As North American continued to manufacture the three X-15s, personnel at the High Speed Flight Station (HSFS) began to plan how they would test the new airplane. During the admittedly brief history of the research airplane program, flight research had been conducted as a cooperative venture of varying degrees with the Air Force and/or Navy. Usually the contractor would first demonstrate the basic flight worthiness of the aircraft and then turn it over to the military service that had funded its development (the Air Force for the X-planes, or the Navy for the Douglas D – 558 series). The military would then conduct a flight-envelope-expansion program with some NACA participation. For instance, the civilian agency normally supplied the instrumentation and research expertise. At some point after the military had obtained the data it desired, it would turn the airplanes over to the NACA, which then conducted a series of purely research-oriented flights to validate wind-tunnel and other predictive techniques. The flight tests at the HSFS had followed a predictable pattern. All flight operations, maintenance, instrumentation, data reduction, research engineering, reporting, and project control were accomplished by NACA personnel. The Air Force supplied support services such as engine overhaul, chase aircraft, carrier aircraft operations, and the usual air base functions (crash trucks, medical services, etc.).[33]

In the case of the X-15, the original memorandum of understanding signed by the members of the Research Airplane Committee stated simply that "upon acceptance of the airplane and its related equipment from the contractor, it will be turned over to the NACA, who shall conduct the flight tests and report the results of same." There was no provision in the MoU for Air Force flights. From experience, however, Walt Williams believed that the Air Force would want to conduct a program of its own. The Air Force substantiated this when it briefly proposed building a fourth airplane for the exclusive use of the AFFTC. The Research Airplane Committee, and others, did not agree and instead believed that the best arrangement would be to operate the three X – 15s on a cooperative basis.-134

The NACA was not sure it really wanted the Air Force to be involved in the flight program. The NACA in general, and Walt Williams in particular, had a severe lack of confidence in the Air Force based largely on the poor management of the X-2 envelope-expansion program, which had resulted in the loss of both airplanes and the death of Mel Apt. It was a long and uphill battle for the AFFTC to establish a relationship wherein the HSFS management would appreciate the necessity for AFFTC support, and thus the need to allow some level of AFFTC participation.-1351 It eventually succeeded.

As the flight program neared, the Air Force wanted to formalize the responsibilities delegated to each organization. The AFFTC, in particular, wanted to expand its role, and expressed on several occasions a desire to change the original MoU. Failing to do that, the AFFTC rather arbitrarily assigned itself the duties of operating the Rocket Engine Test Facility and the carrier aircraft. Williams did not consider this "of any serious consequence" since similar arrangements had worked satisfactorily in the past.-36 was unable to attend. Williams, true to form, bluntly asked the Air Force representatives exactly what the Air Force’s desires were. Kincheloe stated that the AFFTC would like to take over the entire job—it wanted everything it could get. On the other hand, Kincheloe stated that he did not believe the AFFTC personnel were technically qualified to conduct such a program, and as a result they wanted to work with the NACA. The underlying tone was that the AFFTC personnel felt uninformed on the progress of the program, something Williams indicated that he would try to correct. All in attendance agreed, however, that Edwards should present a unified view to the outside world, and that the AFFTC and HSFS should internally coordinate their answers before publicly announcing them.[37]

Actually, the two groups had already taken this tack during the mockup inspection. Engineers from the AFFTC and HSFS had met prior to and again during the inspection to discuss what items needed to be changed. Several other items, particularly the switch from a B-36 to a B-52 carrier aircraft, had also resulted from pre-coordinated joint action. In spite of this, the AFFTC representatives still felt that they were not receiving sufficient consideration on the program and wanted a more formal agreement finalized.[38]

This led to a discussion of missions and objectives. The AFTTC pointed out that it wanted its engineering staff to benefit from active participation in the entire program. This would allow the Air Force engineers to become familiar with advanced technology for evaluating future weapons systems, which was, after all, their primary job. Of particular concern was that the NACA was specifying the research instrumentation without AFFTC input. Williams pointed out that the main reason NASA had not consulted AFFTC personnel concerning instrumentation was that they lacked the experience to make any significant contributions. Indeed, the researchers at the HSFS were largely dependent on the scientists at Langley and Lewis for advice since nobody had ever designed instrumentation to measure the aero-thermo environment expected for the X-15.[39]

At the end of the meeting, Williams pointed out that the NACA was primarily responsible for research into structures, handling qualities, and flight techniques, and therefore needed to have the primary responsibility for the X-15 program. This had been the rationale behind the original MoU. Nevertheless, Williams was smart enough to know that he needed the support of the AFFTC personnel, and besides, many of their observations were valid-they needed to be involved in the program in order to sharpen their skills for evaluating future weapons systems. Much more so than NASA, the NACA existed primarily to provide data that were useful to the industry and the military services that paid most of the bills.[40]

In an attempt to satisfy everybody concerned, Williams agreed to set up the X-15 Flight Test Steering Committee as a logical successor to similar committees used on previous programs. However, Williams emphasized that the NACA "had no intention whatsoever" of relinquishing the technical direction of the program per the original MoU. To ensure that this was the case, Williams appointed himself chairman of the committee and reserved the controlling vote. Other members of the committee were the AFFTC X-15 project engineer, the HSFS X-15 project engineer, and a test pilot from each organization. Initially the NACA personnel were Kenneth S. Kleinknecht, Joseph A. Walker, and Hubert M. Drake, respectively; Richard Harer, Iven Kincheloe, and an undetermined engineer represented the Air Force. The Air Force’s Paul Bikle would act as an advisor to the committee, somewhat countering Williams and his unilateral veto authority. It was the beginning of a long association between Bikle and the X-15.[41]

intent of the [X-15] program have to be cleared with NACA Headquarters. It is presumed there are similar restrictions on the [Air Force] Flight Test Center. It should be understood at the outset, therefore, that the steering committee would have jurisdiction only in regard to matters that would normally come under the jurisdiction of the Flight Test Center or the High-Speed Flight Station." At the same time, Dryden wrote to Lieutenant General Donald L. Putt on 2 October 1957 indicating he had authorized Williams to participate in such a committee, and urged Putt to authorize the AFFTC to participate. Eventually this group morphed into the X-15 Joint Operations Committee and was responsible for coordinating most of the X-15 flight program. Some references indicate that the Navy had membership on the X-15 Joint Operations Committee.*42!

Soon after Dryden wrote this, the Soviet Union launched Sputnik, diverting the attention of Headquarters elsewhere. At the end of 1957, NASA disbanded the Interlaboratory Research Airplane Projects Panel; for the next decade, oversight for the X-15 would come from the Research Airplane Committee run by Hugh Dryden.

Although the development of the X-15 had carried the System 644L designation, the initial flight program was designated System 605A. An R&D project card was prepared that outlined the extent of the test series, as well as the anticipated funding requirements. At the time, the AFFTC optimistically expected that 300 flights would be made over a five-year period beginning in July 1959 from air-launch sites located above Cuddeback Lake, Silver Lake, Mud Lake, Jakes Lake, and the Bonneville Salt Flats. The anticipated funding was $2,400,000 in FY60, $2,386,000 in FY61, and $2,325,000 for each of the next three years.*43!

