Transiting from Air to Space
The North American X-15


The conclusion of the X-15's flight test program brought an era in flight testing history to a close. In 199 flights, the X-15 spent eighteen hours above Mach 1, twelve hours above Mach 2, nearly nine hours above Mach 3, nearly six hours above Mach 4, one hour above Mach 5, and scant minutes above Mach 6. It flew to Mach 6.72 (4,520 mph) and an altitude of 67 miles. Twelve pilots flew it, and one of them died. Beginning as a hypersonic aerodynamics research tool, the X-15 eventually became much more than that. What, then, did it accomplish?

In October 1968 John Becker enumerated 22 accomplishments from the research and development work that produced the X-15, 28 accomplishments from its actual flight research, and 16 from testbed investigations. As of May 1968, the X-15 had generated 766 technical reports on research stimulated by its development, flight testing, and test results, equivalent to the output of a typical 4,000-man federal research center working for two years. As the X-I had provided a focus and stimulus for supersonic research, the X-15 furnished a focus and stimulus for hypersonic studies. A sampling of its accomplishments indicates their scope: 1 The X-15 also made its mark in many other ways. When NACA began its development, the science of hypersonic aerodynamics was in its infancy; the few existing hypersonic tunnels were used largely for studies in fluid mechanics. Aerodynamicists feared that there might be a hypersonic ”facility barrier,” much like the earlier transonic tunnel trouble that led to the Bell X-1 and Douglas D558, so that hypersonic tunnel tests might prove of little value in predicting actual flight conditions. The X-15 disproved this; predicted wind tunnel data and data flight testing of the airplane generally showed remarkable agreement. Proving that hypersonic laminar flow conditions did not develop led to the disappearance of this ”technical superstition,” and recognition that the small surface irregularities that prevent laminar flow at low speed also prevent its formation at hypersonic speeds. Like the earlier X-l, the X-15 encouraged a great deal of ground research and simulation techniques. So successful were these methods and so great was the engineers' confidence in these methods and the X-15's flight results that the X-15 wound up actually decreasing the likelihood of NASA's developing any future hypersonic research aircraft with the prime justification being the generation of unique and otherwise unobtainable data. Any future research aircraft would be built more for ”proof of concept” purposes than for acquiring information unobtainable by other means. At the conclusion of the X-15 program, the German Society of Aeronautics and Astronautics presented the NASA X-15 team with the Eugen Sanger Medal – a fitting and appropriate honor. In his acceptance address on behalf of the team, John Becker stated that "no new exploratory research airplane can ever again be successfully promoted primarily on the grounds that it will produce unique flight data without which a successful technology cannot be achieved.” 2

Nearly ten years after Becker's assessment, Capt. Ronald G. Boston of the U.S. Air Force Academy's history department reviewed the X-15 program for ”lessons learned” that might be applied or benefit the development of the National Hypersonic Flight Research Facility Program, an effort that itself died shortly thereafter. Boston's study, presented in clipped outline style, offers an interesting perspective on the X-15 both from the vantage point of history, as well as giving an inkling of the state of the art in hypersonic studies in the mid-1970s on the eve of the Shuttle in light of the X-15's experience. Reprinted here in full, it provides an interesting complementary viewpoint to that of X-15's originator John Becker: 3


This outline presents a synopsis of X-15's contributions to aerospace technology and is intended as a preliminary report on the X-15 historical study conducted as part of the National Hypersonic Flight Research Facility (NHFRF) feasibility study. Specifically, this study looks to see of what value the developments and lessons of the X-l5 program have been. It is a case study of the X-15 program intended to show the value of research aircraft.

Covered in this study are two general types of contributions made by the X-15: revolutionary and evolutionary. Revolutionary contributions are those technological breakthroughs that open new fields, that are dependent upon the advanced capabilities of the research aircraft, and that are sometimes totally unexpected. Evolutionary contributions include those for which the research vehicle represents the latest and most advanced stage in the developmental process. While the latter may not be dependent upon the particular aircraft's capabilities, the demands of the research program nonetheless drive the technology toward a greater degree of perfection. The two types are often confused; yet, only the former provides legitimate justification for undertaking a research program. But in an evaluation in retrospect, both forms of contribution make up the ultimate worth of a program.

The study begins with the~X-15 program's goals and examines the degree of success achieved. It covers the lessons learned, both intentional and unintentional in origin. It then looks to the present time to see what, if any, uses have been made of the knowledge gained. Lastly, this study poses the questions raised but left unanswered in the conduct of this program.
  1. Program Overview:

    1. Goals and Design Philosophy. Using near-state-of-the-art (1954) technology to propel a conservative Mach 2 design out to Mach 6 and 250,000 feet to explore the hypersonic and near-space environments:
      (1) To verify existing theory and wind-tunnel techniques.
      (2) To study aircraft structures under high (1,200°F) heating.
      (3) To investigate stability and control problems associated with high-altitude boost and reentry.
      (4) To investigate the biomedical effects of both weightless and high-g flight.

