Category Hypersonics Before. the Shuttle

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

It was not until the mid-1940s that it became apparent to aerodynamic researchers in the United States that it might be possible to build a flight vehicle capable of hypersonic speeds. Until that time, propulsion systems capable of generating the thrust required for such vehi­cles had simply not been considered techni­cally feasible. The large rocket engines that had been developed in Germany during World War II allowed concept studies to be initiated with some hope of success.

Nevertheless, in the immediate post-war peri­od, most researchers believed that hypersonic flight was a domain for unmanned missiles. When an English translation of a technical paper by German scientists Eugen Sanger and Irene Bredt was provided by the U. S. Navy’s Bureau of Aeronautics (BuAer) in 1946, this preconception began to change. Expanding upon ideas conceived as early as 1928, Singer and Bredt had concluded during 1944 that a rocket-powered hypersonic aircraft could be built with only minor advances in technology. The concept of manned aircraft flying at hypersonic speeds was highly stimulating to researchers at the National Advisory Committee for Aeronautics (NACA).’ But although there were numerous paper studies exploring variations of the Sanger and Bredt proposal in the late 1940s, none bore fruit and no hardware construction was undertaken at that time. It was from this background, how­ever, that the concept for a hypersonic research airplane would emerge.2

At the time, there was no established need for a hypersonic aircraft, and it was assumed by many that no operational military3 or civil requirement for hypersonic vehicles would be forthcoming in the foreseeable future. The need for hypersonic research was not over­whelming, but there was a growing body of opinion that it should be undertaken.

The first substantial official support for hyper­sonic research came on 24 June 1952 when the NACA Committee on Aerodynamics passed a resolution to “… increase its program dealing with the problems of unmanned and manned flight in the upper stratosphere at altitudes between 12 and 50 miles,4 and at Mach num­bers between 4 and 10.” This resolution was ratified by the NACA Executive Committee when it met the following month. A study group consisting of Clinton E. Brown (chair­man), William J. O’Sullivan, Jr., and Charles H. Zimmerman was formed on 8 September 1952 at the Langley5 Aeronautical Laboratory. This group endorsed the feasibility of hyper­sonic flight and identified structural heating as the single most important technological prob­lem remaining to be solved.

An October 1953 meeting of the Air Force’s Scientific Advisory Board (SAB) Aircraft Panel provided additional support for hyper­sonic research. Chairman Clarke Millikan released a statement declaring that the feasi­bility of an advanced manned research aircraft “should be looked into.” The panel member from Langley, Robert R. Gilruth, played an important role in coordinating a consensus of opinion between the SAB and the NACA.

Contrary to Sanger’s conclusions, by 1954 it was generally agreed within the NACA and industry that the potential of hypersonic flight could not be realized without major advances in technology. In particular, the unprecedent­ed problems of aerodynamic heating and high-temperature structures appeared to be so formidable that they were viewed as “barriers” to sustained hypersonic flight.

Fortunately, the successes enjoyed by the sec­ond generation X-ls and other high-speed research programs had increased political and philosophical support for a more advanced research aircraft program. The large rocket engines being developed by the long-range missile (ICBM) programs were seen as a way to provide power for a hypersonic research vehicle. It was now agreed that manned hypersonic flight was feasible. Fortunately, at the time there was less emphasis than now on establishing operational requirements prior to conducting basic research, and perhaps even more fortunately, there were no large manned space programs with which to compete for funding. The time was finally right for launch­ing a hypersonic flight research program. s

The specific origins of the hypersonic research program occurred during a meeting of the NACA inter-laboratory Research Airplane Panel held in Washington, DC, on 4­5 February 1954. The panel chairman, Hartley A. Soule, had directed NACA research air­craft activities in the cooperative USAF – NACA program since 1946 and was well versed in the politics and personalities involved. The panel concluded that a wholly new manned research vehicle was needed, and recommended that NACA Headquarters request detailed goals and requirements for such a vehicle from the research laboratories.

