Category X-15 EXTENDING THE FRONTIERS OF FLIGHT

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

Charles Henderson Feltz was born on 15 September 1916 on a small ranch near Channing, Texas. He graduated from Texas Technological College in 1940 with a bachelor of science degree in mechanical engineering. He joined the Los Angeles Division of North American Aviation the same year, serving in a variety of engineering positions for the design and production of the B-25. Following the war, Feltz became the design group engineer in charge of the design of the wing structure for the F-82 and F-86 series of aircraft. Between 1948 and 1956 Feltz was the assistant project engineer for the development of the FJ2 Fury and the F-86D Sabre, and in 1956 he became the project engineer and program manager for the X-15.-115

In 1962 Feltz became the chief engineer for the Apollo command and service module, and advanced to vice president and deputy program manager by 1970. When the Apollo program ended, he became the vice president and deputy program manager for the Space Shuttle orbiter. Between 1974 and 1976, Feltz was the vice president and technical assistant to the president of the North American Rockwell Space Division. In 1976 he became vice president of the North American Aerospace Operations Division of Rockwell International, and in 1980 he became president of the Space Transportation System Development and Production Division of Rockwell International. Feltz retired from Rockwell International in 1981.

Texas Tech named Feltz a distinguished engineer in 1967, and a distinguished alumnus in 1972. During his career Feltz received a variety of medals and honors from NASA and industry groups, including a NASA Distinguished Public Service Medal in 1981. Charlie Feltz, the common-sense engineer, passed away on 3 January 2003.

Initially the primary justification for a manned research airplane was the choking problems of the wind tunnels, but, as it turned out, this limitation disappeared prior to the beginning of high­speed flight tests. Although this largely eliminated the need for the X-planes, it is unlikely that the progress in developing transonic ground facilities would have occurred without the stimulus begun by the X-1 and D-558. Clearly, there was an important two-way flow of benefits. Stimulated by the problems encountered by the research airplanes during flight, researchers created new ground facilities and techniques that in turn provided the data necessary to develop yet faster airplanes. Comparing the results of flight tests at ever-increasing speeds allowed the wind tunnels to be refined, producing yet better data. It was a repetitive loop.-122

The programs proceeded remarkably rapidly, and the first supersonic flights showed nothing particularly unexpected, much to the relief of the researchers. The most basic result, however, was dispelling the myth of the "sound barrier." The fearsome transonic zone became an ordinary engineering problem, and allowed the designers of operational supersonic aircraft to proceed with much greater confidence.-1231

When people think of X-planes, record-setting vehicles like the X-1 generally come to mind. In reality, most X-planes investigated much more mundane flight regimes, and there were only a handful of high-speed manned experimental aircraft, built mainly during the late 1940s and early 1950s. Specifically, there were five designs (only three of which carried X" designations) intended for the initial manned assault on high-speed flight: the Bell X-1 series, the Bell X-2, the Douglas D-558-1 Skystreaks, the Douglas D-558-2 Skyrockets, and the North American X-15. Of the five, one probed high subsonic speeds, two were supersonic, and one pushed the envelope to Mach 3. The fifth design would go much faster.-124

The X-planes gave aviation its first experience with controlled supersonic flight. On 14 October 1947, Air Force Captain Charles E. Yeager became the first human to break the sound barrier in level flight when the XS-1 achieved Mach 1.06 at 43,000 feet. It took six additional years before NACA test pilot A. Scott Crossfield exceeded Mach 2 in the D558-2 Skyrocket on 20 November 1953. The Bell X-2 proved to be the fastest and highest-flying of the "round one" X-planes and the most tragic, with the two X-2s logging only 20 glide and powered flights between them. Nevertheless, Captain Iven C. Kincheloe, Jr., managed to take one of the airplanes to 126,200 feet on 7 September 1956. Twenty days later, Captain Milburn G. Apt was killed during his first X-2 flight after he reached Mach 3.196 (1,701 mph), becoming the first person to fly at three times the speed of sound, albeit briefly.1251

The contributions of the early high-speed X-planes were questionable, and the subject of great debate within the NACA and the aircraft industry. Opinions on how successful they were depend largely on where one worked. The academics and laboratory researchers, and a couple of aerospace-industry designers, are on record indicating the contributions of the X-planes were minimal. On the other side, however, many of the hands-on researchers and pilots are certain the programs provided solid, real-world data that greatly accelerated progress in the design and manufacture of the Mach 1 and Mach 2 combat aircraft that followed.-1261

For instance, the X-1 was the first aircraft to purposely break the sound barrier in level flight, but other aircraft were doing so in shallow dives soon afterwards.1271 The first combat type designed from the start as a supersonic fighter—the Republic XF-91 "Thunderceptor’—made its maiden flight only 19 months after Yeager’s flight. How much the X-1 experience contributed to Alexander Kartveli’s design is unknown.1281 The same thing happened at Mach 2. By the time Scott Crossfield took a D-558-2 to twice the speed of sound, Kelly Johnson at Lockheed had already been developing what would become the F-104 Starfighter for over a year. It is unlikely that the rocket-powered X-planes actually assisted Johnson much—something he would make clear during later deliberations.1291

The X-1E complemented the heating research undertaken by the X-1B, but the F-104 was already flying and could more easily acquire data at Mach 2. Even at the Flight Research Center (FRC), there was debate over how appropriate this exercise was. FRC research engineer Gene Matranga later recalled, "We could probably fly the X-1E two or three times a month, whereas Kelly [Johnson] was flying his F-104s two or three times a day into the same flight regimes, so it really didn’t make sense for us to be applying those kinds of resources to [obtain] that kind of information." However, it is unfair to judge the X-1E program too harshly since its major purpose was simply to keep a cadre of rocket-powered experience at the FRC in anticipation of the upcoming X-15.1301

Even John Becker recognized the dichotomy represented by the experience: "[T]he cooperative research-airplane program pursued by the Air Force, NACA, and Navy had not been an unqualified success…. Some had lagged so seriously in procurement that their designs had become obsolescent before they were flown. In a few cases tactical designs superior to the research aircraft were in hand before the research aircraft flew." It was not anybody’s fault— technology was simply changing too fast. Trying to sort out the detailed story is nearly impossible and well beyond the scope of this book.1311

Nevertheless, although most believed that the concept of a dedicated research airplane still held promise, researchers decided that the next design would need to offer a significant increment in performance to leapfrog the combat types then in development. Chuck Yeager’s October 1947 assault on the sound barrier had ignited a billion-dollar race to build ever-faster aircraft, and directly affected every combat aircraft design for the next two decades. However, a few

aeronautical researchers had always been certain that the sound barrier was simply a challenge for the engineers, not a true physical limitation. The X-1 had proven it was possible for humans to fly supersonically. The next goal was so much faster.