    2. Achievements and Ultimate Utilization. All design goals were met; most were surpassed: Mach 6.7, 354,200 feet, 1,300 degrees F, and 2,000 pounds per square foot (psf). In addition, once the original research goals were accomplished, the X-15 became a handy high-altitude, hypersonic testbed for which 46 follow-on experiments were designed--majority flown before the program was abruptly terminated in 1968. Many proposals for modifying or optimizing the basic airframe surfaced during the course of the program, and the X-15 was envisioned as a hypersonic facility for the 1970s. Due to the absence of a subsequent hypersonic mission, aircraft applications of X-15 technology have been few. In space, however, the X-15 paved the way for manned, orbital and lunar flight.

  2. Hypersonic Aerodynamics:

    1. Hypersonic Flow. The X-15 program remains the most thoroughly tested aircraft program to date and offered an excellent opportunity to compare actual flight data with theory and wind tunnel predictions. The X-15 verified existing wind-tunnel techniques for approximating interference effects for high-Mach, high-angle-of-attack hypersonic flight, thus giving increased confidence in small scale techniques for hypersonic design studies. Wind-tunnel drag measurements were also validated, except for the 15 percent discrepancy found in base drag--masked by the "sting" support used in the tunnel. The laminar boundary layer theory for hypersonic flight was disproven, the flow actually being almost entirely turbulent. X-15 flight-test data indicated that hypersonic flow phenomena are linear above Mach 5, allowing us to design with confidence craft like the Mach 25-30 Shuttle Orbiter that must fly as expected without the cautious "buildup" program of the X-15.

    2. Stability and Control. X-15's experience disproved the existence of ”barriers” to hypersonic flight as were suspected after the X-1 and X-2 aircraft encountered extreme, high-supersonic instability.
      (1) ”Wedge Tail.” A redesigned vertical stabilizer reduced the instability that plagued the X-1 series and X-2 aircraft.
      (2) ”Rolling Tail.” Differentially deflected horizontal stabilizers gave precise roll control and allowed for elimination of ailerons out on a hot wing section. This design concept was later incorporated into the ”swing wing” of the B-1 bomber to simplify wing construction.
      (3) Tunnel Parameter Verification. X-15 data verified wind-tunnel parameters used for aerodynamic stability prediction above Mach 2.Flight test results also pointed out the need for an ”error band” or degree of uncertainty to be put on such predictions. AFFTC and NASA Dryden Flight Research Center have both made inputs to the Shuttle program in this regard based on past flight test experience, the X-15 providing the only parameter's experience above Mach 2.
      (4) Side-Stick Controller. The first modern application of the side-stick concept for more precise, ”wrist-action” control – as now comes standard in the F-16.
      (5) Augmentation. Some phases of flight, such as reentry, were marginally stable, and pilots required artificial augmentation (damping) to achieve satisfactory stability. The X-15 necessitated the development of one of the earliest stability augmentation systems (SAS). Originally equipped with a simple fail-safe, fixed gain system, one of the three ships was later equipped with a triple redundant adaptive flight control system (AFCS). Here the pilot flew via inputs to the electrical augmentation system. Though a point of continuing debate, the X-15 did not incorporate ”fly-by-wire” if meant to denote a non-mechanically linked control system. A purely electric side stick had been developed under contract for the X-15 and test flown in a F-101B. Thus the X-15 did advance ”fly-by-wire” technology.

    3. Simulation Techniques. The art of simulation grew with the X-15 program, not only for pilot training and mission rehearsal, but for research into controllability problems. Subject to continuous updating based on flight-test results, the simulator was programmed to ”fly” like the aircraft. Thus the simulator could be used to explore those areas of the flight envelope too risky for actual flight. The demands of the X-15's wide velocity and altitude envelope necessitated development of the first full six-degree-of-freedom flight simulator. The X-15 program showed the value of good wind-tunnel testing and simulation in maximizing the knowledge gained from each of the 199 short, expensive test flights.

    4. Aerodynamic Heating Effects. In a major discovery, the existing Sommer-Short and Eckert T-prime heating prediction theories (laminar flow) were found to be 30 to 40 percent in excess of flight-test results. (Hence the X-15's structure was over designed for heating effects.) This discovery led to renewed wind-tunnel testing leading to NASA-Langley's choice of the empirical Spaulding-Chi model for hypersonic heating. Lighter, more optimum vehicles are now possible, the Apollo command and service modules being a case in point. Based on their X-15 experience, Rockwell International devised a computerized mathematical model for aerodynamic heating called HASTE--Hypersonic and Supersonic Thermal Evaluation – which gives a workable ”first cut” approximation for design studies. HASTE was, for example, used directly in the initial Apollo design study.

