Category X-15 EXTENDING THE FRONTIERS OF FLIGHT

Hugh Latimer Dryden was born 2 July 1898 in Pocomoke City, Maryland. He earned his way through Johns Hopkins University, completing the four-year bachelor of arts course in three years and graduating with honors. Influenced by Dr. Joseph S. Ames, who for many years was chairman of the NACA, Dryden undertook a study of fluid dynamics at the Bureau of Standards while taking graduate courses at Johns Hopkins. In recognition of his laboratory work, the university granted him a doctor of philosophy degree in 1919.[12]

Dryden became head of the bureau’s aerodynamics section in 1920. With A. M. Kuethe, in 1929 he published the first of a series of papers on the measurement of turbulence in wind tunnels and the mechanics of boundary-layer flow. He advanced to chief of the Mechanics and Sound Division of the Bureau of Standards in 1934, and in January 1946 became assistant director. Six months later he became associate director.

In 1945 Dryden became deputy scientific director of the Army Air Forces Scientific Advisory Group. In 1946 he received the nation’s second highest civilian decoration, the Medal of Freedom, for "an outstanding contribution to the fund of knowledge of the Army Air Forces with his research and analysis of the development and use of guided missiles by the enemy."

In 1947 Dryden resigned from the Bureau of Standards to become director of aeronautical research at the NACA. Two years later the agency gave him additional responsibilities and the new title of director. Dryden held this post until he became deputy administrator of the new National Aeronautics and Space Administration (NASA) in 1958. The National Civil Service League honored Dryden with the Career Service Award for 1958. He served as the deputy administrator of NASA until his death on 2 December 1965.

The primary objective of the flight program was to explore the hypersonic flight regime and compare the results against various analytical models and wind-tunnel results. The physical X-15 configuration was of only passing interest and was not an attempt to define what any future operational aircraft might look like; it was simply a means to obtain the necessary thermal environment and dynamic pressures. The researchers wanted to understand heating rates, stagnation points, laminar and turbulent flow characteristics, and stability and control issues.

Later, the X-15 would become a carrier for various experiments, and the airplane configuration would be of even less interest.

and X-15-3 (56-6672). The second airplane became X-15A-2 after North American extensively modified it following an accident midway through the flight program. The two carrier aircraft were an NB-52A (52-003) and an NB-52B (52-008); although not identical, they were essentially interchangeable.-19!

The program used a three-part designation for each flight. The first number represented the specific X-15 ("1" was for X-15-1, etc.). There was no differentiation between the original X-15- 2 and the modified X-15A-2. The second position was the flight number for that specific X-15 (this included free flights only, not captive carries or aborts); the first flight was 1, the second was 2, etc. If the flight was a scheduled captive carry, the second position in the designation was a C; if it was an aborted free-flight attempt, it was an A. The third position was the total number of times that either NB-52 had carried aloft that particular X-15, including captive carries, aborts, and actual releases. A letter from Paul Bikle established this system on 24 May 1960 and retroactively redesignated the 30 flights that had already been accomplished.-119!

The requirement to measure flight loads on aircraft flying at supersonic and hypersonic speeds led the FRC to construct the High-Temperature Loads Calibration Laboratory in building 4820 during 1964. The facility allowed researchers to calibrate strain-gage installations and test structural components and complete vehicles under the combined effects of loads and temperatures. The laboratory was a hangar-type structure with a small shop and office area attached to one end. A door measuring 40 feet high and 136 feet wide allowed access to an unobstructed test area that was 150 feet long by 120 feet wide and 40 feet high.222

The High Temperature Loads Calibration Laboratory was established at the Flight Research Center to allow researchers to calibrate strain-gage installations and test structural components and complete vehicles under the combined effects of loads and temperatures. The facility was equipped with a programmable heating system that used infrared quartz lamps available in various lengths from 5 inches to 32 inches. Reflector arrangements were available for heating rates from 0 to 100 Btu per second per second and temperatures up to 3,000 degrees Fahrenheit. This photo shows an X-15 horizontal stabilizer being tested under the lamps. (NASA)

