X-15 Research Results
IN 120 FLIGHTS during the past five years, the X-15 has
achieved its mission research objectives in 110, or 92 percent of the
total. This remarkably high degree of mission success is in striking
contrast to that of unmanned space vehicles of the X-15's own
design era. As a result, the X-15 program has often been thrust
into the running debate over manned-versus-unmanned-vehicles as
proof of the superiority of piloted aircraft over automatically
However, the X-15 program alone cannot disprove the merits of
unmanned vehicles, since it contributes to only one side of the
argument. Nor, on the other hand, does it glorify the role of the pilot,
for it was only through the use of automatic controls for some operations
that the full potential of the X-15 was utilized. Rather, the real
significance of its excellent mission reliability is that it has shown
that the basic philosophy of classical, piloted aircraft operation is
just as applicable to the realm of hypersonic and space flight as it is
to supersonic flight. That philosophy decrees that the pilot is
indispensable, and that he must be able to override any automatic
control, bringing his skill and training to bear upon deficiencies of
This concept was not universally accepted at the time the X-15
was designed. Many aeronautical experts were afraid that the pilot
might be taking too large a step into unknown areas, and that automatic
devices and systems could better accomplish his task. Airplanes and
control systems have changed radically since the Wright
Flyer, they argued, but pilots have not.
Those who pioneered the X-15 concept were well aware of the
limitations of the human operator. They had no illusions that research
pilots, no matter how well-trained, could get along without
aid if called upon to control a rapidly oscillating system. Neither
had the pilots, for they were no less engineers than pilots. Where
the X-15 pioneers and pilots differed from engineers arguing for
unmanned systems was in fully understanding the advantages of the
By utilizing man's capabilities, the X-15 systems were made much
simpler than automatic operations would have been, notably for
launching, maneuvering, and landing. Beginning with the earliest
studies, the suggestions of experienced research pilots have been an
integral part of the program. One objective was to remove as many
unknowns as possible for the pilot before the flight program began.
Another was to make sure that the pilot's task in flight tests would
become a realistic continuation of his previous experience and
training. The question of whether or not a pilot could control the X-15
while sustaining a force of 6 G's became one of how to provide this
capability, so that the pilot could maintain control and not restrict
aircraft performance. In shepherding the X-15 through "normal"
flights that start, at zero-G at launch and often end with a 10-G
landing impact, pilots have had to learn new tricks and approach
old procedures warily.
Pilots who were destined to be first to fly the X-15 were selected
soon after the program got underway. In keeping with the joint
nature of the project, representatives of North American Aviation,
the Air Force, the Navy, and NASA were assigned to the program
as project pilots. North American Aviation selected A. S. Crossfield,
a former rocket-plane pilot for NACA, to make the contractor
demonstration flights. The Air Forces assigned Capt. I. C. Kincheloe,
of X-2 fame, and Capt. (now Lt. Col.) R. M. White.
NASA named J. A. Walker, Chief Research Pilot at the Flight
Research Center; N. A. Armstrong; and J. B. McKay, each an
experienced rocket-plane pilot. To this early group was added
Lt. Comdr. (now Cmdr.) F. S. Petersen, of the Navy, in mid-1958.
The untimely death of Capt. Kincheloe (one of the earliest and
most vigorous X-15 proponents), in late 1958, elevated Capt. White
to the position of Air Force project pilot, and Capt. (now Maj.)
R. A. Rushworth came into the program. M. O. Thompson, of
NASA, and Capt. J. H. Engle, of the Air Force, joined the original
group in 1962. The X-15 team also benefited from the contributions
of many pilots not assigned to the program, who were active
in the early studies of NASA, Air Force, and Navy.
A vital link between X-15 pilots and the accomplishment of their
various research missions is the craft's instrument display. The pilots
accomplish the major phase of every flight solely by reference to
cockpit instruments. Thus, the instruments are no less important
than the control system. In spite of the X-15's large range of
operating conditions, its cockpit display is rather conventional. Some
instruments were consolidated, new instruments were added, and
there have been later modifications, but basically the cockpit is
representative of 1957-58 instrumentation techniques.
