Category X-15 EXTENDING THE FRONTIERS OF FLIGHT

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.

X-15 flights generally began early in the morning; indeed, most flight-testing at Edwards began early in the morning when the temperatures and winds in the high desert were lower. The ground crew had mated the X-15 to the NB-52 the day before and stayed all night or arrived early to prepare the airplane for the flight. Floodlights lit the scene as propellants and gases were loaded onto both the carrier aircraft and research airplane, and liquid-oxygen vapor drifted around the area, lending a surreal fog. When the X-15 pilot arrived, he generally went straight to the physiological support van to get into the David Clark full-pressure suit. Getting the suit on and hooking up the biomedical instrumentation took about 15 minutes once the program switched to the A/P22S-2 suits; the MC-2 suits had taken considerably longer.-1^5

When the ground crew was ready for the pilot to enter the cockpit, two technicians carried a portable cooling system and other equipment while they escorted the pilot from the van to the airplane-a scene vaguely similar to Cape Canaveral before a space flight. Oddly, the driver of the physiological support van in which the pilot donned the pressure suit made no particular effort to park near the X-15, forcing the pilot to walk across the ramp. A large ladder and platform were located alongside the X-15 to allow the pilot and his handlers easy access to the cockpit. The cockpit itself was large for a single-seat airplane, but the bulk of the pressure suit made it seem somewhat smaller. Nevertheless, most pilots found it had more than adequate room and some of the smaller pilots even had difficulty reaching all of the controls mounted far forward, since the seat was not adjustable. Once the pilot was in the cockpit, the ground crew hooked up a myriad of lines, hoses, and straps that provided life support and monitored the pilot’s biomedical data.

While this was happening, the pilot began going through the preflight checklist to verify the status of all the aircraft systems. Once this was completed (usually a 30-minute process), the ground crew closed the X-15 canopy. The cockpit suddenly seemed smaller since the canopy fit snugly around the pressure-suit helmet.

While this was happening, the ground crew was disconnecting the servicing carts used to prepare the NB-52 and X-15 for flight. At this point, the NB-52 started its engines and the carrier aircraft pilots went through their preflight checklist, taking about 10 minutes to complete the activity. The ground crew then closed up the NB-52 hatches and the mated pair taxied toward the runway accompanied by a convoy of a dozen or so vehicles. Edwards is a large base, and the aircraft had to taxi for 2 or 5 miles depending on which runway was active. One of the H-21 helicopters took off and performed a visual check of the runway to make sure no debris was present, then took up a position beside and slightly behind the bomber, preparing to follow it down the runway for as long as possible.

At the end of the runway, the ground crew removed the safety pins from the X-15 release hooks. When everybody signaled they were ready, the NB-52 took off and climbed to 25,000 feet while circling over Edwards to make sure the X-15 could make an emergency landing on Rogers Dry Lake. Once above 25,000 feet, the NB-52 turned toward the launch lake and began climbing to 45,000 feet, since at this altitude the X-15 could glide to an alternate lakebed if necessary. The NB-52 supply topped off the X-15 liquid-oxygen tank, and the inertial platform was receiving alignment data, but otherwise things were quiet. Chase-1 flew in formation with the B-52, observing the X-15 for leaks or other anomalies that might signal a potential problem.

The mission rules dictated that if a serious problem occurred on the NB-52 while the mated pair was on the way to the launch lake, the carrier aircraft would jettison the research airplane since the extra 30,000 pounds of dead weight under the right wing would undoubtedly be detrimental to saving the NB-52. Similarly, if something happened on the X-15 that looked like it would endanger the NB-52, the research airplane would be jettisoned. As Scott Crossfield later observed, "It was not heroics; it was simple mathematics. Better to lose one man than four." In reality, the X-15 stood a chance of surviving if it was jettisoned, especially if the X-15 pilot had some advanced notice. The major problem was that neither of the APUs aboard the X-15 was running during captive carry since there was not enough propellant to last more than about 30 minutes. During the climb-out, the NB-52 supplied all of the X-15’s electrical needs, as well as breathing oxygen and pressurization gas. If the carrier aircraft jettisoned the X-15, the pilot would have his hands full trying to get the APUs started using a small emergency battery since without the APUs the pilot had no flight controls, no radio, no anything. If the APUs started, the pilot could try to fly (with or without the engine) to a lakebed. Of course, the ejection seat was always an option. Fortunately, the program never had to find out what would happen in that scenario.-1161

The need for the various lakebeds was largely driven by a program requirement to always have a landing site available to the X-15 pilot if he needed it for any reason. Therefore, each flight was planned such that the X-15 could glide to an emergency landing site from any point on the flight path, although frequently the nearest site was behind the airplane and required the pilot to turn around, as illustrated in this drawing. The program used the emergency landing sites 10 times. (NASA)

At 12 minutes before the scheduled launch time, things began to happen. The X-15 pilot started both APUs and began to run through the prelaunch checklists. The pilot checked all of the X-15 systems, exercised the flight controls, tested the ballistic control system, and set all switch positions. Chase-1 observed the results of these tests and reported them back to the X-15 pilot. During this time, radar and radio communication with NASA-1 guided the carrier aircraft into position near the launch lake. Eight minutes before launch the NB-52 began a long sweeping turn back toward Edwards, coming onto the final heading about 4 minutes later. At the same time, the X-15 pilot began activating the propulsion system. At 2 minutes prior to launch the X-15 pilot started the data recorders, checked the ball nose one last time, and turned on the cameras. One minute prior to launch the XLR99 was set to precool and the igniter was set to idle. More checks were performed to make sure the engine looked ready to fire. The X-15 pilot took a deep breat h.[l7]

Three, two, one: launch. The X-15 separated from the NB-52 and began to fall. The launch was harder than most pilots initially expected because the X-15 went from normal 1-g flight while attached to the carrier aircraft to 0-g flight instantaneously. The X-15 also wanted to roll to the right because of downwash from the NB-52 wing and interference from the fuselage. The X-15 pilots usually had left roll input applied at the moment of launch, but the airplane still rolled – more so on some occasions than others.

The XLR99 start sequence was remarkably simple, a necessary attribute in the days before computerized control systems. The first step was to pressurize the propellant tanks with gaseous helium to ensure a smooth flow of propellants to the turbopump. Then the oxidizer system was precooled to ensure that the liquid oxygen did not vaporize between the propellant tank and the turbopump (vaporized liquid oxygen caused the turbopump to cavitate and go into an overspeed condition that resulted in an automatic shutdown). It required about 10 minutes to chill the oxidizer system. Next was the engine prime sequence that fed a small amount of liquid oxygen and ammonia to the turbopump. The igniter-ready light came on when the prime cycle began and the pilot turned on the igniter switch. This all happened before the X-15 dropped away from the NB-52. As the X-15 was falling, the pilot continued the engine start procedure. There were about 10 seconds available to light the engine before the pilot had to abort to the launch lake; that was time for two ignition attempts.-118

Pressing the pump-idle button to start the turbopump initiated the ignition cycle. The turbopump spooled up quickly and forced propellants into the first-stage igniter, where a spark plug ignited them. The propellants were then fed into the second-stage igniter chamber, where the flame generated by the first-stage igniter caused them to combust. The second-stage igniter produced 1,500 lbf-as much as one chamber on the XLR11.