The X-15 Joint Operations Committee coordinated the flight program and could call on support from other organizations as needed. The FRC was responsible for the maintenance and logistics of the three X-15s, while the AFFTC maintained the two NB-52s. An exception to this was that the FRC maintained the unique launch equipment on the NB-52s. Technically, flying the NB-52s was a joint project, but in reality a NASA pilot never flew the airplanes; the FRC did, however, supply the launch panel operators. NASA was responsible for data collection and analysis, with support from the Air Force as needed (or desired by the Air Force). All aircraft instrumentation, as well as High Range operation and maintenance, was the responsibility of NASA. The AFFTC was responsible for the biomedical instrumentation package, and maintained the David Clark full – pressure suits and rescue apparatus (parachutes, etc.). The Air Force provided most support aircraft (C-130s, H-21s, and chase aircraft), although NASA began to provide more chase aircraft as the program continued, and the Navy briefly contributed a Douglas F5D Skyray. It was not unusual for NASA pilots to fly AFFTC chase planes.*441

The AFFTC supplied all of the propellants and gases necessary for X-15 and NB-52 operation, and was responsible for all maintenance of uninstalled engines (XLR11 and XLR99) and engine overhauls. The Air Force maintained and operated the Rocket Engine Test Facility used for ground-engine runs. NASA was responsible for installing engines in the X-15s, performing maintenance and inspections of installed engines, and conducting the ground runs using the AFFTC test stands. As the program continued, NASA began to perform more maintenance on the XLR99 engines, including recoating the nozzles. The AFFTC maintained the X-15 APUs and was responsible for all engine, APU, and stable-platform logistics.*451

The Air Force marked and maintained the lakebeds; provided inter-agency coordination (e. g., with the FAA); supplied medical, fire, and security personnel as needed; and operated the long-range camera facilities. The Air Force also operated and maintained several radar facilities that were not part of the High Range but nonetheless generated data to support the flight program. NASA provided maintenance for the stable platform (and later the inertial systems) and the ball nose, since both of these were considered research instrumentation. It was a complicated agreement but it worked remarkably well.[46]

The AFFTC expected the ARDC (and later the Systems Command) to "establish, fund, and monitor an open call type contract with North American Aviation, Inc. to furnish such articles and supplies and perform for the Government such services as may be required." Similar contracts existed with Reaction Motors for the engines, and with Sperry for the stable platform.-47

Getting Ready for Maximum Speed

The general cautiousness that was beginning to permeate NASA was also affecting the X-15 program, and the buildup to the maximum speed flight was unusually conservative. Although the program had never been "wild and crazy," it had previously taken reasonable risks when it understood the problems and their consequences. This was not the case during preparations for the maximum speed flight, which really did not represent that large an increment over the Mach 6+ speeds already attained. Nevertheless, in preparation, the program dealt with each individual piece separately.

Pete Knight flew the next X-15A-2 flight (2-49-86) with the ventral on, primarily to familiarize himself with the handling qualities, since all of his previous flights had been with the ventral off. All future X-15A-2 flights would use either the ventral or the ramjet. Flight 2-50-89 was the first flight where the external tanks operated (knowingly) successfully, including the improved instrumentation that let the ground crew and pilot know the propellants were transferring correctly.[319]

Getting Ready for Maximum Speed

The NB-52s required a modification to strengthen the wing in order to carry the X-15A-2 and its external tanks. On 27 June 1066, Bob Rushworth was in the cockpit of a scheduled captive-carry flight (2-C-80) to test the X-15A-2 with full external tanks. (NASA)

Edwards in early April, in time for flight 2-51-92. In addition, NASA relocated the thermocouple recording system from the center-of-gravity compartment to the main instrument bay since it had failed to operate on the previous two flights because of the cold environment. By May 1967, three dummy ramjet shapes had arrived at Edwards, and wind-tunnel tests in the JPL 21-inch hypersonic tunnel had verified the mated ramjet configuration. One of the three dummy ramjets was sent to Inglewood to have a thermal protection system installed, and engineers at the FRC instrumented the other two in preparation for flight. Researchers had already calibrated the flow – field cone probes at Mach numbers of 3.5 and 4.4 in the Ames 1 by 3-foot wind tunnel, with additional tests scheduled at Mach numbers of 5.0 and 7.4.[320]

Pete Knight evaluated the handling qualities of the X-15A-2 with the dummy ramjet installed under the fixed portion of the ventral stabilizer on flight 2-51-92. This flight did not include the external tanks and reached Mach 4.8. Knight jettisoned the ramjet just before landing, much like the ventral rudder, to provide the necessary clearance for the landing gear. Next up for X-15A-2 was a flight with the ablative coating and dummy ramjet, but without the external tanks.[321]

Skylight Compartment

Several of the proposed experiments needed to expose telescopes or other devices to the atmosphere at high altitudes. To accommodate this, North American devised the "skylight" modification, which consisted of a hatch that opened at high altitude to give a portion of the instrument compartment free access to the outside environment. This required the installation of pressure bulkheads around a portion of the instrument elevator to allow the lower portion of the instrument compartment that held the data recorders to remain pressurized. The proposed hatch was 18 by 12 inches, with the 18-inch dimension lengthwise of the aircraft, and a pair of 6-inch­wide doors split along the centerline to open. Several of the experiments also required a stabilized platform inside the compartment. Since the University of Wisconsin had the first experiment that needed such a platform, NASA awarded the university a contract to develop a star-tracking, gyro – stabilized platform that would replace the upper portion of the instrumentation elevator. The university estimated that this platform could be available about six months after it received the go-ahead.[62]

Skylight Compartment

Several of the proposed experiments needed to expose telescopes or other devices to the atmosphere at high altitudes. To accommodate this, North American devised the "Skylight" modification that consisted of a hatch that opened at high altitude to give a portion of the instrument compartment free access to the outside environment. A skylight compartment was installed during the rebuilding of X-15A-2 and, somewhat later, on X-15-1. This is the Ultraviolet Stellar Photography Experiment (#1) on X-15A-2. (NASA)

North American anticipated that it would take about two months to perform the modification to X-15-2, with most of that time required for rerouting wiring in the instrument compartment and building the pressure bulkheads. A change order was prepared for the modification and was awaiting approval when Jack McKay’s landing accident damaged X-15-2 and put the entire effort on hold. The Air Force decided to press ahead with the skylight modification as part of rebuilding X-15-2 into the advanced configuration, but for some reason the actual implementation changed somewhat. The hatch became slightly larger, with two upward-opening doors that were 20 inches long by 8.5 inches wide. Otherwise, the changes were mostly the same as originally conceived.[63]

North American also installed a similar but slightly smaller compartment on X-15-1 in early 1966 to carry the "Western Test Range (WTR) launch-monitoring" experiment (#20). Only the WTR and MIT experiments used this X-15-1 capability.-64


Although the X-15 would eventually play a major role in the Hypersonic Research Engine (HRE) project, researchers had not considered air-breathing propulsion at all during its conceptual development in 1954. At the time, most researchers believed that hypersonic air-breathing engines were improbable or, more likely, impossible. Several military programs undertook subsonic-burning ramjet development, but there appeared to be fundamental obstacles to their use at hypersonic speeds. In 1955, William H. Avery at the Applied Physics Laboratory (APL) of The Johns Hopkins University conducted a survey of ongoing ramjet development efforts and concluded that Mach 4 was about the highest speed achievable by ramjets. Two problems arose at higher speeds: the lack of structural materials for the combustor, and a serious energy loss due to dissociation of the propulsive airflow and the failure of this plasma to recombine in the nozzle.*230*

A remarkable change from pessimism to optimism occurred during the late 1950s and early 1960s. By 1964, researchers believed they could solve the problems encountered previously. In particular, a hydrogen-powered supersonic combustion ramjet appeared to have the dramatic potential of useful performance up to near-orbital velocities. No single breakthrough created this newfound optimism-it sprang from a confluence of results from a number of unrelated research efforts. The first important contribution was a series of external burning studies conducted at the NACA Lewis Flight Propulsion Laboratory and the Marquardt Corporation in Van Nuys, California. These studies appeared to confirm that combustion in a supersonic flow was possible. Similar results had been produced by the APL and Antonio Ferri at the Brooklyn Polytechnic Institute in the late 1950s. The next innovation was the development of hydrogen as a fuel, which was also mainly accomplished at Lewis. Beginning in 1954, Richard J. Weber at Lewis began to think about the possibility of using supersonic combustion, internally, in a ramjet engine. Although he doubted that shock-free combustion would be possible in a supersonic combustor, Weber decided to analyze the ideal performance that would be attainable in a ramjet. The work had a low priority and proceeded slowly, but resulted in the first definitive analytical assessment of a supersonic ramjet.-1231

