Category Hypersonics Before. the Shuttle

During the summer of 1961, a new research initiative was proposed by the Air Force’s Aeronautical Systems Division at Wright – Patterson AFB and NASA Headquarters; using the X-15 to carry a wide range of sci-

On 4 November 1960, the program attempt­ed to launch two X-15 flights in a single day. Here X-15-1 is mount­ed on the NB-52B and X-15-2 is on the NB-52A. Rushworth was making his first flight in X-15-1, a low (48,900 feet) and slow (Mach 1.95) familiar­ization. The X-15-2, with Crossfield as pilot, aborted due to a failure in the No. 2 APU. (NASA photo E-6186)

entific experiments unforeseen when the air­craft was conceived in 1954.

Researchers at the FRC wanted to use the X-15 to carry high-altitude experiments related to the proposed Orbiting Astronomical Observatory; others suggested modifying one of the airplanes to carry a Mach 5-f – ram­jet for advanced air-breathing propulsion studies. Over 40 experiments were suggested by the scientific community as suitable can­didates for the X-15 to carry. In August 1961 NASA and the Air Force formed the “X-15 Joint Program Coordinating Committee” to prepare a plan for a follow-on experiments program. The committee held its first meet­ing on 23-25 August 1961 at the FRC.26

Many experiments suggested to the commit­tee related to space science, such as ultravio­let stellar photography. Others supported the Apollo program and hypersonic ramjet stud­ies. Hartley Soule and John Stack, then NASA’s director of aeronautical research, proposed the classification of experiments into two groups: category A experiments consisted of well-advanced and funded experiments having great importance; cate­gory В included worthwhile projects of less urgency or importance.2’

In March 1962 the committee approved the “X-15 Follow-on Program,” and NASA announced that an ultraviolet stellar photog­raphy experiment from the University of Wisconsin’s Washburn Observatory would be first. The X-15’s space science program eventually included twenty-eight experi­ments including astronomy, micrometeorite collection (using wing-top pods on the X-15- 1 and X-15-3 that opened at 150,000 feet), and high-altitude mapping. The micromete­orite experiment was unsuccessful, and was ultimately cancelled. Two of the follow-on programs, a horizon definition experiment from the Massachusetts Institute of Technology, and test of insulation material for the Saturn launch vehicle, directly bene­fited the Apollo program. The Saturn insula­tion was applied to the X-15’s speed brakes, which were then deployed at the desired speed and dynamic pressure to test both the insulating properties and the bonding materi­al. By the end of 1964, over 65 percent of data being returned from the three X-15 air­craft involved follow-on projects; this per­centage increased yearly through conclusion of the program. ™

As early as May 1962, North American had proposed modifying one of the X-15s as a flying test bed for hypersonic engines. Since the X-I5s were being fully utilized at the time, neither the Air Force nor NASA expressed much interest in pursuing the idea. However, when the X-15-2 was damaged during a landing accident on 9 November 1962 (seriously injuring Jack McKay, who would later return from his injuries to fly the X-15 again), North American proposed mod­ifying the aircraft in conjunction with its repairs. General support for the plan was found within the Air Force, which was will­ing to pay the estimated $6 million.31

On the other hand, NASA was less enthusias­tic, and felt the aircraft should simply be repaired to its original configuration.32 Researchers at NASA believed that the Mach 8 X-l 5 would prove to be of limited value for propulsion research. However, NASA did not press its views, and in March 1963 the Air Force authorized North American to rebuild the aircraft as the X-15A-2. Twenty-nine inches were added to the fuselage between the existing propellant tanks. The extra vol­ume was to be used by a liquid hydrogen tank to power the ramjet, but the LH2 tank could be replaced by other equipment as needed. In fact, the compartment was frequently used to house cameras to test reconnaissance con­cepts, or to observe the dummy ramjet during flight tests, through three heat-resistant win­dows in the lower fuselage. The capability to carry two external propellant tanks was added to provide additional powered flight with the XLR99. The right wingtip was also modified to allow various wingtip shapes to be carried interchangeably, although it appears that this capability was never used.33

Forty weeks and $9 million later, North American delivered the X-15A-2,’4 The air­craft made its first flight on 25 June 1964 piloted by Bob Rushworth. Early flights demonstrated that the aircraft retained satis­factory flying qualities at Mach 5, although on three flights thermal stresses caused por­tions of the landing gear to extend at Mach 4.3, generating “an awful bang and a yaw.’"5 In each case Rushworth landed safely, despite the blow-out of the heat-weakened tires in one instance. On 18 November 1966, Pete Knight set an unofficial world’s speed record of Mach 6.33 in the aircraft. The drop tanks had been jettisoned at Mach 2.27 and 69,700 feet. A nonfunctional dummy ramjet was constructed in order to gather aerodynamic data on the basic shape in preparation for possible flight tests in the early 1970s. The first flight with the dummy ramjet attached to the ventral was on 8 May 1967. Although providing a pronounced nose-down trim change, the ramjet actually restored some of the directional stability lost when the lower ventral rudder had been removed.

NASA had evaluated several possible coat­ings that could be applied over the X-15’s Inconel X hot-structure to enable it to with­stand the thermal loads experienced above Mach 6. The use of such coatings could be beneficial since various ablators were being investigated by the major aerospace contrac­tors during the early pre-concept phases-16 of the Space Shuttle development." Such a coat­ing would have to be relatively light, have good insulating properties, and be easy to apply, remove, and reapply before another flight. The selected coating was MA-25S, an ablator developed by the Martin Company in connection with some early reusable space­craft studies. Consisting of a resin base, a cat­alyst, and a glass bead powder, it would pro­tect the hot-structure from the expected 2,000 degrees Fahrenheit heating at Mach 8. Martin esumated that the coating, ranging from 0.59 inches thick on the canopy, wings, vertical, and horizontal stabilizers, down to 0.015 inch­es on the trailing edges of the wings and tail, would keep the skin temperature below 600 degrees Fahrenheit. The first unpleasant sur­prise came, however, with the application of the coating to the X-15A-2: it took six weeks. Getting the correct thickness over the entire surface proved harder than expected. Also, every time a panel had to be opened to service the X-15, the coating had to be removed and reapplied around the affected area.

