On this day in engineering history, the American public
first learned details about the X-15, a rocket-powered, hypersonic, manned
spaceplane that would set unofficial world records for both speed and altitude.
Built by North American Aviation, the X-15 was a joint project of the National
Advisory Committee for Aeronautics (NACA), the United States Air Force (USAF),
and the United States Navy (USN). NACA's successor, the National Aeronautics
and Space Administration (NASA), applied lessons learned from over a decade of X-15
research to the Mercury, Apollo, and Space Shuttle programs.
Designed by Charles Feltz, a North American Aviation
engineer who was regarded as one of the best manufacturing managers in the airframe
business, the X-15 was a single-seat, mid-wing craft that used both conventional
aerodynamic controls and a rocket-based reaction-control system. During flight
in the relatively dense air of Earth's atmosphere, the X-15 used rudder-surface
vertical stabilizers to control yaw, and canted horizontal surfaces on the tail
to control both pitch and roll. During flight in the thinner air outside of
Earth's atmosphere, the X-15 used a specialized reaction-control system. Hydrogen
peroxide rockets on the spaceplane's nose controlled pitch and yaw, while thrust
rockets on the wings offered roll control.
The X-15 was designed to be air-launched from a B-52 bomber flying
at an altitude of 5,000 ft and at speeds greater than 500 mph. Although the spaceplanes
were supposed to be powered by a single 57,000-lb-thurst rocket engine, program
delays at the Reaction Motors Division of Thiokol Chemical forced the first
X-15s to use a pair of XLR-11 engines instead. After the first 30 or so X-15 missions,
however, Thiokol Chemical finished work on the XLR-99, the first large, restartable,
throttleable liquid-propellant rocket engine. With a throttle setting that
could be varied from 50 - 100 % of thrust, the XLR-99 was fueled with two
propellants, liquid oxygen (LOX) and anhydrous ammonia, both of which were fed
via turbine pumps at a flow rate of more than 4,500 kg per minute.
Depending upon the mission's parameters, the XLR-99 provided
thrust for only the first 80 to 120 seconds of flight. The remaining 8 to 12
minutes of the manned missions were unpowered, ending in a 200-mph glide-landing
on a dry lakebed in the California
desert. Capable of speeds above Mach 5, the X-15 had an outer skin made of a special
nickel-chrome alloy (Inconel X) that could withstand extreme aerodynamic
heating. The cabin was made of aluminum and kept isolated for cooling purposes.
Although the X-15 had its share of emergency landing and accidents (including
at least one fatality), almost 200 flights were made with this experimental