In a September 1958 report, Weber and John S. Mackay identified several beneficial features of a supersonic combustion ramjet (scramjet), noting that it could relax inlet/diffuser requirements, reduce combustor heating, minimize the nozzle-dissociation problem, alleviate variable-geometry inlet requirements, and provide the potential for performance levels much higher than any other air-breathing engine at speeds above Mach 7. The effects of the combustor area ratio, thermal compression, and other design parameters were determined for the first time. Several other researchers generally confirmed the results of this research by 1960. Ironically, by the time NASA finally published the Weber-Mackay paper in 1958, the authors had moved on to other research, believing there would be little interest in the scramjets and few (if any) applications for them.[232]

This was an interesting time for the NACA laboratories. On 1 October 1958, the NACA ceased to exist and the new NASA came into being. Lewis was beginning to abandon all work on air­breathing engines in favor of rocket engines. Thus, it surprised Weber when in early 1959 he was invited to speak at the 2nd Symposium on Advanced Propulsion Concepts as a specialist in supersonic combustion. Weldon Worth, then technical director of the Aero-Propulsion Laboratory at Wright Field, organized an entire session on the subject. Worth had many interests, including Aerospaceplane, a large single-stage-to-orbit vehicle powered by scramjets. Shortly after the beginning of the Mercury program in 1959, most of the major aerospace companies participated in studies of the Aerospaceplane, although in retrospect it is easy to see that the concept could never have worked given the technology of the era. Nevertheless, the concept was exciting, and Alexander Kartveli at Republic Aviation, for example, enlisted the services of Antonio Ferri to collaborate on orbital concepts. In addition to scramjets, the companies began working on imaginative new schemes, such as the air collection engine system (ACES) and the liquid air collection and enrichment system (LACES), that would extract air from the atmosphere on the way up to be used as oxidizer by rocket engines when the vehicle left the sensible atmosphere.-1233

The first public discussion of hypersonic propulsion and its possible applications was held at the 4th AGARD Colloquium in Milan during April 1960. Ferri fired the imagination of his audience with the prospects of air-breathing engines that worked all the way to orbit. Many of the older researchers were politely skeptical. The Aerospaceplane concepts would survive for several years within the Air Force before everybody came to realize they were simply too advanced for the state of the art. For a brief time, however, they influenced the road taken by some of the early space shuttle studies.-1233

The scramjet concept, however, survived relatively intact. Between 1959 and 1963, the military spent $10 million on scramjet research, and researchers failed to uncover any "concept killer" obstacles. Perhaps equally important was the appearance of a rapidly growing cult of ardent scramjet enthusiasts of which Ferri was the chief spokesman and Worth the chief benefactor. By all appearances, it seemed that practical scramjet applications were just around the corner. One

1964 summary stated that "scramjets are passing into the development stage" and listed no less than 19 institutions that were working on the concept, including five that were "testing complete engine models."[235]

In 1964 the Air Force released Project Forecast, one of the periodic studies the military conducted into possible advanced concepts for future applications. Largely through the lobbying efforts of Ferri and Worth, the scramjet became an area that merited special emphasis. Consequently, General Bernard A. Schriever, the commander of the Air Force Systems Command, established a special task force to examine scramjet technology and its potential. However, the cards were stacked in favor of the technology because the majority of the members of the task force were members of Worth’s staff or representatives of the various contractors working under $22 million worth of scramjet contracts. The final report published in April 1965 envisioned "no unforeseen problems" and recommended initiating a high-priority national development program. There were, of course, skeptics who did not believe the technology was nearly as advanced as its proponents claimed. However, few of them were willing to buck the Air Force hierarchy, which apparently had decided to embark on a crusade.-1236

The origination of large government research and development projects is seldom a logical process, and the HRE was no exception. North American Aviation was always interested in new business, and during the early 1960s it was particularly keen on finding new ways to exploit the X-15 research airplane. Given the newfound interest in scramjet propulsion, project aerodynamicist Bill Johnston decided to marry the two. During May 1962, Johnston visited various Air Force and NASA centers with a proposal to modify an X-15 for use as a flying test bed for hypersonic air-breathing engines. To many researchers, including some at the FRC, the X-15 seemed like an ideal test bed for such a propulsion system. As envisioned by the FRC in 1961, this idea was "an extensive air-breathing engine development program… in which one or more sub-scale modular experimental engines would be flown in a true flight environment aboard the X-15." Surprisingly, there were no takers, and the proposal floundered until November 1962 when Jack McKay made an emergency landing in X-15-2, injuring himself and seriously damaging the airplane.-1237-

North American took this opportunity to dust off Johnston’s concept and reiterated its proposal to modify the airplane for propulsion testing. The Air Force supported the plan and was willing to pay the estimated $4.75 million to rebuild and modify the aircraft. Many within NASA, however, were not in favor of the idea since they considered the proposed Mach 8 speed to be of limited value for propulsion research. Nevertheless, NASA did not press its objections and the Air Force authorized North American to modify the airplane. It thus appeared that a Mach 8 carrier vehicle would be available within a couple of years; however, the propulsion test engines themselves were completely undefined.-1238

To correct this illogical situation, the FRC quickly launched a study aimed at determining what type of engine would be appropriate for testing on the X-15. Recognizing that the expertise for monitoring such a study would be found chiefly at other NASA facilities, the FRC solicited comments on its draft procurement documents; no support was forthcoming. In fact, Kennedy F. Rubert at Langley expressed his opposition to any flight program as "an unwise expenditure of government funds" since engine research "is better done on the ground." Undaunted, the FRC continued with its procurement and stated firmly that it planned "to take an active role in advanced air-breathing propulsion and the X-15 should prove very useful in this regard." This course of action seemed to both prolong the X-15 program and increase the participation of the FRC in basic research projects, despite philosophical misgivings on the part of many at the FRC, including Paul Bikle.[239]

After a brief proposal period, the FRC awarded a four-month study contract to the Marquardt Corporation to generate requirements applicable to 1) subsonic combustion ramjets, 2) LACES, 3) scramjets, 4) ducted rockets, and 5) turboramjets.[240]

When the final report appeared in December 1963, the results bore little resemblance to the dummy ramjet that would ultimately fly on X-15A-2. Marquardt determined that the X-15 was a viable platform for testing ramjets in the speed range of Mach 4 to Mach 8, providing a useful complement to ground testing. The company proposed to test three different types of ramjets that required 33 months to develop and build. Surprisingly, the study investigated using one of the basic X-15s for preliminary testing while North American rebuilt X-15A-2.4241

The three proposed engines included a subsonic combustion ramjet, a scramjet, and a "convertible ramjet," all sharing a common external design referred to as MA-131. The convertible engine operated as a subsonic combustion engine between Mach 3 and 5, and with supersonic combustion between Mach 7 and 8; it transitioned from one mode to the other between Mach 5 and 7. Researchers investigated three other engine types—the turboramjet, ducted rocket, and LACE/ACES-and ruled them out, although the ducted rocket returned as part of the later air – augmented rocket propulsion system (AARPS) concept.42424

Marquardt considered regenerative, radiation, and ablative cooling schemes, and settled on the latter as being the most cost-effective and lowest risk. Unfortunately, this probably doomed the Marquardt proposal since most hypersonic-engine researchers were firmly convinced that testing was not worthwhile unless the engine used regenerative cooling. Another controversial aspect of the Marquardt proposal was the plan to forego a true variable-geometry inlet design in favor of an inlet that could be set to various positions on the ground before flight. In retrospect, this seems like a good compromise. Undoubtedly, any production engine would use a variable-geometry inlet, but it would be expensive and time-consuming to develop one. In addition, given how short a period the X-15 could maintain steady flight conditions, there would be little opportunity to adjust the inlet in flight in any case. Ground personnel could adjust it to allow data to be gathered on successive flights at different geometries. Still, the researchers at Langley and Lewis believed the solution was inelegant and rejected it out of hand.-42434