Because the ablator would char and emit a residue in flight, North American had installed an “eyelid” over the left cockpit window; it would remain closed until just before landing. During launch and climbout, the pilot would use the right window, but residue from the ablator would render it opaque above Mach 6. The eyelid had already been tested on several flights.™

Late in the summer of 1967, the X-15A-2 was ready for flight with the ablative coat­ing. The weight of the ablator—125 pounds higher than planned—together with expected increased drag reduced the theoretical maxi­mum performance of the airplane to Mach 7.4, still a significant advance over the Mach

6.3 previously attained. The appearance of the X-15A-2 was striking, an overall flat off – white finish, the external tanks a mix of sil­ver and orange-red with broad striping. On 21 August 1967, Knight completed the first flight in the ablative coated X-15A-2, reach­ing Mach 4.94 and familiarizing himself with its handling qualities. His next flight was destined to be the program’s fastest flight, and the last flight of the X-15A-2.’9

On 3 October 1967, 43,750 feet over Mud Lake, Knight dropped away from the NB-52B. The flight plan showed the X-15A-2 would weigh 52,117 pounds at separation, more than 50 percent heavier than originally conceived in 1954* The external tanks were jettisoned 67.4 seconds after launch at Mach

2.4 and 72,300 feet; tank separation was satis­factory, however, Knight felt the ejection was “harder” than the last one he had experienced (2-50-89). The recovery system performed satisfactorily and the tanks were recovered in repairable condition. The XLR99 burned for

140.7 seconds before Knight shut it down. Radar data showed the X-15A-2 attained Mach 6.70 (4,520 mph) at 102,700 feet, a winged-vehicle speed record that would stand until the return of the Space Shuttle Columbia from its first orbital flight in 1981.41

The post-landing inspection revealed many things. The ability of the ablative material to protect the aircraft structure from the high aerodynamic heating was considered good except in the area around the dummy ramjet where the heating rates were significantly higher than predicted. The instrumentation on the dummy ramjet had ceased working approximately 25 seconds after engine shut­down, indicating that a bum through of the ramjet/pylon structure had occurred. Shortly thereafter the heat propagated upward into the lower aft fuselage causing the hydrogen – peroxide hot light to illuminate in the cock­pit. Assuming a genuine overheat condition, William Dana in the NASA 1 control room had requested Knight to jettison the remain­ing peroxide. The high heat in the aft fuse­lage area also caused a failure of a helium check valve allowing not only the normal helium source gas to escape, but also the emergency jettison control gas supply as well. Thus, the remaining residual propel­lants could not be jettisoned. The aircraft was an estimated 1,500 pounds heavier than normal at landing, but the landing occurred without incident.

wave impinged on the ramjet and its sup­porting structure. The heat in the ramjet pylon area was later estimated to be ten times normal, and became high enough at some time during the flight to ignite 3 of the 4 explosive bolts holding the ramjet to the pylon. As Knight was turning downwind in the landing pattern, the one remaining bolt failed structurally and the ramjet separated from the aircraft. Knight did not feel the ramjet separate, and since the chase aircraft had not yet joined up, was unaware that the ramjet had separated.

The position of the X-15 at the time of sepa­ration was later established by radar data and the most likely trajectory estimated. A ground search party discovered the ramjet on the Edwards bombing range. Although it had been damaged by impact, it was returned for study of the heat damage.

The unprotected right-hand windshield was, as anticipated, partially covered with ablation products. Since the left eyelid remained closed until well into the recovery maneuver, Knight flew the X-15 using on-board instru­ments and directions from William Dana in the NASA 1 control room. The eyelid was opened at approximately Mach 1.6 as the air­craft was over Rogers Dry Lake, and the visi­bility was considered satisfactory. Knight landed at Edwards 8 minutes and 12 seconds after launch.

Engineers had not fully considered possible shock interaction with the ramjet shape at hypersonic speeds. As it turned out, the flow patterns were such that a tremendous shock

The ablator obviously was not totally success­ful; in fact this was the closest any X-15 came to structural failure induced by heating. Post­flight inspection revealed that the aircraft was

charred on its leading edges and nose. The ablator had actually prevented cooling of some hot spots by keeping the heat away from the hot-structure. Some heating effects, such as where shock waves impinged on the ramjet had not been thoroughly studied. To John Becker the flight underscored.. the need for maximum attention to aerothermodynamic detail in design and preflight testing.”42 To Jack Kolf, an X-15 project engineer at the FRC, the post-flight condition of the airplane “… was a surprise to all of us. If there had been any question that the airplane was going to come back in that shape, we never would have flown it.”1-1

Some of the problems encountered with the ablator were nonrepresentative of possible future uses. The X-15 had been designed as an uninsulated hot structure. Any future vehicle would probably be designed with a more conventional airframe, eliminating some of the problems encountered on this flight. But some of the problems were very
real. The amount of time it took to apply the ablator was unacceptable. Even considering that the learning curve was steep, and that after some experience the time could be cut in half or even further, the six weeks it took to coat the relatively small X-15 bode ill for larger vehicles. Nevertheless, ablators would continue to be proposed on various Space Shuttle concepts, in decreasing quantity, until 1970 when several forms of ceramic tiles and metal “shingles” would become the preferred concepts.44

It was estimated that repairing the X-15A-2 and refurbishing the ablator for another flight near Mach 7 would have taken five weeks. The unexpected airflow problems around the ramjet ended any idea of flying it again. NASA sent the X-15A-2 to North American for general maintenance and repair, and although the aircraft returned to Edwards in June 196S, it never flew again. It is now on exhibit—in natural black finish—at the Air Force Museum, Wright-Patterson AFB, Ohio.

Ultimately, Garrett did deliver a functioning model of the ramjet, and it was successfully tested in a wind tunnel in late 1969. In this case successful meant that supersonic com­bustion was achieved, although for a very short duration and under very controlled and controversial conditions.45

The X-15-3 featured specialized flight instrumentation and displays that rendered it particularly suitable for high-altitude flight research. A key element was the Minneapolis Honeywell MH-96 “adaptive” flight control system originally developed for the X-20 Dyna-Soar. This system automatically com­pensated for the airplane’s behavior in vari­ous flight regimes, combining the aerody­namic control surfaces and the reaction con­trols into a single control package. This was obviously the way future high-speed aircraft and spacecraft would be controlled, but the technology of the 1960s were severely taxed by the requirements for such a system.

By the end of 1963, the X-15-3 had flown above 50 miles altitude. This was the altitude that the Air Force recognized as the mini­mum boundary of space flight, and five Air Force pilots were awarded Astronaut Wings for their flights in the X-15.4* All but one of these flights was with X-15-3 (Astronaut Joe Engle’s first space flight was in Х-15-t). NASA did not recognize the 50 mile criteria, using the international 62 mile standard instead. Only a single NASA pilot went this high; Joe Walker set a record for winged space flight by reaching 354,200 feet (67 miles), a record that stood until the orbital flight of Columbia nearly two decades later. By mid-1967, the X-15-3 had completed sixty-four research flights, twenty-one at altitudes above 200,000 feet. It became the primary aircraft for carrying experiments to high altitude.