A state-of-the-art control room was provided to operate the heating and loads equipment remotely, and a data acquisition system occupied the second floor over the office spaces. Large windows overlooked the hangar floor, and the room included a closed-circuit television system. A high-capacity hydraulic system could operate up to 34 actuators to apply loads to test specimen or entire aircraft. Perhaps more importantly for the X-15 program, the facility had a programmable heating system that used infrared quartz lamps available in various lengths from 5 inches to 32 inches. Reflector arrangements were available for heating rates from 0 to 100 Btu per second per second and temperatures up to 3,000°F.[223]

NASA used this facility for a variety of purposes during the remainder of the flight program. This included testing a set of X-15 horizontal stabilizers as part of the loads program undertaken late in the flight program, and the laboratory proved to be critical for solving the inadvertent landing- gear extension problem suffered by X-15A-2 when it began its envelope expansion program. NASA later used the laboratory to test portions of the XB-70A and Lockheed Blackbirds.

In mid-1965 the X-15 program was roughly three-quarters of the way through its eventual flight program, and flight surgeons at the AFFTC published a report on their findings to date. At the time, nine different pilots had flown the X-15 (Adams, Dana, and Knight had not yet flown); however, researchers only collected data from the six who had flown a sufficient number of flights to be statistically relevant to the analysis (omitting Armstrong, Petersen, and Thompson). The researchers noted that a "potentially very useful comparison of pilot performance and concurrent physiologic response is not possible because the X-15 flight test program is not structured as a psycho-physiological experiment. The aforementioned variability in flight profiles and unpredictable aircraft malfunctions makes possible only a general, qualitative comparison, rather than a specific, quantitative one."[37]

It is important when considering the physiological data obtained during the X-15 program to keep in mind the conditions under which researchers collected the data. In addition to the normal variability of physiologic responses, no two X-15 flights were the same. Different flight profiles and random aircraft malfunctions varied the physiological and psychic stresses to which the pilots were exposed.[38]

During an altitude mission, immediately after launch, the X-15 rotated to a preplanned climb angle and accelerated at 3-3.5 g. The pilot experienced a front-to-back ("eyes-in") inertial force that increased the apparent weight of the body, particularly the chest area, and resulted in a prompt increase in respiratory rate that continued until the acceleration subsided. After approximately 40 seconds of acceleration, the pilot pushed over to a "zero-normal" acceleration. The pulse rate, which had been increasing up to and throughout the launch operation, tended to decline during this period. At engine burnout, approximately 80 seconds after launch, the longitudinal acceleration dropped abruptly to zero, followed by a variable period in which all accelerations were essentially zero. The pilot was in an essentially weightless state during this period and the respiration rate showed a prompt decline.[39]

Immediately after engine burnout, the pilot invariably immediately experienced an increased heart rate that tended to decrease during the zero-g period. The increased heart rate at this point was probably a psychic response to the abrupt transition from a hypergravic state (in the normal plane) to a hypogravic ("weightless") state. From this point to the landing phase, the pilot was busily engaged in usually complex flight maneuvers, including "accomplishing deliberate aircraft perturbations in roll, pitch, and yaw for the purpose of collecting stability and control information." Heart rates and respiratory rates tended to reflect the difficulties the pilot encountered in managing the flight.*40

After the aircraft passed over the top of its trajectory and was descending at a steep angle, the pilot had to pull out of the dive into level flight. This pull-up generated a positive normal acceleration that the pilot experienced as increased body weight. At this point, the anti-g portion of the David Clark full-pressure suit activated to counteract these forces. Nevertheless, during this maneuver, the blood tended to pool in the lower parts of the body, the carotid arteries experienced decreased pressure, and the cardio-accelerator reflex produced a prompt increase in heart rate. The heart rate lowered as the accelerations decreased. The landing maneuver produced only mild accelerations, and the small increase in both heart rate and respiratory rate during this phase was entirely a psychic response to the task of accomplishing the landing. Since steady-state conditions existed for only a few seconds at a time during the brief 8-10-minute flights, physical and psychic stimuli were usually occurring concurrently and independently. This meant that "only the grossest correlation with heart rate and respiratory rate responses [could] be made."*41

In most professions, including piloting, there is a general trend for heart rates and respiratory rates to decrease as an individual gains experience in performing a task. However, an analysis showed "no statistically significant difference" between early flights and later flights for each of the six pilots analyzed. Researchers believed a number of factors could explain this failure to adhere to the expected trend. The first, and probably most important, was that there were no "easy" X-15 flights. Trying to obtain the maximum amount of data on each flight kept the pilot very busy performing the required maneuvers at the proper time while maintaining the desired flight profile. This required intense concentration, and the pilot also had to monitor aircraft systems during this period.-*42*