In responding to the NACA Headquarters, all of the NACA laboratories set up small ad hoc study groups during March 1954. Langley had been an island of hypersonic study since the end of the war and chose to deal with the problem in more depth than the other labora­tories. After the new 11-inch hypersonic wind tunnel at Langley became operational in 1947, a research group headed by Charles H. McLellan was formed to conduct limited hypersonic research.7 This group, which reported to the Chief of the Langley Aero – Physics Division, John V. Becker, provided verification of newly developed hypersonic theories while investigating such important phenomena as hypersonic shock-boundary – layer interaction. The 11-inch tunnel later served to test preliminary design configura­tions that led to the final hypersonic aircraft configuration. Langley also organized a paral­lel exploratory program into materials and structures optimized for hypersonic flight.

Given this, it was not surprising that a team at Langley was largely responsible for defining the early requirements for the new research airplane. The members of the Langley team included Maxim A. Faget in propulsion; Thomas A. Toll in configuration, stability, and control; Norris F. Dow in structures and mate­rials; and James B. Whitten in piloting. All four fell under the direction of Becker. Besides the almost mandatory elements of stability, control, and piloting, a fourth objective was outlined that would come to dominate virtual­ly every other aspect of the aircraft’s design— it would be optimized for research into the related fields of high-temperature aerodynam­ics and high-temperature structures. Thus it would become the first aircraft in which aero – thermo-structural considerations constituted the primary research problem, as well as the primary research objective.

The preliminary specifications for the research aircraft were surprisingly brief: only four pages of requirements, plus six addition­al pages of supporting data. A new sense of urgency was present: “As the need for the exploratory data is acute because of the rapid advance of the performance of service air­craft, the minimum practical and reliable air­plane is required in order that the develop­ment and construction time be kept to a mini­mum."* In other versions of the requirements this was made even more specific: “It shall be possible to design and construct the airplane within 3 years.”9 As John Becker subsequent­ly observed, "… it was obviously impossible that the proposed aircraft be in any sense an optimum hypersonic configuration.”

In developing the general requirements, the team developed a conceptual research aircraft that served as a model for the eventual X-15. The aircraft they conceived was “… not pro­posed as a prototype of any of the particular

The first Bell X-2 (46-674) made its ini­tial unpowered glide flight on 5 August 1954. This aircraft made a total of 17 flights before it was lost on 27 September 1956. Its pilot, Air Force Captain Milburn Apt had flown to a record speed 2,094 mph, thereby becom­ing the first person to exceed Mach 3.

(NASA/DFRC)

concepts in vogue in 1954 … [but] rather as a general tool for manned hypersonic flight research, able to penetrate the new regime briefly, safely, and without the burdens, restrictions, and delays imposed by opera­tional requirements other than research.” The merits of this approach had been convincing­ly demonstrated by the successes of the X-l and other dedicated research aircraft of the late 1940s and early 1950s.10

Assuming that the new vehicle would be air launched like the X-l and X-2, Langley estab­lished an aircraft size that could conveniently be carried by a Convair B-36, the largest suit­able aircraft available in the inventory. This translated to a gross weight of approximately

30.0 pounds, including 18,000 pounds of fuel and instrumentation.11 A maximum speed of 4,600 mph and an altitude potential of

400.0 feet were envisioned, with the pilot subjected to approximately 4.5g (an accelera­tion equal to 4.5 times the force of gravity) at engine burnout.12

The proposed maximum speed was more than double that achieved by the X-2, and placed the aircraft in a region where heating was the
primary problem associated with structural design, and where very little background information existed. Hypersonic aerodynam­ics was in its infancy in 1954. The few small hypersonic wind tunnels then in existence had been used almost exclusively for fluid mechanics studies, and they were unable to simulate either the high temperatures or the high Reynolds numbers of actual flight. It was generally believed that these wind tunnels did not produce valid results when applied to a full-scale aircraft. The proposed hypersonic research airplane, it was assumed, would pro­vide a bridge over the huge technological gap that appeared to exist between laboratory experimentation and actual flight.12

One aspect of the Langley proposal caused considerable controversy. The Langley team called for two distinct research flight profiles. The first consisted of a variety of constant angle-of-attack, constant altitude, and maneu­vering flights to investigate the aerodynamic and thermodynamic characteristics and limi­tations of then-available technology. These were the essential hypersonic research flights. But the second flight profile was designed to explore some of the problems of manned

space flight by making "… long leaps out of the sensible atmosphere.” This included inves­tigations into high-lift and low-L/D (lift over drag; commonly called a drag coeffi­cient) during the reentry pull-up maneuver” which was recognized as a prime problem for manned space flight from both a heating and piloting perspective.14