The engineers at the FRC begged to differ, noting that "the elimination of complex and unproven inlet and exhaust nozzle control systems from the test engine also provides operational simplicity with reasonable assurance for success without a long and costly X-15 flight test program." Nevertheless, the engineers worried that the drag created by the ramjet installation might result in much slower-than-desired acceleration while on the way to the intended flight conditions. Marquardt modified the design so that the inlet would close during the acceleration phase and open only when the correct test conditions were available.-42444

Instead of the round cross-section dummy ramjet that ultimately hung from the ventral stabilizer on X-15A-2, Marquardt proposed a rectangular shape that fit flush against the lower fuselage. The unit would be 188 inches long, 24 inches wide, and 21 inches high. A boundary-layer fence between the inlet and the lower fuselage ensured clean airflow into the inlet. Researchers expected the hydrogen-fueled ramjet could produce up to 1,000 lbf gross thrust. Several mockups were produced and fitted to X-15A-2 at various times, including one airshow at Edwards.-12454

estimated that the Typhon installation would weigh 803 pounds, while the three Marquardt engines would average about 900 pounds. The engineers believed all of these were within the capability of the X-15 to handle, although they shifted the center of gravity dangerously rearward.[246]

In addition to the engine package itself, North American needed to install an engine control system, engine jettison system, liquid-fuel storage and transfer system, and fuel pressurization system, as well as a new fire detection system. The engine package would replace the ventral stabilizer on the basic X-15, but would hang from a stub ventral on X-15A-2.[247]

On the basic airplane the entire ventral stabilizer and lower rudder actuator would be removed, a "roller support system" would be installed that actually supported the ramjet, a new frame would be added in the fuselage at station 483.5, and a pair of conformal liquid-hydrogen slipper tanks would be added over the aft portions of the side tunnels. Since North American was including a liquid-hydrogen tank in the center-of-gravity compartment of the rebuilt X-15A-2, the slipper tanks would be unnecessary. The longer rear skids on the advanced airplane would also allow the ramjet to be mounted on the stub ventral stabilizer. This had a couple of desirable effects: the inlet would be farther away from the flow disturbance caused by the X-15 fuselage, and at least a small ventral stabilizer would remain if the ramjet had to be jettisoned at high speeds. North American also suggested using JP-Pentaborane as an alternate fuel, and proposed installing a system that could handle either fuel as necessary.-1248

North American did express a couple of concerns. The modifications would increase landing-gear loads significantly, reducing the factor of safety below the 150% normally maintained. The company suggested beefing up the landing gear on the basic X-15 at a minimum, and perhaps even strengthening the already beefier gear on the advanced airplane. A more disturbing concern was the aft center of gravity that would be created by the ramjet installation, particularly in the basic airplane where the liquid hydrogen (and, more importantly, its tankage) would be mounted far aft. North American advised performing a new series of stability and control wind-tunnel tests to determine how bad the situation might really be. Ultimately, researchers at the JPL conducted these tests during 1966 on both the basic airplane and the advanced X-15A-2.-249

Assuming a go-ahead in February 1965, North American estimated it would take three months to design the modifications to X-15-1, four months to fabricate the modification kit, and four months to install it, meaning that X-15-1 would be available to support hypersonic engine testing in January 1966. The advanced X-15A-2 would become available in July 1966. Researchers expected that the flight program would encompass approximately 25 flights spread over a two – year period.-1258

Hypersonic Research Engine

None of the NASA reviewers, excepting the FRC, believed a research program based on the Marquardt engine concept was justifiable. They pointed out that the emphasis on low cost would result in overly simplified designs that provided little valid test data. In addition, the use of ablative materials would produce contaminants that might strongly affect the combustion process. While discussing the problem with Kennedy Rubert, John Becker suggested that Rubert offer an alternative that might be worthwhile. Rubert then described, in general terms, a concept very close to what would eventually become the HRE-a sophisticated dual-mode engine that was thoroughly researched on the ground and used a clean internal metallic structure without ablative coatings. Becker argued that this was a superior alternative.-1251!

After the final Marquardt briefing at the FRC, Douglas E. Wall, who was in charge of X-15 research engine activities, called an informal meeting of the NASA participants to discuss the next move. The outside center reviewers were unanimously against the Marquardt engines, and generally against any flight program. Wall argued convincingly that financial support for an extensive scramjet program was unlikely to be forthcoming unless it was tied to an X-15 flight experiment. Although almost everybody still viewed a flight program as unnecessary, all agreed that Wall was probably correct. Everybody recognized, however, that Lewis would present a formidable obstacle; not only was Lewis traditionally unsympathetic to research airplanes, but the center had also recently abandoned almost all air-breathing engine research. To bypass the expected objections, the researchers decided to propose the Hypersonic Research Engine (HRE) program as a joint FRC – Langley effort, with Langley managing the ground phase and the FRC being responsible for the flight phase.-1252

Although he was personally unconvinced, Paul Bikle endorsed this concept and verbally presented it to NASA Headquarters, with Rubert recommended as the program manager. The initial Langley reaction, however, was unfavorable, mainly because Lawrence K. Loftin, Jr., one of the assistant center directors, had recently recommended against such a program. In Becker’s Aero Physics Division there was a very different reaction. For years, propulsion-related fluid mechanics and hypersonic inlet/diffuser work in the division had suffered from a dearth of real-life applications. The prospect of involvement with a real engine for X-15 testing offered an exciting infusion of much needed vitality. Becker also pointed out that the HRE project would reveal whether any of the performance claims for the scramjet were valid, something that appealed to many Langley managers. Another important consideration was the complete lack in 1964 of ground-test facilities for true-temperature simulation with clean air above Mach 5. Therefore, researchers viewed the X-15 as a unique test facility, and eventually Langley management came around and began supporting the project.-1252

NASA asked Rubert to develop a detailed plan for the HRE program, and on 17 March 1964 he released a preliminary proposal that outlined a three-phase program. Phase I was to define a practical, high-performance, Mach 3-8 hypersonic engine, and to design, develop, and build such an engine; Phase II was to measure the performance of the engine in the laboratory; and Phase III was to measure the performance of the engine in maneuvering flight and to validate the ground – test results. Significantly, the proposal did not discuss the need for scramjet research, assuming (incorrectly) that this was already well known. Rubert stated flatly that the "gaps" in component technology "had been filled," leaving only uncertainties "which can be discovered and resolved only by design and construction of a truly practical research engine." Nobody at Langley challenged these claims, which mainly demonstrated the inflated technical confidence in the concept that existed at the time. Rubert’s plan required four years at a cost of $30.4 million, plus the operations costs of the X-15. The proposal "sailed through" its approval process at NASA Headquarters "with no opposition and few questions asked." Significantly, Lewis director Abe Silverstein did not oppose the project, although he stopped short of actually supporting it. Phase I funds were released on 13 June 1964.[254]

As Wall had indicated, tying the HRE proposal to the X-15 lent credibility. The X-15 was still a successful program that enjoyed almost universal approval within NASA; anything related to it, by default, usually enjoyed similar approval. Of course, there should have been questions. Both the schedule and budget were hopelessly optimistic for developing an entirely new type of engine.

