X-15 Research Results

Chapter 8

Man-Machine Integration



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 controlled devices.

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 machinery.

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 human operator.

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.


photo of X-15 test pilots and the X15.

The entire present team of X-15 research pilots includes, from left to right, John B. McKay (NASA),
Joseph A. Walker (NASA), Milton O. Thompson (NASA), Maj. Robert A. Rushworth (USAF), and
Capt. Joe H. Engle (USAF). Previous X-15 pilots at various times were A. Scott Crossfield (NAA),
Neil A. Armstrong (NASA), Lt. Comdr. Forrest S. Petersen (USN), and Maj. Robert M. White (USAF).


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 conditions.

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 pressures.

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.


photo of Joseph A. Walker 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 ground.


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 aircraft motions.

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.

photo of a damaged X15

The 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 unrelated
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 touchdown,
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).

photo of an X15 inverted and crashed.


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 crushed vertebrae.

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.


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