This brought other concerns: “.., As the speed increases, an increasingly large portion of the aircraft’s weight is borne by centrifugal force until, at satellite velocity, no aerodynamic lift is needed and the aircraft may be operated completely out of the atmosphere. At these speeds the pilot must be able to function for long periods in a weightless condition, which is of considerable concern from the aeromed – ical standpoint.” By employing a high altitude ballistic trajectory to approximately 250,000 feet, the Langley group expected the pilot would operate in an essentially weightless condition for approximately two minutes. Attitude control was another problem, since traditional aerodynamic control surfaces would be useless at the altitudes proposed for the new aircraft; the dynamic pressure would be less than 1 pound per square foot (psf). The
use of small hydrogen-peroxide thrusters for attitude control was proposed.

While the hypersonic research aspect of the Langley proposal enjoyed virtually unani­mous support, it is interesting to note that the space flight aspect was viewed in 1954 with what can best be described as cautious toler­ance. There were few who believed that any space flight was imminent, and most believed that manned space flight in particular was many decades in the future, probably not until the 21st century. Several researchers recom­mended that the space flight research was pre­mature and should be removed from the рто – gram. Fortunately, it remained.15

Hypersonic stability was the first problem of really major proportion encountered in the study. Serious instability had already been encountered with the X-l and X-2 at Mach numbers substantially lower than those expected with the proposed hypersonic research aircraft, and it was considered a major challenge to create a solution that would permit stable flight at Mach 7.

Researchers at Langley discovered through

The notional research airplane designed by John V. Becker’s group at Langley shows the basis for the eventual X-15. Note the bullet­shaped fuselage (similar to the X-1) and the configuration of the empennage. This was the shape most of the early wind tunnel and analytical studies were per­formed against. (NASA)

wind tunnel testing and evaluating high speed data from earlier X-planes that an extremely large vertical stabilizer was required if the thin sections then in vogue for supersonic aircraft were used. This was largely because of a rapid loss in the lift-curve slope of thin sections as the Mach number increased. The solution devised by McLellan, based on theoretical considerations of the influence of airfoil shape on normal force characteristics, was to replace the thin supersonic-airfoil section of the vertical stabilizer with a 10 degree wedge shape. Further, a variable-wedge vertical sta­bilizer was proposed as a means of restoring the lift-curve slope at high speeds, thus per­mitting much smaller surfaces, which were easier to design structurally and imposed a smaller drag penalty on the airframe. McLellan’s calculations indicated that this wedge shape should eliminate the disastrous directional stability decay encountered by the X-l and X-2.

edge, very similar to the one eventually used on the Space Shuttle orbiters. Both the brak­ing effect and the stability derivatives could be varied through wide ranges by variable deflection of the wedge surfaces. The flexibil­ity made possible by variable wedge deflec­tion was thought to be of great value because a primary use of the airplane would be to study stability, control, and handling charac­teristics through a wide range of speeds and altitudes. lfi

Two basic structural design approaches had been debated since the initiation of the study—first, a conventional low-temperature design of aluminum or stainless steel protect­ed from the high-temperature environment by a layer of assumed insulation; and second, an exposed hot-structure in which no attempt would be made to provide protection, but in which the metal used and the design approach would permit high structural temperatures.17

Becker’s group also included speed brakes as part of the vertical stabilizers to reduce the Mach number and heating during reentry. Interestingly, the speed brakes originally pro­posed by Langley consisted of a split trailing

It was found from analysis of the heating pro­jections for various trajectories that the air­plane would need to accommodate tempera­tures of over 2,000 degrees Fahrenheit on the lower surface of the wing. At the time, there

This chart was used by Becker to demon­strate the relative dif­ferences between the nominal recovery tem­perature, compared to the temperatures expected to be sus­tained by an insulated structure and an appropriately designed heat-sink skin (hot-structure). Inconel X was the material of choice very early in the study.