The X-15-3 would also make the most tragic flight of the program. At 10:30 in the morning on 15 November 1967, the X-15-3 dropped away from the NB-52B at 45,000 feet over Delamar Dry Lake. At the controls was Major Michael J. Adams, making his seventh X-15 flight. Starting his climb under full power, he was soon passing through 85,000 feet. Then an electrical disturbance distracted him and slightly degraded the control of the aircraft; having adequate backup controls, Adams con­tinued on. At 10:33 he reached a peak altitude of 266,000 feet. In the NASA 1 control room, mission controller Pete Knight monitored the mission with a team of engineers. As the X-15 climbed, Adams started a planned wing-rock­ing maneuver so an on-board camera could scan the horizon. The wing rocking quickly became excessive, by a factor of two or three. At the conclusion of the wing-rocking portion of the climb, the X-15 began a slow drift in heading; 40 seconds later, when the aircraft reached its maximum altitude, it was off head­ing by 15 degrees. As Adams came over the top, the drift briefly halted, with the airplane yawed 15 degrees to the right. Then the drift began again; within 30 seconds, Adams was descending at right angles to the flight path. At 230,000 feet, encountering rapidly increas­ing dynamic pressures, the X-15 entered a Mach 5 spin.47

In the NASA 1 control room there was no way to monitor heading, so nobody suspect­ed the true situation that Adams now faced. The controllers did not know that the air­plane was yawing, eventually turning com­pletely around. In fact, Knight advised Adams that he was “a little bit high,” but in “real good shape.” Just 15 seconds later, Adams radioed that the aircraft “seems squirrely.” At 10:34 came a shattering call: “I’m in a spin, Pete.” Plagued by lack of heading information, the control room staff saw only large and very slow pitching and rolling motions. One reaction was “disbelief; the feeling that possibly he was overstating the case.” But Adams again called out, “I’m in a spin.” As best they could, the ground controllers sought to get the X-15 straight­ened out. There was no recommended spin recovery technique for the X-15, and engi­neers knew nothing about the aircraft’s

realizing that the X-15 would never make Rogers Dry Lake, went into afterburner and raced for the emergency lakes; Ballarat and Cuddeback. Adams held the X-15’s controls against the spin, using both the aerodynamic control surfaces and the reaction controls. Through some combination of pilot tech­nique and basic aerodynamic stability, the airplane recovered from the spin at 118,000 feet and went into an inverted Mach 4.7 dive at an angle between 40 and 45 degrees.48

Adams was in a relatively high altitude dive and had a good chance of rolling upright, pulling out, and setting up a landing. But now came a technical problem; the MH-96 began a limit-cycle oscillation just as the airplane came out of the spin, preventing the gain changer from reducing pitch as dynamic pressure increased. The X-15 began a rapid pitching motion of increasing severity, still in a dive at 160,000 feet per minute, dynamic pressure increasing intolerably. As the X-15 neared 65,000 feet, it was diving at Mach 3.93 and experiencing over 15-g vertically, both positive and negative, and 8-g laterally.

The aircraft broke up northeast of the town of Johannesburg 10 minutes and 35 seconds after launch. A chase pilot spotted dust on Cuddeback, but it was not the X-15. Then an Air Force pilot, who had been up on a delayed chase mission and had tagged along on the X-15 flight to see if he could fill in for an errant chase plane, spotted the main wreckage northwest of Cuddeback. Mike Adams was dead; the X-15-3 destroyed.49

NASA and the Air Force convened an acci­dent board. Chaired by NASA’s Donald R. Bellman, the board took two months to pre­pare its report. Ground parties scoured the countryside looking for wreckage; critical to the investigation was the film from the cock­pit camera. The weekend after the accident, an unofficial FRC search party found the camera; disappointingly, the film cartridge was nowhere in sight. Engineers theorized that the film cassette, being lighter than the camera, might be further away, blown north by winds at altitude. FRC engineer Victor Horton organized a search and on 29 November, during the first pass over the area, Willard E. Dives found the cassette.

Most puzzling was Adams’ complete lack of awareness of major heading deviations in spite of accurately functioning cockpit instru­mentation. The accident board concluded that he had allowed the aircraft to deviate as the result of a combination of distraction, misin­terpretation of his instrumentation display, and possible vertigo. The electrical distur­bance early in the flight degraded the overall effectiveness of the aircraft’s control system and further added to pilot workload. The MH-96 adaptive control system then caused the airplane to break up during reentry. The board made two major recommendations: install a telemetered heading indicator in the control room, visible to the flight controller; and medically screen X-15 pilot candidates for labyrinth (vertigo) sensitivity.5" As a result of the X-15 ’s crash, the FRC added a ground – based “8 ball” attitude indicator in the control room to furnish mission controllers with real time pitch, roll, heading, angle of attack, and sideslip information.

Mike Adams was posthumously awarded Astronaut Wings for his last flight in the X-15-3, which had attained an altitude of

266,0 feet—50.38 miles. In 1991 Adams’ name was added to the Astronaut Memorial at the Kennedy Space Center in Florida.

The X-15 program would only fly another eight missions. The X-15A-2, grounded for repairs, soon remained grounded forever. The X-15-1 continued flying, with sharp dif­ferences of opinion about whether the research results returned were worth the risk and expense.

A proposed delta wing modification to the X-15-3 had offered supporters the hope that the program might continue to 1972 or 1973. The delta wing X-15 had grown out of stud­ies in the early 1960s on using the X-15 as a hypersonic cruise research vehicle.

Essentially, the delta wing X-15 would have made use of the third airframe with the adap­tive flight control system, but also incorporat­ed the modifications made to the X-15A-2— lengthening the fuselage, revising the land­ing gear, adding external propellant tanks, and provisions for a small-scale experimen­tal ramjet. NASA proponents, particularly John Becker at Langley, found the idea very attractive since: “The highly swept delta wing has emerged from studies of the past decade as the form most likely to be utilized on future hypersonic flight vehicles in which high lift/drag ratio is a prime requirement i. e., hypersonic transports and military hypersonic cruise vehicles, and certain recoverable boost vehicles as well.”51

Despite such endorsement, support remained lukewarm at best both within NASA and the Air Force; the loss of Mike Adams and the X-15-3 effectively ended all thought of such a modification.

As early as March 1964, in consultation with NASA Headquarters, Brigadier General

James T. Stewart, director of science and tech­nology for the Air Force, had determined to end the X-15 program by 1968.52 At a meeting of the Aeronautics/Astronautics Coordinating Board on 5 July 1966, it was decided that NASA should assume total responsibility for all X-15 costs (other than incidental AFFTC support) on 1 January 1968.51 This was later postponed one year. As it turned out, by December 1968 only the X-15-1 was still fly­ing, and it cost roughly $600,000 per flight. Other NASA programs could benefit from this funding, and thus NASA did not request a continuation of X-15 funding after December 1968.54 During 1968 William Dana and Pete Knight took turns flying the X-15-1. On 24 October 1968, Dana completed the X-15’s 199th, and as it turned out the last, flight reaching Mach 5.38 at 255,000 feet. A total of ten attempts were made to launch the 200th flight, but a variety of maintenance and weather problems forced cancellation every time. On 20 December 1968, the X-15-1 was demated from the NB-52A for the last time. After nearly a decade of flight operations, the X-15 program came to an end.