The basic flight-guidance instrument is an indicator that displays
the three airplane-attitude angles together with angle of attack and
angle of yaw. Grouped around this instrument are a G-indicator,
altitude and speed indicators, and a stop watch for timing
rocket-engine operation. A coarse-and-fine-attitude indicator and an
angle-of-attack indicator are also required.
entire present team of X-15 research pilots includes, from left to right,
John B. McKay (NASA),
Emphasis was placed upon backup or alternate displays rather
than sophisticated guidance schemes. The pilot controls the airplane
to achieve a programed, memorized flight plan. Since this
does not include precise trajectory guidance, accurate instrument
and display sensitivity was not originally provided. This technique
has been adequate for the exploratory flight program, and actual
flight conditions proved to be within about ten percent of desired
Joseph A. Walker (NASA), Milton O. Thompson (NASA), Maj. Robert A. Rushworth
Capt. Joe H. Engle (USAF). Previous X-15 pilots at various times were A. Scott
Neil A. Armstrong (NASA), Lt. Comdr. Forrest S. Petersen (USN), and Maj. Robert
M. White (USAF).
Later flights, however, have required more precise control, and
several special pseudo-guidance and display systems have been utilized.
The low-altitude, high-heating flights have demanded very
precise flight-path control to arrive at desired test conditions. This
is especially critical during the first 40 seconds. If those initial
conditions are in error, the pilot doesn't have adequate time to correct
the flight path. The original cockpit display wasn't adequate for
accomplishing these flights with repeatable precision. Modifications
to provide the pilot with additional information, such as airflow
temperature and air pressures, have been explored with some success.
These instruments necessitated the development of new procedures
for measurement and computation as well as for cockpit display.
Another important adjunct to integrating the X-15 pilot with his
airplane is a pressure suit, to protect against reduced atmospheric
pressure at high altitude. For the human body, space flight begins
at an altitude of about 55 000 feet, and at that height a pilot has to
have a pressure suit to survive in case something goes wrong with the
cockpit pressurization system. It was highly desirable to use proven
equipment for this critical item, but a suitable pressure suit at first
was not available. While suits that provided the desired pressure
protection had been developed, they were very cumbersome. When
pressurized, they practically immobilized the pilot. The X-15 pilot
would need to operate the controls when his suit was pressurized.
Moreover, the suit would be an integral part of the escape system
and would have to be able to withstand high air temperatures and
A suit that met these requirements was developed by the David C.
Clark Co., which had created a means of giving the wearer high
mobility. The key to its design is a link-net type of material, which
covers a rubberized pressure garment. The suit is not just a protective
garment that the pilot dons, like a parachute, but an integral
part of his environment. It provides both cooling and ventilation,
supplies breathing oxygen, and contains parachute harness,
earphones, microphone, pressure regulators, electrical leads for
physiological equipment, and a system to prevent visor-fogging.
Chief Research Pilot Joseph A. Walker, of NASA's Flight
Research Center, Edwards, Calif., stands beside an X-15 in full-pressure
suit, the type that provides all X-15 pilots with livable atmosphere during
flight. The dark tube attached to Walker leads to a portable unit that
supplies each pressure-suit wearer with essential air-conditioning on the
The pressure suit began as another major undeveloped subsystem
for the X-15. Its advanced form today represents a state-of-thc-art
improvement. At one time the pace of the X-15's flight program
depended on the course of the suit's development.
Along with other X-15 systems, the pressure suit has undergone
continuous improvement and updating. It has operated satisfactorily
on several flights in which partial cockpit pressurization was
lost at altitudes above 100 000 feet. Although the suit was designed
specifically for the X-15, its technology has been utilized in
other programs, notably Mercury and Gemini. An adaptation of the
X-15 suit has become standard apparel for fighter squadrons of the
Air Force's Air Defense Command.
