X-15 Research Results

Chapter 4

Flight Research



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 of time.

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 altittude.

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.

illustration of the X15's flight path.

Profile 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 procedures.

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 component failures.

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 corridor.

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.


photo of an X15 pilot in a flight simulator

Before each 10-12-minute research mission, X-15 pilots train as long as 10 hours in the
electronic simulator at Edwards AF Base. Chief Research Pilot Walker is sitting in its
cockpit here. The simmulator duplicates the X-15's cockpit, instruments, and control system,
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 foreseen.


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 flights.

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 acceleration levels.

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 landing sites.

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.


map showing Edwards AFB, and lakes nearby.

This 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 from takeoff,
attached to a B-52 drop plane, to landing. Launch always occurs near one of the many
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 landing sites.

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 speeds.


Previous Next Index