Category X-15 EXTENDING THE FRONTIERS OF FLIGHT

The WADC evaluation of the NACA proposal arrived at ARDC Headquarters on 13 August. Colonel Victor R. Haugen, director of the WADC laboratories, reported that his organization believed the proposal was technically feasible. The only negative comment referred to the absence of a suitable engine. The WADC estimated that the development effort would cost $12,200,000 and take three or four years. The cost estimate included $300,000 for studies, $1,500,000 for design,

$9,500,000 for the development and manufacture of two airplanes, $650,000 for engines and other government-furnished equipment, and $250,000 for modifications to a carrier aircraft. Somewhat prophetically, one WADC official commented informally: "Remember the X-3, the X-5, [and] the X-2 overran 200%. This project won’t get started for twelve million dollars."-81

A four-and-a-half-page paper titled "NACA Views Concerning a New Research Airplane," released in late August 1954, gave a brief background of the problem and attached the Langley study as a possible solution. The paper listed two major problems: "(1) preventing the destruction of the aircraft structure by the direct or indirect effects of aerodynamic heating; and (2) achievement of stability and control at very high altitudes, at very high speeds, and during atmospheric reentry from ballistic flight paths." The paper concluded by stating that the construction of a new research airplane appeared to be feasible and needed to be undertaken at the earliest possible opportunity.-^

A meeting between the Air Force, NACA, Navy, and the Office of the Assistant Secretary of Defense for Research and Development took place on 31 August 1954. There was general agreement that research was needed on aerodynamic heating, "zero-g," and stability and control issues at Mach numbers between 2 and 7 and altitudes up to 400,000 feet. There was also agreement that a single joint project was appropriate. The group believed, however, that the selection of a particular design (referring to the Langley proposal) should not take place until mutually satisfactory requirements were approved at a meeting scheduled for October.-101

Also on 31 August, and continuing on 1 September, a meeting of the NACA Subcommittee on High-Speed Aerodynamics was held at Wallops Island. Dr. Allen E. Puckett from the Hughes Aircraft Company was the chair. John Stack from Langley gave an overview of the proposed research airplane, including a short history of events. He reiterated that the main research objectives of the new airplane were investigations into stability and control at high supersonic speeds, structural heating effects, and aeromedical aspects such as human reactions to weightlessness. He also emphasized that the performance of the new airplane must represent a substantial increment over existing research airplanes and the tactical aircraft then under development. In response to a question about whether an automatically controlled vehicle was appropriate, Stack reiterated that one of the objectives of the proposed program was to study the problems associated with humans at high speeds and altitudes. Additionally, the design of an automatically controlled vehicle would be difficult, delay the procurement, and reduce the value of the airplane as a research tool.-11 design of the airplane" and that the establishment of a design competition was the most desirable course of action. The subcommittee forwarded the recommendation to the Committee on Aerodynamics for further consideration.-1121

Major General Floyd B. Wood, the ARDC deputy commander for technical operations, forwarded an endorsement of the NACA proposal to Air Force Headquarters on 13 September 1954, recommending that the Air Force "initiate a project to design, construct, and operate a new research aircraft similar to that suggested by NACA without delay." Wood reiterated that the resulting vehicle should be a pure research airplane, not a prototype of any potential weapon system or operational vehicle. The ARDC concluded that the design and fabrication of the airplane would take about 3.5 years. In a change from how previous projects were structured, Wood suggested that the Air Force should assume "sole executive responsibility," but the research airplanes should be transferred to the NACA after a short Air Force airworthiness demonstration program.-121

During late September, John R. Clark from Chance-Vought met with Ira H. Abbot at NACA Headquarters and expressed interest in the new project. He indicated that he personally would like to see his company build the aircraft. It was ironic since Chance-Vought would elect not to submit a proposal when the time came. Many other airframe manufacturer representatives would express similar thoughts, usually with the same results. It was hard to see how anybody could make money building only two airplanes.141

The deputy director of research and development at Air Force Headquarters, Brigadier General Benjamin S. Kelsey, confirmed on 4 October 1954 that the new research airplane would be a joint USAF-Navy-NACA project with a 1-B priority in the national procurement scheme and $300,000 in FY55 funding to get started.15

At the same time, the NACA Committee on Aerodynamics met in regular session on 4 October 1954 at Ames, with Preston R. Bassett from the Sperry Gyroscope Company as chairman. The recommendation forwarded from the 31 August meeting of the Subcommittee on High-Speed Aerodynamics was the major agenda item. The following day the committee met in executive session at the HSFS to come to some final decision about the desirability of a manned hypersonic research airplane. During the meeting, various committee members, including De Elroy Beeler,

Walt Williams, and research pilot A. Scott Crossfield, reviewed historic and technical data. Williams’s 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 was a very strong influence in getting the X-15 program launched in the right direction." Williams would later do the same for Project Mercury.161