The instrument panel of the X-15-3 with the MH-96 adaptive con­trol system installed. The dark panel imme­diately ahead of the center control stick allowed the pilot to control how the MH-96 reacted.

(NASA photo E63-9834)

The year 1999 marked the 40th anniversary of the first flight of the X-15; this anniversary occurred more than 30 years after the program ended. The X-15 was the last high-speed research aircraft to fly as part of the research airplane program. The stillborn X-30 of the 1980s never took flight, and the verdict is still out on the fate of the Lockheed Martin X-33 demonstrator. Neil Armstrong, among others, once called the X-15 “the most successful research airplane in history.”1

Twelve men flew X-15. Scott Crossfield was first; William Dana was last. Pete Knight went 4,520 mph (Mach 6.70); Joe Walker went 67 miles (354,200 feet) high. Five of the pilots were awarded Astronaut Wings. Mike Adams died. What was learned? What should have been learned?

In October 1968 John V. 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 experiments carried by the X-15. Becker’s comments have been well documented elsewhere, but are quoted here as appropriate.2

Nearly ten years after Becker’s assessment, Captain Ronald G. Boston of the U. S. Air Force Academy’s history department reviewed the X-15 program for “lessons learned” that might benefit the development of the X-24C National Hypersonic Flight Research Facility Program, an effort that was cancelled shortly afterwards. Boston’s paper offered an interesting perspective on the X-15 from the vantage point of the mid-1970s?

In 1999, the historian at the Dryden Flight Research Center, J. D. “Dill” Hunley, wrote a lessons-leamed paper on the X-15. Drawing heavily but not uncritically upon Becker’s and Boston’s insights, it too pro­vides an interesting perspective, and is quot­ed several times in the pages that follow.4

Lessons Learned (or not)

The X-15 was designed to achieve a speed of Mach 6 and an altitude of 250,000 feet to explore the hypersonic and near-space envi­ronments. More specifically, its goals were:

(1) to verify existing (1954) theory and wind tunnel techniques;

(2) to study aircraft structures under high (1,200 degrees Fahrenheit) heating;

(3) to investigate stability and control problems associated with high-altitude boost and reentry; and

(4) to investigate the biomedical effects of both weightless and high-g flight.

All of these design goals were met, and most were surpassed. The X-15 actually achieved Mach 6.70, 354,200 feet, 1,350 degrees Fahrenheit, and dynamic pressures over 2,200 pounds per square foot.5 In addition, once the original research goals were achieved, the X-15 became a high-altitude hypersonic testbed for which 46 follow-on experiments were designed.

Unfortunately due to the absence of a subse­quent hypersonic mission, aircraft applica­tions of X-15 technology have been few. Given the major advances in materials and computer technology in the 30 years since the end of the flight research program, it is

unlikely that many of the actual hardware lessons are still applicable. That being said, the lessons learned from hypersonic model­ing, simulation, and the insight gained by being able to evaluate actual X-15 flight test results against wind tunnel and predicted results, greatly expanded the confidence of researchers during the 1960s and 1970s.

In space, however, the X-15 contributed sig­nificantly to both the Apollo and Space Shuttle programs. Perhaps the major contribu­tion was the final elimination of a spray-on ablator as a possible thermal protection sys­tem for the Space Shuttle. This would likely have happened in any case as the ceramic tiles and metal shingles were further developed, but the operational problems encountered with the (admittedly brief) experience on X-15A-2 hastened the departure of the abla­tors. Although largely intangible, proving the value of man-in-the-loop simulations and pre­cision “dead-stick” landings have also been invaluable to the Space Shuttle program.

The full value of X-15’s experience to designing advanced aircraft and spacecraft can only be guessed at. Many of the engi­neers (including Harrison Storms) from the X-15 project worked on the Apollo space­craft and the Space Shuttle. In fact, the X-15 experience may have been part of the reason that North American was selected to build later spacecraft. Yet X-15’s experience is overshadowed by more recent projects and becomes difficult to trace as systems evolve through successive programs. Nonetheless, many of those engineers are confident that they owe much to the X-15, even if many are at a loss to give any concrete examples.

John V. Becker, arguably the father of the X-15, once stated that the project came along at “ … the most propitious of all possible times for its promotion and approval.” At the time it was not considered necessary to have a defined operational program in order to conduct basic research. There were no “glamorous and expensive” manned space projects to compete for funding, and the gen­eral feeling within the nation was one of try­ing to go faster, higher, or further. In today’s environment, as in 1968 when Becker was commenting, it is highly unlikely that a pro­gram such as the X-15 could gain approval.6

This situation should give pause to those who fund aerospace projects solely on the basis of their presumably predictable outcomes and their expected cost effectiveness. Without the X-15’s pioneering work, it is quite possible that the manned space program would have been slowed, conceivably with disastrous consequences for national prestige.7

According to Becker, proceeding with a gen­eral research configuration rather than with a prototype of a vehicle designed to achieve a specific mission as envisioned in 1954 was critical to the ultimate success the X-15 enjoyed. Had the prototype route been taken, Becker believed that “… we would have picked the wrong mission, the wrong struc­ture, the wrong aerodynamic shapes, and the wrong propulsion.” He also believed that a second vital aspect to the success of the X-15 was its ability to conduct research, albeit for very short periods of time, outside the sensi­ble atmosphere.®

The latter proved to be the most important aspect of X-15 research, given the contribu­tions it made to the space program. But in 1954 this could not have been foreseen. Few people then believed that flight into space was imminent, and most thought that flying humans into space was improbable before the next century. Fortunately, the hypersonic aspects of the proposed X-15 enjoyed “virtu­ally unanimous approval,” although ironical­ly the space-oriented results of the X-15 have been of greater value than its contributions to aeronautics.9

A final lesson from the X-15 program is that success comes at a cost. It is highly likely that researchers can never accurately predict the costs of exploring the unknown. If you under-

stand the problems well enough to accurately predict the cost, the research is not necessary. The original cost estimate for the X-15 pro­gram was $10.7 million. Actual costs were still a bargain in comparison with those for Apollo, Space Shuttle, and the International Space Station, but at $300 million, they were over almost 30 times the original estimate.10 Because the X-15’s costs were not subjected to the same scrutiny from the Administration and Congress that today’s aerospace projects undergo, the program continued. One of the risks when exploring the unknown is that you do not understand all the risks. Perhaps politi­cians and administrators should learn this par­ticular lesson from this early and highly suc­cessful program.