The throttle quadrant was a "backwards L" slot located on the left console. The outer corner was the "idle" position, the bottom inside corner of the L corresponded to minimum throttle, and the most forward position was 100% power. Moving the throttle inboard opened the main propellant valves and forced 30 gallons per second of propellants into the main chamber, where they were ignited by the flame from the second-stage igniter. The pilots found early in the flight program that the engine did not ignite reliably at low-power settings, so they usually immediately

advanced the throttle to the 100% position. Although the XLR99 proved to be a remarkably reliable engine, it really did not like to throttle. Still, the capability provided a certain amount of research utility that would not otherwise have been available, although it also contributed to several in-flight emergencies when the engine decided it no longer wanted to work as its designers had intended.-1191

In an attempt to ensure that the entire propulsion system was in working order, NASA conducted a ground run before almost every flight. Although it was comforting to the X-15 pilot to know the engine indeed seemed to work, these tests also added a great deal of wear and tear to the engines and other systems. During ground runs, the pilot would allow the engine to stabilize at 100% thrust for 8 seconds, and then retard it to idle for 5 seconds before shutting the engine down. The pilot would then perform an emergency restart sequence that relit the main chamber at 75% thrust. The pilot would stabilize the engine for 5 seconds, reduce it to idle for a couple of seconds, and then shut it down. It all became routine.

Energy management started the instant the NB-52 released the X-15. If the XLR99 did not light in two attempts, the pilot would make an emergency landing at the launch lake. If the engine died within the first 30-40 seconds of flight, the pilot would turn around and make an emergency landing at the launch lake. After about 40 seconds of burn, the airplane would be too far away to make it back to the launch lake, but if the engine burned less than 70 seconds, it was unlikely the pilot could make it to Rogers Dry Lake. The 30-second period in between was why the program had a large assortment of intermediate lakebeds available.

Unfortunately, the technology did not exist to provide the pilot with an energy-management display, although NASA installed a rudimentary unit in X-15-3 toward the end of the flight program. It was up to the ground controller (NASA-1) to advise the pilot where to land if a problem developed during the flight. As flight attendants are fond of saying on every commercial airline flight, the nearest exit may be behind you. In many cases, the best landing site was an intermediate lake that the airplane had already passed at hypersonic velocities.

The intermediate lakes were more important for the high-speed flights than for the altitude flights. Given enough altitude, the X-15 could glide for over 400 miles-more than enough distance to make it back to Edwards from almost any point on the High Range. Every flight was supposed to have excess energy as the airplane arrived over Edwards, allowing some flexibility during the final approach. Nevertheless, part of why the program had an excellent safety record was that the pilots and flight planners always had contingency plans—even for the contingency plans.

Most X-15 flights were essentially in the vertical plane, and it was important to establish the proper heading back toward Edwards during the first 20 seconds after launch. Once the engine shut down, the ballistics was pretty well established for the next few minutes of flight. If there was a wind at the launch altitude (and usually there was), the NB-52 would crab as necessary to maintain the proper ground track during the final 10-minute turn. At 1 minute to launch, the NB – 52 pilot would turn to the desired launch heading and allow the carrier aircraft to drift. Since winds at launch did not seriously affect the X-15 trajectory, this minimized the workload on the X-15 pilot to obtain, and hold, the desired heading. After launch, any necessary heading corrections were made by the X-15 pilot using small bank angles while performing a 2-g rotation and accelerating from Mach 1 to 2 in about 20 seconds. Once the X-15 reached the desired pitch angle, the g-level was less than one, and no further turning corrections could be made until after completion of the reentry.-120

The design speed mission was flown at relatively low altitudes – from 100,000 to 110,000 feet. These were the essential heating flights that were used to validate the various theoretical and experimental (wind tunnel) results. The time at maximum speed was not spent flying straight and level since the pilot was conducting a series of rudder pulses and other maneuvers to optimize the heating on the side of the aircraft that was instrumented. The ability to exactly repeat these maneuvers from one flight to the next was critical for the ultimate success of the flight program and a tribute to the flying skills of the pilots. (NASA)

The thrust from the XLR99 could be terminated by one of two routine ways at the nominal end of burn. The most frequently used method was called "shutdown." When a specific set of flight conditions had been reached, the pilot would manually shut down the engine. Normally the pilot did this after a precalculated amount of time based on a stopwatch in the cockpit that started when the main propellant valves opened. After NASA installed the X-20 inertial systems later in the program, the pilot could also use inertial velocity to shut down the engine, and several of the high-altitude flights used the altitude predictor installed in X-15-3. The other type of thrust termination was called "burnout." In this method the pilot just let the engine burn until the propellants were exhausted and the engine quit.-121

The high-speed flights were conducted at fairly low altitudes (a relative term since the altitudes would have been considered extraordinary before the X-15 program began). For these flights, the X-15 was essentially an airplane; its wings generated lift, maneuvering was accomplished via a set of aerodynamic control surfaces, and the atmosphere created a great deal of drag and friction on the airframe. The pilot would begin a 2-g rotation to the desired pitch angle immediately after the engine lit. During this rotation the primary piloting task was to adjust the bank angle to attain and hold the desired heading back to Edwards. As he approached 70,000 feet, the pilot initiated a gentle pushover to come level at something between 100,000 and 110,000 feet. As the airplane came level, the pilot either stabilized his speed at some preset value to conduct various research maneuvers, or continued to accelerate to attain more speed. The X-15 liked to accelerate; even at the top end, it took only 6 seconds to accelerate from Mach 5 to Mach 6. The research maneuvers continued after engine burnout until the airplane decelerated to the point that no more useful data were forthcoming. These were the essential heating flights.[22]

Altitude flights began much the same way, except that the pilot continued a steep climb out of the atmosphere. The engine shut down on the way up and the airplane coasted over the top on a ballistic trajectory. The pitch angle, in conjunction with the shutdown velocity, established both the range and maximum altitude of the arc that would follow. As the airplane continued on the ballistic trajectory, it was committed to a steep descent back into the atmosphere. The pilot set up the angle of attack for reentry, performed a pullout to level flight after reentry, and then began a shallow descent during the glide back to Rogers Dry Lake. A combination of dynamic pressure (q), load factor (g), and structural temperature limited the reentry since the relaxation of one parameter resulted in an excess of one of the others. These flights spent between 2 and 5 minutes outside the atmosphere, much of that time in a weightless (i. e., no accelerations) environment. The ballistic control system allowed the pilot to maintain attitude control, but he could not change the flight path of the airplane. Contrary to popular lore, as fast as it was, the X – 15 never flew anywhere near fast enough to attain orbital velocities or altitudes.-1231

For the next few minutes, the calls from NASA-1 were primarily comparisons between the planned profile (on the plot boards) and the actual radar track of the airplane. These let the X-15 pilot know how well he had flown the exit phase and, more importantly, what maneuvers might be required during reentry. If he was "high and long," he would expect to make an immediate turn and apply the speed brakes during the latter part of the reentry. If he was "low and short," he would expect a straight-ahead glide with brakes closed. A "left of course" call would alert him to expect a right turn to a new heading after reentry. The ability to comprehend some of these energy-management subtleties while simultaneously controlling the aircraft’s attitude and subsystems, and accomplishing test maneuvers was one of the goals of the X-15 simulator training.-1241