In psychological terms, the pilot had a fast-moving, intensive task to perform continuously during his 8-10-minute flight, plus a few minutes on each end. Added to this was the psychic stress of actual or potential system failures, which were not uncommon during the flight program. Another factor was the variation in flight profiles, which meant that the pilots had little or no opportunity to develop a routine for a familiar flight. Furthermore, there was often a considerable interval between flights flown by individual pilots. In the end, the researchers found that it was "not surprising that a rapid reduction in responsiveness" was not seen. The researchers found that, overall, "the spectrum of physiological response of the pilots to X-15 flights, in terms of heart rate, respiratory rate, blood pressure, and pulse pressure, has remained quite stable throughout the X-15 Program regardless of pilot experience level. This pattern of physiological response may be tentatively considered the norm for this type of operation."*431

In the end, the X-15 program was both a contributor to and a recipient of biomedical instrumentation. It was the first program to generate meaningful requirements in airborne biotelemetry and was the impetus for the development of several pieces of instrumentation that later found their way into standard clinical practice. Although Mercury and Gemini gathered better data, the X-15 nevertheless contributed to the physiological database that helped establish baselines for future programs. However, perhaps the most significant contribution of the X-15 program from a biomedical perspective was "the unequivocal, and at times dramatic, demonstration of the capabilities of the human pilot in managing a vehicle and a flight profile from launch to landing, which is a true space flight in miniature.’444*

The ASD sponsored an experiment that was essentially a product evaluation program of materials selected for use on the Dyna-Soar. Researchers bonded cermet (ceramic-metallic composite) runners to the rear landing-gear skids on the X-15 for five flights using X-15-3 in early 1964. Two additional flights were conducted using X-15A-2 to evaluate Inconel X skids. Engineers compared these data with those obtained on five earlier flights that used standard 4130-steel skids but carried additional instrumentation to measure landing loads:*206!

One of the outcomes of the study was an evaluation of skid wear. The amount of skid wear depended on the speed of the sliding, the hardness of the skid material, the strength of the surface material, and the sliding distance. For this evaluation, engineers measured the thickness of the X-15 skids after each flight, generally near the point of attachment to the main strut. The difficulties involved in removing and reinstalling the skids in a timely manner precluded weighing them. The cermet skids experienced a considerable amount of wear during the first landing because of the soft outer layer of copper-nickel, but showed less wear on later landings because the tungsten-carbide chips were uncovered.-1207

The data for the 4130-steel skids showed an increasing amount of skid wear as the sliding distance increased beyond 6,400 feet. The wear characteristics of the Inconel X skids were not determined because of the difficulty of measuring the chemically milled areas inside the skid. However, preliminary data indicated a wear resistance superior to that of the 4130 steel, with or without a cermet coating.-208

Joe Engle has the unique honor of having flown the X-15 and the Space Shuttle, bringing lifting – reentry vehicles full circle. Engle flew the X-15 for 24 months from 7 October 1963 until 14 October 1965, making 16 flights with the XLR99 engine. Engle reached Mach 5.71, a maximum speed of 3,888 mph, and an altitude of 280,600 feet.

Joe Henry Engle was born on 26 August 1932 in Abilene, Kansas, and graduated from the University of Kansas at Lawrence with a bachelor of science degree in aeronautical engineering in 1955. After graduation he worked at Cessna Aircraft as a flight-test engineer before being commissioned through the Air Force ROTC program in 1956. Engle earned his pilot wings in 1958 and flew F-100s for the 474th Fighter Squadron (Day) and later the 309th Tactical Fighter Squadron at George AFB, California.

Engle graduated from the Test Pilot School in 1962 and attended the ARPS at Edwards for training as a military astronaut. He graduated from the ARPS in 1963 and became a project pilot for the X – 15 program in June 1963. Engle received an Air Force astronaut rating for making a flight above 50 miles in the X-15.-113

In 1966, at the age of 32 years, Engle became the youngest person selected to become an astronaut. First assigned to the Apollo program, he served on the support crew for Apollo X and then as backup lunar module pilot for Apollo XIV. In 1977 he was commander of one of two crews that conducted the approach and landing tests with the Space Shuttle Enterprise. In November 1981 he commanded the second flight (STS-2) of the Columbia and manually flew the reentry, performing 29 flight-test maneuvers from Mach 25 through landing rollout. This was the first (and so far only) time a pilot has flown a winged aerospace vehicle from orbit through landing. He accumulated the last of his 224.5 hours in space when he commanded Discovery during mission 51-I (STS-27) in August 1985.