(NASA)

was no known insulating technique that could meet this requirement. The Bell “double­wall” concept where a non Toad-bearing metal sandwich acted as the basic insulator, would later undergo extensive development, but in 1954, it was in an embryonic state and not applicable to the critical nose and leading edge regions. Furthermore, it required a heavy and space-consuming supplemental liquid cooling system. However, the study group felt that the possibility of local failure of any insu­lation scheme constituted a serious hazard. Finally, the problem of accurately measuring heat-transfer rates—one of the prime objec­tives of the new research aircraft program— would be substantially more difficult to accomplish with an insulated structure.

At the start of the study it was by no means obvious that the hot-structure approach would prove practical either. The permissible design temperature for the best available material was about 1,200 degrees Fahrenheit, which was far below the estimated equilibrium temperature peak of about 2,000 degrees Fahrenheit. It was clear that some form of heat dissipation would have to be employed—either direct internal cooling or heat absorption into the structure
itself. It was felt that either solution would bring a heavy weight penalty.

The availability of Inconel Xі* and its excep­tional strength at extremely high temperatures, made it, almost by default, the structural mate­rial preferred by Langley for a hot-structure design. During mid-1954, an analysis of an Inconel X structure was begun by Becker’s group; concurrently, a detailed thermal analy­sis was conducted. A subsequent stress study indicated that the wing skin thickness should range from 0.05 to 0.10 inches—about the same values found necessary for heat absorp­tion in the thermal analysis.

Thus it was possible to solve the structural problem for the transient conditions of a Mach 7 aircraft with no serious weight penal­ty for heat absorption. This was an unexpect­ed plus for the hot-structure. Together with the fact that none of the perceived difficulties of an insulated-type structure were present, the study group decided in favor of an uninsulat­ed hot-structure design.

Unfortunately, it later proved that the hot – structure had problems of its own, particularly

Inconel X was easily the best high-tempera­ture alloy available during the 1950s. It possessed a rare combination of high tensile strength and the ability to withstand high temperatures. Although it proved somewhat difficult to work with, it did not impose some of the problems encountered with titanium on other high-speed aircraft projects. (NASA)

in the area of nonuniform temperature distri­bution. Detailed thermal analyses revealed that large temperature differences would develop between the upper and lower wing skin during the pull-up portions of certain tra­jectories. This unequal heating would result in intolerable thermal stresses in a conventional structural design. To solve this new problem, wing shear members were devised which did not offer any resistance to unequal expansion of the wing skins. The wing thus was essen­tially free to deform both spanwise and chord – wise with asymmetrical heating. Although this technique solved the problem of the gross thermal stresses, localized thermal-stress problems still existed in the vicinity of the stringer attachments. The study indicated, however, that proper selection of stringer pro­portions and spacing would produce an acceptable design free from thermal buckling.

During the Langley studies, it was discovered that differential heating of the wing leading edge produced changes in the natural torsion­al frequency of the wing unless some sort of flexible expansion joint was incorporated in its design. The hot leading edge expanded faster than the remaining structure, introduc­ing a compression that destabilized the sec­tion as a whole and reduced its torsional stiff­ness, To negate this phenomenon, the leading edge was segmented and flexibly mounted in an attempt to reduce thermally induced buck­ling and bending.

With its research objectives and structure now essentially determined, the Langley team turned its attention to the questions of propulsion by examining various existing rocket propulsion systems. The most promis­ing configuration was found to be a grouping of four General Electric A1 or A3 Hermes rocket engines, due primarily to the “thrust stepping” (a crude method of modulating, or throttling, the thrust output) option this con­figuration provided.

The studies prompted the NACA to adopt the official policy that the construction of a manned hypersonic research airplane was fea­sible. In June 1954, Dr. Hugh L. Dryden sent a letter to Lieutenant General Donald Putt at Air Force Headquarters stating that the NACA was interested in the creation of a new manned research aircraft program that would explore hypersonic speeds and altitudes well in excess of those presently being achieved. The letter also recommended that a meeting between the NACA, Air Force Headquarters, and the Air Force SAB be arranged to discuss the project. Putt responded favorably, and also recom­mended that the Navy be invited to participate.