Perhaps surprisingly, the altitude flights required a longer ground track than the high-speed flights. This was primarily because the airplane covered many miles while it was outside the atmosphere. Let us use the two maximum performance flights as comparative examples. Joe Walker’s 354,200-foot altitude flight required a ground track of 305 miles to climb out of the atmosphere, coast to peak altitude, reenter, make the pullout, and then slow to land. On the other hand, Pete Knight’s 4,520-mph speed flight only took 225 miles, mainly because the airplane slowed quickly after engine burnout since the speed flights occurred in a relatively dense atmosphere.-1251

Surprisingly, the X-15 spent more time at higher Mach numbers during the altitude missions than it did during the speed missions. This is because there is less aerodynamic drag at high altitudes and the airplane coasted at high velocities for longer while it was outside the atmosphere. The altitude missions were particularly demanding on the pilot since even small deviations from the planned profile could result in overshooting the target altitude. The ballistic control system was required to control the attitude of the airplane during these missions, most of which were flown by X-15-3 since the MH-96 adaptive control system provided more redundancy. (NASA)

During the envelope expansion and heating flights, the pilots performed a specific set of maneuvers (rudder pulses, angle-of-attack changes, and rolls) to evaluate the stability and control of the airplane in various flight regimes. Many times these maneuvers were near the limits of controllability for the airplane, and well-practiced contingency plans were always at the ready. Other tests provided information on control effectiveness, aerodynamic performance, lift-to-drag ratio, and aero-thermo loads. All of these maneuvers required that the pilot fly at a specific speed, attitude, and altitude while gathering the data. Often, the program needed to exactly duplicate the profile on subsequent flights to eliminate variables from the data, a decided challenge before the advent of computerized flight-control systems.-126

As Milt Thompson later observed, "This is the kind of thing a research pilot is required to do to earn his money-accomplishing good maneuvers for data purposes. Flying the airplane is just something the pilot does to get the desired test maneuver. He can be the greatest stick and rudder pilot in the world, but if he cannot do the required data maneuvers, he is worthless as a research pilot." Most of the X-15 pilots were very good research pilots.-127

Assuming all went as planned, the X-15 arrived back at Edwards and set up a high key (the highest point of the final approach to a runway) at approximately 35,000 feet and 290 to 350 mph. As he approached Edwards, the X-15 pilot began dumping any residual propellants to lower the landing weight and to get rid of potentially explosive substances. It also made a convenient way for the chase planes to find the small X-15 in the vast skies over the high desert. The X-15

then entered a 35-degree banking turn while maintaining 250 to 300 knots. The pilots normally turned to the left, although each pilot seemed to develop a preference, and it really did not matter much. At the completion of the turn, the X-15 was approximately 4 miles abeam of the intended touchdown point at 18,000 feet altitude headed in the opposite direction of the landing runway (this was low key). The pilot then continued around 180 degrees, turned onto final at about 8,000 feet and 300 knots, and flared at around 1,000 feet. The pilot jettisoned the ventral rudder, lowered the landing flaps as the airplane came level at about 100 feet, and deployed the landing gear at 215-225 knots. Touchdown was generally made at around 190-200 knots. The pilot judged the possible crosswinds by the simple expedient of looking at the smoke from flares beside the runway.-1281

Unsurprisingly, not all flights arrived at high key exactly as planned. At least one flight arrived at high key at 80,000 feet and over Mach 3.5, another made high key at only 25,000 feet, and one made a straight-in approach because it was too low on energy when it arrived at Edwards. Despite these variances, the majority of X-15 touchdowns were made within 2,000 feet of the intended spot, although a couple of flights missed by over 4,000 feet. Neil Armstrong managed to miss by 12 miles-fortunately, Rogers is a large dry lake. The X-15 generally slid for 8,000-10,000 feet before coming to a stop, chased by a convoy of rescue and support vehicles.-291

The general concept was similar to that ultimately adopted as the terminal area energy – management maneuver used by the Space Shuttle. The proven ability of the X-15 (and later the heavyweight lifting bodies) to make unpowered approaches was one reason the Space Shuttle program decided it could eliminate the complexity of landing engines and make the Orbiter a glider. It is another enduring legacy of the X-15 program.-301

After Jack McKay’s emergency landing in X-15-2 on 9 November 1962, North American proposed modifying X-15-2 to an advanced configuration capable of reaching Mach 8 velocities. NASA in general and Paul Bikle in particular were not particularly enthusiastic and felt the Air Force should simply repair the aircraft to its original configuration or retire it altogether. Many researchers believed that the Mach 8-capable X-15A would be of limited value for aero-thermo research. However, NASA did not press its views, and on 13 May 1963 the Air Force directed North American to repair and modify the aircraft at a cost of $4.75 million. The advanced aircraft was intended to evaluate an air-breathing hypersonic research engine (HRE) being developed at Langley. It was designed to reach 8,000 fps at an altitude of 100,000 feet, and a dynamic pressure of 1,000 psf. Heating rates up to 210 Btu per square foot per second were expected, with peak structural temperatures approaching 2,400°F.[231

The modifications did not significantly alter the physical appearance of X-15A-2. The wingspan was still 22.36 feet, but the airplane was 29 inches longer due to a plug in the center of gravity compartment between the propellant tanks. Perhaps the most obvious change was the addition of external propellant tanks on each side of the fuselage below the wings. These allowed the airplane to carry approximately 70% more propellant, a necessary ingredient in raising performance to 8,000 fps. The tanks provided an additional 60 seconds of engine burn time, for a total of 150 seconds at 100% power. Other modifications included adding hydrogen-peroxide tanks within extended aft-side fairings to supply the turbopump for the longer engine burn times, and additional pressurization gas in a spherical helium tank just behind the vertical stabilizer.12321

The fuselage extension provided additional internal volume for experiments, and the center-of – gravity compartment access doors could accommodate optical windows looking up or down. The compartment could also accommodate a liquid-hydrogen tank, with a total capacity of 48 pounds, to fuel the ramjet mounted under the fixed portion of the ventral stabilizer, but it appears NASA never actually installed the tank. Perhaps the most difficult part of incorporating this extension was moving the B-52 pylon attach points to maintain the vertical stabilizer in the appropriate position under (through) the NB-52 wing.-12331

North American strengthened the landing gear using a strut that was 6.75 inches longer than the original. This provided 33 inches of ground clearance to the bottom of the fixed ventral stabilizer; the expected ramjet was 30 inches in diameter. The new strut provided a 1,000-pound increase in allowable landing weights, but there was some concern over the effects of the longer strut on nose-wheel and forward fuselage loads. In an attempt to provide an additional margin of safety, the nose-gear trunnion was mounted 9 inches lower (effectively lowering the nose gear by the same amount), allowing an attitude at nose-gear touchdown that was similar to that of the basic airplanes.12341

Instead of the trapezoidal windows on the original airplanes, North American installed elliptical windows that used three panes of glass to withstand the increased temperatures at Mach 8. The outer pane was 0.65-inch-thick fused silica, the middle pane was 0.375-inch alumino-silicate, and the inner pane was 0.29-inch laminated soda lime glass. This time the company mounted the outside windshield-retaining frame flush with the glass to prevent the reoccurrence of the flow heating experienced early in the program.-12331