Engle has flown more than 180 different types of aircraft and logged nearly 14,000 flight hours. Among his many honors, Engle has been awarded the Distinguished Flying Cross (1964), the AIAA Lawrence Sperry Award for Flight Research (1966), the NASA Distinguished Service Medal and Space Flight Medal, and the Harmon International, Robert J. Collier, Lawrence Sperry, Iven C. Kincheloe, Robert H. Goddard, and Thomas D. White aviation and space trophies. In 1992 he was inducted into the Aerospace Walk of Honor.-114

X-15 flights did not begin with a pilot waking up and deciding he wanted to fly that day. Weeks or months before, a researcher would develop requirements for data gathered under specific conditions. One of the flight planners (Johnny Armstrong, Dick Day, Bob Hoey, Jack Kolf, or John Manke, among others) would take these requirements and lay out a flight plan that defined the entire mission. The term "flight planner" does not begin to describe the expertise of the engineers who performed this function. These engineers lived in the simulator and were experts on the airplane. They determined the thrust settings, climb angles, pushover times, and data-gathering maneuvers; they also evaluated stability and control issues and heating concerns. In addition to laying out specific flights, the flight planners performed parametric studies that were not related to a particular flight or pilot training. Some of these included glide performance, peak altitude versus pitch angle, speed-optimization techniques, and reentry trades involving dynamic pressure, load factors, angle of attack, and temperatures.!11!

The flight planners would then present their plan to the pilot selected for the flight. The flight planners and the pilot would spend the next week or month, depending on the complexity of the mission, in the simulator choreographing every second of the flight. After extensive practice with the nominal mission, the pilot flew off-design missions to acquaint himself with the overall effect of changes in critical parameters, including variations in engine thrust or engine shut-down times.!12!

At this point, the primary ground controller (called "NASA-1") joined the flight planners and pilot for additional simulations so that they could all become familiar with the general timing of the flight. After practicing the off-design missions, the team evaluated various anomalous situations, including failures of the engine, stable platform, ball nose, radio and dampers, and variations in the stability derivatives. For instance, the flight planners would insert simulated premature engine shutdowns at critical points to acquaint the pilot with the optimum techniques for returning to the lake behind him or flying to an alternate lake ahead of him. Normally the failure of the velocity or altitude instrument would not affect a flight; however, in the event of an attitude presentation failure during the exit phase of an altitude mission, the pilot had to initiate an immediate pushover from about 30 degrees pitch attitude to 18 degrees so that he could visually acquire the horizon. Failures of the ball nose were usually not terminal since the pilot could still fly the mission using normal acceleration, attitudes, and stabilizer-position indications, but the results were not as precise. Radio failure meant the pilot had to be self-sufficient-an undesirable situation, but not a tremendous problem for most test pilots.-113

A simple flight would encompass 15-20 hours of simulator time, and a complex mission could easily double that. Given that each flight was only 8-10 minutes long, this represented a lot of training. By far, these were the most extensive mission simulations attempted during the X-plane program, and would point the way to how the manned space program would proceed. Although the drill at times seemed tedious and time-consuming to all involved, it undoubtedly played a major role in the overall safety and success of what was unquestionably a potentially dangerous undertaking. All of the pilots praised the flight planners and the simulators, and nobody believes the program would have succeeded nearly as well without it. Milt Thompson later observed, "[W]e were able to avoid many pitfalls because of the simulation. It really paid off. I personally do not believe that we could have successfully flown the aircraft without a simulation, particularly in regard to energy management." Simulation and mission planning are some of the enduring legacies of the X-15 program.[14]

NASA held the fourth and last conference on the progress of the X-15 program at the FRC on 7

October 1965. This conference was considerably smaller than the previous ones, with only 13 papers written by 25 authors. The FRC employed 18 of the authors, while four came from other NASA centers, one from the AFFTC, and the remaining two from other Air Force organizations. Approximately 500 persons attended the event. At this point, the program had conducted approximately 150 flights over 6 years.[227]