NACA representatives met with members of the Air Force and Navy research and develop­ment groups on 9 July 1954 to present the proposal for a hypersonic research aircraft as an extension of the existing cooperative research airplane program. It was soon dis­covered that the Air Force SAB had been making similar proposals to Air Force Headquarters, and that the Office of Naval Research had already contracted with the Douglas Aircraft Company to determine the feasibility of constructing a manned aircraft capable of achieving 1,000,000 feet altitude. Douglas had concluded that 700,000 foot alti­tudes would be possible from the reentry deceleration standpoint, but that the thermo­structural problem had not been thoroughly analyzed. It was agreed that a cooperative pro­gram would be more cost effective and likely lead to better research data at an earlier time.19

The Navy and Air Force representatives viewed the NACA proposal with favor, although each had some reservations. At the close of the meeting, however, there was agreement that both services would further study the details of the NACA proposal, and that the NACA would take the initiative to secure project approval from the Department of Defense.2"

Less than a month later, the Air Force identi­fied the principal shortcoming of the original Langley proposal—the apparent lack of a suitable rocket engine. In early August the Power Plant Laboratory at the Wright Air Development Center (WADC) pointed out

that “no current rocket engines” entirely satis­fied the NACA requirements, and emphasized that the Hermes engine was not designed to be operated in close proximity to humans—that it usually was fired only when shielded by concrete walls. Other major objections to the Hermes engine centered around its relatively early state of development, its limited design life (intended for missile use, it was not required to operate successfully more than once), and the apparent difficulty of incorpo­rating the ability to throttle it during flight.21 WADC technical personnel who visited Langley on 9 August drew a firm distinction between engines intended for piloted aircraft and those designed for missiles; the NACA immediately recognized the problem, but con­cluded that although program costs would increase, the initial feasibility estimates would not be affected.22

WADC’s official reaction to the NACA pro­posal was submitted to the Air Research and Development Command (ARDC) on 13 August.2’ Colonel V. R. Haugen reported “unanimous” agreement among WADC par­ticipants that the proposal was technically fea­sible; excepting the engine situation, there was no occasion for adverse comment. The evaluation forwarded by Haugen also con­tained a cost estimate of $12,200,000 “distrib­uted over three to four fiscal years" for two research aircraft and necessary government – furnished equipment. Estimated costs includ­ed: $1,500,000 for design work; $9,500,000 for construction and development, including flight test demonstration; $650,000 for gov­ernment furnished equipment, including engines, $300,000 for design studies and specifications; and $250,000 for modification of a carrier aircraft.24 Somewhat prophetically, one WADC official commented informally: “Remember the X-3, the X-5, [and] the X-2 overran 200 percent. This project won’t get started for $12,000,000.”"

On 13 September, the ARDC issued an endorsement of the NACA proposal, and rec­ommended that the Air Force “… initiate a project to design, construct, and operate a new research aircraft similar to that suggested by NACA without delay.” The aircraft, empha­sized ARDC, should be considered a pure research vehicle and should not be pro­grammed as a weapon system prototype. On 4 October 1954, Brigadier General Benjamin S. Kelsey, Deputy Director of Research and Development at Air Force Headquarters, stat­ed that the project would be a joint Navy – NACA-USAF effort managed by the Air Force and guided by a joint steering commit­tee. Air Force Headquarters further pointed out the necessity for funding a special flight test range as part of the project:4′

The NACA Committee on Aeronautics met on 5 October 1954 to consider the hypersonic research aircraft. During the meeting, historic and technical data were reviewed by various committee members including Walter C. Williams, De E. Beeler, and research pilot A. Scott Crossfield from the High-Speed Flight Station (HSFS). Williams’ support was cru­cial. Crossfield would later describe Williams as.. the man of the 20th Century who made more U. S. advanced aeronautical and space programs succeed than all the others together. … He had no peer. None. He was a very strong influence in getting the X-15 program launched in the right direction.”27

Although one Committee member expressed opposition to the proposed hypersonic research aircraft as an extension to the on­going test programs, the rest of the Committee supported the project. The Committee formal­ly adopted a resolution to build a Mach 7 research airplane (attached as an appendix to this monograph).2*

Because the anticipated cost of the project would require support from Department of Defense contingency funds as well as Air Force and Navy R&D funds, a formal Memorandum of Understanding (MoU) was drafted and sent around for signatures begin­ning in early November 1954. The MoU was originated by Trevor Gardner (Air Force Special Assistant for Research and Development), and was forwarded, respec-

tively, for the signatures of J. H. Smith Jr. H (Assistant Secretary of the Navy [Air]) and Hugh L. Dryden (Director of the NACA). Dryden signed the Moll on 23 December 1954, and returned executed copies to the Air Force and Navy.’0