The X-15A-2 modification also included a "skylight" hatch. Two upward-opening doors, 20 inches long by 8.5 inches wide, were installed above the instrument compartment behind the cockpit to expose cameras and other experiments. North American revised the normal research instrumentation elevator so that the upper shelf could extend upward through the open hatch if needed. The modifications also included additional data recorders: five 36-channel oscillographs, eight three-channel oscillographs, two 14-track tape recorders, one 24-cell manometer, and one cockpit camera. In addition, a new 86-channel PDM telemetry system was used to transmit data to the ground in real time.-12361

The accident had seriously damaged the outer portion of the right wing. North American found that it could adequately repair the main wing box, but the outer 41 inches were a total loss. With the government’s concurrence, the company modified the wing box to support a replaceable outer panel that allowed the testing of various materials and structures during hypersonic flight. The panel provided with the airplane (and the only one that apparently ever flew) was similar in construction and materials to the standard X-15 wing except that it was equipped with a 26.7 by 23-inch access panel that allowed access to an extensive amount of research instrumentation.-12321

TANKS

After Jack McKay’s emergency landing at Mud Lake, the Air Force had North American rebuild the X-15-2 with several modifications intended to allow flights to Mach 8 to support ramjet propulsion research. The X-15A-2 was designed to reach 8,000 fps at an altitude of 100,000 feet, and a dynamic pressure of 1,000 psf. Heating rates up to 210 Btu per square foot per second were expected, with peak structural temperatures approaching 2,400 degrees Fahrenheit.

A 29-inch fuselage extension provided a larger center-of-gravity compartment to hold a liquid hydrogen fuel tank for the proposed ramjet, and external tanks carried additional propellants for the XLR99. (NASA)

NASA conducted wind-tunnel tests of the X-15A-2 during the summer and fall of 1963. The tests indicated that there was little aerodynamic difference between the modified X-15A-2 without external tanks and the basic X-15. Despite the anticipated similarities, engineers decided it was prudent to conduct an abbreviated flight series to verify that the airplane still handled satisfactorily. Stability and control maneuvers conducted during the initial flights of the X-15A-2 largely verified the wind-tunnel predictions. However, the verification took significantly longer than expected when the program encountered trouble with the modified landing-gear system.-1238

North American completed final assembly of X-15A-2 in Inglewood on 15 February 1964, and the Air Force accepted the airplane on 17 February, three weeks ahead of schedule and slightly below budget. The airplane was, however, 773 pounds overweight, a condition that was expected to reduce the maximum velocity somewhat. The design launch weight was 49,640 pounds, and propellants accounted for 32,250 of these pounds (18,750 pounds internally and 13,500 pounds in the external tanks). Subsequent modifications would add another few hundred pounds in empty weight. North American delivered the airplane to the FRC on 18 February and an "official" government acceptance ceremony took place on 24 February.-239

flight was to check out the various systems, evaluate the handling qualities of the modified airplane, and gain preliminary experience with the ultraviolet stellar photography experiment

(#1).[240]

It was not bad for a "checkout" flight. Using 77% thrust, Bob Rushworth reached Mach 4.59 and 83,300 feet. In an ironic twist, Jack McKay, who had been injured on the flight that damaged the airplane, was the NASA-1 controller. As expected, Bob Rushworth reported that the modified X- 15A-2 handled much like a basic X-15. Static longitudinal stability remained about the same, despite a 10% forward shift in the center of gravity. The already low directional stability of the unmodified airplane was somewhat lower in X-15A-2, but Rushworth did not think it posed a significant threat to safety. The longitudinal trim characteristics of the modified airplane were essentially unchanged up through Mach 3 at angles of attack up to 15 degrees. The modified airplane had a trim capability reduced by approximately 3-5 degrees at higher angles of attack and Mach numbers.*241

Things got more exciting on the second flight (2-33-56). Shortly after a maximum Mach number of 5.23 was obtained, the nose gear unexpectedly extended as the airplane decelerated through Mach 4.2. After the flight Rushworth wrote, "Everything was going along fine and just about the time I was ready to drop it over [lower the nose] I got a loud bang and… the resulting conditions that I had gave me quite a little bit of concern because the airplane began to oscillate wildly and I couldn’t seem to catch up with it. I put the dampers back on and stuffed the nose down to about 5 degrees angle of attack and it seemed to be normal then except I had a sideslip and I was then required to use left roll to hold the airplane level. A couple of seconds later I realized that this sound that I had heard was very much similar to the nose gear coming out in the landing pattern so that was the only thing I could think of. I announced that and then a few seconds later I began to get smoke in the cockpit, quite a little bit more than I had ever seen before. This partially confirmed that the nose gear, at least the door was open. I wasn’t sure that the gear was out but it was; there was enough of an explosion there to make me think that the gear was out."[242]

For the time being, the chase planes were of no help in confirming the problem since they were some 10 miles below the X-15. Jack McKay as NASA-1 could not help much either, since no emergency procedures existed for this particular failure. McKay did advise Rushworth that it would probably be best if the X-15 remained at high altitude until it had slowed considerably, thereby easing the aero-thermo loads on the extended nose gear. At one point NASA-1 advised Rushworth to use the brakes to slow down a bit, but Rushworth had other ideas: "No, I don’t want to get brakes out, I want to get the damn thing home." Fifty miles away from Edwards, the X-15 was still traveling at Mach 2.5 and McKay advised Rushworth, "Let’s go max L/D, Bob. You’re looking OK now. Your heading is good. You’re on profile. Looks like you’ve got plenty of energy."*241

The chase planes finally spotted the X-15 as it was descending 20 miles northeast of Edwards. Despite the degraded control and increased drag resulting from the extended nose gear, Bob Rushworth was doing fine. Joe Engle in Chase-3 verified that the nose gear appeared to be structurally sound and in the locked-down position. As for the tires, Engle reported, "OK, Bob, your tires look pretty scorched; I imagine they will probably go on landing." There was a worry, however, that the oleo strut had also been damaged by the heat and dynamic pressure; if it failed on landing, the X-15 could break in half or worse. There seemed to be little choice. Engle was right-the tires disintegrated shortly after the nose gear came down, but Rushworth managed to stop the airplane without serious difficulty.*244*

An investigation revealed that aerodynamic heating was the cause of the failure. The expansion of the fuselage was greater than the amount of slack built into the landing gear release cable. This caused an effective pull on the release cable that released the uplock hook. An outward bowing of the nose gear door imposed an additional load on the uplock hook. The load from both of these sources caused the uplock hook to bend, allowing the gear to extend. Engineers duplicated this failure in the High-Temperature Loads Calibration Laboratory by simulating the fuselage expansion and applying heat to the nose-gear door.[245]

The same stability and control data flight plan was duplicated for the next X-15A-2 light (2-34­57) on 29 September 1964. Again, shortly after reaching a maximum Mach number of 5.20, Bob Rushworth experienced a similar but less intense noise and aircraft trim change at Mach 4.5-the small nose-gear scoop door had opened. In his post-flight report Rushworth noted, "Yes, I sensed it was the little door, because the magnitude of the bang when it came open wasn’t as large as the other experience." During the normal gear-extension sequence, air loads on the small door pulled the nose gear door open to assist in the extension of the nose gear. Although not as serious a failure as that on the previous flight, it again precluded obtaining dampers-off stability data. NASA redesigned the nose-gear door to provide positive retention of the scoop door regardless of the thermal stresses. Engineers also modified the other two airplanes since the basic failure mode was common.[246]