The X-1E was the last rocket-powered X-plane at the NACA High-Speed Flight Station until the arrival of the three X-15s. There is considerable debate over the economics of flying the X-1E given that some jet-powered aircraft could attain the same velocities, but the primary purpose of the X-1E was to maintain a cadre of rocket experience at the HSFS pending the arrival of the X – 15. (NASA)

The engineers never expected that the design proposed by North American would be the one actually built—it seldom works that way even for operational aircraft, much less research vehicles. True to form, the design evolved substantially over the first year of the program, and on 14-15 November 1955 researchers gathered in Inglewood to resolve several issues. For instance, the North American proposal used 1,599 psf for the minimum design dynamic pressure, while the NACA wanted at least 2,100 psf and preferably 2,500 psf. It would take 100 pounds of additional structure to accommodate the higher pressure. On the other hand, increasing the design load factor from 5.25 g to 7.33 g would cost another 135 pounds, but everybody agreed that raising the design dynamic pressure was a better use of the weight. Nevertheless, as built, the X-15 was rated at 7.33 g, and the change was incorporated when it became obvious that the additional weight was rather trivial after various other upgrades were incorporated.-1191

Researchers also spent considerable effort on evaluating the structural materials proposed by North American, but a lack of detailed information made it impossible to reach a final decision on the wing leading-edge material. The group discussed various ceramic-metallic (cermet), copper, fiberglass, plastic, and titanium carbide materials without conclusion. North American had proposed a wing leading edge that was easily detachable, and the researchers considered this a desirable capability even though it drove a slightly more complex structure and a little additional weight. A weight increase of 13 pounds allowed the use of Inconel X sandwich construction for the speed brakes and provided additional speed brake hinges to handle the higher dynamic pressure already approved. The use of 0.020-inch titanium alloy for the internal structure of the wings and stabilizers instead of 24S-T aluminum gained support, although it involved a weight increase of approximately 7 pounds.

Other structural discussions included changing the oxygen tank to Inconel X due to the low – impact strength of the original titanium at cryogenic temperatures. At the same time, researchers reviewed the need to include a pressurization system to stabilize the propellant tanks. Initially the engineers had considered this undesirable, and North American had not provided the capability in the original design. However, the additional stresses caused by increasing the design dynamic pressure made it necessary to accept a large increase in structural weight or include a pressurization system, and the attendees endorsed the latter. In fact, during the flight program, pilots routinely repressurized the propellant tanks after they jettisoned any remaining propellants to provide an extra margin of structural strength while landing.-128

When the researchers considered a random-direction, 1-inch thrust misalignment, it became obvious that the original large dorsal vertical stabilizer was unsatisfactory for the altitude mission profile. Based on experience with the X-1, the researchers knew that an installed engine could be a couple of degrees out of perfect alignment, although aerodynamic trim easily corrected this. However, in the case of the X-15, the thrust of the engine and the extreme velocities and altitudes involved made the issue a matter of some concern, and the government and North American agreed to include provisions correcting potential thrust misalignment. Along with several other issues, this caused engineers to modify the configuration of the vertical stabilizer.-121

Researchers also concluded that the design would suffer from some level of roll-yaw coupling, and agreed upon acceptable limits. The government also pointed out the need for a rate damping (stability augmentation) system in pitch and yaw for a weight increase of 125 pounds. The need to make the dampers redundant would be the subject of great debate throughout the development phase and early flight program, with the initial decision being not to. Attendees also decided the ballistic control system did not require a damping system, something that would change quickly during the flight program.-122

North American agreed to provide redundant ballistic control systems and to triple the amount of hydrogen peroxide originally proposed. Engineers agreed to provide separate sources of peroxide for the ballistic controls and auxiliary power units (APUs) to ensure that the power units always had propellant. These changes added about 117 pounds.-123

The configuration of the pilot’s controls was finally established. A conventional center stick mechanically linked to a side-controller on the right console operated the aerodynamic control surfaces, while another side-controller on the left console above the throttle operated the ballistic control system. These were among the first applications of a side-stick controller, although these were mechanical devices that bore little resemblance to the electrical side-sticks used in the much later F-16.[24]

In an unusual miscommunication, the attendees at the November meeting believed the WADC had already developed a stable platform and would provide this to North American as government – furnished equipment. Separately, the NACA agreed to supply a "ball nose" to provide angle-of – attack and angle-of-sideslip data. The ball nose, or something functionally similar, was necessary because the normal pitot-static systems would not be reliable at the speeds and altitudes envisioned for the X-15. Although North American proposed a system based on modified Navaho components, the NACA believed that the ball nose represented a better solution.-123

Per a recent service-wide directive, the Air Force representative had assumed that the X-15 would be equipped with some sort of encapsulated ejection system. On the other hand, North American had proposed a rather simple ejection seat. The company agreed to document their rationale for this selection and to provide a seat capable of meaningful ejection throughout most of the expected flight envelope, although all concerned realized that no method offered escape at all speeds and altitudes.-1261

The November meetings ended with a presentation by Douglas engineer Leo Devlin detailing their second-place proposal. A presentation on the advantages of HK31 magnesium alloy for structural use was interesting but provided no compelling reason to switch from Inconel X. Afterwards, Rocketdyne presented a 50,000-lbf rocket engine concept based on the SC-4 being designed for a high-altitude missile; this was a matter of only passing interest, given that a modified XLR30 was already under contract. Separately, Hartley Soule and Harrison Storms discussed the proposed wind-tunnel program, attempting again to agree on which facilities would be used and when.-123