  3. Structures:

    1. Development. X-15 was designed as a ”heat sink” structure to absorb heat pulses, not to withstand hypersonic cruise heating. Development showed the validity of ground ”partial simulation” testing of primary members stressed under high temperature. A facility was since built at DFRC for heat-stress testing of the entire structure. X-15's development pioneered the use of corrugations and beading to relieve thermal expansion stresses (as now used on YF-12/SR-71, though Lockheed disclaims any X-15 inputs). Metals with dissimilar expansion coefficients were also used to alleviate stresses. The leading edges were segmented, much like a concrete sidewalk, to allow for expansion. The X-15 required the perfection of fabrication (milling and welding) techniques for high temperature alloys: Inconel X (skin) and titanium (structural members) had heretofore not been extensively used to such fine specifications. Such is now routine in aircraft and spacecraft construction.

    2. Flight Stresses. Though the primary structure proved sound, several surface design problems were uncovered during early flight tests.
      (1) Local Hot Spots. A surprise lesson came with the discovery of heretofore unconsidered local heating phenomena. Tiny slots in the leading edge material, the abrupt contour change along the canopy, and the wing root caused flow disruptions that produced excessive heating and adjacent material failure. The X-15, tested in ”typical” panels or sections, demonstrated the problems encountered when those sections are joined and thus precipitated an analytical program designed to predict such local heating stresses. Today, from this experience, Rockwell engineers are closely scrutinizing the segmented, carbon-carbon composite leading edge of the Shuttle Orbiter's wing. The bimetallic ”floating retainer” concept designed to dissipate stresses across the X-15's windshield carried over to Rockwell's Apollo and Shuttle windshield designs as well.
      (2) Hot Air Leaks. Hot boundary-layer air on several occasions seeped into the nose-gear compartment, damaging gear and compartment and causing high-speed extension of the gear. The need for very careful examination of all seals thus became apparent, and closer scrutiny of surface irregularities, small cracks, and areas of flow interaction became routine. Consequently, Rockwell engineers are now examining the seal around the Orbiter's thermal surface tiles.
      (3) Panel Flutter. Incidences of X-15's panel flutter led to an industry-wide reevaluation of panel flutter design criteria in 1961–62. Stiffeners and reduced panel sizes alleviated the problems on the X-15's upper vertical stabilizer and side fairings. Similar techniques later found general application in the high speed aircraft of the 1960s.
      (4) Boundary Layer Noise. The X-15 provided the first opportunity to study the effects of acoustical fatigue over a wide range of Mach and dynamic pressures. In these first inflight measurements, ”noise” related stresses were found to be a function of g-force, not Mach number. Such fatigue was determined to be no great problem for a structure stressed to normal inflight loading. This knowledge has allowed for more optimum structural design of missiles and space capsules that experience high velocities.

    3. Fabrication Techniques. Working with the hard nickel alloy Inconel X required new fabrication techniques. New welding, drilling, forming, and milling methods were perfected and are commonplace with the tough aerospace alloys now in use. The ”Chem-Mill,” or chemical milling, was a North American Aviation development that got its first test in reducing the center portions of skin panels to reduce weight. North American also pioneered a new spar construction to combat thermal expansion: the X-15's ”hat” – spar construction, which gave compressive strength while reducing secondary stresses, has evolved into the ”sine wave” spar used on the B-1 and other supersonic aircraft. To remedy the thermal buckling along the side fairings, North American also pioneered the use of expansion joints that nonetheless retained fuselage structural integrity. Indeed, the fuselage itself was used as the fuel tanks, advancing the concept of integral tankage to reduce weight.

  4. Manned Flight:

    1. Bioastronautics. Coming at a time when serious doubts were being raised concerning man's ability to handle complex tasks in the high-speed, weightless environment of space, the X-15 program became the first program for repetitive, dynamic monitoring of pilot heart rate, respirations, and EKG under extreme stress over a wide range of speeds and forces. When preexisting, theoretical limits for heart rate were exceeded, all estimates of man's ability to endure stress had to be revised upward. Accelerated heart rates therefore caused no undue alarm or mission aborts for the subsequent manned space program. In fact, X-15's success gave the confidence to go ahead with early manned Mercury flights – the downrange ballistic shots being similar to the X-15's mission profile-at a time of great political concern over the success of America's first space program. Biomedical monitoring as begun with the X-15 has continued at DFRC. Pilot functions are being studied with an eye to devising the means to monitor pilot response and alertness from the ground as a function of vital measurements.
      (1) Instrumentation. The bio-instrumentation developed for the X-15 program has allowed similar monitoring of all subsequent flight test programs. Incorporated in the pressure suit, pickups are unencumbering and compatible with aircraft electronics. The flexible, spray-on wire leads have since found use in monitoring cardiac patients in ambulances.
      (2) pressure Suit Development.. The A/P-22S-2, the first single-piece, full pressure suit, was developed for the X-15 program. Later it was refined as the A/P-22S-6 suit, which remains the standard USAF operational suit for high-altitude flight.