The lack of coordination with the Air Force and Navy should have been disturbing. Moreover, nobody asked how Langley was going to compensate for its lack of experience in engine development and testing (traditionally a Lewis task). It would not be as easy as it appeared.-1252

The proposed HRE engine was described as a "truly practical complete engine" that would provide "factual" performance data under real-world conditions. If researchers could have fulfilled all of the hopeful claims of the original proposal, this single engine project would have advanced the technology from the early exploratory research stage to an operational system. An elaborate subscale prototype engine was obviously required to fulfill such claims. Unfortunately, this was not what would be specified for the Phase I competition. Instead, the statement of work called for the "best possible research engine." While the Phase I engine would be required to deal with realistic internal flow conditions, it would not be required to consider the critically difficult high – temperature regeneratively cooled structure or to worry about external drag. In fact, the structure and external features were to be "refined only to the extent necessary for compatibility" with the internal flow performance requirements.12561

Rubert never openly acknowledged the downgrading of the engine concept from the advanced prototype referred to in the original proposal to an aerothermodynamic boilerplate. The rationale for the change was that project personnel soon realized that the limited funding they had requested could not possibly pay for a prototype engine; unfortunately, they continued to portray the engine as a prototype throughout the project. Ironically, the engine specified in the Phase I statement of work closely followed the 18-inch-diameter pod-type engine that had been suggested by Marquardt during the initial FRC study and was uniformly rejected by Langley and the other NASA centers.-12571

The decision to seek the highest possible internal performance and to impose no thrust-minus – drag requirement had several unfortunate consequences. In order to comply, the contractor that eventually won the Phase I competition used a higher-than-optimal degree of external compression, which caused extremely high external cowl drag. In the final design, designers thickened the cowl to house research instrumentation, aggravating the drag problem even further. In the end, the engine was capable of producing essentially zero net thrust (thrust minus drag). When outside researchers realized this, most of the already lukewarm support for the project vanished.12581

A Phase I request for proposals was issued to 35 potential bidders, calling for a nine-month, 27,500 man-hour (roughly 20 people) study to develop a concept and a preliminary design; determine performance, life, weight, and safety data; and provide a development plan, manufacturing plan, and costs for the Phase II effort. When the proposals arrived on 28 May 1964, only four companies had responded. A team of 50 engineers and researchers from Ames, Langley, Lewis, and the FRC convened to pick a winner. Significantly, there was only a single Air Force representative on the technical review panels. NASA awarded parallel study contracts to Garrett, General Electric, and Marquardt in October. The evaluators never thoroughly understood why Pratt & Whitney had chosen to submit a proposal, since nobody thought it was a "serious effort to compete."12591

Nine months later, the same small army of evaluators reconvened to look over the results of the three studies. The concepts were essentially unchanged by the nine months of effort, and the results seemed to favor Garrett. John Becker later said that there were "some flaws in the deliberations which led to Garrett’s selection," although he maintained they were honest mistakes and not deliberate attempts to mislead.12601

The Garrett engine was the smallest, simplest, and easiest to cool, and had the best structural approach of the three designs. The evaluators also believed that the engine had a very high research potential because of the quasi-two-dimensional nature of the flow in its shallow annular combustor. Researchers thought that this would simplify the analysis of the combustor data for use in any future two-dimensional combustor design. There was another powerful consideration in favor of Garrett: under the leadership of Anthony duPont, Garrett had exhibited an energy and zeal unmatched by the other companies. Drawing on $250,000 in company funds, it had built a full-scale HRE combustor model and later operated it successfully at the Navy Ordnance Aerophysics Laboratory in Daingerfield, Texas, under simulated Mach 6 conditions. This made it seem like the design was well along, and that developing an engine based on it would be a quick and inexpensive process.-261

The General Electric design, on the other hand, did not appear to offer a two-dimensional approach because its combustor annulus was too deep. The engine was also large, heavy, and hard to cool. The evaluation team also penalized General Electric because of its long development schedule and high cost, although in retrospect they were much more realistic than Garrett’s.261

The evaluators ranked the Marquardt engine developed by Ferri last because of its complex three­dimensional flows and a general lack of substantiation of the claims made for it. The most serious question revolved around the thermal compression effect used to avoid having a variable – geometry inlet.[263]

As it turned out, there were three serious flaws in the evaluation. The supposedly simple two­dimensional flow expounded by Garrett was illusory. The complex boundary layers, focused shock waves, and resulting separations and complex interactions made the actual flow virtually impossible to analyze, unique to this particular engine and undesirable from all standpoints. The generally better performance actually obtained with the General Electric combustor suggests that it should have received at least an equal rating. It is interesting to note that during his oral briefings to the evaluation board, Antonio Ferri had called attention to the problems of axisymmetrical design, including "focused shocks," "high losses," and "high cowl drag"-the same problems that were actually encountered by the Garrett engine. The evaluators unfortunately dismissed these comments as prejudiced.264

The second flaw in the evaluation was the belief that the study would reveal true time and cost estimates in a situation where the government told the contractors what the times and costs should be. The two contractors that fed back what the government wanted to hear were credited with "responsiveness," while the one (Marquardt) that provided more realistic (but unpopular) estimates was penalized. The last flaw was perhaps the most unfortunate. Garrett gave the impression that the already-developed model combustor could be easily made to work. However, its apparent success during early tests was the result of researchers not understanding the test conditions, and in reality the development of the combustor would prove to be the primary problem of the HRE. Garrett later blamed its misleading experience with this model for its gross underestimates of the true time and cost of the actual engine.261

There were also questions, even prior to Phase I, regarding whether Garrett had sufficient expertise to undertake the development of the HRE. The company had little experience with scramjet aerothermodynamics, and both management and technical personnel were inexperienced in this type of development effort. The fact that the company was proceeding with the development of the combustor model at its own expense was the primary consideration that overruled its inexperience.261

On 11 July 1966, NASA awarded Garrett the Phase II contract, but things quickly began to unravel during the negotiations for a final contract. Modifications and ground support equipment for X – 15A-2 would cost a staggering $8.7 million, vastly more than had been expected. Within six weeks, Garrett was proposing a $10 million increase in overall costs. Given the overall reductions

in funding caused by the conflict in Southeast Asia, NASA managers began to question the necessity for the program.[267]

At a meeting of the Aeronautics and Astronautics Coordinating Board on 5 July 1966, the Air Force announced it would stop funding the X-15 program in 1968 and that NASA would take sole responsibility for the majority of operational support (the military would continue to provide minor base support services). The withdrawal of Air Force support for the X-15 program was a serious setback because it meant that the NASA budget would need to find an additional $8 million per year, an enormous increase that was unlikely to occur. At the same time, there was another adverse development on the Air Force side: a gradual decrease in support for hypersonic technology in general, and scramjets in particular. Just before retiring from a long and notable career, General Bernard A. Schriever was unsuccessful in his attempt to obtain $50 million for an ambitious scramjet program.-1268

Finding additional NASA funding would not be easy. This raised significant questions about how to proceed to Phase II. Interestingly, the question of whether to proceed seems not to have been raised at all. In September 1966, Langley established a formal project office to oversee Phase II, with Rubert designated as the project manager, and by year’s end approximately a dozen people staffed the office.-1269

On the contractor side, the president of Garrett, Harry Wetzel, was getting "politely impatient" with the delay in getting started. In a letter to NASA administrator James E. Webb on 16 December 1966, Wetzel indicated that he might invoke "pre-project costs" under a provision of the contract if delays should continue. This appeared to have the desired effect because a plan was soon devised that split the former Phase II effort into Phases IIA and IIB. The first part would cover development and the manufacture of one pre-prototype, flight-weight engine. Phase IIB would subsequently produce six prototype engines for ground tests, qualification testing, and later flight tests as part of Phase III. NASA formally approved Phase IIA with a target cost of $15.6 million (including fee), and researchers estimated that Phase IIB would cost $13 million. However, nobody attempted to define a cost estimate for Phase III because it contained too many unknowns. In addition, project management did not address how to extend the X-15 program long enough to allow the engine to be developed, or how to pay for X-15 operations.-1279