To check out the modifications to the nose gear, the program decided on a low-speed flight (2­35-60) to a maximum Mach number of 4.66. The flight planners decided to give Bob Rushworth a break after the two previous adventures, so Jack McKay made this flight, which went off without a problem. The nose gear performed normally.-12471

Rushworth was in the cockpit again for the next fight (2-36-63) of X-15A-2 on 17 February 1965. In a run of bad luck that is hard to fathom, this time the right main skid extended at Mach 4.3 and 85,000 feet. In his post-flight report Rushworth wrote, "Jack [McKay, NASA-1] was talking away and things were going along real nice and I couldn’t seem to get a word in there to tell him that I had a little problem. It took several seconds to get the airplane righted and dampers back on, very much similar to the nose gear coming out. Once I got it righted, I realized that I had a tremendous sideslip, I guess 4 degrees, and it took a lot of rudder deflection to get sideslip to zero. This persisted all the way down until I got subsonic. Once I had gone subsonic the airplane handled reasonably well." Again, the chase pilot was able to verify that the gear appeared structurally sound, and Rushworth managed to make a normal landing. When Rushworth finally got out of the airplane, he turned around and kicked it-enough was enough. Post-flight inspection revealed that the uplock hook had bent, allowing the gear to deploy. Again, aerodynamic heating was determined to be the source of the high load on the uplock hook.[248]

NASA flew five more X-15A-2 flights (2-38-66 through 2-42-74) before the envelope expansion program was begun. These flights were primarily conducted to study stability and control, but they also included landing-gear performance tests. Each flight carried the ultraviolet stellar photography experiment but obtained little useable data because of problems in maintaining the precise attitudes required for the experiment. Fortunately, the landing gear seemed to behave throughout these flights, but it had caused this portion of the flight program to take longer than expected.[2491

The engineers had always had some concerns about operating the X-15A-2 with the 23.5-foot – long, 37.75-inch-diameter external tanks. These attached to the airplane structure within the side fairings at fuselage stations 200 and 411. Propellant and gas interconnects ran through a tank pylon that was located between stations 317 and 397 and was covered by a set of retractable doors after the tanks were jettisoned. The left tank contained about 793 gallons of liquid oxygen in one compartment and three helium bottles with a total capacity of 8.4 cubic feet. The right tank contained about 1,080 gallons of anhydrous ammonia in a single compartment. The empty left tank weighed 1,150 pounds and the empty right tank weighed only 648 pounds; when they were full of propellants, they weighed 8,920 pounds and 6,850 pounds, respectively. Note that the left tank was over 2,000 pounds heavier than the right tank when they were full. To minimize weight and cost, the government had opted not to insulate the liquid-oxygen tank. As a result, the evaporation rate was high enough that the engineers considered the NB-52 top-off supply to be marginal. If a flight encountered a long hold time prior to launch, it might prove necessary to abort the mission and return to Edwards due to excessive liquid-oxygen boil – off.[250]

The use of external tanks on the X-15A-2 was unique in that the pilot had to jettison the tanks from the aircraft. The structural limitations of the aluminum tanks and the degraded handling qualities dictated that the maximum allowable Mach number with the external tanks was 2.6, so the pilot had to jettison the tanks before reaching that speed. In addition, a landing was not possible with the tanks installed because of the increased drag and a lack of ground clearance. Hence, the program expended considerable effort to ensure the tanks would jettison when commanded.-125^

Each tank was forcibly ejected from the airplane during flight through the use of fore and aft gas – cartridge ejectors and a forward solid-propellant sustainer rocket that imparted pitching and rolling moments to the tank after it had been ejected. For a normal empty tank jettison, both sets of gas cartridges fired and the nose rocket ignited. In the case of an emergency jettison when the tanks were full, only the nose gas cartridge fired.

The tanks were relatively expensive, so they were equipped with a recovery system that included a drogue and a main parachute. The drogue chute deployed from its nose compartment immediately after separation, and the main descent parachute deployed when a barometric sensor detected the tanks passing through 8,000 feet. Although the engineers expected some impact damage, they believed it was possible to refurbish the tanks at a reasonable cost.-252!

Wind-tunnel tests indicated that satisfactory separation characteristics existed when the dynamic pressure was less than 400 psf and the angle of attack was less than 10 degrees; acceptable separation probably existed for dynamic pressures up to 600 psf. At higher angles of attack and dynamic pressures, researchers expected the tanks to roll excessively and to pitch up within close proximity to the airplane. Tank separation characteristics with partly expended propellants were unknown and were a potential problem since there were no slosh baffles or compartments for center-of-gravity control. The researchers expected the full-tank ejection characteristics to be satisfactory for any reasonable flight conditions that might occur within 15 seconds of launch.-253!

Prior to the first flight using external tanks, the Air Force conducted two dummy tank jettison tests with X-15A-2 located over a 10-foot-deep pit in the ground beside the ramp. Technicians constructed a pair of beams with similar mass and inertia properties to simulate empty tanks. Preloaded cables attached to the beams applied simulated aerodynamic drag and side loads. The first test used a single set of ejector cartridges at simulated air loads of 400 psf, 5 degrees angle of attack, and 3 degrees of sideslip. The second test used two sets of ejector cartridges at a simulated dynamic pressure of 600 psf. Both tests were successful, and high-speed motion pictures showed good separation characteristics. During the tests, the X-15 APU supplied hydraulic and electrical power, and engineers engaged the SAS to observe its reaction to the separation, which was satisfactory.-2541

LEFT HAND LOX (3920 fb FULL)

The external tanks allowed carriage of approximately 70 percent more propellant, a necessary ingredient in raising performance to 8,000 fps. The tanks provided an additional 60 seconds of engine burn time, for a total of 150 seconds at 100-percent power. The tanks were 23.5 feet long and 37.75 inches in diameter. The left tank contained about 793 gallons of liquid oxygen in one compartment and three helium bottles with a total capacity of 8.4 cubic feet. The right tank contained about 1,080 gallons of anhydrous ammonia in a single compartment. The empty left tank weighed 1,150 pounds and the empty right tank weighed only 648 pounds; when full, they weighed 8,920 pounds and 6,850 pounds, respectively. (NASA)

Despite this, during a review leading up to the first flight, engineers expressed concern over the possible separation of partially filled tanks during an emergency. The wind-tunnel tests and the tank separation system only covered full and empty scenarios. What would happen if the pilot had to abort the flight during the first 60 seconds of powered flight while the engine was siphoning propellants from the external tanks? The initial response was that the tanks, as designed, would not withstand the loads imposed during a separation with a partial load. Engineers at the FRC and AFFTC considered installing a rapid propellant-dump system, installing a set of baffles in the tanks, or even providing a system that would allow the pilot to refill the tanks using internal propellants. All seemed too complicated given the time and money available to the program.[255]

After a great deal of consultation among the engineers, flight planners, and pilots, management decided to continue, at least for the time being. The risk was considered reasonable because the XLR99 had never encountered a premature shutdown from 100% thrust-all failures had occurred either during ignition or while throttling. If a failure happened during ignition, the tanks would be full and would not present a problem, and the plan called for no throttling during the high-speed runs. Nevertheless, engineers decided to add a third jettison button in the cockpit. This one, intended for use with partial tanks, would fire the forward gas cartridge and ignite the separation rocket-sort of a middle ground between the other scenarios.-1256

Although there were physical differences between the basic X-15 and the X-15A-2 without external tanks, their aerodynamic qualities were similar. With external tanks on the airplane,
however, some rather dramatic differences existed, with the general trend toward unfavorable characteristics. The offset center of gravity caused by the external tanks further complicated the overall control task. At launch with full tanks, the vertical center of gravity was approximately 9 inches below the aircraft waterline, moving upward as the engine consumed propellants. The pilot had to use additional nose-up stabilizer trim to counteract the nose-down pitch at engine ignition caused by this offset below the thrust vector. The heavier liquid-oxygen tank on the left side displaced the center of gravity 2 inches to that side, causing a left rolling moment that the pilot also had to counteract.-257!