The session at the HSFS stirred more emotion than the earlier meeting in Washington. First, Beeler discussed some of the more general results obtained previously with various research airplanes. Then Milton B. Ames, Jr., the committee secretary, distributed copies of the NACA "Views" document. Langley’s associate director, Floyd Thompson, reminded the committee of the major conclusion expressed by the Brown-O’Sullivan-Zimmerman study group in June 1953: that it was impossible to study certain salient aspects of hypersonic flight at altitudes between 12 and 50 miles in wind tunnels due to technical limitations of the facilities. Examples included "the distortion of the aircraft structure by the direct or indirect effects of aerodynamic heating" and "stability and control at very high altitudes at very high speeds, and during atmospheric reentry from ballistic flight paths." The study admitted that the rocket-model program at Wallops Island could investigate aircraft design and operational problems to about Mach 10, but this program of subscale models was not an "adequate substitute" for full-scale flights. Having concluded that the

Brown group was right, and that the only immediate way known to solve these problems was to use a manned aircraft, Thompson said that various NACA laboratories had then examined the feasibility of designing a hypersonic research airplane. Trying to prevent an internal fight, Thompson explained that the results from Langley contained in the document Milton Ames had just distributed were "generally similar" to those obtained in the other NACA studies (which they were not), but were more detailed than the other laboratories’ results (which they were).[17]

Williams and Crossfield followed with an outline of the performance required for a new research airplane and a discussion of the more important operational aspects of the vehicle. At that point, John Becker and Norris Dow took over with a detailed presentation of their six-month study.

Lively debate followed, with most members of the committee, including Clark Millikan and Robert Woods, strongly supporting the idea of the hypersonic research airplane.

Surprisingly, Clarence L. "Kelly" Johnson, the Lockheed representative, opposed any extension of the manned research airplane program. Johnson argued that experience with research aircraft had been "generally unsatisfactory" since the aerodynamic designs were inferior to tactical aircraft by the time research flights began. He felt that a number of research airplanes had developed "startling performances" only by using rocket engines and flying essentially "in a vacuum" (as related to operational requirements). Johnson pointed out that "when there is no drag [at high altitude], the rocket engine can propel even mediocre aerodynamic forms to high Mach numbers." These flights had mainly proved "the bravery of the test pilots," Johnson charged. The test flights generated data on stability and control at high Mach numbers, Johnson admitted, but aircraft manufacturers could not use much of this information because it was "not typical of airplanes actually designed for supersonic flight speeds." He recommended that they use an unmanned vehicle to gather the required data instead of building a new manned airplane. If aeromedical problems became "predominant," Johnson said, a manned research airplane could then be designed and built, and it should have a secondary role as a strategic reconnaissance vehicle.[18]

Clarence L "Kelly"Johnson, the legendary founder of the Lockheed Skunk Works, was the only representative on the NACA Committee on Aerodynamics to vote against proceeding with the development of the X-15. Previous X-plane experience had left Johnson jaded since the performance of the research airplanes was not significantly advanced from operational prototypes. As it turned out, the X-15 would be the exception, since no operational vehicle, except the Space Shuttle, has yet approached the velocity and altitude marks reached by the X-15. (Lockheed Martin)

into flight research in the shortest time possible." In comparing manned research airplane operations with unmanned, automatically controlled vehicles, Crowley noted that the X-1 and other research airplanes had made hundreds of successful flights despite numerous malfunctions.-1191 In spite of the difficulties—which, Crowley readily admitted, had occasionally caused the aircraft to go out of control—research pilots had successfully landed the aircraft an overwhelming percentage of the time. In each case the human pilot permitted further flights to explore the conditions experienced, and in Crowley’s opinion, automated flight did not allow the same capabilities.-1291

After some further discussion, and despite Johnson’s objections, the committee passed a resolution recommending the construction of a hypersonic research aircraft:1211

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The "requirements" of the resolution conformed to the conclusions from Langley, but were sufficiently general to encourage fresh approaches. Appended to the specification under the heading of "Suggested Means of Meeting the General Requirements" was a section outlining the key results of the Becker study.1221

Kelly Johnson was the only member to vote nay. Sixteen days after the meeting, Johnson sent a "Minority Opinion of Extremely High Altitude Research Airplane" to Milton Ames with a request that it be appended to the majority report, which it was.1231

On 6 October 1954, Air Force Headquarters issued Technical Program Requirement 1-1 to initiate a new manned research airplane program "generally in accordance with the NACA Secret report, subject: ‘NACA Views Concerning a New Research Aircraft’ dated August 1954." The entire project was classified Confidential. The ARDC followed this on 26 October with Technical Requirement 54 (which, surprisingly, was unclassified).1241

In the meantime, Hartley Soule and Clotaire Wood held two meetings in Washington on 13 October. The first was with Abraham Hyatt at the Navy Bureau of Aeronautics (BuAer) to obtain the Navy’s recommendations regarding the specifications. The only significant request was that provisions should exist to fly an "observer" in place of the normal research instrumentation package. This was the first (and nearly the only) official request from the Navy regarding the new airplane, excepting the engine. In the second meeting, Soule discussed the specifications with Colonel R. M. Wray and Colonel Walter P. Maiersperger at the Pentagon, and neither had any significant comments or suggestions.