The MoU (attached as an appendix to this monograph) provided that technical direction of the research project would be the responsi­bility of the NACA, acting.. with the advice and assistance of a Research Airplane Committee” composed of one representative each from the Air Force, Navy, and the NACA. Administration of the design and construction phases of the project was assigned to the Air Force. The NACA would conduct the flight research, with extensive support from the Air Force Flight Test Center. The Navy was essen­tially left paying 25 percent of the bills with little active roll in the project, although it would later supply biomedical expertise and a single pilot. The NACA and the Research Airplane Committee were charged with the responsibility for disseminating the research results to the military services and aircraft industry as appropriate based on various secu­rity aspects. The concluding statement on the MoU was: “Accomplishment of this project is a matter of national urgency.’’11

It should be noted that it was not unusual in the late 1940s and early 1950s for the military services to fund the development and con­struction of aircraft for the NACA to use in its flight test programs. This was how most of the testing on the X-l and others had been accom­plished. The eventual X-l5 would be the fastest, highest-flying, and most expensive of these joint projects.12

After the signed copies of the MoU were returned to all participants, the Department of Defense authorized the Air Force to issue invitations to contractors having experience in the development of fighter-type aircraft to participate in the design competition. After the Christmas holidays, on 30 December, the Air Force sent invitation-to-bid letters to 12 prospective contractors; Bell, Boeing,

Chance-Vought, Consolidated (Convair), Douglas, Grumman, Lockheed, Martin, McDonnell, North American, Northrop, and Republic. The letter asked those interested in bidding to notify Wright Field by 10 January 1955, and to attend a bidder’s conference on 18 January 1955.”

Attached to the letter were a preliminary out­line specification, an abstract of the Langley preliminary study, a discussion of possible engines, a list of data requirements, and a cost outline statement. Each bidder was required to satisfy various requirements set forth, except in the case of the NACA abstract which was presented as "… representative of possible solutions.”14

Grumman, Lockheed, and Martin expressed little interest in the competition and did not attend the bidder’s conference, leaving nine possible competitors. At the bidders’ confer­ence, representatives from the contractors met with NACA and Air Force personnel to discuss the competition and the basic design requirements.

During the bidders’ conference, the airframe manufacturers were informed that one prime proposal and one alternate proposal (that might offer an unconventional but superior solution to the problems involved) would be accepted from each company. It also was noted that an engineering study, only, would be required for a modified aircraft where an observer could be substituted for the research instrumentation (a Navy require­ment); that a weight allowance of 800 pounds, a volume of 40 cubic feet, and a power requirement of 2.25 kilowatts (kW) needed to be provided for research instru­mentation; and that the winning design would have to be built in 30 months and be capable of attaining speeds of Mach 6 and altitudes of 250,000 feet. Following the pre­liminary statements concerning the bidding, NACA personnel briefed the various compa­nies in attendance on new information that had resulted from late 1954 wind tunnel research that had taken place at Langley.

Subsequently, between the bidders’ confer­ence and the 9 May submission deadline, Boeing, Chance-Vought, Convair, Grumman, McDonnell, and Northrop notified the Air Force that they did not intend to submit for­mal proposals. This left Bell, Douglas, North American, and Republic. During this period, representatives from these companies met with NACA personnel on numerous occa­sions and reviewed technical information on various aspects of the forthcoming research airplane. The NACA also provided these con­tractors with further information gained as a result of wind tunnel tests in the Ames 10-by – 14 inch supersonic tunnel and the Langley Mach 4 blowdown tunnel.

On 17 January 1955, NACA representatives met with Air Force personnel at Wright Field and were informed that the research airplane was identified as Air Force Project 1226 and would be officially designated X-15.