The nominal flight profile for the speed missions was to maintain the airplane at a 12-degree angle of attack until it reached a pitch attitude of 34 degrees. The pilot held this climb attitude until the external propellant was depleted. Tank ejection occurred at approximately Mach 2.1 and

67.0 feet, and the pilot maintained an angle of attack of 2 degrees until the airplane reached

100.0 feet. The airplane then accelerated to maximum velocity.258

As was the case with the basic airplane, the simulator predicted poor handling qualities at high angles of attack, due primarily to the large negative dihedral effect caused by the presence of the ventral rudder. For a yaw damper failure with the speed brakes out, a divergent sideslip oscillation persisted above 6 degrees angle of attack. Although the pilot could damp this divergence, it required almost continuous attention and left little time for other tasks. The simulator showed that turning off the roll damper would eliminate the divergent yaw oscillation, but then the pilot would have to fly the airplane with less lateral directional stability. From the simulator studies it was determined that, because of the relatively low-altitude profiles required, the airplane could be safely flown after a roll and/or yaw damper failure if an angle of attack of less than 8 degrees was maintained. The program accepted this restriction for the initial envelope expansion flights. However, for the projected ramjet tests, which required flights at high dynamic pressures, a divergence of this type could occur too rapidly for the pilot to take corrective action. Hence, NASA decided to provide a redundant yaw damper, similar to the ASAS used for the roll axis. The FRC began initial design work, but the flight program ended before the system was completed.-1259!

The final ground-based external tank test took place on the Rocket Engine Test Facility where X – 15A-2 had completed a full-duration engine run with the external tanks installed on the aircraft. Engineers had already corrected deficiencies uncovered during several earlier tests.268

The expected performance of X-15A-2 represented a significant improvement over the demonstrated 6,019 fps of the basic aircraft. With external tanks and the ventral rudder, the estimated velocity was between 7,600 and 7,700 fps at 120,000 feet. Replacing the ventral with an assumed ramjet configuration would decrease that to about 7,200 fps at 118,000 feet, a result of increased weight and drag for the ramjet configuration. This performance was, however, appreciably less than the design goal of 8,000 fps at 100,000 feet. As a result, Reaction Motors was investigating the development of a new injector and nozzle to provide additional thrust in an attempt to bring the performance back up to 8,000 fps. Again, the end would come before the company completed the work.261

The length of time the airplane could remain at high velocity and dynamic pressure determined the amount of useful data about the ramjet that could be obtained. Researchers expected that X – 15A-2 could stay above 7,000 fps for 50 seconds and above 6,000 fps for 110 seconds per flight. For ramjet tests that required steady conditions (that is, at a relatively constant velocity and dynamic pressure), the pilot would throttle the XLR99 to minimum and extend the speed brakes so that low acceleration existed. The expected stabilized test time for this configuration was approximately 14 seconds at 7,000 fps and 40 seconds at 6,000 fps.268

The first flight (2-43-75) with empty external tanks was on 3 November 1965, the only flight launched from Cuddeback, about 60 miles north of Edwards. Bob Rushworth jettisoned the tanks at Mach 2.25 as the airplane passed through 70,300 feet, and took the airplane to Mach 2.31 and 70,600 feet before landing at Rogers Dry Lake after a flight of only 5 minutes and 1 second (the shortest non-emergency powered flight of the program). Post-flight analysis indicated that the handling qualities were essentially as predicted by the simulator. Rushworth, who for a change was flying without deploying part of the landing gear, commented that he thought the "roll stability was significantly less than I had expected," but the "longitudinal control wasn’t quite as bad" as he had anticipated.-126^

Two ground-based mobile trackers, each with 150-inch lenses on 35-mm Mitchell cameras running at 72 and 48 frames per second, provided photographic coverage of the tank separation. In addition, six Askania tracking cameras recorded the tank recovery system. Because the events took place at quite a distance, the resulting image size was small and researchers could only make a qualitative analysis of the event. The tanks separated cleanly from the aircraft; however, it appeared that the tanks did not rotate nose down as much as expected. They exhibited a tumbling action during flight with the drogue chutes attached, and tended to trim at an angle of attack of about -110 degrees. The drogue chutes occasionally collapsed during flight, so the engineers lengthened the drogue chute riser for future flights. Impact with the desert destroyed the liquid-oxygen tank after the nose cone containing the main descent chute did not separate properly. The Air Force recovered the ammonia tank in repairable condition. The tumbling action of the tanks increased the total drag and the tanks fell short of their predicted impact points-the ammonia tank landed 2.3 miles short and 0.6 mile to the left, while the liquid-oxygen tank landed 2.7 miles short and 1.6 miles to the left. This was still well inside the bounds of the Edwards impact range and did not represent a problem.-264

Joe Engle ended up being the only X-15 pilot who would get to fly the next lifting-reentry vehicle, the Space Shuttle. He also has the distinction of being the only person to fly back from orbit, on the second Space Shuttle flight (STS-2). Milt Thompson said that "Joe Engle seemed to have a charmed relationship with the X-15" because for the most part all of Engle’s flights went according to plan. However, not everybody would agree with that assessment of his 15th flight.

On 10 August 1965, Engle took X-15-3 to 271,000 feet-his second flight above 50 miles.

Mission rules stated that the X-15 pilot should fly an alternate low-altitude mission if the yaw damper channel on the MH-96 failed during the first 32 seconds of flight. This was because it was unlikely the airplane could make a successful reentry with a failed yaw damper. On this flight (3-46-70), the yaw channel failed 0.6 seconds after the X-15 dropped off the pylon. Engle reset the damper and did not feel obligated to fly the alternate profile since the damper successfully reset. It was a temporary reprieve, however. The damper failed again 19 seconds later; the reset was successful for at least 10 seconds until it failed again. The damper failed three times in the first 32 seconds of flight. Remarkably, Engle successfully flew the mission, although he missed some of the profile for various reasons, including a preoccupation with resetting the failing yaw damper.264

At the end of 1965, NASA could see that the end of the X-15 program was in sight. Researchers had long since completed the originally envisioned basic flight research, and the aircraft were now primarily experiment carriers, although X-15A-2 was still extending the flight envelope somewhat. However, plans to use X-15A-2 as a hypersonic ramjet test bed began to unravel when, on 6 August 1965, Secretary of Defense Robert S. McNamara disapproved the funding necessary for the effort.266