The XLR99 was the first large man-rated rocket engine that was capable of being throttled and restarted in flight. This com­plexity resulted in many aborted missions (approximately one-tenth of all mission aborts) and significantly added to the devel­opment cost of the engine. When the X-15 program ended, many felt that the throt­tleable feature might have been a needless luxury that complicated and delayed the development of the XLR99.

But in the mid-1960s these attributes were considered vital to the development of a rocket engine to power the Space Shuttle. At the time, Shuttle was to consist of two total­ly reusable stages—essentially a large hyper­sonic aircraft that carried a smaller winged spacecraft much like the NB-52s carried the X-15s. The same basic engine was going to power both stages; the pilots therefore need­ed to be able to control its thrust output. At some points in the early Shuttle concept development phases, the same engines would also be used on-orbit to effect changes in the orbital plane. So the original concept for the Space Shuttle Main Engines (SSME) included the ability to operate at 10 percent of their rated thrust, and to be restarted mul­tiple times during flight.11

In the end, the production SSMEs are throt­tleable within much the same range as the XLR99—65 to 109 percent, in one percent increments. In actuality about the only rou­tine use of this ability is to throttle down as the vehicle reaches the point of maximum dynamic pressure during ascent, easing stresses on the vehicle for a few seconds on each flight. Even this would not have been necessary with a different design for the solid rocket boosters.12 So the complexities required to enable the engine to throttle may, again, have been a needless luxury. Nevertheless, the development pains experi­enced by Reaction Motors provided insight for Pratt & Whitney and Rocketdyne (the two main SSME competitors) during the design and development of the SSMEs.

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 became the first program for repetitive, dynamic mon­itoring of pilot heart rate, respiration, and EKG under extreme stress over a wide range of speeds and forces. The Bioastronautics Branch of the AFFTC measured unusually high heart and breathing rates on the parts of the X-15 pilots at points such as launch of the X-15 from the NB-52, engine shutdown, pull­out from reentry, and landing. Heart rates averaged 145 to 160 beats per minute with peaks on some flights of up to 185 beats per minute. Despite the high levels, which caused initial concern, these heart rates were not associated with any physical problems or loss of ability to perform piloting tasks requiring considerable precision. Consequently, theo­retical limits had to be re-evaluated, and Project Mercury as well as later space pro­grams did not have to be concerned about such high heart rates in the absence of other symptoms. In fact, the X-15’s data provided some of the confidence to go ahead with early manned Mercury flights—the downrange bal­listic shots being not entirely dissimilar to the X-15’s mission profile.’3

The bio-instrumentation developed for the X-15 program has allowed similar monitor­ing of many subsequent flight test programs. Incorporated into 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.

Another contribution of the X-15 program was the development of what John Becker calls the “first practical full-pressure suit for pilot protection in space.”14 The David Clark Company had worked with the Navy and the HSFS on an early full-pressure suit for use in high-altitude flights of the Douglas D-558- II; the suit worn by Marion Carl on his high – altitude flights was the first step. This suit was made of a waffle-weave material and had only a cloth enclosure rather than a hel­met. It should be noted that Scott Crossfield was heavily involved in the creation of this suit, the success of which Crossfield attrib­utes to “… David Clark’s genius.”15

The David Clark Company later developed the A/P-22S-2 pressure suit that permitted a higher degree of mobility.16 It consisted of a link-net material covering a rubberized pres­sure garment. Developed specifically for the X-15, the basic pressure suit provided part of the technological basis for the suits used in the Mercury and Gemini programs. It was later refined as the A/P-22S-6 suit that became the standard Air Force operational suit for high altitude flight in aircraft such as the U-2 and SR-71. However, it should be added that the space suit for Project Mercury underwent further development and was pro­duced by the B. F. Goodrich Company rather than the David Clark Company, so the line of development from X-15 to Mercury was not entirely a linear one, and security surround­ing the U-2 and Blackbird programs have obscured some of this history.17

X-15 pilots practiced in a ground-based sim­ulator that included the X-15 cockpit with all of its switches, controls, gauges, and instru­ments. An analog computer converted the pilot’s movements with the controls into instrument readings and indicated what the aircraft would do in flight to respond to con­trol actions. After a flight planner had used the simulator to lay out a flight plan, the pilot and flight planner worked “for days and weeks practicing for a particular flight.” The X-15 simulator was continually updated with data from previous flights to make it more accurate, and eventually a digital computer allowed it to perform at higher fidelity.18

Much has been made of the side-stick con­troller used on the X-15. Although the con­cept has found its way onto other aircraft, it has usually been for reasons other than those that initially drove its use on the X-15. The X-15 designers feared that the high g-loads encountered during acceleration would make it impossible for the pilot to use the conven­tional center stick; such worries are not the reason Airbus Industries has used the con­troller on the A318-series airliners. And although the side-stick controller has proven very popular in the F-16 fighter, it has not been widely adopted. Nevertheless, the X-15 experience provided a wealth of data over a wide range of flight regimes.

Some phases of X-15 flight, such as reentry, were marginally stable, and the aircraft required artificial augmentation (damping) systems to achieve satisfactory stability. The X-15 necessi­tated the development of an early stability aug­mentation system (SAS). The first two X-15s were equipped with a simple fail-safe, fixed – gain system. The X-15-3 was equipped with a triple-redundant adaptive flight control system; the pilot flew via inputs to the augmentation system. Although a point of continuing debate, the X-15 did not incorporate a “fly-by-wire” system if meant to denote a nonmechanically linked control system. Nevertheless, the SAS system did “fly” the X-15-3 based on pilot input rather than the pilot flying it direcdy. This basic concept would find use on an entire generation of aircraft, including such high performance fighters as the F-15. The advent of true fly-by­wire aircraft, such as the F/A-18, would advance the concept even further.

In 1954, the few existing hypersonic wind tunnels were small and presumably unable to simulate the conditions of actual flight at speeds above Mach 5, The realistic fear at the time was that testing in them would fail to produce valid data. The X-15 provided the earliest, and so far most significant, valida­tion of hypersonic wind tunnel data. This was of particular significance since it would be extremely difficult and very expensive to build a large-scale hypersonic wind tunnel.

This general validation, although broadly con­firmed by other missiles and spacecraft, came primarily from the X-15; it made the conven­tional, low-temperature, hypersonic wind tun­nel an accepted source of data for configura­tion development of hypersonic vehicles.

The X-15 program 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 a 15 percent discrepancy found in base drag— caused by the sting support used in the wind tunnel. All of this greatly increased the con­fidence of engineers as they set about design­ing the Space Shuttle.

