X-15 Research Results
THE HEART of an exploratory research program is planning. For the X-15, it
is nearly endless, and in a constant state of flux. This work started with a
feasibility study, which revealed that major changes in flight-operations procedures
from those of previous research airplanes would be required. This grew into
a program of ever-increasing detail and variety to explore the many facets of
flight within the corridor as well as in the space-equivalent and reentry regions.
With a performance capability of Mach 6 and 250 000 feet, the X-15 had outgrown
the type of operation that had suited the X-1 and X-2. The expanded requirements
were evident in the B-52 launch airplane, pilot training, emergency-rescue facilities,
emergency-landing facilities, and in a facility to coordinate and control each
flight, as well as a radar and communications network. All these had to be developed
and integrated into an over-all plan that would provide maximum support for
the pilot on each flight. Little wonder, therefore, that preparations for flight
operations started almost as early as the design studies began, in 1956. Work
was underway at various facilities of NACA, the Air Force, and North American
Aviation. Most of it was being done by the two groups that would carry forward
the flight-research program: the NACA High Speed Flight Station and the Air
Force Flight Test Center, both at Edwards Air Force Base, California. These
two organizations had worked together in a spirit of cooperation and friendly
competition since the X-1 and D-558-I research programs of 1947. They were experienced
in the peculiarities of rocket-airplane operations and the techniques for exploring
new aerodynamic conditions in flight. To them, the X-15 was more than just the
newest of the X-series of research airplanes. The advanced nature of the program,
airplane, systems, and region of exploration would require a supporting organization
as large as the combined staff needed for all previous rocket airplanes.
North American Aviation played a major role, of course, during the initial phases
of the flight program. Its demonstration and de-bugging of the new airframe
and systems comprised, in many respects, the most arduous and frustrating period
of flight operations. The first year, in particular, was full of technical problems
and heartbreak. One airplane split open on landing. Later, its hydrogen-peroxide
tank exploded, and its engine compartment was gutted by fire on the ground.
Another X-15 blew apart on the rocket test stand. Flight research has never
been painless, however, and these setbacks were soon followed by success. Inevitably,
the NAA flights and the research program overlapped, since not only were two
of the three airplanes in operation but an interim rocket engine was in use
for the early flights. (The XLR-99 engine was delayed, and two RMI XLR-11 rocket
engines, having a total thrust of 16 000 pounds, were installed and flown for
30 flights of the X-15.) In addition, exploratory research flights to determine
practical operating limits merged with many of the detailed research flights,
and even with some flights carrying scientific experiments. Such flexibility
is normal, however, since flight research does not consist of driving rigidly
toward fixed goals.
The X-15 program progressed from flight to flight on foundations laid upon freshly
discovered aerodynamic and operational characteristics. This research approach
requires preflight analysis of all constraints on aerodynamic and stability-and-control
characteristics, on structural loads, and on aerodynamic-heating effects to
determine the boundary within which the flight can be made with confidence.
The constraints are regarded as critical limits, and the delicate balance between
adequacy and inadequacy can most easily be found by approaching a limit yet
never exceeding it.
Operational considerations require an answer to every question of "What if this
malfunctions?" before a pilot is faced with it, perhaps critically, in flight.
Often the success of a mission depends upon the pilot's ability to switch to
alternate plans or alternate modes of operation when a system or component fails.
And flight research requires a certain wariness for unanticipated problems and
the inevitable fact that they become obvious only when a system or component
is exposed to them at a critical time. Yet some risk must be taken, for a too-conservative
approach makes it almost impossible to attain major goals in a practical length
These factors have always been important to flight research, but they were severely
compounded in the case of the X-15. In investigating the reentry maneuver and
conditions of high aerodynamic heating, the airplane is irrevocably committed
to flight in regions from which the pilot cannot back off in case he encounters
an unforeseen hazard. The complicating factor is that the load-carrying ability
of the heat-sink structure is not so closely associated with specific speed-altitude-load
conditions as it is in most other airplanes. Instead, it depends largely upon
the history of each flight up to the time it encounters the particular condition.
Therefore, it wasn't at all easy to predict margins of safety for the X-15's
structural temperatures in its initial high-heating flights. Moreover, since
both airframe and systems were being continuously modified and updated as a
result of flight experience, many limiting conditions changed during the program.