By the time of the conference, the X-15 had essentially met or exceeded all of its revised performance specifications. The future would bring no additional altitude marks, and additional speed of less than a Mach number. For the most part, the government was using the X-15 as an experiment carrier, although X-15A-2 continued some additional aero-thermo-dynamic research. Jim Love noted that 10 pilots had used the three X-15s to accumulate almost 1 hour of flight above 200,000 feet and almost 4 hours at speeds in excess of Mach 4.[228]

The follow-on experiments were taking on unanticipated importance. Love observed, "The use of the airplane as an experimental test bed is one of the most significant extensions in the research capability of the X-15 airplanes. They have been utilized to carry various experimental packages to required environments, obtaining measurements with these packages, and then returning the experiment and results to the experimenters… several experiments were installed on each aircraft for better flight utilization. For this reason, on the X-15-1 airplane, specially constructed [wing] tip pods and tail-cone box have been installed… to accommodate the experiments… Three experiments have been completed, five are in progress, and three more are planned for next year."[229]

Love noted that "the X-15 program has never settled down to a routine operation because of the continued increase in complexity and the nature of experiments and research performed by each aircraft. This attribute is probably characteristic of research programs." The lack of routine, however, undoubtedly increased the cost of the program and placed a heavy burden on personnel to maintain safety.*230

Although it was an integral part of X-15-3, the Minneapolis-Honeywell MH-96 adaptive flight – control system was also an experiment and hence part of the research program. In 1956, researchers performing in-house studies at the Flight Control Laboratory of the Aeronautical Systems Division (ASD) at Wright-Patterson AFB determined that it was feasible to design a self­adaptive flight-control system. As the name implies, such a system automatically adapts itself to provide essentially constant damping in flight conditions of varying control-system effectiveness. In other words, a given movement of the control stick would always result in the same airplane response, regardless of how far the control surfaces had to move to accomplish the maneuver. At the time, most aircraft still had simple mechanical linkages to the flight controls, with manually set trim tabs. The new supersonic fighters had more sophisticated system that adjusted their gains as a function of measured and computed air data. However, these functions required extensive flight-testing to perfect, and generally resulted in complex and unreliable systems. Researchers expected that future vehicles would be operating in flight regimes where air data might not be available, and decided to develop a new approach. The Air Force awarded a number of study contracts in 1957 that led to flight-testing of a variety of adaptive concepts on several Lockheed F-94 Starfires by the Massachusetts Institute of Technology (MIT) and the Minneapolis – Honeywell Regulator Company. When government funding ended, Minneapolis-Honeywell continued its effort with a company-funded flight program using a McDonnell F-101A Voodoo. The Air Force subsequently provided limited funding for the F-101 trials, and future astronaut

Virgil Grissom flew some of the evaluation flights.*45*

By 1958 the Flight Control Laboratory was convinced of the potential of self-adaptive techniques; however, the performance of the available aircraft was insufficient to test the concepts, particularly the first expected application in the Boeing X-20 Dyna-Soar orbital glider. The logical choice of test platforms was the X-15 because its flight profile was the closest approximation to the Dyna-Soar that was available. Unfortunately, the X-15 program was already in high gear and the Air Force was reluctant to delay the critical hypersonic testing planned for the three airplanes.*461

Despite the lack of an available test platform, the Flight Control Laboratory continued with its development effort. The Air Force released invitations to bid in late 1958, evaluated proposals during early 1959, and awarded to contract to Minneapolis-Honeywell in June 1959. Although the primary purpose of the program was to test the self-adapting technique in a true aerospace environment, researchers also decided to evaluate several features that were recognized as desirable for any production system. These included dual redundancies for reliability, the integration of aerodynamic and ballistic control systems, rate-command control, and simple outer-loop hold modes for attitude and angle of attack. Within a few months, Honeywell flew the prototype MH-96 in the F-101A at Minneapolis and in the X-15 fixed-base simulator in Inglewood.*47*