    2. Manned Flight Operations. America's space and advanced manned vehicle programs are all indebted to the X-15 for some aspect of their training, command and control, or recovery procedures. The X-15 not only demonstrated the value of man at the controls, but provided the accepted methodology for experimental manned programs.
      (1) pilot-in-the-Loop. The X-15 provided for no ground-based control input or override; the pilot remained constantly ”in the loop,” controlling and correcting aircraft attitude. He provided a highly sophisticated onboard ”computer” and also served as the primary backup system for redundancy. Statistics show that without a pilot in control, the 3 aircraft would have sustained 15 losses on the first 47 flights alone. Overall mission success rate stood at 96 percent, versus 80 percent for component reliability. The pilots were able to recognize and override malfunctions to complete the primary or alternate missions to greatly enhance the worth of the program.
      (2) Crew Training. The opportunity to observe the pilot's performance under high-stress and high g-forces also dictated that an extensive ground training program be instituted to prepare pilots to handle the complex tasks and mission profiles. The result was a simulation program that became the foundation for crew training for all manned space work. The program depended on four types of training simulation.
        (a) Six Degree-of-Freedom Fixed-Base. A static cockpit mockup provided the means for extensive mission rehearsal-averaging 20 hours per 10 minute flight. Such preparation was directly responsible for the high degree of mission success achieved as pilots rehearsed their primary, alternate, and emergency diversion mission profiles.
        (b) Dynamic Simulation. Prior to the first X-15 mission, the ability of the pilot to function under the high g-forces expected on boost and reentry was tested in a closed-loop, six degree-of-freedom simulation using the centrifuge at the Naval Air Development Center, Pa. This simulation ”first” had the pilot controlling the g-forces and demonstrated pilot ability to function under 12 to 15 g's – more than ever experienced on actual flights. This project became the prototype for programs set up at the Ames Research Center and the Manned Spacecraft Center at Houston.
        (c) Variable Stability Aircraft. X-15 pilots maintained proficiency and adaptability by practicing on T-33 and F-100 aircraft whose handling characteristics could be varied in flight, simulating the varied response of the X-15 traversing a wide range of velocities and atmospheric densities.
        (d) Approach and Landing. Pilots practiced the exacting, low L/D landing maneuver in F-104 aircraft. With gear and speed brakes extended, the F-104's power-off glide ratio approximated that of the unpowered X-15. Shuttle Orbiter crews continue this same practice.
      (3) Command and Control. The ”NASA 1” control room located atop DFRC was the model for establishing the Mission Control Center (MCC) at Houston. Back up systems monitors and flight trackers were duplicated. Astronaut Capsule Communicators, ”Cap-Comms,” were a direct outgrowth of the X-15's practice of using an X-15 pilot as the ground communicator for all X-15 missions. Of course, all subsequent work at Edwards relied on X-15's spawned methodology. The X-15 program required an elaborate tracking network known as ”High Range.” Operational techniques were established for real-time monitoring and trajectory correction. These were carried over to the space program – the very same NASA personnel went on to set up the world-wide MCC tracking system.
      (4) Reentry and Landing. By demonstrating the operational feasibility of high angle-of-attack, ”lifting” reentries to unpowered, low L/D recoveries and landings, the X-15 paved the way for the lifting-body programs and the current Shuttle Orbiter concept. Accordingly, landing-assist rockets intended to ease the touchdown of the Shuttle Orbiter were ultimately eliminated from the Orbiter design. X-15 pilots routinely landed within 1,000 .feet of target with 70 percent reliability. The techniques for ground-monitored energy management to arrive overhead the landing spot at a ”high key” originated with the X-15 program. Here the extreme altitudes and distances from touchdown exceeded the pilot's ability to make a visual, ”deadstick” recovery as in preceding rocket aircraft programs. The terminal approach for the Shuttle Orbiter is a variation of the 360-degree, overhead pattern flown by the X-15: the Orbiter will enter figure-eight ”energy-dissipation circles” overhead the approach end of the field until energy is reduced to within landing limits. Thus X-15 operations experience, more than any other source, provides the basic framework for the research programs of the 1970s and 80s. In fact, in 1958 North American Aviation proposed launching an X-15 into orbit for subsequent recovery.

  5. Component Systems: The extreme speed and altitude demands of the X-15 program forced development of a number of advanced subsystems that continue to yield dividends long after the program's termination.