The research instrumentation for the X-15 was the subject of a two-day meeting between personnel from Langley and the HSFS on 16-17 November. The group concluded that strain gauges would be required on the main wing spars for the initial flights, where temperatures would not be extreme, but that wing pressure distributions were not required. The HSFS wanted to record all data in the aircraft, while Langley preferred to telemeter it to the ground. Unfortunately, a lack of funds prevented the development of a high-speed telemetry system. The day following the NACA meeting, representatives from North American drove to the HSFS and participated in a similar meeting. Charlie Feltz, George Owl, and D. K. Warner (North American chief of flight test instrumentation) participated along with Arthur Vogeley, Israel Taback, and Gerald M. Truszynski from the NACA. The participants quickly agreed that the NACA would provide the instruments and North American would install them. The first few flights would use a more or less standard NACA airspeed boom on the nose of the X-15 instead of the yet-to-be-completed ball nose. North American desired to have mockups of the instrumentation within nine months to facilitate the final design of the airplane, and the NACA indicated this should be possible.[28]

The debate regarding engine fuels flared up again briefly at the end of November when John Sloop at Lewis wrote to Captain McCollough recommending the use of a hydrocarbon fuel instead of ammonia. Lewis had concluded that it would be no more difficult to cool a hydrocarbon fuel than ammonia, and the fuel would be cheaper, less toxic, and easier to handle. No information was available on repeated starts of a JP-4-fueled rocket engine, but researchers at Lewis did not expect problems based on recent experience with a horizontally mounted 5,000-lbf engine. The researchers repeated their warning that anhydrous ammonia would attack copper, copper alloys, and silver, all of which were standard materials used in research instrumentation. At the same time, the HSFS wrote that tests exposing a standard NACA test instrument to anhydrous ammonia vapor had proven disastrous. Both NACA facilities repeated their request for a change to a hydrocarbon fuel.[29]

Later the same day, Captain McCollough notified Hartley Soule that the Power Plant Laboratory had reviewed the data submitted by Reaction Motors on the relative merits of substituting a hydrocarbon fuel for ammonia. The laboratory concluded that Reaction Motors had grossly underestimated the development time for conversion, and recommended the continued use of anhydrous ammonia as the most expeditious method of meeting the schedule. A meeting on 1 December at Wright Field brought all of the government representatives together to finalize the fuel issue. The conclusions were that 1) one fuel had no obvious advantage over the other insofar as performance was concerned, 2) the corrosive character of anhydrous ammonia was annoying but tolerable, 3) it would take 6 to 12 months to switch fuels, and 4) the engine development program should continue with anhydrous ammonia. This finally put the issue to rest, although the NACA facilities still believed the requested change was justified.[30]

November also saw an indication that Inconel might have unforeseen problems. A test of the tensile strength of the alloy was published by Langley, and the results differed significantly (in the wrong direction) from the specifications published by the International Nickel Company, the manufacturer of Inconel. NACA Headquarters asked Langley to explain the discrepancies. The reason was unknown, but researchers though it could be related to variations in the material, milling procedures, heat treatment, or testing procedures. Fortunately, further testing revealed that the results from the first test were largely invalid, although researchers never ascertained the specific reasons for the discrepancy. Still, the episode pointed out the need to precisely control the entire life cycle of the alloy.-131

In December, North American engineers visited both Ames and Langley to work out details of the wind-tunnel program. The participants agreed that Langley would perform flutter tests on the speed brakes using the 1/15-scale model. The PARD would make a second flutter investigation, this one of the wing planform, since North American required data from a large-scale model at Mach 5 and a dynamic pressure of 1,500 psf—something no existing tunnel could provide. North American was supplied with additional requirements for a rotary-derivative model to be tested at Ames, and NACA personnel suggested that two 1/50-scale models be constructed—one for testing at Ames and one for Langley. The North American representatives agreed to consider the suggestion, but pointed out that no funds existed for two models. Ames also announced that they would take the 10 by 14-inch hypersonic tunnel out of service on 1 May for several months of modifications. The location was important since the tunnels were not identical and researchers could not directly compare the results from the two facilities.-1321

Ultimately, funds were found to build two 1/50-scale models—one for use at Langley in the 11- inch hypersonic and 9-inch blowdown tunnels, and one for the North American 16-inch wind tunnel. It was decided not to use the Ames tunnel prior to its closing. Langley also tested a 1/15- scale high-speed model while Ames tested a rotary-derivative model. The wind-tunnel investigations included evaluating the speed brakes, horizontal stabilizers, vertical stabilizer, fuselage tunnels, and rolling-tail. Interestingly, the tests at Langley confirmed the need for control system dampers, while North American concluded they were not necessary. This was not the final answer, and researchers would debate the topic several more times before the airplane flew.[33]

Various wind tunnels around the country participated in the X-15 development effort. This 1956 photo shows an original "high tail" configuration. Note the shock waves coming off the wing leading edge and a separate showck wave just behind it coming off the front of the landing skid. Very soon, this configuration would change substantially as the fuselage tunnels were made shorter, the vertical surfaces reconfigured, and the skids moved further aft. (NASA)

North American had based its design surface temperatures on achieving laminar flow during most of the flight profile. However, most of the heat-transfer theories in general use at the time assumed fully turbulent flow on the fuselage. Researchers had previously raised the same issue with no particular solution. Ultimately, researchers used the Unitary Plan tunnel at Langley and the Air Force Arnold Engineering Development Center at Tullahoma, Tennessee, to resolve the discrepancy. These tests provided heat-transfer coefficients that were even higher than the theoretical values, particularly on the lower surface of the fuselage. Because of these results, the Air Force directed North American to modify the design to withstand the higher temperatures.

