The top speed of the X-15 was 4,520 mph (7,274 km/h)

The North American X-15 Top Speed

The North American X-15 was the first operational spaceplane in the world. It still holds the record for the fastest crewed, powered flight in history.

Tobias Holm
Tobias Holm

Table of Contents

The X-15 is a rocked-powered, experimental aircraft made by North American Aviation and operated by the United States Air Force and NASA. When the X-15 went into service, it became the first operational spaceplane in the world.

Propelled by its rocket engine, the X-15 is known for its capability of reaching very high speeds. But what is the North American X-15 top speed?

Worlds Fastest Crewed, Powered Plane

As an experimental, hypersonic aircraft, the X-15 was built with hypersonic speed in mind, and it broke several speed and altitude records in the 1960s. The X-15 reached the edge of space and returned valuable data used for later air- and spacecraft designs.

North American Aviation designed the X-15 to be carried by a Boeing B-52 bomber aircraft that would propel the rocket plane to a speed of around 500 mph (805 km/h) at an altitude of roughly 8.5 miles (13.7 km). From there, the B-52 would drop the X-15, and the rocket engine would power up, shooting the X-15 to hypersonic speeds near the edge of space.

Scott Crossfield, a test pilot for North American Aviation, performed the X-15's first flight (an unpowered glide) on June 8, 1959. Crossfield also flew the X-15's first powered flight on September 17, 1959, marking the rocket plane's official entry into service. During its service life, 12 pilots flew the X-15 on 199 flights. Among the pilots was Neil Armstrong, who later became the first person to walk on the moon.

The X-15 performed its last flight on October 24, 1968, and retired from service. North American built a total of three X-15s.

X-15 Development and Design

In November 1955, North American Aviation was awarded a contract to develop the X-15 airframe. The following year, The United States Air Force made a deal with Reaction Motors to construct the engine.

Instead of taking off like a conventional aircraft, the X-15 was designed to be carried by a Boeing B-52 Stratofortress. At the target altitude, the B-52 would drop-launch the X-15 from under its wings. This saved fuel so the rocket plane could achieve its high speed and altitude targets. The B-52 would release the X-15 at an altitude of about 8.5 miles (13.7 km) at about 500 miles per hour (805 km/h).

At very high altitudes, the air density is too low for normal aerodynamic flight control surfaces (like rudders, ailerons, or elevators) to work. The X-15 had to be maneuverable in these altitudes, so North American equipped it with a reaction control system (RCS) that used small rocket thrusters instead of moveable control surfaces.

Despite the incredible speeds of the X-15, it was equipped with an ejection seat for the pilot to eject in case of an emergency. It was a special ejection seat designed to operate at a speed of up to Mach 4 (4,500 km/h; 2,800 mph) at 120,000 feet (37 km) altitude. Fortunately, the pilots never used the ejection seat during the X-15 program.

While the initial 24 flights of the X-15 were powered by an earlier engine (the Reaction Motors XLR11), the remaining flights were powered by the Reaction Motors XLR99 liquid-fuelled rocket engine. The XLR99 engine burned 15,000 pounds (6,804 kg) of propellant in 80 seconds. The engines used ammonia and liquid oxygen as fuel.

What Is the North American X-15 Top Speed?

The North American X-15 was built for hypersonic speed and everything about the design centered around pushing the rocket plane to speeds never reached before. At the same time, the X-15 program gathered a lot of invaluable data, which formed the basis for many other technological advances.

The top speed of the X-15 was 4,520 mph (7,274 km/h) Mach 6.7. It reached this speed during flight number 188 on October 3, 1967, at an altitude of 19.3 mi (31.1 km). Piloted by William J. "Pete" Knight, this flight established the official world record for the fastest speed ever attained by a powered crewed aircraft. The record remains unbroken to this day.

The record-breaking flight was made in the second of the three X-15s built, the X-15A-2. It also marked the final flight of this aircraft, and the X-15A-2 retired after setting the record.

Wrapping It Up

The North American X-15 is a rocket-powered, experimental aircraft built by North American Aviation in the 1950s. The X-15 holds the record for the fastest ever recorded flight by a crewed, powered aircraft.

During its total of 199 flights, the aircraft broke several speed and altitude records in the 1960s. North American built three X-15s. The plane flew for the first time on June 8, 1959, and the last one retired on October 24, 1968.

The top speed of the X-15 is 4,520 mph (7,274 km/h) Mach 6.7. It set that record on October 3, 1967, at an altitude of 19.3 mi (31.1 km) during flight number 188. This flight also set the record for the fastest crewed, powered flight in history. The record remains to this day.

Another speed record, this time for the fastest air-breathing crewed aircraft (the X-15 did not use air-breathing engines), is held by the iconic Lockheed SR-71 Blackbird used by the United States Air Force. Read more about the SR-71 and its top speed here:

How Fast Is the SR-71? - Planenerd
The iconic Lockheed SR-71 Blackbird was designed for record-breaking speeds but how fast is the SR-71?
Military Aviation

Tobias Holm Twitter

Founder of Planenerd, based in Denmark. Got a LEGO plane as a kid. Obsessed with aviation since. None of my friends want to talk about airplanes.