    1. Flight Data Systems. The X-15 required a choice be made between four possible approaches to flight data: 1) pressure instruments; 2) ground-based radar monitoring; 3) simple gyroscopic instruments; or 4) true inertial systems. The inertial approach, then very primitive, augmented with pressure instruments and radar, was selected.
      (1) Air-Data Sensors. For subsonic flight the X-15 relied on simple pilot-static pressure instruments. (Later in the program, an extendable pitot tube was added when the velocity envelope was expanded beyond Mach 6.) Mach, dynamic pressure, static pressure, and altitude for hypersonic flight were telemetered from the ground where "High Range" computers evaluated radar inputs and ambient atmospheric conditions gathered by sounding rockets. Angle of attack and yaw were derived from a null-seeking "ball nose" which measured the pressure differentials felt across ports in the ball. The ball nose was later modified to measure static pressure to monitor dynamic pressure [q = f(Pt)] which gave the pilot the ability to limit or hold a constant dynamic pressure. Thus far the ball nose has not found subsequent application. The Shuttle Orbiter will rely on redundant, onboard inertial systems backed up by ground radar.
      (2) Inertial Flight Data Systems (IFDS). Onboard measurement of velocity was handled by inertial systems. All three aircraft were initially equipped with analog-type systems which proved to be highly unreliable. Later, two aircraft, including the one aircraft with the adaptive control system, were modified with digital systems. In the subsequent parallel evaluation of analog versus digital IFDS, the latter was found to be superior. It was far more flexible and could make direct inputs to the adaptive flight control system; it was also subject to less error. This type is now the accepted approach, as will be used on the Shuttle Orbiter.

    2. Landing Gear. The main landing gear represented a marked departure from the standard pneumatic tire and retractable strut-as were retained in the nose-gear assembly. To reduce storage and heating problems, Inconel X skids were spring-loaded along the aft underside of the fuselage. This highly successful arrangement was programed for the X-20 Dyna-Soar and will be seen on the Rockwell HiMAT (High Maneuvering Aircraft Technology) RPV. One surprise lesson on the slap-down loading problems that low L/D aircraft with extremely aft-mounted main gear can experience was learned: the nearly immediate loss of lift as the nose lowered on touchdown caused unexpectedly high gear loads which resulted in gear failure and a major accident in 1962.

    3. Aerospace Hardware. The combination of high aerodynamic heating and cryogenic liquids posed severe problems for the X-15's designers. From their efforts have come thermal insulators for hydraulic lines and actuators which are used today in the Shuttle Orbiter, high-temperature hydraulic fluids, cryogenic tubing as used directly in Apollo components, and experience with Inconel and titanium pressure vessels to withstand extreme temperature and pressure gradients. By way of costly aborts, engineers learned almost embarrassing lessons such as the need to pressurize the gear boxes of auxiliary power units taken to the low ambient pressure of space – where foaming of the lubricant caused material failures.

    4. Cabin Environmental Systems. The X-15 presented the first requirement for full space-environment human engineering. While life support was provided by the full pressure suit, cockpit and electronics bay air-conditioning used the first cryogenic (liquid nitrogen) cooling system – designed by Garrett-Air Research, who went on to do the environmental controls for the Mercury capsules.

    5. Reaction Controls. The X-15 provided the first operational test of hydrogen-peroxide reaction controls outside the earth's atmosphere. Designed by Bell Aerospace and flown on their X-1B to 75,000 feet in 1958 (not outside aerodynamic control effects), this system represented a true technological leap when included in the X-15 design in 1956. It later went into the Mercury spacecraft as the primary control system.

    6. Propulsion. The X-15 was powered by the XLR99 liquid fueled rocket motor. Produced specifically for the X-15 mission, this complicated motor pioneered the concept of a throttleable, restartable motor with an idle-power feature. At idle, the XLR99 could complete 55 percent of its start and light-off sequence before drop. This complexity also resulted in many aborted missions (approximately one-tenth of all mission aborts). The requirement for a ”man-rated” fail-safe system further compromised reliability. Through hindsight a number of X-15 engineers now feel the throttleable future to have been a needless luxury that complicated and delayed the development of the XLR99 – this feature has not been used on subsequent motors. However, the production effort did give confidence in the concept, and six XLR99 throttleable motors yet remain in storage for some future reuse.

    The full value of X-15's experience to the designing of sub-systems for advanced aircraft and especially spacecraft can only be guessed at. At Rockwell International Corporation (Los Angeles Aircraft Division) many of the same people from the X-15 project worked on the Shuttle Orbiter. Yet X-15's experience is overshadowed by more recent projects and becomes exceedingly difficult to trace as systems evolve through successive programs. Nonetheless, those engineers are confident that they owe much to the X-15, even though many are at a loss to give any concrete examples.