One of the widely held beliefs in the mid – 1950s was the theoretical presumption that the boundary layer (the thin layer of air close to the surface of an aircraft) would be highly stable at hypersonic speeds because of heat flow away from it. This presumption fostered the belief that hypersonic aircraft would enjoy laminar (smooth) airflow over their surfaces. At Mach 6, even wind tunnel extrapolations indicated extensive laminar flow. However, flight data from the X-15 showed that only the leading edges exhibited laminar flow and that turbulent flow occurred over most surfaces. Small surface irregularities, which produced turbulent flow at transonic and supersonic speeds, also did so at Mach 6.20 Thus, engineers had to aban­don their hopeful expectations. Importantly, X-15 flight test data indicated that hyperson­ic flow phenomena were linear above Mach 5, allowing increased confidence during design of the Space Shuttle, which must rou­tinely transition through Mach 25 on its way to and from space. The basic X-15 data were also very useful to the NASP designers while that program was viable.

In a major discovery, the Sommer-Short and Eckert T-prime aerodynamic heating predic­tion theories in use during the late 1950s were found to be 30 to 40 percent in excess of flight test results. Most specialists in fluid mechanics refused to believe the data, but repeated in-flight measurements completely substantiated the initial findings. This led the aerodynamicists to undertake renewed ground-based research to complete their understanding of the phenomena involved— highlighting the value of flight research in doing what Hugh Dryden had predicted for the X-15 in 1956: that it would “separate the real from the imagined.”21

Subsequent wind tunnel testing led to Langley’s adopting the empirical Spaulding – Chi model for hypersonic heating. This eventually allowed the design of lighter vehi­cles with less thermal protection that could more easily be launched into space. The Spaulding-Chi model found its first major use during the design of the Apollo com­mand and service modules and proved to be quite accurate. In 1999 the Spaulding-Chi model was still the primary tool in use.

Based on their X-15 experience, North American devised a computerized mathe­matical model for aerodynamic heating called HASTE (Hypersonic and Supersonic Thermal Evaluation) which gave a workable “first cut” approximation for design studies. HASTE was, for example, used directly in the initial Apollo design study. Subsequent

versions of this basic model were also used early in the Space Shuttle design evolution.

At the time of the first Mach 5 X-15 flight, perhaps its greatest contribution to aeronau­tics was to disprove the existence of a “sta­bility barrier" to hypersonic flight that was suspected after earlier research aircraft encountered extreme instability at high supersonic speeds. Although of little conse­quence today, the development of the “wedge” tail allowed the X-15 to successful­ly fly above Mach 5 without the instability that had plagued the X-l series and X-2 air­craft at much lower speeds. The advent of modem fly-by-wire controls and stability augmentation systems based around high speed digital computers have allowed designers to compensate for gross instabili­ties in basic aerodynamic design, and even to tailor an aircraft’s behavior differently for different flight regimes. The era of building a vehicle that is dynamically stable has passed, and with it much of this lesson.

The art of simulation grew with the X-15 pro­gram, not only for pilot training and mission rehearsal, but for research into controllability problems. The same fixed-based simulator used by the pilots could also be used to explore those areas of the flight envelope deemed too risky for actual flight. The X-15 program showed the value of combining wind tunnel testing and simulation in maximizing the knowledge gained from each of the 199 test flights. It also provided a means of com­paring “real” flight data with wind tunnel data. It is interesting to note that the man-in – the-loop simulation first used on X-15 found wide application on the X-30 and the X-33. In fact, DFRC research pilot Stephen D. Ishmael has flown hundreds of hours “in” the X-33, which ironically is an unpiloted vehicle.

Before the X-15, high-speed research air­craft flown at Edwards could be monitored and tracked from Edwards. The trajectory of the X-15 extended much farther from

Edwards than those of the previous research aircraft, requiring two up-range stations where tracking, communications, and telemetry equipment were installed and inte­grated with the control room back at the FRC. Along the X-15 flight route, program personnel also surveyed a series of dry lakebeds for emergency landings and tested them before each flight to ensure they were hard enough to permit the X-15 to land.22 In many ways this parallels the tracking and communications network and the transat­lantic abort sites used by the Space Shuttle.

The opportunity to observe pilot perform­ance under high stress and high g-forces indicated that an extensive ground training program was needed to prepare pilots to han­dle the complex tasks and mission profiles of space flight. The result was a simulation pro­gram that became the foundation for crew training for all human space flight. The pro­gram depended on four types of simulation.

Prior to the first X-15 mission, the abil­ity of the pilot to function under the high g-forces expected during launch and reentry was tested in a closed-loop, six – degree-of-freedom centrifuge at the Naval Air Development Center, Johnsville, Pennsylvania. This project became the prototype for programs set up at the Ames Research Center and the Manned Spacecraft Center at Houston (now the Johnson Space Center).23

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 mission profiles. Similar, but much more elabo­rate, rehearsals are still used by astro­nauts preparing for Space Shuttle flights.

X-15 pilots maintained proficiency by flying an NT-33 or JF-100C variable – stability aircraft whose handling charac-

teristics could be varied in flight, simu­lating the varied response of the X-15 traversing a wide range of velocities and atmospheric densities. Much of this training is now conducted in advanced motion-based simulators, although the Air Force still operates a variable-stabil­ity aircraft (the VISTA F-16).

Pilots practiced the approach and land­ing maneuver in F-104 aircraft. With landing gear and speed brakes extended, the F-104’s power-off glide ratio approximated that of the unpowered X-15. Shuttle crews continue this same practice using modified Gulfstream Shuttle Training Aircraft (STA).

Astronaut “capsule communicators,” (cap – comms) were a direct outgrowth of the X-15’s practice of using an experienced pilot as the ground communicator for most X-15 mis­sions.24 This practice existed through Mercury, Gemini, and Apollo, and continues today on Space Shuttle missions. It is still believed that a pilot on the ground makes the best person to communicate with the crew, especially in stressful or emergency situations.25

Subsequent flight test work at Edwards relied heavily on the methodology developed
for the X-15. There are no fewer than three high-tech control facilities located at Edwards today; the facility at Dry den, the Riddley Control Center complex at the AFFTC, and the B-2 control complex locat­ed on South Base. Each of these control cen­ters has multiple control rooms for use dur­ing flight test. The X-33 program has built yet another control room, this one located near the launch site at Haystack Butte.26

The X-15 program required a tracking net­work known as “High Range.” Operational techniques were established for real-time flight monitoring which were carried over to the space program. The experience of setting up this control network became something of a legacy to Mercury and later space projects through the personnel involved. Gerald M. Truszynski, as Chief of the Instrumentation Division at the FRC, had participated in set­ting up the High Range, as had Edmond C. Buckley, who headed the Instrument Research Division at Langley, The Tracking and Ground Instrumentation Group at Langley had the responsibility for tracking the Mercury capsules, and it was headed, briefly, by Buckley.27

Buckley soon transferred to NASA Headquarters as assistant director for space
flight operations, with Truszynski joining him in 1960 as an operations engineer. Both con­tinued to be involved in instrumentation and communication until a reorganization under NASA Administrator James Webb created an Office of Tracking and Data Acquisition with Buckley as director. Buckley named Truszynski as his deputy, and in 1962 appointed him to lead the Apollo Task Group that shaped the Apollo tracking and data net – work. JK Much of this same infrastructure was used early in the Space Shuttle program.