In the high desert, plans to fly a "dummy" ramjet shape on the modified X-15A-2 were taking shape. In order to gain basic aerodynamic data and investigate the effects of carrying a generic ramjet shape on X-15A-2, the FRC had several dummy ramjets constructed. None of these resembled the engine mockups that engineers had hung under the X-15 in the past. The FRC fabricated the dummy shapes, about 7 feet long and 2 feet in diameter, from a series of truncated cones. The program flew two different nose configurations: a 20-degree cone on flights 2-51-92 and 2-52-96, and a 40-degree cone on flight 2-53-97.-1271

To accommodate the dummy ramjet, NASA significantly modified the ventral stabilizer on X-15A – 2 by removing 2.8 feet from the front and adding a blunt, unswept leading edge. In addition, engineers removed approximately 3 inches of the lower surface for the first 3.3 feet of the ventral to allow the ramjet to be mounted in a semi-submerged location. Ten impact pressure probes were installed on the leading edge. Most of these protruded approximately 5 inches in front of the ventral, although the three closest to the ramjet were progressively shorter. The probes extended through the pylon standoff shock wave except near the ramjet, where they were made shorter to measure pylon-ramjet interference effects.-12721 smooth cylindrical surface of the lower fuselage of X-15A-2. This was the same camera window used for the Hycon/Mauer experiments (#27) and it protruded a maximum of 1.75 inches below the fuselage. This protrusion was located approximately 13 feet ahead of the leading edge of the pylon, or about 10.25 feet ahead of the tip of the 40-degree nose cone. For these tests, researchers installed a Millikan 16-mm movie camera to photograph the ramjet.-12731

Surface static-pressure orifices were located on the right side of the dummy ramjet and pylon. All orifices were normal to the surface and flush with the metal skin. When the ablative coating was used, an insert of higher-density ablative material at each orifice location maintained a sharp edge at the outer surface. The first flight with the 20-degree nose cone did not use any nose probes. The second flight with the 20-degree cone, and the only flight with the 40-degree cone used a rake with two 40-degree cone probes protruding from the extreme nose in an "L" shape. The top cone probe was on the ramjet centerline, and the lower cone was 8 inches below.12741

A wind-tunnel study conducted after the last X-15A-2 flight showed that shock waves generated by the wing leading edge, lower-fuselage camera window, and fuselage side fairing all impinged on the dummy ramjet and pylon. Researchers found that these were very sensitive to the angle of attack, with a 1% increase in free-stream angle of attack resulting in a 10% increase in impact pressure at Mach 6.5.12751

At the beginning of 1967, the program planners, who had originally expected the program to be completed by the end of 1969, did not expect to begin flight tests of an operable ramjet before 1971 at the earliest. The schedule had begun to slip even before the start of Phase I, when NASA extended the original four-year project 15 months just to accommodate the procurement cycle. However, there was little actual concern among those involved at the time since they believed the Air Force and NASA would extend the X-15 program as required. By 1967, that prospect was beginning to look less likely.12761

Faced with the long schedule extension, greatly increased costs, the loss of Air Force X-15 funding, waning interest in hypersonic technology in general, and the prospect for austere R&D funding in the years ahead, managers began to doubt they could complete the HRE program at all. In particular, they considered it unlikely that the necessary continuation of the X-15 program could be obtained using only NASA funding. In retrospect, it was obvious to most of those involved that the HRE should have died a natural death at this point. As is often the case, however, the project had developed a life of its own. Some believed that Phase IIA would develop useful scramjet technology regardless of what transpired in the future. Since there was still a minor chance that the X-15 program would continue with the development of the delta-wing cruise vehicle, Langley launched HRE Phase IIA on 3 February 1967 with the signing of the final contract with Garrett.12221

Any chance of flight-testing a real HRE vanished on 15 November 1967 when X-15-3 crashed, ending the proposed delta-wing program at the same time. Surprisingly, perhaps because researchers had long anticipated it, the actual demise of the X-15 portion of the program seems to have caused only minor distress in the HRE project office. Researchers suggested that the X-15 had served a very useful purpose by imposing "real" design requirements for the engine, but surmised they could realize some 90% of the program objectives even without actual flight tests.

By this time roughly half of the Phase IIA costs had been committed, and these would be lost if the HRE program was terminated. There would be other costs associated with terminating the project, possibly totaling the entire cost of Phase IIA; the decision was to let Phase IIA continue unchanged.-12781

Given the lack of a flight vehicle, NASA decided to reorient the HRE into a ground-based program. Ground testing moved from the Navy Daingerfield Facility to a newly completed test stand at the Lewis Plumbrook installation. In early 1968, Rubert told Garrett to stop work on the X-15 modification package and other items related to flight-testing, and the dummy ramjet eventually tested on X-15A-2 had little relation to the HRE developed by Garrett. As it turned out, stopping this work saved little money since the development of most flight subsystems continued in an effort to achieve a "realistic" engine. As John Becker later observed, "And thus was HRE adroitly decoupled from the X-15 which gave it birth and left to make its own way, apparently unchanged but actually now stripped of its glamour and its principle reason to exist."279

The HRE had survived the loss of the X-15 flight phase by only a few months when mounting cost and schedule overruns forced the abandonment of the original plan to develop and test a complete hydrogen-burning engine. Nevertheless, the program continued-with dubious results – until 22 April 1974, when NASA finally terminated it.2801

Although the original promoters of the HRE had oriented Phase I only toward an internal aerothermodynamic performance test model of a scramjet, the combination of the ambitious general claims made for the project and the X-15 flight requirements forced it in the direction of a much more costly subscale prototype with realistic structural and other subsystems. Much later John Becker estimated that the total cost of the entire HRE program, including 25 X-15 flights, would have exceeded $125 million, or about four times the original estimate. The total actually expended was $50.8 million, including $7.5 million charged to the rebuilding of X-15A-2 and the construction of the dummy ramjets at the FRC.2811


John Manke was the last NASA pilot assigned to the X-15 program, but he never flew the airplane. Manke was born on 13 November 1931 in Selby, South Dakota. He attended the University of South Dakota before being selected for the NROTC program in 1951, and graduated from the Marquette University in Milwaukee in 1966 with a bachelor of science degree in electrical engineering. Following graduation, Manke entered flight training and served as a fighter pilot with the Marine Corps. He left the service in 1960 and worked for Honeywell for two years.

NASA hired Manke on 25 May 1962 as a flight research engineer, and he served as an X-15 flight planner. Along with Mike Adams, Manke completed X-15 "ground school" and conducted a test run of the XLR99 in the Rocket Engine Test Facility. Manke left the X-15 program after the X-15- 3 accident that claimed Mike Adams’s life. On 28 May 1968 he flew the HL-10, the first of his 42 flights in a heavyweight lifting body.

After the X-15 program ended, Manke became chief of flight operations at the FRC in October 1981 and continued in that capacity until he retired on 27 April 1984.


As it turned out, the initial flight plan was modified somewhat as the program progressed. The envelope-expansion program was eventually broken into two parts: the basic research program and the basic program extension. The first category consisted mostly of the original plan that covered the aerodynamic, stability and control, and structural aspects of the basic X-15. The government expected that it would take only 17 flights to reach the design conditions of Mach 6 and 250,000 feet; the rest of the early flights would be for pilot familiarization.

Nevertheless, intermediate progress deviated considerably from the plan, since during the course of the program observations sometimes indicated the need for extreme caution and at other times permitted larger increments than planned. In the end, partially because of the delay resulting from X-15-3 blowing up at the Rocket Engine Test Facility, it took 45 flights to reach Mach 6, and 52 flights to reach 246,700 feet (close enough to 250,000).