  6. Follow-on Experiments: By roughly 1963 the X-15 had completed its original research objectives. There was talk of terminating the program entirely; there was even talk of closing DFRC for want of further flight research programs. New life was given to both as proposals for research needing either the speed or altitude of the X-15 surfaced. In the early 1960s the X-15 alone had the capability to carry a payload of much weight (or size) above the atmosphere. And unlike in missile research, the X-15 returned equipment and results for reevaluation, recalibration, and reuse. Perhaps the earliest true ”follow-on” experiment came in 1961: a coating material designed to reduce the infrared emissions of the B-70 was tested to Mach 4.43 (525°F) on the exterior surface of an X-15 stabilizer panel. Thus began a series of 46 additional experiments concerning the physical sciences, space navigation aids, reconnaissance studies, and advanced aerodynamics--many of the 46 were left unfinished when the X-15 program ended in 1968.

    1. Physical Sciences. Of special concern to scientists was the X-l5's ability to carry experiments above the attenuating effects of the earth's atmosphere.
      (1) Ultraviolet Stellar Photography. This astronomical study required photometering of the ultraviolet brightness of several of the brighter stars to study the material make-up of stars. The X-15 carried four cameras (on a gimbaled platform in the instrument bay behind the pilot) above the filtering effects of the ozone layer – approximately 40 miles up. Conducted in 1963 and again in 1966, this work was subsequently continued on improved sounding rockets.
      (2) Atmospheric Density Measurement. The X-15 was ideally suited to measure densities of the 30 to 74 kilometer altitudes, crosschecking measurements on ascent with those on descent. Using the ball nose to take measurements, flow-angularity errors were eliminated. The X-15 provided atmospheric density profiles of seasonal variation.
      (3) Micrometeorite Collection. Designed to collect samples at various altitudes, this experiment was part of a larger NASA study to build a particle-impact data base for spacecraft design criteria. Only on the last of six flights did this experiment ”catch” any particles, those being so contaminated by reaction control jet particles that the project was canceled.
      (4) Rarefied Gas Flow. This experiment failed to provide any useful information despite repeated attempts.
      (5) Solar Spectrum Measurements. The X-15 provided the first direct measurement from above the atmosphere of the sun's irradiance. A scientific revelation, this data allowed refinement of the Solar Constant of Radiation which was revalued 2.5 percent lower than existing ground-based determinations. This vital constant provides a measure of thermal energy incident on the earth and upon which all photochemical processes depend. It is also useful for the design of thermal protection for spacecraft.

    2. Space Navigation:
      (1) Horizon Definition. The X-15 supported two – MIT and NASA-Langley – projects to determine the earth's infrared horizon radiance profile. This information has been used in attitude referencing systems for orbiting spacecraft. The MIT work was part of an Apollo support program seeking alternative means for earth's orbit reinsertion guidance in case of radar or communications failure. The space sextant designed for this task was checked enroute on Apollo missions 8, 10, and 11 with relatively good accuracy when compared to radar position.
      (2) High-Altitude Daytime Sky Brightness. This successful program to collect data on radiation characteristics of the daytime sky background was part of an effort to develop a ”star tracking” navigational system. Such an automatic electro-optical tracking system is now used on SAC reconnaissance planes and has applications in satellite positioning and space travel.

    3. Reconnaissance Systems. The X-15's speed and altitude combined to make it an ideal testbed for high-speed aircraft and satellite systems development.
      (1) Ultraviolet Studies. Ultraviolet (UV) sensors were studied as ICBM early warning detectors. This three-part project yielded promising results, but to date UV systems remain overshadowed by the more advanced infrared systems.
        (a) UV Earth's Background. Good data was obtained on the UV background against which the UV signature of an ICBM's exhaust could be detected.
        (b) Exhaust Plume Characteristics. To determine the signature of a typical rocket exhaust above the ozone layer, the exhaust plume of the X-15 itself was scanned.
        (c) Pacific Missile Range Monitor. To test the feasibility of detecting a missile launch by its UV signature, an actual launch from Vandenberg AFB was to be monitored. However, due to equipment malfunctions, scheduling problems, and ultimately a snow storm which prevented the last scheduled X-15 flight, this test was never possible.
      (2) Infrared Studies. Infrared (IR) work was devoted to two separate projects:
        (a) Space Detection Systems. The current satellite detection systems began as X-15 IR experiments. Early (1963) experiments studied the IR exhaust plume characteristics of the X-15. The follow-up project to measure the earth's IR background using an IR scanner never flew before the X-15 program ended. Nonetheless, the equipment developed therein contributed directly to successful tests later carried by U-2 aircraft and thus to the eventual satellite program.
        (b) IR Scanner. This experiment produced the first IR picture taken through a "hot" window. Though only a crude, two dimensional image was obtained, the notion that hypersonic IR reconnaissance was impossible was disproven. This work also advanced the development of operational line scanners for mapping carried on RF-4, EF-111, and Navy aircraft. The Earth's Resources Development Agency (ERDA) even uses this technology to monitor pollution levels.
      (3) Optical Background. This effort to determine daytime background interference effects to laser optics produced good data showing the feasibility of high-altitude laser surveillance. No actual pictures or images resulted, and this work has moved on to satellite testbeds.
      (4) Aerial Photographs. Optical degradation experiments determined that the shock wave, boundary-layer flow, and temperature gradients across windows caused negligible degradation to visual, near-IR, and radar aerial photography to Mach 5.5 and 125,000 feet. However, improved photographic equipment and much faster-speed films may very well invalidate these findings, hence the need for renewed flight testing. Toward the end of this experiment several tests of near-IR color photography produced the first successful inflight use of color films. Such were later used in reconnaissance work over Southeast Asia where colored emissions could denote enemy activity under dense foliage. ERDA now uses this technique via satellites to study the earth's resources.