The basic system consisted of an adaptive controller that contained the various electronic modules and redundant rate gyro packages (each containing three rate gyros-one for each axis). The system also required an attitude reference (i. e., an inertial platform) and angle-of-attack and angle-of-sideslip information. The electronics modules were programmed with an ideal response rate (the "model") for the aircraft, and the MH-96 adjusted the damper gains automatically until the aircraft responded at the ideal rate. Essentially, the gain changer operated by monitoring the limit-cycle amplitude and adjusting the gain to maintain a constant amplitude. A tendency for the amplitude limit cycle to increase resulted in a gain reduction, whereas loss of the limit cycle initiated a gain increase. Lead compensation largely determined the limit-cycle frequency, which had to be higher than the aircraft’s natural frequency but lower than its structural frequency. On the X-15, that worked out to about four cycles per second.*48*

The MH-96 adaptive flight control system installed in X-15-3 was an early attempt at a fly-by­wire concept, although the linkages were still largely mechanical. Pilot inputs were compared to an ideal model running on a small computer, and the MH-96 commanded the control surfaces to move an appropriate amount based on speed, dynamic pressure, and other variables. The system generally worked well, and the MH-96 was used for all of the program’s high-altitude flights since it provided better redundancy than the standard stability-augmentation system installed in the other two airplanes. (NASA)

Early on, this model presented a problem that was first seen in the X-15 simulator: a quick decrease in gain was necessitated by the rapid buildup of control surface effectiveness during reentry. Delays in the gain reduction, partly caused by the lag in the mechanical control linkages, resulted in temporary oscillations as high as 3 degrees, peak to peak, at the servo. Modifications to the gain computer improved the situation but never eliminated it. However, since the X-20 was going to be a fly-by-wire vehicle, it would not have suffered from this problem. A different issue proved easier to resolve. A control problem existed whenever motions about one axis were coupled to another. To address this, the MH-96 contained cross-control circuitry that commanded a roll input proportional to the yaw rate in order to combat the unfavorably high negative dihedral effect demonstrated by the X-15 during wind-tunnel testing. This was, essentially, the MH-96 flying the beta-dot technique.-49

When a ground test of the XLR99 severely damaged X-15-3, the Air Force agreed to modify the airplane to accommodate the prototype MH-96. The installation of the system into the airplane began in December 1960 and presented something of a challenge. Although it allowed the removal of the original Westinghouse stability augmentation system, the MH-96 required an even greater volume. NASA installed most of the system electronics on the lower instrument-elevator shelf, but this required a "rather extensive revision of the original instrument-recording configuration" since the data recorders normally occupied this area.-501

requirement due to the low probability of a successful reentry from high altitude without damping. The ability to fail safe was equally important since a large transient introduced in a high-dynamic-pressure region would result in the destruction of the vehicle. The MH-96 provided completely redundant damper channels, such that either channel could control the vehicle. The adaptive feature of the circuitry permitted one channel to be lost with little or no loss in system performance, since the remaining gain changer would attempt to provide the additional gain required to match the limit cycle. The gain computers were interlocked, when operative, to prevent overcritical gain following a limit-cycle circuit failure, and to provide the desired limiting effect for hard-over failures.[51]

In the case of a model or variable-gain amplifier failure, conventional monitor circuits would disengage either or both channels when required. Combined with the desire of NASA for increased system flexibility, this led to the addition of parallel fixed-gain channels with fail-safe passive circuitry. Since these channels operated simultaneously with the adaptive channels to avoid the time-lag penalties of switching, they effectively limited the minimum gain for adaptive operation. The fixed-gain circuits had to be sufficiently powerful for satisfactory emergency performance throughout the flight envelope, but below the critical level in the high-dynamic-pressure regions. A successful compromise was elusive, and X-15-3 spent most of its career with some restrictions on its flight envelope.-1521

As installed in X-15-3, the MH-96 provided stability augmentation in the pitch, roll, and yaw axes. The MH-96 controlled both the aero surfaces and the ballistic control system, blending the two as needed to achieve the desired control responses. In addition, autopilot modes provided control-stick steering, pitch and roll attitude hold, heading hold, and angle-of-attack hold. Pilot commands to the system were electrical signals proportional to the displacement of the right side controller or the center stick, and the rudder pedals. Nevertheless, the left-hand controller remained in the cockpit and the pilot could use it if desired to manually control the ballistic control thrusters. Provisions were also incorporated to electrically trim the pitch and roll axes. Trimming presented some problems of its own, mainly because the MH-96 was adapted to use existing X-15 hardware instead of its own newly developed hardware. In order to keep the pitch servos centered, the "trim follow-up" system used a low-rate trim motor to adjust the center point of the pilot’s stick. This not only centered the servos, it also physically moved the stick. If the pilot was performing a precise task while the trim motor was moving the stick, it was easy to get out of phase with the trim motor and it would become saturated, oscillating at low amplitude all by itself at about 0.5 Hz, with no pilot input for several seconds. The pilots found this disconcerting.-1521