This proved particularly costly in terms of weight and performance, adding almost 2,000 pounds of additional heat-sink material to the airframe. This is when the program changed its advertising. Instead of using 6,600 fps (Mach 6.5) as a design goal, the program began talking about Mach 6; it was obvious to the engineers that the airplane would likely not attain the original goal. Later, measurements from the flight program indicated that the skin temperatures of the primary structural areas of the fuselage, main wing box, and tail surfaces were actually several hundred degrees lower than the values predicted by the modified theory; in fact, they were below predictions using the original theories. However, resolving these types of uncertainties was part of the rationale for the X-15 program in the first place.[34]

By January 1956, North American required government guidance on several issues. A meeting on 18 January approved the use of a removable equipment rack in the instrument compartment.

North American would still permanently mount some instrumentation and other equipment in the fuselage tunnels, but everybody agreed that a removable rack would reduce the exposure of the majority of research instruments and data recorders to ammonia fumes during maintenance.135

It soon became evident, contrary to statements at the November meeting, that no suitable stable platform existed, although the WADC had several units under development. It was a major blow, with no readily apparent solution.-1361

Other topics discussed at the 18 January meeting included the speed brake design and operation. Full extension of the speed brakes at pressures of 2,500 psf would create excessive longitudinal accelerations, so North American revised the speed brakes to open progressively while maintaining 1,500-psf pressure until they reached the full-open position. All in attendance thought that this was an appropriate solution.-1371

Pilot escape systems came up again during a 2-3 May 1956 meeting at Wright Field among Air Force, NACA, Navy, and North American personnel. WADC personnel pointed to a recent Air Force policy directive that required an encapsulated escape system in all new aircraft. Researchers from the WADC argued that providing some sort of enclosed system would comply with this policy and allow the gathering of research data on such systems. (This seemed an odd rationale in that it appeared to assume that the pilot would use the capsule at some point—an entirely undesirable possibility.) Those opposed to the Air Force view objected to any change because it would add weight and delay development. The opposing group, including Scott Crossfield, believed that the safety features incorporated in the X-15 made the ejection seat acceptable. After the meeting, the Air Force directed North American to justify its use of an ejection seat, but did not direct the company to incorporate a capsule.138

During a 24 May meeting at Langley, representatives from Eclipse-Pioneer briefed researchers from the NACA, North American, and the WADC on a stable platform that weighed 65 pounds and could be ready in 24 months. Later events would show that these estimates were hopelessly optimistic.-1391

On 11 June 1956, the government approved a production go-ahead for the three X-15 airframes, although North American did not cut metal for the first aircraft until September. Four days later, on 15 June 1956, the Air Force assigned three serial numbers (56-6670 through 56-6672) to the X-15 program. The Contract Reporting and Bailment Branch furnished this data by phone on 28 May and confirmed it in writing on 15 June.1401

The TR-139 engine proposed by Reaction Motors was an extensively modified version of the Navy-developed XLR30-RM-2. Reaction Motors liked to call it a "turborocket" engine because it used turbopumps to supply its propellants, a relatively new concept. The XLR30 dated back to 1946 when Reaction Motors initiated the development of a 5,000-lbf engine to prove the then – new concepts of high-pressure combustion, spaghetti-tube construction, and turbine drive using main combustion propellants. By 1950, engineers believed these principles were sufficiently well established to initiate the development of a 50,000-lbf engine. The turbopump and its associated valves completed approximately 150 tests, and Reaction Motors considered it fully developed, with the exception of additional malfunction-detection and environmental tests that were required before a flight-approval test could be undertaken. The evaluation of a "breadboard" engine had demonstrated safe and smooth thrust-chamber starting, achieved 93-94% of the theoretical specific impulse, and shown satisfactory characteristics using film cooling.-126

The engine consisted of a single thrust chamber and a turbopump to supply the liquid oxygen and liquid anhydrous ammonia propellants from low-pressure tanks on the aircraft. These propellants had boiling points of -298°F and -28°F, respectively. That meant that after the propellants were loaded into the X-15 tanks, they would immediately begin to boil off at rates that were dependent upon the nature of the tank design and ambient conditions. In an uninsulated tank, liquid oxygen has a boil-off rate of approximately 10% per hour on a standard day. Even the crudest insulation significantly lowers this, and a well-insulated tank can experience less than 0.5% per hour of boil- off. Reaction Motors pointed out that insulating a tank usually required a great deal of volume, and that the airframe manufacturer would need to conduct a trade study to find the best compromise between volume and boil-off. Since the B-36 carrier aircraft had sufficient volume to carry additional liquid oxygen to top off the X-15, this was not a major issue. Anhydrous ammonia, on the other hand, has a relatively high boiling point and very low evaporation losses. Simply sealing the tank by closing the vent valve would minimize losses to the point that the ammonia would not have to be topped off before launch.-127

Reaction Motors did have some cautions regarding the hydrogen peroxide that powered the TR – 139 turbopump and the X-15 ballistic control system. It was necessary to maintain the propellant below 165°F to prevent it from decomposing, and Reaction Motors believed that it would be necessary to insulate all the valves, lines, and tanks. North American thought that only the main storage tank required insulation, because of the relatively short exposure to high temperatures. However, not insulating the entire system allowed small quantities of propellant (such as found in the lines supplying the reaction control system) to potentially reach elevated temperatures. To counter this, Reaction Motors recommended installing a continuous-circulation system whereby the propellant was kept moving through the lines in order to minimize its exposure to high compartment temperatures, particularly in the wings. If the engineers found the circulation system to be insufficient, it was possible to install a rudimentary cooling system on the main tank.-1281

20 40 ЄО 80 100 а ALTITUDE X 1000 FT.