    4. Advanced Aerodynamic Research. The X-15 served to carry aloft aerodynamic projects that were impractical for wind-tunnel study.
      (1) Several tests of flow distortion over surface irregularities were run to verify wind-tunnel studies; little disparity between the two was noted.
      (2) Attempts to measure cold-wall effects on coefficients of heating produced only marginal results, and this effort is still underway using the YF-12/SR-71 at DFRC. a
      (3) The feasibility of using fluidic (cavity) temperature probes to measure total temperature at high Mach, where standard probes burn away, was demonstrated.
      (4) A complete reentry guidance system for onboard, computerized energy management incorporating digital inputs to the adaptive flight control system was under study until the one aircraft so equipped was destroyed in 1967.

    5. X-15A-2 Modification Program. The 1962 crash of aircraft number two opened the door for extensive modification since considerable rebuilding was required. The resultant modification, as the X-15A-2, was primarily aimed at providing a testbed for development of a Mach 8 hypersonic, air breathing engine – the Hypersonic Ramjet Engine (HRE). Then, as now, no tunnel facility existed wherein such an engine could be realistically tested, and rocket boosters could not give steady-state tests or return the equipment.
      (1) HRE Program. The actual prototype engine was to be carried attached to the lower ventral of the X-15. Twenty-nine inches were added to the fuselage' between the existing tanks for the liquid hydrogen to power the HRE. This compartment could also be used to carry other experiments and included a three-panel, high-heat resistant window in the belly. Two external fuel tanks were added alongside the fuselage and tucked under the wings to increase rocket-boost time to attain Mach 8. These tanks were jettisoned at about Mach 2. To withstand the added heating due to increased velocity, the entire aircraft surface was coated with an ablative-type insulator.
        (a) Flight Program. Garrett-Air Research contracted to provide six prototype engines by mid-1969. In the meantime flight-test evaluations were made of the modified aircraft itself and of a dummy or mock-up HRE attached to the X-15A-2. On the first and only maximum-speed test of the X-15A-2 in 1967, shock impingement off the dummy HRE caused severe heating damage to the lower empennage, and very nearly resulted in loss of the aircraft. Though quickly repaired, the X-15A-2 never flew again as the X-15A-2's already cautious supporters abandoned the project. Hindsight would place the blame for this design oversight on haste and insufficient flow interaction studies. A key lesson learned from this episode was not to hang external stores or pylons on hypersonic aircraft, at least not without far more extensive study of underside flow patterns. The HRE was eventually tunnel tested in 1969, and the primary objective of achieving supersonic combustion was met, though the thrust produced was less than the drag created. HRE engineers nonetheless claim a success in that the objective was supersonic combustion, not a workable engine. The X-15 program can claim credit for spawning the HRE project, which has been continued on to the present at NASA-Langley. Though no realistic testbed yet exists, futuristic designs for a hypersonic research aircraft now envision internally mounted engine test facilities.
        (b) Ablator Tests. Since Mach 8 exceeded the heating limits of the Inconel X, a spray-on ablator of silicone-based elastomeric material was chosen to protect the aircraft. The ablator was to limit skin temperatures to 500°F in the 1,900°F environment of Mach 7.4 in this first-ever test of such insulation for an aircraft. Except where HRE pylon shock impingement caused a ten-fold rise in temperature, the ablator worked successfully to Mach 6.7. However, this approach was found to be operationally infeasible. Extensive man hours (approximately 20 days) were required to refurbish the charred ablator surface, and then the integrity of the ablator-to-skin bonding was of concern for subsequent flights. Other operational problems argued against spray-on ablatives: the crew could not walk on the vehicle; access panels were hard to remove and recover without leaving surface cracks; liquid oxygen if spilled on the ablator damaged the surface, requiring a coat of white paint to seal the ablative material's surface.
        (c) Replaceable Wingtip. Though not a part of the HRE project, the right wing tip, damaged in 1962, was rebuilt to allow interchangeable wing-tip shapes. This facility portended valuable studies in the future, but was never utilized.