The reliability of the adaptive system on the X-15 was excellent, despite the fact that it was not a production system. The mean time between failures of the dampers was 360 hours, and the entire system averaged 200 hours, mainly because some servos were not redundant. The adaptive electronics had a failure time of 100,000 hours. All of these figures compared favorably with the 100 hours demonstrated by the Westinghouse SAS in the other two airplanes.-1541

The MH-96 experienced only two persistent failures during its 65 flights, and each involved only a single axis. It also experienced several other momentary failures on one or more axes; however, in each instance the adaptive mode reengaged following the transient disturbance. Milt Thompson observed that "it appears obvious that black-box technology is capable of providing a high degree of reliability through redundancy and improved electronic hardware." Thompson admitted, however, that the quality of maintenance and the caliber of technical support might have had something to do with the reliability.1551

Although it was not known at the time, 50 years later almost all high-performance aircraft would use some sort of adaptive flight controls. Many of the problems would be eliminated, or at least minimized, by the incorporation of digital fly-by-wire technology, quick-acting hydraulic and electric actuators, and fast digital computers. Nevertheless, given the technology available at the time, the MH-96 was an important first step.

The X-15 offered investigators a unique opportunity to measure heat transfer and skin friction under quasi-steady flight conditions at high Mach numbers and low wall-to-recovery temperature ratios. This allowed them to make a direct comparison between measured flight data and calculated values. A considerable amount of heat-transfer data and some skin-friction data were obtained during the flight program, and these data indicated that the level and rate of change of turbulent skin friction and heat transfer were lower than predicted by the most widely used theories, such as those of Van Driest and Eckert. However, comparisons of the X-15 data and the theory were inconclusive due to uncertainties about the boundary layer conditions because of

non-uniform flow and conduction losses. To evaluate the problem, researchers wanted to use a highly instrumented panel in a location with known flow characteristics. They also wanted the panel to be shielded from aerodynamic heating until the airplane was in a steady-state cruise condition.[209]

Researchers selected the X-15-3 with the sharp-leading-edge modification on the dorsal rudder to carry the experiment. The test panel was located just behind the right-side leading-edge boundary-layer trips 15.1 inches below the top of the rudder, and was constructed of 0.0605- inch-thick Inconel X. Researchers installed a removable panel on the left side of the rudder to provide access to the instrumentation used for the test panel. To obtain the desired wall-to – recovery temperature ratios and ensure an isothermal test surface when the airplane reached the desired speed and altitude, it was necessary to insulate the test panel during the initial phase of the flight. Explosive charges jettisoned the insulating cover from the test panel in approximately 50 milliseconds, resulting in an instantaneous heating of the test panel (the so-called "cold wall" effect). Researchers instrumented the test panel with thermocouples, static-pressure orifices, and a skin-friction gage with the data recorded on tape by a PCM data acquisition system at a rate of 50 samples per second. A Millikan camera operating at 400 frames per second was in the upper bug-eye camera bay to record the events. The measurements obtained were in general agreement with previous X-15 data.12101

Researchers used the same general location for another test panel, but without the cold wall. This panel, which was flush with the normal surface of the rudder, had a microphone and static – pressure orifice mounted flush, and an "L"-shaped total pressure probe sticking out and forward. The microphone was located 28.8 inches from the original rudder leading edge (not the sharp extension) and 20.3 inches from the top of the rudder. The data was recorded onboard the airplane and evaluated after the flight. The intent of the experiment was to determine when the boundary layer transitioned to turbulent flow. The highest noise levels occurred during reentry as the Reynolds numbers reached their peak value. The data gathered provided a qualitative indication of the end of the transition that agreed reasonably well with wind-tunnel data. Interestingly, researchers also recorded some data while the NB-52 carried the X-15, and described the noise level as "very high" due to aerodynamic interference with the carrier aircraft. This confirmed predictions made before the first glide flight.12111