OVERVIEW: This 700+ page book is part of the NASA HISTORY SERIES. A
history of the design and achievements of the high-speed, 1950s-era
X-15 airplane. FROM THE FORWARD BY WILLIAM H. DANA: The X-15 was an
airplane of accelerations. When an X-15 pilot looks back on his
X-15 flights, it is the accelerations he remembers. The first of
these sensations was the acceleration due to B-52 lift, which held
the X-15 at launch altitude and prevented it from falling to Earth.
When the X-15 pilot hit the launch switch, the B-52 lift was no
longer accessible to the X-15. The X-15 fell at the acceleration
due to Earth's gravity, which the pilot recognized as "free fall"
or "zero g." Only when the pilot started the engine and put some
"g" on the X-15 was this sensation of falling relieved. The next
impression encountered on the X-15 flight came as the engine lit,
just a few seconds after launch. A 33,000-pound airplane was
accelerated by a 57,000-lbf engine, resulting in a chest-to-back
acceleration of almost 2 g. Then, as the propellant burned away and
the atmosphere thinned with increasing altitude, the chest-to-back
acceleration increased and the drag caused by the atmosphere
lessened. For a standard altitude mission (250,000 feet), the
weight and thrust were closer to 15,000 pounds and 60,000-lbf at
shutdown, resulting in almost 4-g chest-to-back acceleration. The
human body is not stressed for 4 g chest to back, and by shutdown
the boost was starting to get a little painful. Milt Thompson once
observed that the X-15 was the only aircraft he had ever flown
where he was glad when the engine quit. On a mission to high
altitude (above 250,000 feet), the pilot did not regain any
sensible air with which to execute a pullout until about 180,000
feet, and could not pull 1 g of lift until 130,000 feet. Flying a
constant angle of attack on reentry, the pilot allowed g to build
up to 5, and then maintained 5 g until the aircraft was level at
about 80,000 feet. There was a deceleration from Mach 5 at 80,000
feet to about Mach 1 over the landing runway, and the pilot
determined the magnitude of the deceleration by the use of speed
brakes. This ended the high-g portion of the flight, except for one
pilot who elected to start his traffic pattern at 50,000 feet and
Mach 2, and flew a 360-degree overhead pattern from that starting
point. Flight to high altitude represented about two-thirds of the
199 X-15 flights. Flights to high speed or high dynamic pressure
accounted for the other third, and those flights remained well
within the atmosphere for the entire mission. The pilot of a
high-speed flight got a small taste of chest-to-back acceleration
during the boost (thrust was still greater than drag, but not by
such a large margin as on the high-altitude flights). The
deceleration after burnout was a new sensation. This condition was
high drag and zero thrust, and it had the pilot hanging in his
shoulder straps, with perspiration dripping off the tip of his nose
onto the inside of his face plate. Milt Thompson collected
anecdotes about the X-15 that remain astonishing to this day. Milt
noted that at Mach 5, a simple 20-degree heading change required 5
g of normal acceleration for 10 seconds. Milt also pointed out that
on a speed flight, the (unmodified) X-15-1 accelerated from Mach 5
to Mach 6 in six seconds. These were eye-opening numbers at the
time of the X-15 program. Those of us in the program at flight 190
thought that the X-15 would continue indefinitely. Then, on flight
191, Major Michael J. Adams experienced electrical irregularities
that made the inertial flight instruments unreliable and may have
disoriented him. In any case, at peak altitude (266,000 feet), the
X-15 began a yaw to the right. It reentered the atmosphere, yawed
crosswise to the flight path, and went into a high-speed spin. It
eventually came out of the spin but broke up during the reentry,
killing the pilot.
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