Thus, while an operational margin of safety has always governed the program,
rather diverse criteria have had to be used to define that margin. Generally,
each flight is a reasonable extrapolation of previous experience to higher speed,
altitude, temperature, angle-of-attack, and acceleration, or to a lower level
of stability. The magnitude of the extrapolation depends on a comparison of
flight results, on wind-tunnel and theoretical analysis, on pilot comments,
and on other pertinent factors. The accuracy of aerodynamic data determined
in flight naturally has a bearing on flight planning. So data-reduction and
analysis are as important considerations as operational and piloting factors.
Through an intensive program of 26 flights in the 1960-62 period (in addition
to flights required for pilot-training or systems-check), the X-15 probed flight
to its design goals of Mach 6, 250 000-feet altitude, and 1200° F structural
temperatures. This was very close to the number of flights originally planned
to reach those goals, but the types of flight differed considerably from those
of the initial plan. Some deviations were made to explore a serious stability-and-control
problem found at high angle of attack. Another was made to explore high-heating
conditions after thermal gradients greater than expected had deformed the structure
to a minor but potentially dangerous extent.
The pace to push past the design goals was slower. Another year and a half passed
before the present maximum altitude of 354 200 feet was attained. Maximum temperature
was raised to 1325° F in a flight to high-heating conditions at Mach 5 and low
A large measure of the success of the program has been due to a research tool
-the X-15 flight simulator- that was not available when planning started, ten
years ago. The flight simulator consists of an extensive array of analog-computing
equipment that simulates the X-15 aerodynamic characteristics and computes aircraft
motions. Linked to the computer are exact duplicates of the X-15 cockpit, instruments,
and control system, including hydraulics and dummy control surfaces.
and pertinent details of a flight in which the X-15 achieved its design goals
in speed and altitude and came very close to that point in structural temperature.
The X-15 flight simulator is somewhat like a Link trainer. But its technology
and complexity are as far advanced beyond those of the Link trainer as the
complexity of a modern high-speed digital computer exceeds that of a desk-top
adding machine. With the simulator, both pilots and engineers can study flight
conditions from launch to the start of the landing maneuver. A flight is "flown"
from a cockpit that is exactly like that of the airplane. Only the actual
motions of pilot and airplane are missing.
Long before the first flight, X-15 pilots had become familiar with the demands
for precise control, especially during the first 85 seconds - the powered
phase, which establishes conditions for the entire flight. They had trained
for the peculiarities of control above the atmosphere with the jet reaction
rockets. They had simulated reentries at high angle of attack over and over
again. The simulator also gave them practice in the research maneuvers and
timing necessary to provide maximum data points for each costly flight. They
had practiced the many flight-plan variations that might be demanded by malfunctions
of rocket engine, subsystems, or pilot display. They thus had developed alternate
methods for completing each mission, and had also developed alternate missions.
Sometimes the flight simulator proved its worth not so much by indicating
exact procedures as by giving the pilot a very clear appreciation of incorrect
Without this remarkable aid, the research program probably would have progressed
at a snail's pace. Yet the flight simulator was not ready-made at the start
of the program. In fact, the complete story of its technology is in large
measure the story of how it grew with the X-15 program. The potential of flight
simulators for aircraft development was just beginning to be appreciated at
the time of the X-15 design. Thus there was interest at the start in using
one to study X-15 piloting problems and control-system characteristics. Early
simulators were limited in scope, though, and concentrated upon control areas
about which the least was known: the exit condition out-of-atmosphere flight,
and reentry. Noteworthy was the fact that angle of attack and sideslip were
found to be primary flight-control parameters, and hence would have to be
included in the pilot's display. One of the chief early uses of the simulator
was to evaluate the final control-system hardware and to analyze effects of
The initial simulations were expanded, and it soon was apparent that the simulator
had a new role, far more significant than at first realized. This was in the
area of flight support; namely pilot training (as already described), flight
planning, and flight analysis. The two last matters are closely interlocked,
which insured that each step in the program would be reasonable and practical.
Pilot training was also closely integrated, since often the margin of safety
was influenced by a pilot's confidence in the results from the flight simulator.
During the exploratory program, the capability of the simulator to duplicate
controllability at hypersonic speeds and high angle of attack was an important
factor in determining the magnitude of each subsequent step up the flight
Even after 120 flights, pilots spend 8 to 10 hours in the simulator before
each 10-12-minute research flight.