ENGINE THRUST ENVELOPE

The final Reaction Motors contract called for an engine capable of being throttled between 15,000 Ibf and 50,000 lbf, although this was later raised to 57,000 lbf. Some engines actually produced more than 60,000 lbf. The engine needed to operate for 90 seconds at full power or 249 seconds at 15,000 lbf. (NASA)

Engineers considered the TR-139 thrust chamber very lightweight at 180 pounds. Furthermore, it used an assembly of "spaghetti tubes" as segments of the complete chamber, and, as it turned out, the spaghetti tubes would prove to be one of the more elusive items during engine development. The thrust chamber used ammonia as a regenerative coolant, but the exhaust nozzle was uncooled and configured to optimize thrust at high altitude. Reaction Motors expected to use a slightly altered XLR30 thrust chamber. The modifications included the incorporation of a liquid propellant igniter (for restarts) and derating to operate at 600 psia instead of 835 psia. The lower chamber pressure was desired to improve local cooling conditions at low thrust levels.-129

In order to improve safety, Reaction Motors proposed the simplest igniter the engineers could think of. The igniter was located along the centerline at the top of the chamber and had two sections. The first section contained a catalyst bed that used activated silver screens to decompose hydrogen peroxide into steam and oxygen at 1,360°F. The second section consisted of a ring of orifices where fuel was injected; when the fuel and superheated oxygen mixed, they combusted. The resulting flame was used to ignite the propellants in the combustion chamber. Reaction Motors believed this simple igniter would not be subject to the kind of failures that could

occur in electrical ignition systems. Despite the apparent desirability of this arrangement, a more traditional electrical ignition system was used in the final engine.[30]

The XLR30 turbopump was a two-stage, impulse-type turbine driving fuel and oxidizer pumps. The turbine operated at a backpressure of 45 psia at full thrust. The designers matched the pump characteristics to allow varying engine thrust over a wide range of thrust simply by varying the power input to the turbine. Varying the flow of hydrogen peroxide to a gas generator controlled the speed of the turbine. The gas generator consisted of a simple catalyst bed that decomposed the hydrogen peroxide into steam. Reaction Motors expected that the engine would need only 2.5 seconds to go from ignition to maximum thrust, and only 1 second to go from minimum to maximum thrust. On the other side, it would take about 1 second to go from maximum to minimum thrust, and not much more to complete a shutdown.-131

However, using a single turbine to drive both the fuel and oxidizer pumps resulted in the XLR30 liquid-oxygen pump operating at too high a speed for the new XLR99. Haakon Pederson, who became the principal designer of the XLR99 turbopumps, modified the original XLR30 oxidizer pump section to have a single axial inlet impeller operating in conjunction with a directly driven cavitating inducer. This required a new impeller design, new casting patterns, a new inducer, and a new pump case. Essentially, this was a new liquid-oxygen pump, and it became one of the major new developments necessary for the XLR99.-132

At this point, Reaction Motors expected to take 24 months to develop the new engine, followed by six months of testing and validation. The company would deliver the first two production engines in the 30th month, and manufacture 10 additional engines at a rate of one per month.-133

All parties finally signed the Reaction Motors contract on 7 September 1956, specifying that the first flight-rated engine was to be ready for installation two years later. The Air Force called the "propulsion subsystem" Project 3116 and carried it on the books separately from the Project 1226 airframe. The final $10,160,030 contract authorized a fee of $614,000 and required that Reaction Motors deliver one engine and a mockup, as well as various reports, drawings, and tools. The 50,000-lbf engine would be throttleable between 30% to 100% of maximum output. The 588- pound engine had to operate for 90 seconds at full power or 249 seconds at 30% thrust.-134

Less than two months after the Air Force issued the letter contract, the NACA began to question the conduct of Reaction Motors. On 11 April 1956, John Sloop from Lewis visited the Reaction Motors facilities and reported a multitude of potential development problems with the ignition system, structural temperatures, and cooling. Sloop reported that approximately 12 engineers were working on the engine, and that Reaction Motors expected to assemble the first complete engine in May 1957. However, Sloop believed that the Reaction Motors effort was inadequate and questioned whether the appropriate test stands at Lake Denmark would be available in late 1956. Sloop suggested that the company needed to assign more resources to the XLR99 development effort.-133

Despite the issues raised by Sloop, the Air Force did not seem to be concerned until 1 August 1956, when the Power Plant Laboratory inquired why scheduled tests of the thrust chamber had not taken place. It was not explained why four months had elapsed before the Air Force questioned the schedule slip.-133
important for maintaining the schedule. Reaction Motors also attributed part of the delay to modifications of two available test chambers to accommodate the high-powered engine.[37]

Simulation in the X-15 program meant much more than pilot training. It was perhaps the first program in which simulators played a major role in the development of an aircraft and its flight profiles. The flight planners used the simulators to determine heating loads, assess the effects of proposed technical changes, abort scenarios, and perform a host of related tasks. In this regard, the term "flight planner" at the AFFTC and FRC encompassed a great deal more than someone who sat down and wrote out a plan for a launch lake and a landing site. It is very possible that the flight planners (such as Elmore J. Adkins, Paul L. Chenoweth, Richard E. Day, Jack L. Kolf, John A. Manke, and Warren S. Wilson at the FRC, and Robert G. Hoey and Johnny G. Armstrong at the AFFTC) knew as much as (or more than) the pilots and flight-test engineers about the airplanes.-12!

The initial group of X-15 pilots worked jointly with research engineers and flight planners to

develop simulations to study the aspects of flight believed to present the largest number of potential difficulties. During late 1956, North American developed a fixed-base X-15 simulator at their Inglewood facility that consisted of an X-15 cockpit and an "iron bird" that included production components such as cables, push rods, bellcranks, and hydraulics. The iron bird looked more or less like an X-15 and used flight-representative electrical wiring and hydraulic tubing, but otherwise did not much resemble an aircraft. The simulator included a complete stability augmentation system (dampers), and ultimately added an MH-96 adaptive flight control system. Controlling the simulator were three Electronics Associates, Inc. (EAI) PACE 231R analog computers that contained 380 operational amplifiers, 101 function generators, 32 servo amplifiers, and 5 electronic multipliers. None of the existing digital systems were capable of performing the computations in real time, hence the selection of analog computers. The simulator could also compute a real-time solution for temperature at any one of numerous points on the fuselage and wing. Simulations were initiated in October 1956 using five degrees of freedom, and the simulator was expanded to six degrees of freedom (yaw, pitch, roll, and accelerations vertically, longitudinally, and radially) in May 1957.[3]