The Power Plant Laboratory had originally listed the Aerojet XLR73, Bell XLR81, North American NA-5400 (an engine in early devel­opment, still lacking a military designation), and the Reaction Motors XLR10 (and its vari­ants, including the XLR30) as engines that the airframe competitors could use in their designs. Early in January, the laboratory had become concerned that the builders of engines other than those listed might protest the exclu­sion of their products. Consequently there emerged an explanation and justification of the engine selection process. It appeared that the engineers had confidence in the ability of the XLR81 and XLR73 to meet airplane require­ments, had doubts about the suitability of the XLR25 (a Curtiss-Wright product), and held the thrust potential of the XLR8 and XLR11 (similar engines) in low repute. For practical purposes, this exhausted the available Air Force-developed engines suitable for manned aircraft. The XLR10 and NA-5400 were the only Navy-developed engines viewed as acceptable in terms of the competition.35

Earlier, the engine manufacturers had been contacted for specific information about the engines originally listed as suitable for the X-15 program,36 and this information was dis­tributed to all four prospective airframe con­tractors.37 Due to its early development status, there was little data available for the North American NA-5400, and the Reaction Motors XLR10 was “not recommended” at the sug­gestion of the engine manufacturer itself. On 4 February each of the prospective engine contractors (Aerojet, Bell, North American, and Reaction Motors) was asked to submit an engine development proposal.38 Based on this, the Air Force very slightly relaxed the rigid limitations on engine selection, instructing competitors that.. if… an engine not on the approved list offers sufficient advantage, the airframe company may, together with the engine manufacturer, present justification for approval…” to the Air Force.35

On 9 May 1955, Bell, Douglas, North American, and Republic submitted their pro­posals to the Air Force. Two days later the technical data was distributed to the evaluation groups with a request that results be returned by 22 June.40 The final evaluation meeting was scheduled for 25 July at Wright Field.41

Shortly thereafter, Hartley A. Soule, as Chairman of the NACA evaluation group, sent the evaluation rules and processes to the NACA laboratories. The evaluation would be based on the technical and manufacturing competency of each contractor, schedule and cost estimates, design approach, and the research utility of each design. In order to expedite the evaluation, each of the NACA laboratories was assigned specific items to consider with responses to be returned to Soule no later than 13 June.

The evaluation of the engine would be made at the same time, but would be conducted sep­arate from that of the airframe contractor, with the possibility that the chosen engine might not be the one selected by the winning air­frame contractor.

On 10 June the HSFS results were sent to Soule, based on the design approach and

research utility aspects of the airframe, flight control system, propulsion unit, crew provi­sions, handling and launching, and miscella­neous systems. The proposals were ranked:

(1) Douglas; (2) North American; (3) Bell; and (4) Republic. The proposals from Douglas and North American were consid­ered almost equal on the basis of points.

The Ames final evaluation, on 13 June 1955, ranked the proposals: fl) North American;

(2) Douglas; (3) Bell; and (4) Republic. The North American structure was considered to be more representative of future aircraft and thus superior in terms of research utility. Douglas retained a simple and conventional magnesium structure, but in so doing avoided the very thermodynamic problems the research effort wished to explore.

The 14 June final evaluation from Langley ranked the proposals; (1) North American; (2) Douglas; (3) Republic; and (4) Bell. Langley felt that while the magnesium wing structure of Douglas was feasible, it was feared that local hot spots caused by irregular aerody­namic heating could weaken the structure and be subject to failure. North American’s use of Inconel X was believed to be an advantage.

The final order representing the overall NACA evaluation was (1) North American; (2) Douglas; (3) Bell; and (4) Republic. All of the laboratories involved in this portion of the evaluation considered both the North American and Douglas proposals to be much superior to those submitted by Bell and Republic.

As with the NACA evaluations, the Air Force found little difference between the Douglas and North American designs, point-wise, with both proposals significantly superior to those of Beil and Republic. The Navy evaluation found much the same thing, ranking the pro­posals: (1) Douglas; (2) North American; (3) Republic; and (4) Bell.

On 26-28 July, the Air Force, Navy, and NACA evaluation teams met to coordinate their separate results. The Air Force and the NACA concluded that the North American proposal best accommodated their require­ments. Accordingly, the Navy decided not to. be put in the position of casting the dissenting vote and after short deliberation, agreed to go along with the decision of the Air Force and the NACA. A combined meeting of the Air Force, Navy, and the NACA was held at NACA Fleadquarters on 12 August for the final briefing on the evaluation. Later, the Research Airplane Committee met, accepted the findings of the evaluation groups, and agreed to present the recommendation to the Department of Defense.