The importance of the flight simulator today reflects the confidence that
pilots and research engineers have gained in simulation techniques. This confidence
was lacking at the start of the program, since the simulator basically provides
instrument flight without motion cues, conditions not always amenable to extrapolation
to flight. However, much has been learned about what can and cannot be established
on a flight simulator, so that even critical control regions are now approached
in flight with much confidence.
each 10-12-minute research mission, X-15 pilots train as long as 10 hours
electronic simulator at Edwards AF Base. Chief Research Pilot Walker is sitting
cockpit here. The simmulator duplicates the X-15's cockpit, instruments, and
including hydraulics and dummy control surfaces, and is nearly as long as
the aircraft itself.
The application of the X-15 simulation techniques to other programs
has accelerated flight-simulation studies throughout the aerospace
industry. Interestingly, this is one of the research results not
Navy's Centrifuge Valuable Aid
A notable contribution to flight simulation was also made by the
Navy, there to fore a rather silent partner in the X-15 program. The
Aviation Medical Acceleration Laboratory at the Naval Air Development
Center, Johnsville, Pa., has a huge centrifuge, capable of
carrying a pilot in a simulated cockpit. The cockpit is contained in
a gondola, which can be rotated in two axes. It is mounted at the
end of a 50-foot arm. By proper and continuous control of the two
axes in combination with rotation of the arm, the forces from high-G
flight can be imposed on the pilot. This centrifuge was an ideal
tool with which to explore the powered and reentry phases of X-15
Another significant aspect of the NADC centrifuge soon became
apparent. Previously, the gondola had been driven along a programed
G pattern, not influenced by the pflot; he was, in effect, a
passenger. But in flight an X-15 pilot not only would have to withstand
high G forces but maintain precise control while being squashed
down in his seat or forced backward or forward. lt was important
to find out how well he could maintain control, especially during
marginal conditions, such as a stability-augmentation failure during
reentry. The latter would superimpose dynamic acceleration forces
from aircraft oscillations on already severe pullout G's. There were
no guidelines for defining the degree of control to be expected from a
pilot undergoing such jostling.
To study this phase, the NADC centrifuge was linked to an electronic
computer, similar to the one used with the X-15 flight simulator,
and the pilot's controls. The computer output drives the
centrifuge in such a manner that the pilot experiences a convincing
approximation of the linear acceleration he would feel while flying
the X-15 if he made the same control motions. (The angular
accelerations may be unlike those of flight, but normally they are of
secondary importance.) This type of closed-loop hookup (pilot
control to computer to centrifuge) had never been attempted before.
It was a far more complex problem than developing the electronics
for the immobile flight simulator.
With this centrifuge technique, pilots "flew" about 400 reentries
before the first X-15 flight. The G conditions on most of these
simulated reentries were more severe than those experienced later
in actual flights. The simulation contributed materially to the
development and verification of the pilot's restraint-support system,
instrument display, and side-located controller. The X-15 work
proved that, with proper provision, a pilot could control to high
Aside from its benefit to the X-15 program, the new centrifuge
technique led to fresh research into pilot control of
aircraft-spacecraft. The Aviation Medical Acceleration Laboratory was soon
deluged with requests to make closed-loop dynamic flight simulations,
particularly for proposed space vehicles. Many of these studies have
now been completed. They have shown that pilot-astronaut control
is possible to 12-15 G's. This research will pay off in the next
generation of manned space vehicles. The X-15 closed-loop program
was also the forerunner of centrifuges that NASA has built for
its Ames Research Center and Manned Spacecraft Center.
In addition to hundreds of hours of training with the flight simulator
and the NADC centrifuge, the X-15 pilots have also trained in
special jet aircraft. These aircraft were used for limited explorations
of some of the new flight conditions. For example, an exploratory
evaluation of the side controller was made as early as 1956
in a T-33 trainer, and later in an F-107 experimental aircraft.
Other tests were made of reaction jet controls, and the reentry
maneuver was explored with two special test aircraft that were in
effect airborne flight simulators. One of the earliest programs, still
in use, is X-15 approach-and-landing training in an F-104 fighter.