X-15 FLIGHT SIMULATION

Simulation in the X-15 program meant much more than pilot training and was the first program where simulators played a major role in the development of the aircraft and its flight profiles. Engineers used the simulators to determine heating loads, the effects of proposed technical changes, and to develop abort scenarios. Controlling the simulator were three Electronics Associates, Inc. (EAI) PACE 231R analog computers that contained 380 operational amplifiers, 101 function generators, 32 servo amplifiers, and 5 electronic multipliers. None of the existing digital systems was capable of performing the computations in real time, hence the selection of analog computers. (NASA)

The simulator covered Mach numbers from 0.2 to 7.0 at altitudes from sea level to 1,056,000 feet (200 miles), although it was not capable of providing meaningful landing simulations. The initial round of simulations at Inglewood showed that the X-15 could reenter from altitudes as high as 550,000 feet as long as everything went well. If done exactly right, a reentry from this altitude would almost simultaneously touch the maximum acceleration limit, the maximum dynamic pressure limit, and the maximum temperature limit. The slightest error in piloting technique would exceed one of these, probably resulting in the loss of the airplane and pilot. An angle of attack of 30 degrees would be required with the speed brakes closed, or only 18 degrees with the speed brakes open. The normal load factor reentering from 550,000 feet would reach 7 g, and a longitudinal deceleration of 4 g would last up to 25 seconds. Simulations in the centrifuge confirmed that pilots could maintain adequate control during these maneuvers, and considerations for the physical well-being of the pilot did not limit the flight envelope.-^

These first simulations indicated the need for a more symmetrical tail to reduce aerodynamic coupling tendencies at low angles of attack, and potential thrust misalignment at high velocities and altitudes. This resulted in the change from the vertical-stabilizer configuration proposed by North American to the one that was actually built. Reentry studies indicated that the original rate – feedback-damper configuration was not adequate for the new symmetrical tail, and an additional feedback of yaw-rate-to-roll-control (called "yar") was required for stability at high angles of attack.-51

Initially, the North American fixed-base simulator was computation-limited, and researchers could only study one flight condition at a time. The first three areas investigated were the exit phase, ballistic control, and reentry. Later, upgrades allowed complete freedom over a limited portion of a mission, and by mid-1957 unlimited freedom over the complete flight regime. By July 1958, the fixed-base simulator at North American already had over 2,000 simulated flights and more than 3,500 hours of experience under various flight conditions, and the airplane would not fly for another year.

As crude as it may seem today, the simulator nevertheless provided the flight planners with an excellent tool. The flight planner first established a detailed set of maneuvers that resulted in the desired test conditions. He then programmed a series of test maneuvers commensurate with the flight time available to ensure that the maximum amount of research data was obtained. Since the simulator provided a continuous real-time simulation of the X-15, it enabled the pilot to fly the planned mission as he would the actual flight, allowing him to evaluate the planned mission from a piloting perspective and to recommend changes as appropriate. Certain data, such as heating rates and dynamic pressures, required real-time computations to verify that the desired maneuvers were within the capability of the airplane.-61

The fixed-base simulator at North American was hardly a fancy affair, just a mocked-up cockpit with a full set of instruments and a television screen. The original cadre of pilots, including Joseph A. Walker, spent a considerable amount of time in the North American simulator before the one at the Flight Research Center was ready. Although crude by today’s standards, the X-15 pioneered the use of simulators not just to train pilots, but also to engineer the aircraft, plan the missions, and understand the results. Not surprisingly, given the involvement of Charlie Feitz, Harrison Storms, and Walt Williams in both the X-15 and Apollo programs, the X-15 pointed the way to how America would conduct its space missions. (NASA)

Engineers also used the simulator to develop vehicle systems before committing them in flight. One of the most notable was the MH-96 adaptive flight control system. Exhaustive tests in the simulator, conducted largely by Neil Armstrong, allowed researchers to optimize system parameters and develop operational techniques. Similarly, engineers used the simulators to investigate problems associated with the use of the dampers, and devised modifications to install on the airplane. Researchers then incorporated the results of flight tests into the simulator.-171

(excepting the computers) to the FRC before turning the first airplane over to the government. Unlike the Inglewood installation, at the FRC the cockpit and analog computers were in the same room: not much to look at, but functional. The Air Vehicle Flight Simulation Facility was located in building 4800 at the FRC in an area that later became the center director’s office. Like many early computer rooms, it used a linoleum-covered plywood false floor to cover the myriad of cables running beneath it. Large air conditioners installed on the building roof kept the computers cool. The X-15 simulator used a set of EAI analog computers procured for earlier simulations at the FRC, including one model 31R, one 131R, and one 231R that were generally similar to the computers used by North American. John P. Smith had begun mechanizing the original equations in the simulator, but Gene L. Waltman completed the task during the last three months of 1960 after Smith was promoted to a new job. The X-15 simulator became operational at the FRC on 3 January 1961. The X-15 simulator was the largest analog simulation ever mechanized at the FRC. The initial Air Vehicle Flight Simulation Facility at the FRC cost $63,000 and upgrades accounted for a further $1,700,000 by the end of 1968.-8

Because the FRC simulator was not yet operational, the flight planning for the first 20 flights used the North American simulator. Dick Day and Bob Hoey spent a considerable amount of time during 1959 and 1960 in Inglewood on flight planning and training the first cadre of pilots.-9 Initially, North American was to transfer the simulator from Inglewood to the FRC in January 1961, but the move was delayed for various reasons, including the need to integrate the MH-96 adaptive flight control system into X-15-3. By March 1961, however, Paul Bikle was becoming concerned: "With the performance envelope expansion program now underway, the requirement of traveling to NAA [North American Aviation] to use the X-15 simulator is becoming unduly restrictive in time and in obtaining the close working relationships essential to a sound flight panning effort." Something needed to change.-10