    Though never labeled as such, the X-15 began to function as a hypersonic, high-altitude ”facility” after the original research work was completed. A high percentage (perhaps half) of the follow-on experiments were failures. Critics have contended that in the rush to extend the life of the X-15 program, and DFRC, experiments of questionable value and hasty preparation were flown on the X-15. As early as 1964, NASA officials did begin questioning the cost effectiveness of the follow-on program. Yet the X-15 was the only facility available at the time, and some of the work produced results that contributed to vital programs of today, such as ballistic early warning. Unfortunately, the X-15 was not designed as a hypersonic facility, and thus was limited in its capability to do experimental work.

  7. Conclusions:

    1. Comment On Contributions. The X-15 was certainly successful in fulfilling its original research goals. Upon X-15's experience rests all subsequent hypersonic study, and the manned space program owes much of its hardware and operations techniques to the X-15. Yet any evaluation of X-15's contributions to technology is tenuous at best. Most systems, knowledge, and especially experience derived from the program have evolved through successive programs over the past decade, and contribution by the X-15 is often obscured. In other cases, the old "X-15 hands" can simply no longer recall what became of the work started on the X-15; this is especially true with the follow-on experiments. Nor is it possible to determine any time factor for the delay between X-15's research and the appearance of useful technology or applications. The nature of the X-15's work was too varied; then too, there has been no subsequent hypersonic requirement outside the laboratory. From almost immediate payoffs in the manned space program on the one hand, X-15's technology sits dormant on the other. The X-15 did, at least, open the door for future hypersonic work, and in so doing sustained interest in manned aircraft at a time when all eyes were turning toward the capsule programs.

    2. Unfinished Work. Thanks to the X-15, hypersonic aerodynamics is well advanced. Thanks also to the X-15, the need for additional work in several key "stopper technologies"--areas which pose serious questions for future hypersonic vehicles--is evident. Since the program's abrupt termination in October 1968, the following areas have stood conspicuously in want of a testbed vehicle.
      (1) Scramjet Testing. Cruise capability is required of any operational hypersonic vehicle, and despite the advances being made in laboratories, development of a hypersonic, air-breathing power plant will require a flight-test facility.
      (2) Structural Cooling. Hypersonic cruise also requires advanced cooling methods, either active or passive, to dissipate the heat buildup. No existing hypersonic wind tunnel can handle sufficiently large prototype hardware and give reasonably accurate stagnation temperatures.
      (3) Aerodynamic Optimization. Despite enhanced ability to do accurate tunnel testing, interference-free testing of aerodynamic shapes can best be done on a hypersonic facility, as envisioned with the replaceable wing tip of the X-15A-2. Validation of proposed designs, such as the delta wing envisioned for the X-15 prior to its termination, ultimately requires flight testing.
      (4) Follow-on Projects. Since 1968, more experiments requiring a hypersonic testbed have been added to the list of projects left unfinished. Though referred to by at least one high ranking ASD officer as ”the first NHFRF,” the X-15 was an ill-suited testbed facility. It had not been designed as such, nor did it provide steady-state flight. It surpassed Mach 6 on only four occasions, the majority of its 199 flights being in the Mach 5 to 5.5 range. Yet what successes it did achieve point to the benefits a well-designed, Mach 6-plus facility could render such fields as hypersonic aerial photography and IR ”hot window” studies.

    3. Value Of The X-15. The commitment to go ahead with a flight-test program drove mid-1950s near-state-of-the-art technology toward perfection. Designed as a pure research vehicle with no operational prototype encumbrances or requirements for optimum design, the X-15 emerged from a short, three-year developmental program to return almost immediate data on the hypersonic environment. It gave the knowledge needed for today's designs for future hypersonic aircraft. Thanks to the X-15, we are able to do far more valuable laboratory research and testing. In a way, the X-15 reduced the urgency for a follow-on vehicle since so much more work can now be done with confidence in the wind tunnel, save the ultimate requirement for flight validation.

Thus wrote Ronald Boston with the perception of ten years after the program concluded.

When the X-15 quit flying, NASA was on the verge of initiating the first Phase A Shuttle studies. Yet, even before the X-15 had flown, a team of developers within the Air Force, industry, and NASA were busily at work on what would have been its immediate successor: an ambitious effort to develop an actual orbital hypersonic lifting reentry vehicle called Dyna-Soar.

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