The basic program extension was essentially similar but was concerned with answering a few lingering questions and conducting the same evaluations of the "advanced" X-15A-2. In the meantime, a separate program began that used the X-15 as a flying test bed and as a carrier for a variety of follow-on experiments.-^

It is interesting to note that the X-15 program lacked much of the drama of the earlier X-planes. Although it was pushing performance levels and the state of art further than any previous airplane, the X-15 did not experience the catastrophic technical problems that had plagued earlier programs. The XLR99 worked, if not perfectly, well enough for its intended purpose, unlike the Curtiss-Wright XLR25 in the X-2. The Inconel X hot structure seemed to suffer little ill effect from its prolonged exposure to high temperatures and dynamic pressures. The inertial coupling phenomena that had caused the loss of the X-2, and almost the X – 1A, had been addressed by a combination of aerodynamic design, an efficient damper system, and some restrictions on flight maneuvers. The explosive effects of Ulmer leather and liquid oxygen were well understood and avoided.-491

However, these conclusions were not obvious as the envelope-expansion program began. The researchers-and pilots-worried about many things. Would the hot structure survive the tremendous heating rates? Would the wings remain attached to the fuselage during a 6-g pullout from high altitude after the structure was heated to 1,200°F? Would the ballistic control system provide sufficient control while outside the atmosphere?

The flight program expanded speed and altitude concurrently. Normally, the speed flights came first to ensure that the airplane was controllable at the velocity necessary for the next altitude flight. During the high-speed flights, the pilot pulled up to an angle of attack that simulated the expected pullout from the next high-altitude flight, allowing a relatively safe evaluation of the effects of the pullout. It took only 12 flights for the X-15 to expand its envelope from the Mach

3.5 and 136,500 feet attained with the XLR11 (and basically representative of the best the earlier

X-planes had managed) to Mach 6.06 and 246,700 feet. It was an amazing feat.

Perhaps not so amazingly to the designers, John Becker and the researchers at Langley had done a lot of basic research, and Charlie Feltz and his team at North American had taken that, added to it, and developed a very robust airframe. North American took Hartley Soule’s comments to Harrison Storms about making errors on the strong side seriously. The airplane ended up a bit overweight, resulting in slightly diminished performance, but it could take a great deal of punishment and survive. The simulation program run by North American and later by the FRC and AFFTC flight planners correctly predicted almost every nuance of the flight program. As the pilots learned to trust the simulator, most of the initial worries disappeared. Still, it was incredible that the program accomplished the envelope expansion so apparently effortlessly.

This is not to say the program did not experience problems. As Bob Hoey remembers, "[T]he X-15 had a significant inertial coupling problem for roll rates that were easily within the capability of the control system. The boundaries were reasonably well established on the simulator, and everyone recognized that there was no need to perform rapid rolls on an X-15 mission, so the pilots were advised ‘don’t do that!’ and they didn’t." The auxiliary power unit provided more than its share of challenges early on, and was never completely satisfactory. The stable platform got off to a marginal start, got better, and then got a lot worse. In the end, a more modern unit originally designed for the canceled X-20 Dyna-Soar replaced it. The ballistic control system was particularly troublesome during the initial flights, so much so that researchers purposely turned it off on some of the early altitude buildup flights. Fortunately, the bugs had been worked out and it performed satisfactorily by the time it was really needed.-50

The XLR99 had its share of minor problems (mainly sensitivity to throttling) and a worrisome habit of shedding some of the insulating coating inside its exhaust nozzle. Then there was the landing gear, which underwent a constant set of modifications right up until the final year of the flight program. In this case, it was not the components’ fault, at least not completely. The airplane was overweight when North American delivered it, and it continued to get heavier over the years. Upgraded struts, skids, nose wheels, tires, and stronger supporting structures never caught up with the weight increases. Still, few of the problems were show-stoppers, and the X-15 program continued at a blistering pace.

Each of the initial X-15 pilots had spent many hours in the fixed-base simulator at North American and had undergone centrifuge training at NADC Johnsville. Prior to his first flight in the X-15, each pilot went through a ground dry run with the X-15 mated to the NB-52 to familiarize himself with the complete prelaunch checklist and cockpit procedures. Each pilot also performed engine runs at the Rocket Engine Test Facility prior to his first X-15 flight. In addition, the pilots flew missions in the NT-33 and JF – 100C variable-stability trainers to become familiar with the low-speed handling characteristics of the X-15. The pilots practiced landings in F-104s, including approaches to each of the uprange lakebeds in service at the time. There should be no surprises.-1511

Ablator Application

There had always been questions about exactly how to apply an ablative coating over the surface of an entire airplane, even one as small as the X-15. Even more questions existed on how to maintain the airplane after applying the coating, and how difficult it would be to refurbish the coating between flights. There appears to have been little actual concern about the effectiveness of the ablator; if it was applied correctly, everyone was relatively sure the concept would work.

As part of its initial contract, Martin Marietta developed a comprehensive procedure for applying the coating, maintaining it, and removing it if necessary. Martin accomplished the first complete application of the ablator in general agreement with the schedule and procedures published earlier. Simply because it represents one of the few attempts to use an ablative coating on an entire airplane, it is appropriate to review the application in detail.-1322

The process began with cleaning the airplane, and Martin admitted the preparatory cleaning was "somewhat overdone" for the first application. Technicians masked all joints, gaps, and openings before the cleaning began to prevent solvent from getting into the airplane. The surface condition of the airplane, with its accumulation of contamination and overabundance of lacquer, necessitated the use of a great deal of solvent during the initial cleaning. Technicians accomplished the final cleaning with powdered cleanser and water using a "water-break-free" test to ascertain when the surface was properly clean. Some areas of the aircraft, especially around fastener heads and skin joints, never did achieve a completely water-break-free condition, and Martin noted that "these areas continually bleed hydraulic fluid or other contamination."[323]

Next, technicians used polyethylene tape to mask all of the seams between panels to keep the ablative material out of the aircraft compartments. The only problem encountered in the initial ablator application was that nobody had anticipated masking the gap between the fixed portion of each vertical stabilizer and the rudders. The installation crew then improvised a solution that was mostly successful. As a means of checking the adequacy of the masking during all phases of ablator operation, technicians placed airborne contamination collectors in nine aircraft compartments before beginning the application process. At the end of the process, quality inspectors from Martin Marietta and NASA checked these collectors and found very little contamination, indicating that the masking worked as expected.-1324

Before turning the airplane over to Martin Marietta, NASA had made a few minor changes to accommodate the ablator installation. The retractable pitot tube (or "alternate pitot" as it was called) was installed, as was a new retaining ring around the ball nose that had a step at its aft end. When the ablator was built up during the application, it would fill up to the top of the step, resulting in a smooth surface.[325]

Next up was installing the molded ablator "details" on the aircraft. This included premolded leading-edge covers made from ESA-3560-NA for the wing and horizontal stabilizers, and covers for various antennas, the canopy leading edge, and the vertical stabilizer leading edge. Although it was not provided as part of the kit, the installation team fabricated a detail for the leading edge of the dummy ramjet instrumentation rake from a spare piece of the vertical stabilizer leading-edge detail.[326]

After technicians glued the details onto the surface of the leading edges, they covered the majority of the airplane with polyethylene sheeting to protect cleaned areas from overspray during the sequential ablator applications. The airplane was broken down into nine distinct areas that technicians would spray in sequence. Technicians installed marker strips (a vinyl foam tape) over the contamination masking and applied a layer of DC93-027 RTV over fastener heads and peripheral gaps of the seldom-removed panels. The installation team then sprayed the MA-25S ablator using a commercial paint spray gun. Controlling the thickness of the ablator was the most significant difficulty encountered during the application process, but the team got much better toward the end as they became more familiar with the deposition characteristics of the material. Some areas, particularly the middle of the wing root and the crown centerline of the fuselage, proved to be too much of a stretch for the technicians standing on the ground. This condition resulted in a "somewhat cheezy" ablator application in those areas, but the layer was deemed adequate to protect the airframe.[327]