Bikle knew that North American did not want to transfer the simulator until the MH-96 integration was complete. In an effort to determine the consequences of moving earlier, Bikle called Dave Mellon at Minneapolis-Honeywell, who said he did not think the move would have an adverse affect on his schedule. Bikle also commented that "if a program delay is inevitable, it is preferable to delay the X-15-3 rather than the present program with the X-15-2." Bikle pushed to have the simulator moved to the FRC during April 1961. "We again want to emphasize that once the transfer has been accomplished, the NASA will make the simulator available for whatever additional simulator effort is required by NAA, M-H [Minneapolis-Honeywell], and other contractors…."-19

At first, the Flight Research Center made do with the crude cockpit that had been used in the centrifuge at NADCJohnsville. This was a cost-saving measure since the X-15 contract required North American to deliver their simulator (excepting the computers) to the FRC before turning the first airplane over to the government. Unlike the Inglewood installation, the cockpit and analog computers were in the same room at the FRC. The Air Vehicle Flight Simulation Facility was located in Building 4800 at the FRC in an area that later became the center director’s office. (NASA)

When the iron bird finally arrived in April 1961, engineers installed it along the east wall of the calibration hangar next door to the computer facility. A wall around the simulator provided some separation from the operations in the hangar. The cockpit faced away from the hangar door, and pilots discovered that sunlight coming through the windows caused visibility issues, so paint soon covered the windows. One of the unfortunate aspects of this installation was that the iron bird was located a little over 200 feet from the computers. This caused a number of signal-conditioning problems that a better grounding system eventually corrected. The hydraulic stand for the iron bird was originally located next to the mockup inside the hangar, but technicians subsequently relocated the unit to a small shed just outside, eliminating most of the noise from the simulator laboratory.-1121

To provide simulations that were more realistic, engineers at the FRC added a "malfunction generator" that could simulate the failure of 11 different cockpit instruments and 23 different aircraft systems. The instruments included a pressure altimeter, all three attitude indicators, and pressure airspeed, dynamic pressure, angle-of-attack, angle-of-sideslip, inertial altitude, inertial velocity, and inertial rate-of-climb indicators. The vehicle systems that could be failed included the engine, ballistic control system, both electrical generators, and any axis in the damper system. Later, the simulator could duplicate the failure of almost any function of the MH-96 adaptive control system. Almost all X-15 flights were preceded by practicing various emergency

The final simulator at the Flight Research Center was functionally identical to the one at North American, and used the same analog computers. The structure behind the cockpit is the "iron bird" that included production components such as cables, push rods, bellcranks, and hydraulics. The iron bird looked more or less like an X-15 and used flight-representative electrical wiring and hydraulic tubing, but otherwise did not much resemble an aircraft. The simulator included a complete stability augmentation system (dampers), and ultimately added an MH-96 adaptive flight control system. (NASA)

Contrary to many depictions of flight simulators in movies, the fixed-base simulator for the X-15 was not glamorous. The iron bird stretched behind the cockpit, but other than in size, it did not resemble an X-15 at all. The cockpit was open, and the sides of the "fuselage" extended only high enough to cover the side consoles and other controls inside of it. A canopy over the cockpit became necessary when researchers installed some instruments and controls (particularly for the experiments) there for later flights, but even then, it was made of plywood.-141

However, unlike most of the previous simulators at the FRC, the X-15 cockpit did have an accurate instrument panel. On one occasion, technicians inadvertently switched the location of the on/off switches for the ballistic control system and the APUs between the simulator and the airplane. It was normal procedure for the pilot to turn off the ballistic controls after reentry, and he practiced this in the simulator before each flight. During the actual flight, the pilot reached for the APU switch instead of the switch he thought was there. Fortunately, he caught himself and avoided an emergency. Everybody redoubled their efforts to ensure that the simulator accurately reflected the configuration of the airplane.15

When X-15-3 came on line with a completely different instrument panel arrangement, it presented some challenges for the simulator. Since the pilots needed to train on the correct instrument panel layout, the simulator support personnel had to swap out instrument panels to accommodate each different airplane. The technicians eventually installed a crank and pulley lift in the ceiling, along with cannon plugs for the electrical connections, to assist in making the change. On at least three occasions the program decided to make the instrument panels in the three airplanes as similar as possible, but they quickly diverged again as new experiments were added.1161

In addition to its simulation tasks, the iron bird found another use as the flight program began. Engineers and technicians at the FRC soon discovered that it was a relatively simple task to remove troublesome components from the flight vehicles and install them on the iron bird in an attempt to duplicate reported problems. Given the initial lack of test equipment available for the stability augmentation system and some MH-96 components, this proved a useful troubleshooting method. The simulator also played an important role in demonstrating the need for advanced display and guidance devices, and found extensive use in the design and development of new systems.-1171

The simulator had a variety of output devices in addition to the cockpit displays, including several eight-channel stripchart recorders and a large X/Y flatbed plotter. The plotter had two independent pens: one showed the X-15 position on a 3-foot-square map of the area, and the other indicated altitude. This plotter was identical to ones used in the control room and at the uprange stations. There were different maps for each launch lake showing the various contingency landing sites and prominent landmarks.1181

Eventually the FRC simulator grew to encompass six analog computers, and the patch panels needed to operate them contained 500 patch cords. The addition of a Scientific Data Systems SDS-930 digital computer in 1964 allowed the generation of nonlinear coefficients for the X – 15A-2. This required an additional analog computer as an interface between the new digital computer and the rest of the simulator. The SDS-930 was somewhat unusual in that it was a true real-time computer, complete with a real-time operating system and a real-time implementation of Fortran.1191

Despite its advanced specifications the SDS-930 was not initially satisfactory, which forced the flight planners to use the modified Dyna-Soar hybrid simulator at the AFFTC for the early X-15A – 2 flights. The SDS-930 was generally unreliable, normally because of memory-parity errors that the computer manufacturer attempted to fix on numerous occasions during 1965, with little success. The problem was not only affecting flight planning for the X-15A-2, it was also delaying simulations needed for the energy-management system scheduled to fly on X-15-3. During early 1966, the SDS-930 was extensively modified to bring it up to the latest configuration, including the addition of two magnetic-tape units and a line printer to assist in the energy-management simulations. While this was going on, the FRC took advantage of the downtime to upgrade the SAS and ASAS implementation on the iron bird, including replacing all of the computer interface equipment for both systems. Technicians also brought all of the mechanical rigging up to the same standard as the three airplanes. However, Johnny Armstrong and Bill Dana both recall that no actual flight planning or flight simulation was "totally digital."1201

