This story originally appeared on The High Frontier and is republished here with permission.
By any measure the North American X-15 was an amazing aircraft. By the end of the decade long, 199 flight programme the three aircraft had pushed airspeed and altitude records way beyond all previous marks. Many X-15 pilots qualified astronauts on their high altitude flights and the wealth of operational knowledge that was gained continues to influence aerospace programmes to this day.
Yet for all this, the X-15 is often overshadowed by NASA's other activities during the 1960s and its legacy overlooked. It could never go as high or as fast as the capsules launched from the Cape, but the fact remains, at the time it was designed the X-15 looked like it would provide America's first forays in human spaceflight and as Tom Wolfe points out in The Right Stuff, they would FLY their vehicle there and back.
Mach 1 and beyond
The story of the X-15 really starts as an extension of the high speed research programs being carried out by the NACA, Air Force and Navy beginning at the end of World War 2. Following the advent of effective liquid-fuelled rocket propulsion, the development of the jet engine and advances in aerodynamics during that conflict, it became clear that aviation would be pushing into new flight regimes. Famously the Bell X-1 (originally the XS-1) marked the first in a long line of experimental aircraft constructed to explore these new regimes and gather data which could later be applied to the design of operational aircraft.
Although research aircraft of the time were not exclusively concerned with high speed flight, many of the early X-planes such as the X-1 series and their Navy equivalents the Douglas Skystreak (D-558-1) and Skyrocket (D-558-2) were designed to examine the transonic and supersonic region. These designs tended to be purely functional and highly conservative serving to access a flight regime and gather the data safely while offering high margins of structural strength to cope with any unknowns that may be encountered. They weren't generally designed to act as prototypes for future operational types – their role was to gather data and extend knowledge. Very rapidly though, the contemporary fighter types of the late 1940s and early 1950s began to match and often exceed the performance of the early research aircraft calling their practicality into question, but while designs such as Kelly Johnson's F-104 Starfighter could now reach speeds in excess of Mach 2 on a routine basis, it was recognised that it would take a rocket powered research craft to probe speeds above Mach 3 and altitudes in excess of 100,000ft.
The much delayed Bell X-2 was anticipated to provide valuable data in these areas, but this troubled aircraft didn't manage to make its first powered flight until 1955. On September 7th 1956, Air Force pilot Iven Kincheloe took the X-2 to a new altitude record of 126,200ft and it looked like the X-2 might start to make good on its promise, but on the very next flight just 20 days later, Milburn Apt was killed after reaching a speed in excess of Mach 3. Apt had fallen victim to a high speed aerodynamic phenomenon known as inertia coupling, where the aircraft's control surfaces lose their ability to counteract the inertia of the fuselage resulting in a loss of control and tumbling in all three axis. Chuck Yeager had managed to survive an encounter with inertia coupling after he exceeded Mach 2.5 in the X-1A, but although Apt was able to recover control and separate the X-2s escape capsule, he became incapacitated and was unable to parachute to safety.
Although the X-2 had provided some data on high altitude flight and aerodynamic heating it was clear that much work remained for the next planned research vehicle.