Once the entire surface was covered, the next task was to go back and remove the trim marker strips. This proved more difficult than had been expected because the tape was "too thick and possessed too high an adhesive tack." Nevertheless, the team eventually accomplished the task, but decided to use a different tape next time. It was important to avoid disturbing the sealing tape under the marker strips, since it would have to protect the compartments from the effects of the sanding operation still to come.[328]

Ablator Application

Applying the MA-25S ablator was more involved than most expected. The airplane had to be scrubbed clean, and then each individual panel had to be taped to ensure ablator did not get into the airplane. The ablator was then sprayed, sanded to a consistent finish, and its depth measured. The amount of time required to coat the relatively small X-15 did not bode well for a large Space Shuttle. (NASA)

The ablator was left to cure at room temperature for a few hours, and then technicians sanded the entire surface to remove overspray and irregularities, and to bring the ablator layer down to within 0.020 inch of its design thickness. This proved to be a very tedious operation. First, the team had to draw grid lines on the airplane to establish precise monitor locations, and a penetrating needle dial gage determined the thickness at each point on the grid. Technicians then sanded the surface. Since this removed the grid lines, they would have to be redrawn and the thickness rechecked. The process continued until the desired thickness was reached. It was evident that there was a need for a better way to establish the grid on the airplane.[329]

When the sanding was finished, the team glued 10 test plugs to the ablator surface and cut through the ablator layer around their periphery. A pull test was performed on the plugs to determine whether the ablator had properly bonded to the skin. The first application successfully passed all of its pull tests. Various "hard point inserts" were then installed around the external tank inboard sway brace attach points and the aircraft jacking points. Inserts of MA-25S-1

material also covered the ram air door in the fuselage nose and the engine compartment fire doors on the aft fuselage.-1330!

MA-25S had a natural pinkish color and somehow this seemed inappropriate for the world’s fastest airplane. Fortunately, the specification called for a layer of Dow Corning DC90-090 RTV over the entire airplane to provide a wear coating and to seal the ablator. The DC90-090 was translucent white and did not completely hide the pink, so NASA asked Martin Marietta to apply an extra coat (or two, in some areas) so that the airplane would have a uniform white finish. This exhausted the available supply of the coating; however, Dow Corning had replaced DC90-090 with a similar product called DC92-007. Martin requested samples of the new product to determine its suitability as a substitute.-333-

At this point, the team applied a limited number of hazard and warning markings to the exterior using standard high-temperature aircraft lacquer paint. The last step was to remove the polyethylene tape that sealed the service panels and install strips of MA-25S-1 around their periphery to provide extra durability during panel removal and replacement. Martin then returned the airplane to the X-15 maintenance crews, who installed instrumentation and prepared it for flight.-1332-

Bug-Eye Camera Bays

As completed, each X-15 had four "bug-eye" structural camera bays, named for their odd shape.-65 Two were located on top of the fuselage just behind the cockpit, and two were under

the center-of-gravity compartment. Originally, each bay held a 16-mm motion picture camera that ground personnel could aim through a limited field of view to observe the fuselage, wings, or stabilizers. Over the course of the program, researchers used these bays to house a variety of other equipment. Sometimes the bug-eye fairings above the fuselage provided a viewing port for the experiments or simply provided extra volume, and at other times flush plates covered the area. The lower bays were usually faired over later in the flight program with the internal space used by experiments or data recorders.

Although it was not truly an experiment, the National Geographic Society occasionally provided cameras for the upper bug-eye camera bays. Photos looking back at the vertical stabilizer of the X-15 with the curve of the Earth in the background are more often than not ones taken by the Society’s cameras.[66]

Early Experiments

During the early portion of the flight program, various small experiments were piggybacked onto the airplanes as time and space permitted. These usually required little, if any, support from the airplane or pilot during the mission since the flight program was concentrating on acquiring aero – thermo and stability and control data.-67

Another Ramjet

Despite the HRE debacle, Marquardt did not give up easily. Although NASA had ruled out a ducted rocket in 1963, Marquardt managed to generate enough interest in the concept to get a study contract from the FRC in early 1964, and the AARPS came back as a separate study. The designers of the AARPS contemplated the use of advanced air-breathing propulsion cycles, such as ducted rockets and ejector ramjets. NASA awarded Marquardt a small contract to define a research and development plan for AARPS and to determine the feasibility and usefulness of flight-testing the system on X-15A-2.2821

On 3 January 1967, the company proposed a different ramjet installation for X-15A-2, actually providing power to allow a "cruise capability of approximately Mach 5." The company proposed installing an "ejector ramjet on X-15A-2 in the area now occupied by the rocket engine." The ramjet would use jet fuel and liquid oxygen as propellants, although hydrogen peroxide was listed as an alternate oxidizer.-12831

The gross weight of the airplane would increase 2,571 pounds (from 35,735 to 38,306 pounds). The amount of liquid oxygen would remain constant at 10,533 pounds, but 9,400 pounds of jet fuel would replace the normal 8,199 pounds of anhydrous ammonia. The propulsion system weight would increase from 910 pounds (XLR99) to 2,280 pounds (1,380 pounds for the engine and 900 pounds for the inlet). On the airplane itself, the liquid-oxygen tank would remain unchanged, but the proposal modified the existing ammonia tank to allow room for the inlet ducting. Unfortunately, Marquardt did not specify how it would accomplish this, given that the ammonia tank was a full-monocoque structural member of the fuselage.-2841

XLR99 (but well above the 16,000 Ibf provided by the interim XLR11s). Marquardt estimated that the acceleration to Mach 5 would take 4.21 minutes at 1.04 g (much slower than the 90 seconds or so it normally took to get to Mach 6), covering approximately 150 miles in the process. Once at Mach 5 the ramjet would provide 14.8 minutes of steady-state cruise, covering 840 miles. This

1,0- mile flight would have necessitated a major extension to the High Range, and might well have exceeded the heat-sink capability of the Inconel structure, even with an ablative coating.[285]

Marquardt also suggested that the engine could be adapted to the delta-wing airplane, and that in the future a modified engine could provide additional cruise performance. In either case, the engine featured a large rectangular inlet located under the fuselage that started slightly ahead of the wing. The inlet duct swept upward into the fuselage just ahead of the ventral stabilizer, explaining the required modifications to the fuel tank. The inlet faired into the ventral, and the ramjet engine was located where the normal XLR99 had been.[286]

It appears that NASA did not take any action based on the study results.


Jack McKay flew the X-15 for 70 months from 28 October 1960 until 8 September 1966, making 29 flights. These included two flights with the XLR11 and 27 with the XLr99. McKay reached Mach 5.65, a maximum speed of 3,938 mph, and an altitude of 295,600 feet. He made three emergency landings in the X-15, and although was seriously injured on one of them, he returned to fly 22 more X-15 missions.

John Barron "Jack" McKay was born on 8 December 1922 in Portsmouth, Virginia, and graduated from Virginia Polytechnic Institute in 1950 with a bachelor of science degree in aeronautical engineering. During World War II he served as a Navy pilot in the Pacific, earning the Air Medal with two oak leaf clusters and a Presidential Unit Citation while flying F6F Hellcats.

He joined the NACA on 8 February 1951 and worked at Langley as an engineer for a brief period before transferring to the HSFS, where he flew the F-100, YF-102, F-102A, F-104, YF-107A, D – 558-1, D-558-2, X-1B, and X-1E. With the exception of Scott Crossfield, McKay accumulated more rocket flights than any other U. S. pilot (46 flights before he joined the X-15 program). As Milt Thompson remembers, "Jack was an excellent stick and rudder pilot, possibly the best of the X-15 pilots." McKay retired from the NASA on 5 October 1971 and died on 27 April 1975, mostly from late complications resulting from his X-15 crash. On 23 August 2005, NASA presented McKay’s family with a set of astronaut wings, honoring MacKay’s high-altitude flight in the X-