Aeromedical aspects of piloting a plane at hypersonic speeds and
in space were early a controversial aspect of the X-15 program.
Some experts in aviation medicine viewed with great concern the
flight environment that X-15 pilots would encounter. In particular,
they were apprehensive of weightless flight, an unknown region
in the mid-1950's.
This concern was not universally shared, especially not by research
pilots. However, everybody agreed that the X-15 pilots would face
the most demanding tasks yet encountered in flight. If the X-15
did not represent the limit of human endurance, it was time to find
out whether or not there was a limit. It was recognized that
whereas techniques to analyze airplane characteristics had been
developed to a high degree of perfection, no means existed for analyzing
the psychomotor performance of a pilot. Thus, a primary research
objective was to fill some of the gaps in knowledge of the
pilot's physiological response.
Physiological measurements and analysis in flight were rather
meager prior to the X-15 program. The limited flight data that
had been obtained had been gathered specifically for aeromedical
analysis. In the X-15 program, by contrast, the aeromedical measurements
would be incidental to the research mission. They would
provide data not only under a true operational flight condition but
in a severe environment.
The work has combined the efforts of the Aeromedical Laboratory
at the Air Force Aeronautical Systems Division, Wright-Patterson
Air Force Base, Ohio; the Bioastronautics Branch of the AFFTC;
and the Air Force School of Aviation Medicine, San Antonio, Texas.
A major portion of it has been the development of instrumentation
techniqucs as an integral part of the pressure suit. Originally the
instrumentation recorded electrocardiograph, skin temperatures,
oxygen flow, and suit pressures. It has undergone continuous
change, the latest development being a means of measuring blood
pressure in flight.
Startling Increase in Heart Rate
The basic measurements of interest for aeromedical analysis are
heart rate, breathing rate, and blood pressure. Since blood pressure
was not measured at the start of the program, the first analysis
centered upon heart rate and breathing rate as measures of the
dynamic response of the body to physiological stresses. The initial
measurements were somewhat startling to aeromedical experts, for
heart rates averaged 145 to 160 beats per minute. On some flights,
they rose as high as 185 beats per minute, and never fell below 145.
When associated with physical stress, such high rates normally have
a grave prognosis. However, as data accumulated from additional
pilots, aeromedical researchers gained insight into the interplay
between psychic and physical stresses of flights of this nature. Most
of the increase in heart rate, they found, occurred before the X-15
was launched from the B-52, and thus reflected a keying up and
anticipation rather than direct physical stress.
Later, analysis of blood-pressure measurements confirmed the
previous conclusions that psychological factors were the primary
influence on heart rate. Aeromedical researchers now have a better
understanding of man's adaptation to hypersonic and space flight.
Significantly, what at first appeared to be excessive heart rates are
now accepted as norms, forming a baseline for pilot response.
The aeromedical investigation has since extended to monitoring
additional cardiovascular dynamics. While these techniques are
being developed, and their data interpreted, groundwork is being
laid for comprehensive analysis of a pilot's psychomotor performance.
Perhaps it may someday make it possible to develop a mathematical
model of a pilot from psychomotor analysis, just as the
aeronautical engineer has arrived at an approximate mathematical
model for aircraft stability from dynamic-response analysis of
The X-15 program achieved another significant first in analyzing
to what degree the pilot contributed to mission success. This work
began as an attempt to find a basis for comparing X-15 reliabilities
with those of unmanned vehicles. While the exploratory work has
not yielded a rigorous technique, it has roused considerable interest
and brought the viewpoints for judging respective reliability of
piloted and unmanned flight vehicles into better focus, if not
agreement. Significantly, the X-15 record of mission success on 92
percent of its flights has been achieved with individual system and
subsystem reliabilities as low as 80 percent. While the use of component
redundancy overcaine some of the shortcomings in critical systems,
a more important contribution to safety and success has been the
capability of the pilot to bypass failed systems or change to alternate
modes of operation.