The hybrid (analog-digital) simulator at the AFFTC initially provided a tool that enabled studies of the performance and handling of the X-20 glider, complete mission planning, and pilot familiarization. It was a logical outgrowth of the analog fixed-base simulators for the X-15. Although they had been ordered long before, the digital computers did not arrive at Edwards until

July 1964, six months after the cancellation of the Dyna-Soar program. The equipment sat mostly unused until the flight planners decided to adapt it to the X-15A-2 Mach 8 flight expansion program. This was done as much to provide Air Force personnel with some hands-on experience as for any demonstrated need for another X-15 simulator.-1211

The analog section of the hybrid simulator used PACE 231R-V and 231R computers similar to those used at the FRC and North American installations. Each computer had approximately 75 operational amplifiers, 170 potentiometers, 36 digitally controlled analog switches, and 26 comparators, and the 231R-V had a mode-logic group that supported an interface to a digital computer. The digital subsystem used a Control Data Corporation DDP-24 that had 8,192 words of ferrite core memory, a 5-microsecond access time, and a 1-MHz clock. Although a Fortran II compiler was available on the machine, engineers coded the real-time programs in assembly language to maximize the performance of the relatively slow machines. Two large patch panels connected the analog subsystem and digital subsystem.-1221

The fixed-base simulators at Inglewood and the FRC consisted of four major parts. The simulator included both controls and displays that were nearly identical to what the pilot found in the X-15 cockpit. The analog computer and malfunction generator were the heart of the system that provided the sequencing and control of the other components. The hydraulic control system was the "iron bird" and actually contained other flight components in addition to the hydraulic system including a complete stability augmentation system (or, later, a complete MH-96 adaptive flight control system). (NASA)

Like the other fixed-base simulators, the AFFTC device had a functional X-15 instrument panel, although it was not as exact as the ones used at the FRC. This was because its intended use was to investigate heating and control problems related to the X-15A-2, not to conduct pilot training. Ultimately, the program did use the AFFTC simulator for some X-15A-2 pilot training, but the final "procedures" training was conducted at the FRC.

Since the X-15 program technically did not need the simulator, the AFFTC engineers were able to develop a "generic" simulation that was usable for other aircraft, not just the X-15. This was an extremely astute idea, and the engineers subsequently used the simulator for the M2-F2, SR-71, X-24A, X-24B, and EF-111. The hybrid simulator was also the only one available to perform heating predictions during reentry simulations of the Space Shuttle Orbiter during the early 1970s, providing valuable input to that program.-123

At the FRC, the simulation team kept busy maintaining the computers and updating the programming to reflect actual flight results. During most of the flight program the simulation lab was busy for at least two shifts, and often three shifts, per day. The first shift performed pilot training and flight planning, the second shift conducted control-system and other studies, and maintenance and reprogramming occupied the third shift as needed. However, the team generally took weekends off. This was not necessarily a good thing for the simulator since it took the analog computers quite a while on Monday morning to warm up.-123

Despite the apparent success of the fixed-base simulators, everybody recognized their limitations. The primary concern was that they were fixed-base and not motion-base, and therefore were inappropriate for landing training. For instance, the lack of a high-quality visual presentation meant that critical visual cues were not available to the pilots. The analog computer also had limitations. For example, the precision needed to calculate altitude and rate of climb for the landing phase was not readily achievable with the parameter scaling used for the rest of the flight. The parameter scaling was critical, and analog computers were accurate to about one part in 10,000. For the X-15 simulation, with the altitude scaled such that 400,000 feet equaled 100 volts, one-tenth of a volt was equal to 40 feet. Any altitude less than this was down in the noise of the analog components and barely detectable. It was simply not possible to calculate accurate altitudes for the landing phase and the rest of the flight profile at the same time. All of this necessitated maintaining a fleet of Lockheed F-104 Starfighters as landing trainers, something the X-15 pilots did not seem to mind at all.-123

Nevertheless, Larry Caw and Eldon Kordes did mechanize a simple four-degrees-of-freedom simulation to study landing loads early in the program. The simulation only covered the last few seconds of a flight, and was not particularly useful as a pilot training tool. However, it allowed Jack McKay and other engineers to look at the variety of forces generated during an X-15 landing, and prompted the first round of landing-gear changes on the airplane.-126

The lack of a motion-base simulator presented several interesting problems. For instance, some phenomena experienced in the JF-100C variable-stability airplane during the summer of 1961 indicated that using the beta-dot technique in the X-15 might be more difficult than anticipated. Consequently, a cooperative program was initiated with NASA Ames to use its three-axis motion – base simulator. The objective was to investigate further the effect of g-loading on the pilot while he performed beta-dot recovery maneuvers. Four pilots-Forest Petersen, Bob Rushworth, Joe

Walker, and Bob White-participated in the tests during September 1961. Paul Bikle reported that, "With fixed-base simulation, the ventral-on condition was uncontrollable, using normal techniques; however, it could be controlled by using the special beta-dot control technique. With the moving cockpit simulation, control using either normal or beta-dot techniques was more difficult for the pilot than with the fixed-base cockpit simulation. These results were in general agreement with the ground and flight tests conducted with the variable-stability F-100 airplane."271

By the end of the X-15 program, the FRC had established simulation as an integral part of the flight program. Today, the Walter C. Williams Research Aircraft Integration Facility (RAIF) provides a state-of-the-art complex of computers, simulators, and iron-bird mockups. As an example of the extent to which simulations were used, during the X-33 program, pilot Stephen D. Ishmael flew countless missions while engineers evaluated vehicle systems, flight profiles, and abort scenarios. What is ironic is that the X-33 was to be an unmanned vehicle— Ishmael was just another computing device, one with a quick sense of reason and excellent reflexes.