Interestingly, the North American proposal was by far the most expensive. The estimat­ed costs for three aircraft plus one static test article and supporting equipment were: Bell, $36.3 million; Douglas, $36.4 million; Republic, $47 million; and North American, $56.1 million.

Because the estimated costs submitted by North American were far – above the amount allocated for the project, the Research Airplane Committee included a recommenda­tion for a funding increase that would need to be approved before the actual contract was signed. A further recommendation, one that would later take on greater importance, called for relaxing the proposed schedule by up to one-and-one-half years. These recommenda­tions were sent to the Assistant Secretary of Defense for Research and Development.

Events took an unexpected twist on 23 August when the North American represen­tative in Dayton verbally informed the Air Force that the company wished to withdraw its proposal. On 30 August, North American sent a letter to the Air Force formally requesting that the company be allowed to withdraw from consideration.43

The Vice President and Chief Engineer for North American, Raymond H. Rice, wrote to the Air Force on 23 September and explained that the company had decided to withdraw

from the competition because it had recently won new bomber and long range interceptor competitions and also had increased activity relating to its on-going F-107 fighter. Having undertaken these projects, North American said it would be unable to accommodate the fast engineering man-hours build-up that would be required to support the desired schedule. Rice went on that, “… due to the apparent interest that has subsequently been expressed in the North American design, the contractor [North American] wishes to extend two alternate courses which have been previ­ously discussed with Air Force personnel: The engineering man-power work load schedule has been reviewed and the contractor wishes to point out that Project 1226 could be han­dled if it were permissible to extend the schedule… over an additional eight month period: in the event the above time extension is not acceptable and in the best interest of the project, the contractor is willing to release the proposal data to the Air Force at no cost.”43

As it turned out, the possibility of extending the schedule had already been approved on 12 August, allowing North American to with­draw its previous letter of retraction once it had been officially informed that it had won the contract.44 Accordingly, on 30 September 1955, the Air Force formally notified North American that its design had been selected as the winner. The other bidders were conse­quently notified of North American’s selec­tion and thanked for their participation.45

By 11 October, the estimate from North American had been reduced from $56,000,000 to $45,000,000 and the maxi­mum annual funds requirement from $26,000,000 to $15,000,000. Shortly there­after, the Department of Defense released the funds needed to start work. More meetings between the Air Force, the NACA, and North American were held on 27-28 October, large­ly to define changes to the aircraft configura­tion. On 18 November, letter contract AF33(600)-31693 was sent to North American, and an executed copy was returned on 8 December 1955.46 The detailed design
and development of the hypersonic research airplane had been underway for just under a year at this point.47

On 1 December 1955, a series of actions48 began that resulted in letter contract AF33(600)-32248 being sent to Reaction Motors, effective on 14 February 1956. Its ini­tial allocation of funds totaled $3,000,000, with an eventual expenditure of about $6,000,000 foreseen as necessary for the delivery of the first flight engine.49

A definitive contract for North American was completed on 11 June 1956, superseding the letter contract and two intervening amend­ments. At that time, $5,315,000 had been committed to North American. The definitive contract allowed the eventual expenditure of $40,263,709 plus a fee of $2,617,075. For this sum, the government was to receive three X-15 research aircraft, a high speed and a low speed wind tunnel model program, a free-spin model, a full-size mockup, propulsion system tests and stands, flight tests, modification of a B-36 carrier aircraft, a flight handbook, a maintenance handbook, technical data, peri­odic reports of several types, ground handling dollies, spare parts, and ground support equip­ment. Exclusive of contract costs were fuel and oil, special test site facilities, and expens­es to operate the B-36. The delivery date for the X-15s was to be 31 October 1958. The quantity of aircraft had been determined by experience; it had been noted during earlier research aircraft programs that two aircraft were enough to handle the anticipated work­load, but three assured that the test pace could be maintained even with one aircraft down.50 This lesson has been largely forgotten in our current budget-conscious era.

A final contract for the engine, the prime unit of government furnished equipment, was effective on 7 September 1956. Superseding the letter contract of February, it covered the expenditure of $10,160,030 plus a fee of $614,000.51 For this sum, Reaction Motors agreed to deliver one engine, a mockup, reports, drawings, and tools.