The Complete Book of Spaceflight: From Apollo 1 to Zero Gravity

X planes
U.S. experimental aircraft designed to answer fundamental
questions about the behavior of aircraft close to, at, or
beyond the speed of sound, and as prototypes for
advanced aerospace vehicles. Most of these planes have
been flown from Muroc Air Field, later named Edwards
Air Force Base.
X-1
The first of the X planes, based on the shape of a .22 caliber
bullet, with revolutionary thin, straight wings. The
rocket-powered Bell X-1 was carried to a height of about
12,200 m under the belly of a Boeing B-29 Superfortress
before being released. Its prime mission was to break the
sound barrier, a feat accomplished for the first time on
October 14, 1947, when Charles “Chuck” Yeager accelerated
his X-1 (christened by him “Glamorous Glennis,”
after his wife) to Mach 1.06 (1,130 km/hr). The X-1 program
was the Air Force’s first foray into experimental
flight research and its first collaborative effort with
NACA. Three X-1s were built, and they completed 157
flights between January 1946 and October 1951. 251
X-2
A swept-wing, rocket plane designed to explore flight at
high supersonic speeds and be the first aircraft to take
man to the edge of space. The Bell X-2 was made of stainless
steel and copper-nickel alloy and was powered by a
two-chamber Curtiss-Wright XLR25 throttleable rocket
engine. Following launch from a B-50 bomber, Bell test
pilot Jean “Skip” Ziegler completed the first unpowered
glide flight on June 27, 1952. This vehicle was subsequently
lost in an explosion during a captive flight in
1953. Lt. Col. Frank “Pete” Everest completed the first
powered flight in a second X-2 on November 18, 1955,
and on his ninth and final flight in late July 1956 he
established a new speed record of Mach 2.87. At high
speeds, Everest reported that the X-2’s controls were only
marginally effective, and data from his flights combined
with simulation and wind tunnel studies suggested that
the aircraft would encounter very severe stability problems
as it approached Mach 3. A pair of young test pilots,
Captains Iven Kincheloe and Milburn “Mel” Apt, were
assigned the job of further expanding the envelope, and
on September 7, 1956, Kincheloe became the first pilot
ever to climb above 100,000 ft (30,500 m), reaching a
X
488
peak altitude of 38,500 m. Twenty days later, Apt made
his first X-2 flight under instructions to follow the “optimum
maximum energy flight path” and avoid any rapid
maneuvers above Mach 2.7. Flying a precise profile, he
became the first human to exceed Mach 3, accelerating to
Mach 3.2 (3,369 km/hr) at 20,000 m. The flight had been
flawless to this point, but shortly after reaching top
speed, Apt attempted a banking turn while still above
Mach 3. The X-2 tumbled violently out of control and
Apt found himself struggling with the same problem of
inertia coupling (the loss of stability at high speeds) that
Yeager had experienced in the X-1 nearly three years
before. Unlike Yeager, however, Apt was unable to
recover, and both he and the aircraft were lost. While the
X-2 had supplied valuable research data on high-speed
aerodynamic heat buildup and extreme high-altitude
flight conditions, this tragic event ended the program
before NACA could start detailed research with the
plane. Finding answers to many of the riddles of high-
Mach flight had to be postponed until the arrival, three
years later, of the most ambitious of all rocket planes: the
X-15. 92
X-15
A rocket plane that set aircraft speed and altitude records
that still stand today. First flown on June 8, 1959, the
X-15 was used to provide data on thermal heating, control,
and stability at extremely high speeds, and on
atmospheric reentry. It was made primarily from titanium
and stainless steel covered with Inconel X nickel, an
alloy that can withstand temperatures up to 650°C. To
sustain even higher temperatures, the X-15 was often covered
with a pink ablative material (MA-25S) that could
boil away, carrying the heat with it. The plane was
dropped from a B-52 bomber at an altitude of 13,700 m
and then ignited its Reaction Motors XLR99-RM-2 throttleable
liquid propellant (liquid hydrogen and anhydrous
ammonia) engine. The rear tail was movable and could
be pivoted for control at altitudes where the air was sufficiently
thick. At greater (nonatmospheric) heights, control
was provided by 12 hydrogen peroxide jets—four in
the wingtips and eight in the nose. The plane was piloted
following a predetermined flight path, and came down
on Rogers dry lakebed using unique landing gear. Just
before landing, the lower half of the bottom tail section
was jettisoned, and two landing skids deployed, together