Under the best-case scenario, the FRC anticipated that the flight program using the basic X-15s would begin winding down at the end of 1967 when X-15-3 began receiving its delta-wing modifications. By the end of 1969, X-15-1 would be retired, leaving only X-15A-2 and the newly redelivered delta-wing X-15-3 in service. X-15A-2 would finish its ramjet tests in mid-1970, transferring all flight activity to the delta wing.-267

Paul Bikle had long believed that any extended operation of the X-15 program beyond its original objectives was unwise and hard to justify in view of the high cost and risk involved. As early as 1961, he had suggested the end of 1964 as a desirable termination date. As time went on, Bikle felt that continued extensions of the program were becoming increasingly hard to justify, and he personally had strong doubts that either the delta wing or the HRE would ever reach flight status on an X-15. In spite of these personal misgivings, Bikle continued to support the program in his public statements.268

1965 FLIGHT PERIOD

January 1966 was much like December 1965 in the high desert-wet. Between 12 November and 1 December 1965 more than 3 inches of rain had fallen, and 2 more inches fell during December. NASA described Rogers, Three Sisters, Silver, and Hidden Hills as "wet," while Mud Lake was only "damp." Over 95% of Cuddeback was under water and there was visible snow at Delamar. A lack of landing sites effectively grounded the X-15 program.268

This gave the program time to do maintenance on the airplanes and incorporate various modifications. For instance, engineers installed the Honeywell IFDS, finally, on X-15-3 along with a new Lear Siegler-developed vertical-scale instrument panel. All of the instrumentation wiring on this airplane was removed and replaced with new four-conductor shielded Teflon wire. It received the modifications necessary to carry the wing-tip experiment pods, and the third skid and stick – kicker needed for higher landing weights were installed.278

The pilots were not greeting the new X-15-3 instrument panel with overwhelming enthusiasm. Paul Bikle opined that "there has been some evidence of reluctance to accept the vertical-scale, fixed-index [tape] instruments." Bikle noted that previously "no objective evaluation of the suitability of the panel for the X-15 mission had been made." To correct this, engineers installed a duplicate of the panel in the fixed-base simulator and conducted runs using "measurable flight control and pilot performance parameters in a comparison of the Lear panel with the traditional panel."278

Of all the performance measures taken, only two showed consistent and significant differences. These were the absolute error in velocity at power reduction and the burnout altitude; in both cases, the statistical results favored the Lear panel. An examination of the altitude and velocity indicators on both panels showed that the differences were the result of high-scale resolution on the Lear instruments, which was almost twice that of the traditional panel instruments. The pilots were still not altogether happy with the new panel, but they no longer mistrusted it.222

For most of the flight program, the X-15 used an instrument panel that contained conventional instrumentation. In 1965, Lear Siegler developed a new panel for X-15-3 that used vertical-scale instruments that were supposed to provide enhanced situational awareness for the pilot. Similar instruments were being incorporated into the latest generation of Air Force fighters about the same time. At first, the new instrument panel was not met with overwhelming enthusiasm from the X-15 pilots, but eventually they came to accept the new instruments, although having two very different cockpit configurations complicated the simulators and training regiments. (NASA)

During this down period, X-15A-2 received a new Maurer camera to replace the Hycon unit in the center-of-gravity compartment. This was not as simple as it sounded and took almost eight weeks of work. X-15-1 received a modification that allowed ground personnel to easily remove or replace the wing-tip pods as needed to support various experiments. NASA could now swap the pods between X-15-1 and X-15-3, and was manufacturing a second set of pods.[273]

As January passed with no relief from the wet lakebeds (another half inch of rain fell at Edwards, with more snow on the upper areas of the High Range), NASA performed more modifications on the airplanes. Because the increase in stiffness of the main skids and the addition of the third skid transmitted higher loads through the structure to the nose gear, engineers decided to reinforce the skin on X-15-1 between fuselage stations 91 and 106. The Air Force sent the NB-52B to Tinker AFB for major maintenance, leaving her older sister to support the flight program, assuming the lakebeds ever dried out. The carrier aircraft returned on 8 April and the AFFTC spent the next five weeks modifying it to carry the heavier X-15A-2.[274]

The X-15A-2 configuration was tested in a variety of wind tunnels, including one at the NASA Jet Propulsion Laboratory in Pasadena, California. The JPL tests centered around determining the effects of shock-wave impingement on the proposed ramjet and finding an alternate vertical – stabilizer configuration to provide enhanced stability at Mach 8 while carrying a ramjet under the ventral stabilizer. (NASA)

NASA also used the time to complete various analyses, including a complete simulation of reentry profiles at the increased weights currently flown by the airplanes. The ground rules were that reentries would be limited to 1,600 psf using an angle of attack of 20 degrees. To avoid exceeding the structural limitations of the airplanes, NASA decided to restrict X-15-1 to altitudes under 265,000 feet and X-15A-2 to less than 250,000 feet. Mostly because it was equipped with the MH-96, NASA allowed X-15-3 to operate up to 360,000 feet. These restrictions were not really a problem since the program had already reached the maximum altitude it was planning on, although the first two airplanes would bump into these limits on several future flights.-1275

The simulations showed that, as currently configured, X-15A-2 should be able to reach a maximum velocity of 7,500 fps without the ramjet and 7,100 fps with the ramjet, both at 120,000 feet. These velocities assumed a launch weight of 51,650 pounds with the use of external tanks and an XLR99 burn time of 152 seconds. Based on these simulations, flight planners decided to conduct the X-15A-2 envelope-expansion program with the ventral on, primarily because it most closely resembled the planned ramjet configuration. However, the program was short of ventral rudders, and it was uncertain whether economic constraints would allow each flight to use one.-1275

More importantly, Langley and the Jet Propulsion Laboratory conducted wind-tunnel tests to investigate shock-wave systems affecting the proposed ramjet installation on X-15A-2. Researchers worried that shock waves impinging on the ramjet could affect inlet and engine performance, structures, and structural heating. The tests provided data for angles of attack between -5 and +20 degrees at Mach numbers between 2.3 and 4.63. A review of the data

showed that a shock wave emanating from the forward tip of the landing-gear skid would impinge on the ramjet inlet at all Mach numbers, and did not significantly vary with the angle of attack. These tests also showed that there was a complex shock impingement around the ventral stabilizer in general. Apparently, these data went unnoticed.-1272

By the beginning of April, the weather had improved considerably. The Air Force was in the process of repairing and re-marking Rogers, Grapevine, and Mud Lake. Cuddeback was dry but still too soft to re-mark. All of the other lakes were drying rapidly and were ready to use by the end of the month.-1278-

During the early 1960s, the X-15 was the only platform that could realistically carry a useful payload above the Earth’s atmosphere and return. Researchers had been making use of various sounding rockets that provided relatively inexpensive access to the upper atmosphere from a variety of locations around the world. However, in general these rockets had very small payload capabilities, could not provide much in the way of power or controlled flight, and were usually not recoverable. On the other hand, although the X-15 was very limited in where it could fly (over southern California and Nevada), it could provide a fair amount of power, it was at least somewhat controllable for aiming purposes, and, most importantly, it was recoverable.

EXPERIMENT ACCOMMODATIONS

Although the X-15 provided some internal space for experiments, many researchers wanted specific views of the world outside or to have their experiments located away from the "noise" of the airplane. This gave rise to several modifications that ultimately affected all three X-15s.