This practice, which involves deliberately inducing as much drag
as possible, has been especially important in maintaining pilot
proficiency in landing, since for any single pilot there are often long
intervals between X-15 flights.
Many flight tests were made to integrate the X-15 with the B-52
launch-airplane operation. The air-launch technique had been
proven, of course, with previous rocket airplanes. The concept has
grown, however, from a simple method for carrying the research aircraft
to high initial altitude, to an integral part of the research-aircraft
operation. For the X-15, the air-launch operation has become in effect
the launching of a two-stage aerospace vehicle, utilizing a
recoverable first-stage booster capable of launching the second
stage at an altitude of 45 000 feet and a speed of 550 mph. As
with any two-stage vehicle, there are mutual interferences. They
have required, among other things, stiffening of the X-15 tail
structure to withstand pressure fluctuations from the airflow around the
B-52 and from the jet-engine noise.
Several of the X-15 systems operate from power and supply
sources within the B-52 until shortly before launch; namely, breathing
oxygen, electrical power, nitrogen gas, and liquid oxygen.
These supplies are controlled by a launch crewman in the B-52, who
also monitors and aligns pertinent X-15 instrumentation and electrical
equipment. In coordination with the X-15 pilot, he helps make
a complete pre-launch check of the latter aircraft's systems. Since
this is made in a true flight environment, the procedure has helped
importantly to assure satisfactory flight operations. The mission
can be recalled if a malfunction or irregularity occurs prior to
second-stage launch. These check-out procedures are also important to
B-52 crew safety, since the explosive potential of the volatile
propellants aboard the X-15 is such that the B-52 crew has little
protection in its .040-inch-thick aluminum "blockhouse."
The launch is a relatively straightforward free-fall maneuver, but
it was the subject of early study and concern. Extensive wind-tunnel
tests were made to examine X-15 launch motions and develop
techniques to insure clean separation from the B-52.
The X-15 required a major change in flight operations from
those of previous rocket airplanes, which had operated in the near
vicinity of Rogers Dry Lake, at Edwards. With a Mach 6 capability,
the X-15 had outgrown a one-base operation, since it may
cover a ground track of 300 miles on each flight. The primary
landing site is at Edwards, which requires launching at varied
distances away from the home base, depending on the specific flight
mission and its required range. A complicating factor in flight
operations is that the launch must be made near an emergency landing
site, and other emergency landing sites must be within gliding
distance as the craft progresses toward home base, for use in the event
of engine failure.
Fortunately, the Califomia-Nevada desert region is an ideal
location for such requirements, because of many flat, barren land
areas, formed by ancient lakes that are now dry and hard-packed.
Ten dry lakes, spaced 30 to 50 miles apart, have been designated for
X-15 use, five as emergency landing sites near launch location, five
as emergency landing sites down-range. The X-15 pilots are
thoroughly familiar with the approach procedures for all emergency
Because of wide variations in the research maneuvers, successive
flights may be made along widely separated ground tracks. The
track will normally pass within range of two or three emergency
sites. The desired research maneuvers often must be altered to make
sure that the flightpath passes near emergency landing sites. These
procedures are studied on the flight simulator, and pilots predetermine
alternate sites and the techniques to reach them for each flight.
On four occasions, rocket-engine malfunctions have necessitated
landing at an emergency site.
drawing shows the flight paths of two typical research missions of the X-15.
Radar stations at Beatty and Ely, Nev., and at home base track each flight
attached to a B-52 drop plane, to landing. Launch always occurs near one of
dry lakes in the region, some of which are indicated here.
Emergency ground-support teams, fire trucks, and rescue equipment are
available at all sites. Airborne emergency teams, consisting of
helicopters with a rescue team and a C-130 cargo airplane
with a pararescue team, are also positioned along the track.
An important adjunct to mission success has been the extensive
support the X-15 pilot receives during a flight from the many people
"looking over his shoulder", both in the air and on the ground. On
hand during a flight are chase aircraft, which accompany the B-52
to the launch point. Although these are soon left far behind after
X-15 launch, other chase planes are located along the intended track
to pick up the X-15 as it nears the primary or alternate emergency
Coordination and control of the farflung operation are carried out
from a command post at the NASA Flight Research Center. Into
it comes information pertinent to the X-15's geographic location,
performance, and systems status, and the status of the B-52, chase
planes, and ground-support teams. Responsibility for the coordination
of this information, as well as for the complete mission, rests
with a flight controller. This function is carried out either by one
of the X-15 pilots or by some other experienced research pilot. The
flight controller is in communication with the X-15 pilot at all times,
to provide aid, since he has far more information available to him
than the pilot has. This information is provided by a team of
specialists who monitor telemetry signals from the airplane. One of
the primary functions of the flight controller is to monitor the X-15's
geographic position in relation to the amount of energy it will need
to reach an intended landing site. The flight controller also provides
navigation information to help the X-15 pilot reach any desired site.
