Hello everyone, welcome back again with us at Military TV. In this episode, we are going to talk
about “X-15: the fastest manned rocket plane ever” The North American X-15 was a plane unlike any
other. Its shaped more like a bullet than a conventional airplane. The aircraft was billed
as the U.S.’ first crewed attempt to ply the outer reaches of Earth’s atmosphere and beyond.
Even though most Americans alive today have likely never heard of it, it marked 61 years since the
X-15’s first powered test flight in California’s high desert. X-15 is rocket-powered research
aircraft built in the 1950s by North American Aviation, Inc., for the U.S military and the
National Aeronautics and Space Administration in order to gather information on flight
conditions beyond the atmosphere. The aircraft was built to find out how aircraft structures,
materials, and control surfaces would perform at hypersonic speeds and very high altitudes.
First flown in 1959, the X-15 set separate unofficial altitude and speed records for
aircraft during the 1960s almost 108 km above the surface of Earth and 6.7 times the speed of sound.
Launched from an in-flight B-52 bomber, the X-15 could not achieve the velocity
and altitude needed for orbital flight. Nevertheless, in 199 flights over a 9-year
period, the aircraft established an extensive database on transonic and supersonic flights
and revealed vital information concerning the upper atmosphere. Of the 12 pilots who flew X-15s,
8 became astronauts including Neil Armstrong. The program has been acknowledged as the most
successful flight research program in history, and it helped make human spaceflight possible.
In terms of design and development, The X-15 was based on a concept study from Walter
Dornberger for the National Advisory Committee for Aeronautics for a hypersonic research aircraft.
The requests for proposal were published on 30 December 1954 for the airframe and
on 4 February 1955 for the rocket engine. The X-15 was built by two manufacturers:
North American Aviation was contracted for the airframe in November 1955,
and Reaction Motors was contracted for building the engines in 1956.
Like many X-series aircraft, the X-15 was designed to be carried aloft and drop launched from under
the wing of a B-52 mother ship. Air Force NB-52A, "The High and Mighty One" (serial 52-0003),
and NB-52B, "The Challenger" (serial 52-0008, also known as Balls 8) served as carrier planes for all
X-15 flights. Release of the X-15 from NB-52A took place at an altitude of about 8.5 miles
and a speed of about 500 miles per hour. Moreover, the X-15 fuselage was long and cylindrical, with
rear fairings that flattened its appearance, and thick, dorsal and ventral wedge-fin stabilizers.
Parts of the fuselage (the outer skin) were heat-resistant nickel alloy,
(Inconel-X 750). The retractable landing gear comprised a nose-wheel carriage and two rear
skids. The skids did not extend beyond the ventral fin, which required the pilot to
jettison the lower fin just before landing. The lower fin was recovered by parachute.
Before we discuss aircraft propulsion, let’s have a look at the cockpit and pilot systems.
Actually, the X-15 was operated under several different scenarios, including attachment to
a launch aircraft, drop, main engine start and acceleration, ballistic flight into thin
air/space, re-entry into thicker air, unpowered glide to landing, and direct landing without a
main-engine start. The main rocket engine operated only for a relatively short part of the flight but
boosted the X-15 to its high speeds and altitudes. Without main engine thrust, the X-15's instruments
and control surfaces remained functional, but the aircraft could not maintain altitude. As the X-15
also had to be controlled in an environment where there was too little air for aerodynamic flight
control surfaces, it had a reaction control system (RCS) that used rocket thrusters. There
were two different X-15 pilot control setups: one used three joysticks, the other, one joystick.
The X-15 type with multiple control sticks for the pilot placed a traditional rudder and stick
between a left joystick that sent commands to the Reaction Control System, and a third joystick on
the right used during high-G maneuvers to augment the center stick In addition to pilot input,
the X-15 "Stability Augmentation System" (SAS) sent inputs to the aerodynamic controls to help
the pilot maintain attitude control. The Reaction Control System (RCS) could be operated in two
modes manual and automatic. The automatic mode used a feature called "Reaction Augmentation
System" (RAS) that helped stabilize the vehicle at high altitude. The RAS was typically used for
approximately three minutes of an X-15 flight before automatic power off. Among the many
controls were the rocket engine throttle and a control for jettisoning the ventral tail fin.
Other features of the cockpit included heated windows to prevent icing and a forward headrest
for periods of high deceleration. The X-15 also had an ejection seat designed to operate at
speeds up to Mach 4 and 120,000 feet altitude, although it was never used during the program.
In the event of ejection, the seat was designed to deploy fins, which were used until it reached
a safer speed/altitude at which to deploy its main parachute. Pilots wore pressure suits, which could
be pressurized with nitrogen gas. Above 35,000 feet altitude, the cockpit was pressurized to
3.5 psi with nitrogen gas, while oxygen for breathing was fed separately to the pilot.
Now let’s talk about its propulsion.
The initial 24 powered flights used two Reaction Motors XLR11 liquid-propellant rocket engines,
enhanced to provide a total of 16,000 pounds-force of thrust as compared to the
6,000 pounds-force that a single XLR11 provided in 1947 to make the Bell X-1 the first aircraft
to fly faster than the speed of sound. The XLR11 used ethyl alcohol and liquid oxygen.
By November 1960, Reaction Motors delivered the XLR99 rocket engine, generating 57,000
pounds-force of thrust. The remaining 175 flights of the X-15 used XLR99 engines, in a single-engine
configuration. The XLR99 used anhydrous ammonia and liquid oxygen as propellants, and hydrogen
peroxide to drive the high-speed turbopump that delivered propellants to the engine.
It could burn 15,000 pounds of propellant in 80 seconds; Jules Bergman titled his book on
the program Ninety Seconds to Space to describe the total powered flight time of the aircraft.
There one more thing we need to discuss, Why X-15 flies steadily at hypersonic speeds?
The answer is because it had a thick wedge tail.
This produced a significant amount of base drag at lower speeds, the blunt end at the rear of
the X-15 could produce as much drag as an entire F-104 Starfighter. A wedge shape was used because
it is more effective than the conventional tail as a stabilizing surface at hypersonic speeds.
A vertical-tail area equal to 60 percent of the wing area was required to give the X-15
adequate directional stability. So, stability at hypersonic speeds was
aided by side panels that could be extended from the tail to increase the overall surface area,
and these panels doubled as air brakes. Thanks for watching, and as always if
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