Wing-Tip Pods

Several experiments (particularly a proposed micrometeorite collection system) had to be located outside the flow field of the X-15 and, it was hoped, outside the zone of contamination from the ballistic control-system thrusters. The most obvious location was the wing tips.

There were several preliminary designs for the wing-tip pods. Initially North American wanted to give the pods a rectangular cross-section, since it would be easier to package the various experiments in them. However, after considering both normal and potential emergency operations in terms of the effects of stability and control, heating, drag, and turbulence, the engineers decided to use circular-cross-section pods constructed of Inconel X. The pods were 8 inches in diameter and 58 inches long, and could weigh a maximum of 96.2 pounds, although the program exceeded this limit on numerous occasions.-1561

There was some initial discussion about equipping the pods with an emergency jettison system in case the micrometeorite collection system stuck open, but the final design had the collection equipment simply burning off during reentry if that occurred. Researchers tested the wing-tip pods in the supersonic and hypersonic wind tunnels at the Jet Propulsion Laboratory (JPL) on 24­25 October 1962 to verify that they had no adverse effect on the airplane. On 5 November 1962, North American tested the configuration in its low-speed wind tunnel to verify that the landing characteristics were not affected. The test results proved to be satisfactory.-1571

Wing tip pods were developed for X-15-1 to house experiments that needed to be located outside the flow field of the X-15. The pods were 8 inches in diameter, 58 inches long, and could weigh a maximum of 96.2 pounds. The first flight (1-50-79) with the pods was on 15 October 1964 with Jack McKay at the controls. Similar pods were later manufactured for X-15-3. (NASA)

Initially, North American manufactured a single set of the pods for X-15-1. NASA installed the modifications necessary to use the wing-tip pods, including the attachment points and wiring routed through the wing, on X-15-1 during March 1964. The first flight (1-50-79) with the pods was conducted on 15 October 1964 with Jack McKay at the controls. The flight reached Mach 4.56 and 84,900 feet, and the pods did not seem to have any major adverse effect on the handling of the airplane. Subsequently, however, some pilots complained that the pods seemed to introduce a buffet at load factors significantly below the previous buffet boundary. Researchers installed accelerometers in the pods to verify this, but failed to uncover any evidence of buffeting. However, the redistribution of mass due to the pod installation appeared to result in a 17-cps vibration tied to the wing-bending mode that was excited by some maneuvers and gusts, which likely explained what the pilots felt.[58]

Researchers subsequently determined that having only one set of pods put an unreasonable constraint on scheduling experiments, and decided to manufacture a second, easily removable set of pods. NASA modified the third airplane to carry wing-tip pods, and could switch the pods between X-15-1 and X-15-3 as needed to support the experiments and flight schedule.

Frequently the rear compartment on one or both pods contained cameras aimed at various parts of the airplane (usually the ablative panels on the stabilizers) or one of the experiments in the tail-cone box. At some point, the pods on X-15-1 also received a small set of drag braces for unspecified reasons (probably an attempt to cure the vibration problem). Despite the original intent, and the best efforts of all involved, the wing-tip pods did not put the experiments outside the contamination zone of the ballistic control thrusters. Residue from the hydrogen peroxide would render several experiments useless. The pods were also inside the nose shock-wave interference zone at certain angles of attack, further hampering some experiments.-1591

Just as the X-15 program was winding down, researchers noted that airplanes flying faster than Mach 3 (the YF-12C/SR-71A and XB-70A) did not generate a sonic-boom noise. NASA had made some measurements at Mach numbers up to 16 during the liftoff and reentry of Apollo spacecraft, but did not consider this representative of future aerospace vehicles, such as the Space Shuttle. Therefore, researchers made measurements during several X-15 flights at Mach numbers up to

5.5 and compared these with results obtained by theoretical methods of determining overpressures.-12121

For flight 1-70-119, instruments were set up at Mud Lake to record the boom generated at Mach 5.3 and 92,000 feet. Researchers obtained satisfactory data even though the airplane was about 6 miles east of the monitoring site. The sonic boom was a typical far-field signature with some slight atmospheric distortion, although this was less than predicted. The boom peak overpressure was about 0.34 psf.12131

environmental conditions based on data obtained at Edwards. When the airplane arrived over Goldstone at Mach 4.8, the engine was operating at 50% thrust and the speed brakes were extended; at Cuddeback the engine had already shut down, the speed brakes had retracted, and the airplane was at Mach 3.5. Although the flight plan called for the airplane to fly directly over the microphone arrays, in reality it passed 1.7 miles south of the array at Goldstone and 7.9 miles south of the Cuddeback array-not an unusual amount of error for an X-15 flight.-1214!

Researchers scaled and corrected the data collected from Goldstone so they could compare it with similar data obtained from an SR-71 flight. The two sets of data were in general agreement. Researchers did not evaluate the data from Cuddeback because of the X-15 miss distance. The results of the experiment also compared favorably to theoretical results, and no unusual phenomena related to the overpressure were encountered.12151

Iven Carl Kincheloe, Jr., was born on 2 July 1928 in Detroit, Michigan. In 1945 he entered Purdue University, where he studied aeronautical engineering as a member of the Air Force ROTC unit. He graduated in 1949 with bachelor of science degree in aeronautical and mechanical engineering.-1161

Kincheloe received his wings at Williams AFB, Arizona, in 1951. In early 1952 he was promoted to captain and entered the Korean War with the 5th Interceptor Wing. He flew 131 missions and shot down five MiG-15s, becoming the 10th jet ace. For his outstanding service he received the Silver Star and the Distinguished Flying Cross with two oak leaf clusters. After he returned to the United States, Kincheloe was a gunnery instructor at Nellis AFB, Nevada, and in 1953 he was accepted into the Empire Test School in Farnborough, England. While in England, he received a master of science degree in aeronautical engineering from Oxford in December 1954.-117

Kincheloe flew the X-2 to an altitude of 126,200 feet, and became famous as "America’s first spaceman." On 27 March Kincheloe was named chief of the manned spacecraft section, fighter operations branch, of the flight-test operations division that was responsible for training the Air Force pilots who were to participate in the X-15 flight program. Kincheloe became the first Air Force project pilot for the X-15; unfortunately, however, he died before he had a chance to fly the airplane. On 26 July 1958, Kincheloe took off on a routine chase mission in an F-104. At 2,000 feet altitude the engine failed. Although Kincheloe was able to roll the airplane inverted to enable the downward-firing ejection seat, he was too low for his parachute to open.

A biography of Kincheloe, First of the Spacemen: Iven C. Kincheloe, Jr., by James J. Haggerty, Jr. (New York: Duell, Sloan and Pearce, 1960), was published in 1960, and a CD-ROM biography of "Kinch" was aboard the Space Shuttle Discovery on STS-70 in July 1995.

During the course of his career, Kincheloe accumulated 3,573 flying hours in 70 American and foreign aircraft. Numerous honors followed his death. One of the most meaningful came from his peers, when the Society of Experimental Test Pilots (SETP) renamed its prestigious Outstanding Pilot Award in his honor. His most public tribute, however, took place far away in his home state when Kinross AFB in Michigan’s Upper Peninsula was renamed Kincheloe AFB in his memory.-1181