Meanwhile, Walter Williams, who had headed the NACA operations at the HSFS/FRC since 1946, was reassigned as Associate Director of the newly formed Space Task Group at Langley in September 1959. He eventually served as the Director of Operations for Mercury, and then as Associate Director of the Manned Spacecraft Center. He also served as operations director in the Mercury Control Center at Cape Canaveral during the Mercury flights of Alan Shepard, Gus Grissom, and John Glenn in 1961 and 1962.2’

Experience from the NASA 1 control room undoubtedly influenced the development of the Mercury Control Center at Cape Canaveral, and perhaps more distantly, even the Mission Control Center (MCC)30 at Houston.31 However, the spacecraft control rooms and their tracking and data acquisition systems drew on many other sources (includ­ing the missile ranges which they shared),32 although the experience setting up the High Range and operating the NASA 1 control room undoubtedly provided some opera­tional perspectives.

An often overlooked area where the X-15 influenced Space Shuttle operations is in the energy management maneuvers immediately prior to landing. By demonstrating that it was possible to make precision unpowered landings with vehicles having a low lift – over-drag ratio, the X-15 program smoothed the path for the slightly later lifting-body program and then for the space shuttle pro­cedures for energy management and landing.

The techniques used by X-15 pilots consist­ed of arriving at a “high key" above the intended landing point. Once he reached the high key, the pilot did not usually need or receive additional information from the con­trol room; he could complete the landing using visual information and his own experi­ence with practice landings in an F-104 con­figured to simulate an X-15 landing. With considerable variation on different missions, the pilot would arrive at the high key on an altitude mission at about 35,000 feet, turn 180 degrees and proceed to a “low key" at about 18,000 feet, where he would turn another 180 degrees and proceed to a landing on Rogers Dry Lake. Depending upon the amount of energy remaining, the pilot could use shallow or tight bank angles and speed brakes as necessary.

Because of their much higher energy, the standard approach for the Space Shuttle con­sists of a variation on this 360-degree approach. As a Shuttle approaches the run­way for landing, if it has excess energy for a normal approach and landing, it dissipates this energy in S-turns (banking turns) until it can slow to a subsonic velocity at about

49,0 feet of altitude some 25 miles from the runway. It then begins the approach and landing phase at about 10,000 feet and an equivalent airspeed of about 320 mph some 8 miles from the runway.33 Early in the Space Shuttle program, a specially-configured T-3834 would accompany the orbiter on the final approach, much as the X-15 chase air­craft did at Edwards. Shuttle pilots practice in a specially-configured Gulfstream Shuttle Training Aircraft, much as the X-15 pilots did in the modified F-104.

It is a beginning. Over forty-five years have elapsed since the X-15 was conceived; 40 since it first flew. And 31 since the program ended. Although it is usually heralded as the most productive flight research program ever undertaken, no serious history has been assembled to capture its design, develop­ment, operations, and lessons. This mono­graph is the first step towards that history.

Not that a great deal has not previously been written about the X-15, because it has. But most of it has been limited to specific aspects of the program; pilot’s stories, experiments, lessons-leamed, etc. But with the exception of Robert S. Houston’s history published by the Wright Air Development Center in 1958, and later included in the Air Force History Office’s Hypersonic Revolution, no one has attempted to tell the entire story. And the WADC history is taken entirely from the Air Force perspective, with small mention of the other contributors.

In 1954 the X-l series had just broken Mach 2.5. The aircraft that would become the X-15 was being designed to attain Mach 6, and to fly at the edges of space. It would be accom­plished without the use of digital computers, video teleconferencing, the internet, or email. It would, however, come at a terrible financial cost—over 30 times the original estimate.

The X-15 would ultimately exceed all of its original performance goals. Instead of Mach 6 and 250,000 feet, the program would record Mach 6.7 and 354,200 feet. And com­pared against other research (and even oper­ational) aircraft of the era, the X-15 was remarkably safe. Several pilots would get banged up; Jack McKay seriously so, although he would return from his injuries to
fly 22 more X-15 flights. Tragically, Major Michael J. Adams would be killed on Flight 191, the only fatality of the program.

Unfortunately due to the absence of a subse­quent hypersonic mission, aeronautical applications of X-15 technology have been few. Given the major advances in materials and computer technology in the 30 years since the end of the flight research program, it is unlikely that many of the actual hard­ware lessons are still applicable. That being said, the lessons learned from hypersonic modeling, simulation, and the insight gained by being able to evaluate actual X-15 flight research against wind tunnel and predicted results, greatly expanded the confidence of researchers. This allowed the development of Space Shuttle to proceed much smoother than would otherwise have been possible.

In space, however, the X-15 contributed to both Apollo and Space Shuttle. It is interest­ing to note that when the X-15 was con­ceived, there were many that believed its space-oriented aspects should be removed from the program since human space travel was postulated to be many decades in the future. Perhaps the major contribution was the final elimination of a spray-on ablator as a possible thermal protection system for Space Shuttle. This would likely have hap­pened in any case as the ceramic tiles and metal shingles were further developed, but the operational problems encountered with the (admittedly brief) experience on X-15A-2 hastened the departure of the ablators.

Many people assisted in the preparation of this monograph. First and foremost are Betty Love, Dill Hunley, and Pete Merlin at the DFRC History Office. Part of this project

was assembling a detailed flight log (not part of this monograph), and Betty spent many long hours checking my data and researching to fill holes. I am terribly indebted to her. Correspondence continues with several of the program principals—John V. Becker, Scott Crossfield, Pete Knight, and William Dana. Dr. Roger Launius and Steve Garber at the NASA History Office, and Dr. Richard Hallion, Fred Johnsen, Diana Comelisse,
and Jack Weber all provided excellent sup­port for the project. A. J. Lutz and Ray Wagner at the San Diego Aerospace Museum archives, Tony Landis, Brian Lockett, Jay Miller, and Terry Panopalis also provided tremendous assistance to the project.

