520 mph, Mach 6.72. It was built to find out how aircraft structures, materials, and control surfaces would perform at hypersonic speeds and very high altitudes. In 199 research flights, the X-15 provided that information and more. The program has been acknowledged as the most successful flight research program in history, and it helped make human spaceflight possible.

X-15 flights were short; each lasted about ten minutes. So that the rocketplane could use all its fuel for acceleration, it was carried beneath the wing of a NASA B-52 to 45,000 feet, where it was dropped. NASA test pilot Milt Thompson remembered the experience in his book At the Edge of Space: “[The launch] was a surprise no matter how many times I went through it. It felt as if the X-15 exploded off the hooks.…

“The pilot did not have much time to waste after launch. He either had to get the engine lit or abort the flight and make a landing at the launch lake. The problem was that he was losing altitude rapidly (about 12,000 feet per minute) while waiting for the engine to light.”

The X-15 that hangs in the Smithsonian Institution's National Air Space Museum is the first of three built by North American Aviation. It was rolled out on October 15, 1958, 15 days after its original sponsor, the National Advisory Committee for Aeronautics, became NASA. Half of its 51-foot fuselage is devoted to propellant tanks for its rocket motor. X-15 number 56-6670 flew 81 missions, including the last eight of the program. It reached a speed of 4,104 mph (Mach 6.06), and an altitude of 266,500 feet. Eric Long/NASM

Inconel X, a ferociously strong nickel alloy, gives the X-15 its gun-metal black color. Inconel was chosen for the airplane's skin because it retained its strength up to 1,2oo degrees Fahrenheit, a temperature the X-15 would routinely experience at high speeds. Eric Long/NASM

Eight small reaction control rockets located at the nose of the aircraft were used to control the attitude of the X-15 when aerodynamic control surfaces became ineffective in the thin air at high altitudes. The thrusters at the nose controlled pitch and yaw. Two 40-pound-thrust rockets were located at each wing tip for roll control. Eric Long/NASM

The rockets circling the X-15's nose produced 113 pounds of thrust, enough power to alter the attitude of the airplane, but not its course. A nose-right command from the pilot, accomplished by moving a hand controller in the cockpit to the right, caused the two thrusters on the left side of the nose to fire. Eric Long/NASM

The 60,000-pound thrust XLR99 rocket engine was the first large rocket motor reliable enough to power a manned craft. Its fuel was anhydrous ammonia, a cryogenic liquid with a boiling point of -33 degrees Fahrenheit. Before the cold ammonia was mixed with liquid oxygen for combustion, it was circulated through small-diameter tubes in the wall of the exhaust nozzle to cool the structure, which was subjected to temperatures of up to 5,000 degrees Fahrenheit. Eric Long/NASM

After early flights showed that the X-15 windshield was subjected to higher temperatures than expected (1000 degrees Fahrenheit, instead of 750), the outer pane of glass was changed to a stronger alumino-silicate glass. The windshield had double inner panes of less heat-resistant soda-lime tempered glass. Twice an outer pane shattered during missions; once when a soda-lime glass pane was mistakenly installed as an outer pane, and once when a retainer frame buckled and created a hot spot. Eric Long/NASM

Built by the Northrop Aircraft Corporation, the X-15's ball nose was an attitude sensing device, an Inconel sphere with small orifices that could measure the airplane's angle of attack and sideslip. Both measurements record the attitude of the aircraftup and down and side to side, respectivelyrelative to the flow of air. Traditional vane-type sensors could not have survived the temperatures during high-speed flights. Eric Long/NASM

Joe Walker reached 354,200 feet, an altitude not exceeded until 2004, when Brian Binnie reached 367,442 feet in SpaceShipOne. The pilots would aim for an altitude but were rarely able to predict exactly how high they would end up. The X-15 climbed at 4,000 feet per second, so if the pilot was even one second late (or early) in shutting down the engine, he would miss the mark by 4,000 feet.

The powered portion of an X-15 flight lasted approximately 85 seconds. Each mission ended in an unpowered landing after a steep descent. The pilots flew a low-lift glide that helped develop the energy management techniques space shuttle pilots would use many years later. Only one X-15 pilot did not make it back for a landing. Air Force Major Michael Adams was killed in 1967 when the aircraft broke apart in a hypersonic spin shortly after reentry. The program ended the following year.