The flight controller's capability to monitor the complete operation
is provided by a radar-telemetry-communications network that extends
400 miles, from Edwards to Wendover, Utah. Ground stations
are located at Edwards; Beatty, Nevada; and Ely, Nevada. Each
station is an independent unit, though all stations are interconnected
by telephone lines or microwave-relay stations. This network is
another joint USAF-NASA facility. Like most other features of
the program, the range has been updated to providc additional
flexibility, accuracy, and/or reliability.
Another integral part of a flight-research program is extensive and
detailed measurements of aircraft behavior. These measurements
enable X-15 pilots to approach critical conditions with confidence,
and also provide data to uncover unforeseen problems. However,
determining suitable instrumentation is not an exact science. In
many cases, although the airplane seemed to be overinstrumented
during design, it was found to be underinstrumented in specific areas
during the flight program. In addition, many compromises had to
be made between the amount of instrumentation for research
measurements and that for systems monitoring. Other compromises
were necessary for measuring and recording techniques. A vast array
of gauges, transducers, thermocouples, potentiometers, and gyros is
required to measure the response of the X-15 to its environment.
Because of the difficulty of measuring pressures accurately in the
near-vacuum conditions of high-altitude flight, an alternate method
for measuring velocity and altitude had to be developed. The system
uses a missile-type inertial-reference system, with integrating
accelerometers to determine speed, altitude, and vertical velocity.
The system also measures airplane roll, pitch, and yaw angle relative to
the Earth, to indicate aircraft attitude to the pilot. Alignment and
stabilization are accomplished during the climb to launch altitude
by means of equipment within the B-52.
Another system development was required for measuring angles of
attack and yaw. Although flight measurements of these quantities
had always been important for analysis of aerodynamic data, they
took on added significance for the X-15 when early simulator studies
showed that they would be required as primary pilot-control parameters
during much of a flight. Rather severe requirements were
placed on the system, since it would have to measure airflow angles
at air temperatures to 2500° F and have satisfactory response for
very low as well as high air pressures. The system consists of a
sphere, 6 1/2 inches in diameter, mounted at the apex of the airplane
nose. This sensor is rotated by a servo system to align pressure
orifices on the sphere with the airflow. The system has been highly
successful for the precise control that the X-15 requires.
A most important contribution to mission success is the "blood,
sweat, and tears" of the men who work to get the X-15 off the
ground. An unsung effort, averaging 30 days in duration, is required
to prepare and checkout the airplane and systems for every
flight. Many of the systems and subsystems were taking a larger
than normal step into unknown areas. Inevitable compromises
during design and construction resulted in an extensive development
effort for many components and subsystems, as part of the
flight-research program. A rigorous program of product improvement
and updating of systems has continued throughout flight operations.
While this work ultimately forced a somewhat slower pace upon the
program, its results are found in the remarkably successful record of
safe flight operations and in-flight reliability.
The flight achievements, of course, are the payoff for the meticulous
preparations that have gone on for the past 10 years. Without
this vast support, the pilots might have taken too large a step
into new flight regimes. While many problems were encountered,
they have been surmounted, some as a result of pilot training, others
as a result of measurements of the response of the airplane to the
new flight environment.
Just as each X-15 flight leaves a few less unknowns for succeeding
flights, so will the X-15 program leave a few less unknowns for
succeeding airplanes. By exploring the limits of piloted flight within
the corridor as well as above it, man has expanded his knowledge in
many fields. The real significance of the four miles of data from
each flight came from tedious analysis of the response, which provided
some insight into basic forces. Sometimes an examination
of gross effects sufficed, but more often it required a penetrating look
into the very core of aerodynamic flow. From this has come the
first detailed picture of airflow around an airplane at hypersonic