Breaking The Sound Barrier: NASA’s History Of Supersonic Flight

Explore the groundbreaking world of NASA's experimental aircraft: Bell X1, Bell X2, and X15. Uncover the pioneering feats that shaped supersonic flight
This article is part 1 of 2 in the NASA X-Planes Program serie.
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NASA’s pursuit of supersonic flight has been a cornerstone of aerospace innovation. Among its notable achievements, the Bell X-1 before and the X-15 after stand out as legendary aircraft that pushed the boundaries of human exploration and covered the way for future advancements in high-speed flight. Let’s explore, with this first episode on our blog, the rich history of NASA’s supersonic endeavors and the groundbreaking program of the agency’s X-Planes.

Early history of supersonic flight

In the early years of NASA, researchers and engineers were determined to conquer the challenges of high-speed flight. Through a series of experimental planes, starting from the iconic X-1 and X-2Starbuster, the agency made significant strides in breaking the sound barrier. Both were developed and produced by Bell Industries, in collaboration with NACA (the old NASA), after the second world war. 

NACA hangar in 1962. Credits: NASA
NACA hangar in 1962. Credits: NASA

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The parasite fighter, Bell X-1

Produced after the end of the Second World War to study the transonic flight regime, the Bell rapidity contributed greatly to laying the foundations for the future US space program of the 1960s. The Bell X-1 was characterized by a particularly shaped fuselage bullet line and the wings were traditionally shaped. The aircraft was equipped with landing gear but was primarily designed for launch from a mother plane (a modified B-29 or B-50 Superfortress bomber), earning it the nickname parasite fighter

Bell X-1 in flight moments after release from Boeing 50 Superfortress mothership. Credits: National Air and Space museum
Bell X-1 in flight moments after release from Boeing 50 Superfortress mothership. Credits: National Air and Space Museum

The prototype was built in 6 models, each profoundly modified from the previous one to research specific flight elements. The 1947 flight was historic as for the first time the Bell X-1. With test pilot Chuck Yeager at the controls, it broke the sound barrier.

Bell X1 during a test fly. The aircraft was the first to reach supersonic speeds. Credits: NASA
Bell X1 during a test flight. Credits: NASA

Even faster, X-2 “Starbuster”

Also developed by Bell in the 1950s, the X-2 went beyond the legacy of its predecessor by exceeding the speed of sound, studying flight between Mach 2 and Mach 3. The aim of the project was mainly dedicated to the development of an extraordinary aircraft equipped with a strong swept wing, guided by studies merely of an aerodynamic nature. These wings, although not very efficient at low speeds, were particularly stable and maneuverable once they exceeded the sound barrier.

Bell X2 on the ground before take off. Credits: NASA
Bell X2 on the ground before take off. Credits: NASA

Unfortunately, the development of the X2 was also characterized by two serious accidents which cost the lives of the two test pilots, Jean “Skip” Ziegler and Milburn G. Apt respectively. The first accident occurred while the plane was still attached to the Boeing B-50: due to a defect in the engine’s liquid oxygen supply system, an explosion occurred, killing both Ziegler and B-50 crew member Frank Wolko. 

Subsequently, on 27 September 1956, the record speed of 3,393 km/h (Mach 3.2) was recorded for the first time at an altitude of 19,800 m, thus setting the speed record. However, the flight turned out to be tragic: the pilot was unable to control the aircraft, failing to jump from the cockpit and dying on impact.

Bell X2 during a test fly when we can see three airbrake under the plane. Credits: NASA
Bell X2 during a test fly when we can see three airbrake under the plane. Credits: NASA

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The X-15 Program

The X-15, developed in the 1950s, was a rocket-powered liquid-fuel aircraft designed to fly at hypersonic speeds and altitudes reaching the edge of space. It was a joint effort between NASA, the U.S. Air Force, and North American Aviation. With its innovative design and advanced technologies, the X-15 became an iconic symbol of pioneering aeronautics.

Three aircraft were built, which performed a total of 199 flights between 1959 and 1968. Altogether 12 pilots flew the X-15 over the years, including Neil Armstrong before being selected by NASA for the Mercury space program and later for the Apollo space program.

Neil Armstrong during his period as test pilot of X15. Credits: NASA

Tutorial: How to set records

The X-15 set numerous records during its operational years from 1959 to 1968. Three aircraft were built in these years, which performed a total of 199 flights in all its career. Altogether 12 pilots flew the X-15. All of them choose from a strict group of highly skilled test pilots. 

With Flight 90 and 91, pilot Joseph A. Walker set height records for altitude reaching 105.9 and 107.8 km respectively. In this way, having exceeded the 100 km altitude set for space flights, Walker became the first astronaut to perform a suborbital flight with a reusable vehicle, as well as the first man to go into space twice. During Flight 188, pilot William J. Knight brought the plane at speeds over Mach 6 (7 274 km/h). These achievements provided invaluable data on aerodynamics, materials, and human factors in extreme flight conditions.

X15 engineering project showing all internal components. Credits: NASA
X15 engineering project showing all internal components. Credits: NASA

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Scientific contributions

Beyond its speed and altitude records, the X-15 contributed significantly to scientific understanding. Its flights gathered data on aerodynamic heating, reentry physics, and the effects of high-speed flight on humans. This knowledge proved instrumental in the development of future spacecraft and supersonic aircraft.

X15 test plane in trans sonic fly. Credits: NASA
X15 test plane in transonic flight. Credits: NASA

Legacy and Impact

The X-15 program’s legacy is immeasurable. Its groundbreaking research and technological advancements directly influenced the design and development of future projects such as the Space Shuttle, the X-30 National Aero-Space Plane (NASP), and other high-speed aircraft like the most recent X-59. The X-15‘s success opened new frontiers for space exploration and laid the foundation for subsequent NASA missions.

X15 on the ground before take off. Credits: NASA
X15 on the ground before take off. Credits: NASA

NASA’s supersonic flight history and the X-plane program represent a relentless pursuit of pushing the limits of human flight. All the early projects and especially the X-15’s achievements, including record-breaking speeds and altitudes, as well as its scientific contributions, have left an indelible mark on aerospace research. By daring to defy the boundaries of speed and altitude, NASA has propelled the field of high-speed flight forward and continues to inspire future generations of explorers, engineers, and aviators.


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Federico Coppola

Federico Coppola

A third-year student of Histoy and Italian Modern literature at Federico II in Naples, passionate about space, writing, and with an incurable dream of flying up through the clouds to reach the stars.
Admin of the Instagram page Italian_space_meme

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