Dennis R. Jenkins Cape Canaveral, Florida February 2000

With the XLR99 engine lagging behind in its development schedule, the X-15 program decided to press ahead with ini­tial flights using two XLR11 engines—the same basic engine that had powered the Bell X-1 on its first supersonic flight. (San Diego Aerospace Museum Collection)

When the Reaction Motors XLR99 engine finally became avail­able, the X-15 began setting records that would stand until the advent of the Space Shuttle. Unlike the XLR11, which was “throttleable” by ignit­ing different numbers of thrust chambers, the XLR99 was a truly throttleable engine that could tailor its output for each specif­ic mission. (San Diego Aerospace Museum Collection)

Hydraulic lifts were installed in the ramp at the Flight Research Center {now the Dryden Flight Research Center) to lift the X-15 up to the wing pylon on the NB-52 mothership. (Jay Miller Collection)

The early test flights were conducted with a long air data probe protruding from the nose of the X-15. Notice the technician manually retracting the nose landing gear on the X-15, some­thing accomplished after the research air­plane was firmly con­nected to the wing of the NB-52 mothership. (San Diego Aerospace Museum Collection)

The X-15 was designed with a hot-structure that could absorb the heat generated by its short-duration flight. Remember, the X-15 seldom flew for over ten minutes at a time, and a much shorter time was spent at the maximum speed or dynamic pressure. Development showed the validity of ground

damaging the landing gear, and in one case causing the gear to extend at Mach 4.2 (flight 2-33-56). Although the landing gear remained intact, the disintegration of the tires made the landing very rough. The need for very careful examination of all seals became apparent, and closer scrutiny of surface irreg­ularities, small cracks, and areas of flow interaction became routine. The lessons learned from this influenced the final detailed design of the Space Shuttle to ensure that gaps and panel lines were adequately protect­ed against inadvertent airflow entry.

Other problems from aerodynamic heating included windshield crazing, panel flutter, and skin buckling. Arguably, designers could have prevented these problems through more extensive ground testing and analysis, but a key purpose of flight research is to discover the unexpected. The truly significant lesson from these problems is that defect in subson­ic or supersonic aircraft that are compara­tively minor at slower speeds become much more critical at hypersonic speeds.37

One of the primary concerns during the X-15 development was panel flutter, evidenced by the closing paper presented at the 1956 industry conference. Panel flutter has proven difficult to predict at each speed increment throughout history, and the hypersonic regime was no different. Although the X-15 was conservatively designed, and incorporat­ed all the lessons from first generation super­sonic aircraft, the fuselage side tunnels and the vertical surfaces were prone to develop panel flutter during flight. This 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, and similar techniques later found general application in the high speed aircraft of the 1960s.’■ The lessons learned at Mach 6 defined criteria later used in the development of the Space Shuttle.

The X-15 provided the first opportunity to study the effects of acoustical fatigue over a wide range of Mach numbers and dynamic pressures. In these first in-flight measure­ments, “boundary layer 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 in-flight loading. This knowledge has allowed for more optimum structural design of missiles and space cap­sules that experience high velocities.

On the X-15, the measurement of velocity was handled by early inertial systems. All three X-15s were initially equipped with analog-type systems which proved to be highly unreliable. Later, two aircraft, includ­ing the X-15-3 with the adaptive control sys­tem, were modified with digital systems. In the subsequent parallel evaluation of analog versus digital inertial systems, the latter was found to be far superior. It was far more flex­ible and could make direct inputs to the adaptive flight control system; it was also subject to less error. Thanks to advances in technology such as laser-ring gyros and dig­ital computers, inertial systems have become inexpensive, highly accurate, and very reli­able.3* In recent years they have been inte­grated with the Global Positioning System (GPS), providing three-dimensional attitude and position information.

During the early test flights, the X-15 relied on simple pilot-static pressure instruments mounted on a typical flight test nose boom. These were not capable of functioning as speeds and altitudes increased. To provide attitude information, the NACA developed the null-sensing “ball-nose” which could survive the thermal environment of the X-15. An extendable pitot tube was added when the velocity envelope was expanded beyond Mach 6. Thus far the ball-nose has not found subsequent application, and probably never will since inertial and GPS systems have evolved so quickly. Interestingly, the Space Shuttle still uses an extendable pitot probe during the landing phase.

The X-15 was the first vehicle to routinely use reaction controls. The HSFS had begun

research on reaction controls in the mid – 1950s using a fixed-base analog control stick with a pilot presentation to determine the effects of control inputs. This was followed by a mechanical simulator to enable the pilot to experience the motions created by reac­tion controls. This device emulated the iner­tial ratios of the X-1B, which incorporated a reaction control system using hydrogen-per­oxide as a monopropellant, decomposed by passing it through a silver-screen catalyst. Because of fatigue cracks later found in the fuel tank of the X-1B, it completed only three flights using the reaction control sys­tem before it was retired in 1958.44

As a result, a JF-104A with a somewhat more refined reaction control system was tested beginning in late 1959 and extending into 1961. The JF-104A flew a zoom-climb maneuver to achieve low dynamic pressures at about 80,000 feet that simulated those at higher altitudes. The techniques for using reaction controls on the X-15, and more importantly, for transferring from aerody­namic controls to reaction controls and back to aerodynamic controls provided a legacy to the space program.41

The X-15-3 was equipped with a Minneapolis Honeywell MH-96 self-adaptive control sys­tem designed for the cancelled Dyna-Soar. The other two X-15s had one controller on the right-hand side of the cockpit for aerodynamic controls and another on the left-hand side for the reaction controls. Thus, the pilot had to use both hands for control during the transition from flying in the atmosphere to flying outside the atmosphere and then back in the opposite direction. Since there was no static stability outside the atmosphere, the pilot had to count­er any induced aircraft motion manually using the reaction controls. The MH-96 had an atti­tude hold feature that maintained the desired attitude except during control inputs. The MH-96 also integrated the aerodynamic and reaction controls in a single controller, greatly improving handling qualities during the transi­tion from aerodynamic to space flight, as well as reducing pilot workload.42

But the basic feature of the MH-96 was auto­matic adjustment of gain (sensitivity) to maintain a desirable dynamic response of the airplane. The MH-96 compared the actual response of the airplane with a preconceived ideal response in terms of yaw, pitch, and roll rates. Initially, Milt Thompson stated that the system was “somewhat unnerving to the pilot” because he was not in “direct control of the aircraft” but was only “commanding a computer that then responded with its own idea of what is necessary in terms of a con­trol output.” However, pilots became “enthu­siastic in their acceptance of it” when they realized that the MH-96 resulted in “more precise command than was possible” with the reaction controls by themselves. Consequently, the X-15-3 with the MH-96 was used for all altitude flights planned above 270,000 feet.4’

There were some problems with the experi­mental system, including one that con­tributed to the death of Mike Adams in X-15- 3 on 15 November 1967. Nevertheless, the MH-96 constituted a significant advance in technology that helped pave the way toward fly-by-wire in the early 1970s. Today, most every aircraft, and several automobiles, fea­ture some variation of a fly-by-wire system with automatic rate-gain adjustment and sta­bility augmentation.44