The North American X-15, most commonly known as the X-15, is a hypersonic, rocket-powered aircraft and undoubtedly one of the most dangerous and ambitious machines ever piloted by man. It has been operated by the United States Airforce and NASA (National Aeronautics and Space Administration) in their series of experimental aircraft.
The X-15 set the altitude and speed records in the 1960s and even reached the edge of space. At its fastest and highest, it traveled at 4,520 miles per hour (7,274 kmh), almost seven times the speed of sound.
To reach this speed it flew at a height of 102,100 feet (31,120 m, or about 19.3 miles up). It was an official world record for the highest speed ever recorded by a powered and crewed aircraft. This record still stands today.
Since the beginning of the program, the 3 X-15s that were built had 12 pilots and were flown approximately 199 times. 8 pilots flew a combined 13 flights which were technically classed as space flights under the US definition, making those pilots technically astronauts.
Origin and Design
The development of the X-15 has origins in engineers who simply wanted to make planes go faster. Scientists in the 1930s discovered that planes powered by piston engines and propellers ran into problems at around the 350 miles per hour (560 kmh) mark.
This indicated that higher speeds would cause many more problems. The research was directed at dealing with those problems. Thus, the X-planes and research began.
The X-1 program first looked at these problems and in 1947 Chuck Yeager piloted the X-1 to faster than the speed of sound. This led to thoughts about hypersonic flight. It was in the 1950s that these efforts became the X-15 program.
It was led by the National Advisory Committee on Aeronautics (NACA) which was the predecessor to NASA, alongside the United States Air Force and Navy. The project was based on a concept study from Walter Dornberger, building on his hypersonic research.
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The prototype for the X-15 was built in 1955 by North American Aviation, which was responsible for the airframe, and Reaction Motors were responsible for building the engines. The plane, with its rockets, did not need air for its propulsion and so could fly higher, which in turn would permit it to fly faster in the thin atmosphere.
The X-15 was designed to be carried to altitude by another plane, and drop-launched from the mothership. These ships became named the Challenger and the High and Mighty One. The plan was to release the X-15 at an altitude of around 8.5 miles (13.6 km) and at a speed of 500 miles per hour (805 kmh).
To achieve this performance, the X-15 looked very different to most contemporary planes. The X-15 had a fuselage that was cylindrical and long and had rear fairings that made the plan flatter and more aerodynamic. Additionally, part of the fuselage was heat resistant, to shield the plane from damage at the speeds it would be flying.
Blue Sky Thinking
The X-15 came from developmental research, and so many suggestions were tabled about how to control the plane. Many different models and programs were used, and it was also put under the test in multiple different scenarios.
This included how the plane would launch, drop, start, and accelerate speeds, re-enter into the atmosphere, and land without a main engine. The main engine was only expected to work for a short part of the flight but was used to dramatically boost the speed and altitude of the X-15.
Without this, the plane would be unable to maintain altitude. It also had multiple different systems to control it at the wildly different speeds at which it would be flying.
Before 1958, the United States Air Force and NACA discussed the idea of a spaceplane that could do an orbital flight. The original plan was the X-15B would launch from the SM-64 Navaho missile. However, this was canceled when the NACA transformed into NASA and changed its focus to Project Mercury and the early space race.
In 1959, there was a space glider program developed and was set to become the United States Air Force’s preferred method for launching a crewed spacecraft into orbit. Though this did not last long, and it was canceled in the early 1960s before a prototype could be built.
The first X-15 flight was an unpowered gliding flight that Scott Crossfield piloted on the 8th of June 1959. Scott would later pilot the first powered flight on the 17th of September that year.
Over the next year, 12 different test pilots flew the X-15. This included Neil Armstrong, the first man on the Moon, and Joe Engle who became a commander of the NASA Space Shuttle missions.
In 1967, the US Air Force test pilot Major Michael J. Adams was unfortunately killed during the X-15 Flight 191 when the plane entered a hypersonic spin upon its descent , violently turning in the air. The airframe broke apart at 60,000 feet (18,300 m). He was later commemorated by a monument in 2004 near his hometown in Johannesburg, California.
During 13 of the total 199 flights of the X-15, eight pilots were able to get above 264,000 feet (80,000 m) which qualified them for recognition of the status as astronauts. Those who belonged to the US Air Force all were awarded military astronaut wings.
However, these were measured using the US definition of space, which was substantially lower than the altitude accepted internationally. Of these 13 flights, only flights 90 and 91 exceeded the internationally recognized line denoting the edge of space known as the Karman line. Both of these flights were piloted by Joseph A. Walker.
The fastest recorded flights were recorded by William J. Pete Knight who managed to travel 4,520 miles per hour (7,274 kmh) and 4,250 miles per hour (6,839 kmh) on flights 188 and 175 respectively. These flights took place in 1967.
The next closest was flight 59 flown again by Joseph A. Walker. Although Walker’s flight approached the same speed, his took place at 23.4 miles (37.6 km) altitude whereas Knight’s flights took place at 19.2 miles and 18.7 miles (30.8 km and 30.1 km) altitude.
The X-15 is an incredible historical legacy for both NASA and the US Air Force. But it is also so much more than this. The program has since formed the foundation of NASA’s New Aviation Horizons which is a variety of new experimental aircraft.
It is a program that hopes to carry on the legacy of displaying advanced technology and pushing the boundaries of aviation. Their goals are hoping to show how airlines can burn less fuel and 75% less pollution. This could be used in commercial flights in the future as well as supersonic flying experiments.
Top Image: The X-15 flew higher and faster than any other plane in history. Source: US Air Force / Public Domain.
By Kurt Readman