the X-15 and hypersonics
flown in 1959 from the NASA High Speed Flight Station (later renamed the
Dryden Flight Research Centre),
the rocket-powered X-15 was developed to provide data on aerodynamics,
structures, flight controls,
and the physiological aspects of high speed, high-altitude flight.
Three were built by North American Aviation for NASA and the U.S. Air
The 1950s was definitely the decade of
speed as far as U.S. aeronautical research was concerned—the U.S. Air
Force and the National Advisory Committee for Aeronautics (NACA) were
convinced that the key to dominating the skies was to fly faster than the
opponent. The X-1 experimental aircraft broke the sound barrier in 1947.
The Navy D-558-II ("D-558 Dash Two") reached Mach 2 on November 20, 1953.
Soon other aircraft were reaching Mach 2.44 (1,650 miles per hour, or
2,655 kilometres per hour) and Mach 3.196 (2,094 miles per hour, or 3,370
kilometres per hour). These high speeds presented new challenges to
People who study how air moves around an
aircraft are called aerodynamicists. Although one of their most useful
tools for decades was the wind tunnel, they could not always provide the
kind of data that they needed. By the 1950s, there was virtually no way to
simulate with a wind tunnel how air flowed around an aircraft at many
times the speed of sound. Aerodynamicists had theoretical models (in this
case a "model" is a set of equations that predict how certain shapes will
act at certain airspeeds), but in order to confirm the models, they would
have to actually fly an aircraft at that speed.
waves festoon a small scale model of the X-15 in NASA's Langley Research
Centre's 4 x 4 Supersonic Pressure Tunnel.
In 1952, the NACA established a goal of
conducting research on aircraft capable of flying at speeds between Mach 4
and Mach 10 and at altitudes between 12 and 50 miles (19 and 80 kilometres).
This speed range was called "hypersonic." On September 30, 1955, North
American Aviation was awarded a contract to develop an aircraft to conduct
this research. The aircraft was designated the X-15. The X-15 developed
numerous technologies associated with high-speed flight. These
technologies were later incorporated into aviation, missile, and space
programs. Of all the X-plane programs (and there have been dozens), the
X-15 is generally considered the most successful because it flew the
longest and greatly expanded the boundaries of flight research.
The X-15 had a long fuselage with short
stubby wings and an unusual tail configuration. A Reaction Motors, Inc.
XLR99 rocket engine generating 57,000 pounds (253,549 Newtons) of thrust
powered the aircraft. This engine used ammonia and liquid oxygen for
propellant and hydrogen peroxide to drive the high-speed turbopump that
pumped fuel into the engine. This rocket could be throttled like an
airplane engine and was the first such throttleable engine that was
"man-rated" or declared safe to operate with a human aboard.
Because the X-15 would operate in
extremely thin air at high altitudes, conventional mechanisms for
controlling the aircraft were not sufficient, and the aircraft was
equipped with small rocket engines in its nose for steering. This was the
first aircraft to use such a steering method, although it was also in
development for the Mercury spacecraft at the same time.
three X-15 rocket-powered research aircraft is being carried aloft under
the wing of its B-52 mothership.
The X-15 was air launched from the B-52 so the rocket plane would have
enough fuel to reach its high speed and altitude test points.
The X-15 designers anticipated that
their biggest problem would be the intense heat that the aircraft would
encounter due to the friction of air over its skin. The upper fuselage
would reach temperatures over 460 degrees Fahrenheit (F) (238 degrees
Celsius [C]). But other parts of the aircraft would reach temperatures of
a whopping 1,230 degrees F (666 degrees C) and the nose would reach a
temperature of 1,240 degrees F (671 degrees C). Designers chose to use a
high-temperature alloy known as Inconel X, which unlike most materials,
remained strong at high temperatures. It was a difficult material to work
with. The wings of the X-15 were constructed of Inconel X skins over
titanium frames and were bolted to the fuselage instead of being mounted
to a main spar as was customary.
November 9, 1962, an engine failure forced Jack McKay, a NASA research
to make an emergency landing at Mud Lake Nevada, in his X-15 aircraft.
The aircraft's landing gear collapsed and the X-15 flipped over on its
McKay was promptly rescued by an Air Forced medical team and eventually
recovered to fly the X-15 again.
The X-15 first flew on June 8, 1959, on
a glide flight. It was dropped from under the wing of a specially modified
B-52 "mothership." The first powered flight took place on September 17.
Once the X-15 fell clear of the B-52, pilot Scott Crossfield ignited the
rocket engine and flew to a relatively pokey Mach .79. But the X-15 was
soon travelling many times the speed of sound. The X-15 continued flying
until October 24, 1968, making 199 total flights among three aircraft and
establishing many records.
During its early years of flight, the
X-15 confirmed the hypersonic models developed by U.S. aerodynamicists.
These models were later used to design other missiles and spacecraft, such
as the Space Shuttle.
Because of its ability to reach such
high speeds and altitudes, the X-15 was a useful test platform for other
research experiments. After its initial test flights it began carrying
micrometeorite collection pods and ablative heat shield samples for the
Apollo program and various other experiments. For approximately the last
six years of its operation, the X-15 was not really conducting the
missions of an X-plane (expanding the frontiers of flight), but was
supporting all kinds of technology programs that required its high speed.
X-15 was configured with a mammoth XLR99 rocket engine providing 57,000
pounds of thrust.
The airplane's skin surfaces were fabricated from a special chrome-nickel
would enable it to withstand the searing 1200-degree Fahrenheit
temperatures predicted in the hypersonic flight environment.
The X-15 pioneered the use of various
materials for high-speed aircraft and spacecraft, as well as the
techniques to construct them. Its rocket engine was also important for the
development of later rocket engines, such as the Space Shuttle Main
Engine. Inconel X was used for some key parts of the Space Shuttle
The X-15 was also the first aircraft to
make extensive use of a "man-in-loop" simulator to explore how the
aircraft would perform in flight. A pilot would sit in the simulator on
the ground and practice his procedures and try to determine what the plane
would do when he later flew it. This was a new use for simulators and is
now common in all experimental programs. Today, long before an aircraft
begins flying, pilots and engineers are using simulators to evaluate its
flying characteristics on the ground.
The X-15 is widely considered by many
aerospace engineers to be the most successful experimental aircraft ever
built. Of the two remaining X-15s, one is in the National Air and Space
Museum in Washington, D.C., and the other is in the Air Force Museum in