In spite of the X-15's excellent mission-reliability record, the
program has had its share of serious malfunctions and operating
problems. These difficulties caused three major accidents, which
required varying degrees of aircraft rebuilding. The X-15 program has
suffered from what has always been a major aircraft problem - complex
reactions to the failure of simple components.
The accidents pointed out the serious consequences of two or more
minor, or unrelated, malfunctions. One X-15 was literally blown
in half when a pressure regulator and a relief valve failed amost
simultaneously during ground tests and pressurized the ammonia
tank beyond the structural limit. The pressure regulator froze because
of an accumulation of moisture and its proximity to liquid-oxygen and
helium lines. The relief valve did not operate when tank pressure
became excessive because of high back-pressure from an ammonia-vapor
disposal system used only for ground operation.
As a result of the explosion, fail-safe concepts have been applied to
ground tests in addition to flight operations.
Two other X-15 accidents occurred during emergency landings
at alternate dry lakes following abnormal shutdown of the rocket
engine. In each case, two unrelated system failures contributed to
a third, which was a major structural failure at touchdown, even
though the pilot had made a satisfactory landing.
X-15's long and valuable research program has been marred by only three serious
accidents, none of which involved a latality. One was an explosion and fire
on a test stand.
The others are shown here. Above: a fuselage split open an landing after two
system failures precipitated
a major structural failure. The plane was back in the air within
three months. Below: another dual failure made the landing gear collapse at
swerving the plane into a crippling, high-speed rollover and injuring the
pilot, John B. McKay.
The pilot fully recovered; the airplane was rebuilt (shown here).
One such landing resulted in abnormally high loads because of a
heavyweight condition from incomplete jettisoning of all unused
propellants, and only partial cushioning of the nose impact by the
nose-gear shock strut. When the nose wheels touched down, the
fuselage buckled just aft of the cockpit, causing it to drag on the
ground. Fortunately, the damage was easily repaired, and the airplane
was back in the air within three months.
The second landing mishap was far more serious. In that instance,
the landing flaps failed to come down, but the pilot, jack McKay,
made a perfect landing for the condition which requires a
high-speed touchdown (in this case, 290 mph). As the airplane rotated
onto the nose gear, the high aerodynamic down loads on the horizontal
tail at that speed, in combination with rebound load following
nose-gear impact, caused the left main landing gear to collapse.
The airplane swerved broadside and rolled over, damaging wings,
demolishing tail surfaces, and injuring McKay, who suffered three
Both pilot and craft have since returned to flight status. McKay,
though shortened by three-quarters of an inch, was back flying
another X-15 within six months. His damaged craft was slower
to return to work. It was modified extensively, and a year and a
half passed before it was back in the air.
These mishaps have forcefully shown that the interplay between
complex systems has to be analyzed down to the smallest detail.
The importance of such analysis has led to exploratory work with
electronic computers in an effort to simulate and study X-15 systems,
and thereby obtain better understanding for the design of the more
advanced vehicles that may follow it.
Other aspects of the X-15 program should also have a far-reaching
influence on the operation of future manned aerospace vehicles.
The fact that the pilot has contributed notably to mission reliability
while in full command should stimulate work toward thoroughly
integrating the pilot's capabilities with future vehicles from their
inception. In addition, man-rating a system has come to mean
more than assurance of safe operations. The use of the pilot to
control many automatic functions not only helps insure safe and
reliable operation but makes less complex systems feasible.
Perhaps the strongest indication of the flexibility obtained by
integrating airplane, pilot, controls, and display is that the X-15 is
now used for research purposes far different from those envisioned
by the men who pioneered the concept. The primary research areas
have been probed until few secrets remain. Researchers have
turned their interest to other intriguing problems that have come into
view with the space age. The X-15 program has embarked on
studies allied to satellites and rocket-borne probes rather than to
aircraft flight research. Thus, not only has the program opened up
to piloted aircraft the realm of hypersonic and reentry flight, it has
also thrust piloted flight into a space-equivalent region, heretofore
the exclusive domain of unmanned systems.