With an endorsement in hand, on 18 October Hugh Dryden conferred with Air Force (colonels Wray and Maiersperger) and Navy (Admiral Robert S. Hatcher from BuAer and Captain W. C.

Fortune from the ONR) representatives on how best to move toward procurement. The parties agreed that detailed technical specifications for the proposed aircraft, with a section outlining the Becker study, should be presented to the Department of Defense Air Technical Advisory Panel by the end of the year. The Navy reiterated its desire that the airplane carry two crew members, since the observer could concentrate on the physiological aspects of the flights and relieve the pilot of that burden. The NACA representatives were not convinced that the weight and cost of an observer could be justified, and proposed that the competing contractors decide what was best.

All agreed this was appropriate. Again, the Air Force requested little in the way of changes.-1251

Hartley Soule met with representatives of the various WADC laboratories on 22 October to discuss the tentative specifications for the airplane. Perhaps the major decision was to have BuAer and the Power Plant Laboratory jointly prepare a separate specification for the engine. The complete specification (airplane and engine) was to be ready by 17 November. In effect, this broke the procurement into two separate but related competitions: one for the airframe and one for the engine.

During this meeting, John B. Trenholm from the WADC Fighter Aircraft Division suggested building at least three airplanes, proposing for the first time more than the two aircraft contained in the WADC cost estimate. There was also a discussion concerning the construction of a dedicated structural test article. It seemed like a good idea, but nobody could figure out how to test it under meaningful temperature conditions, so the group deferred the matter.

Also on 22 October, Brigadier General Benjamin Kelsey and Dr. Albert Lombard from Air Force Headquarters, plus admirals Lloyd Harrison and Robert Hatcher from BuAer, visited Hugh Dryden and Gus Crowley at NACA Headquarters to discuss a proposed Memorandum of Understanding (MoU) for conducting the new research airplane program. Only minor changes to a draft prepared by Dryden were suggested.-261 The military representatives told Dryden that a method of funding the project had not been determined, but the Air Force and Navy would arrive at a mutually acceptable agreement for financing the design and development phases. During the 1940s and 1950s it was normal for the military services to fund the development and construction of aircraft (such as the X-1 and D-558, among others) for the NACA to use in its flight research programs. The aircraft resulting from this MoU would be the fastest, highest-flying, and by far the most expensive of these joint projects.

The MoU 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. The New Developments Office of the Fighter Aircraft Division at Wright Field would manage the development phase of the project. The NACA would conduct the flight research, and the Navy was essentially left paying part 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 would disseminate the research results to the military services and aircraft industry as appropriate based on various security considerations. The concluding statement on the MoU was, "Accomplishment of this project is a matter of national urgency."[27]

The final MoU was originated by Trevor Gardner, Air Force Special Assistant for Research and Development, in early November 1954 and forwarded for the signatures of James H. Smith, Jr., Assistant Secretary of the Navy for Air, and Hugh L. Dryden, director of the NACA, respectively. Dryden signed the MoU on 23 December 1954 and returned executed copies to the Air Force and Navy.[28]

John Becker, Norris Dow, and Hartley Soule made a formal presentation to the Department of Defense Air Technical Advisory Panel on 14 December 1954. The panel approved the program, with the anticipated $12.2 million cost coming from Department of Defense contingency funds as well as Air Force and Navy research and development funds.-129

After the Christmas holidays, on 30 December, the Air Force sent invitation-to-bid letters to Bell, Boeing, Chance-Vought, Convair, Douglas, Grumman, Lockheed, Martin, McDonnell, North American, Northrop, and Republic. Interested companies were asked to attend the bidders’ conference on 18 January 1955 after notifying the procurement officer no later than 10 January. An abstract of the NACA Langley study was attached with a notice that it was "representative of possible solutions" but not a requirement to be satisfied.-129

Also accompanying the invitation-to-bid letters was a simple chart that showed the expected flight trajectory for the new research airplane. It was expected that each flight would provide about 130 seconds of good research data after engine burnout. This performance was almost exactly duplicated by the X-15 over the course of the flight program. (NASA)

This was undoubtedly the largest invitation-to-bid list yet for an X-plane, but many contractors were uncertain about its prospects. Since it was not a production contract, the potential profits were limited. Given the significant technical challenges, the possibility of failure was high. Of course, the state-of-the-art experience and public-relations benefits were potentially invaluable. It was a difficult choice even before Wall Street and stock prices became paramount. Ultimately, Grumman, Lockheed, and Martin expressed little interest and did not attend the bidders’ conference, leaving nine possible competitors. At the bidders’ conference, representatives from the remaining contractors met with Air Force and NACA personnel to discuss the competition and

the basic design requirements. The list of participants read like a Who’s Who of the aviation world. Robert Woods and Walter Dornberger from Bell attended. Boeing sent George Martin, the designer of the B-47. Ed Heinemann from Douglas was there. Northrop sent William Ballhaus.-131

During the bidders’ conference the Air Force announced that each company could submit one prime and one alternate proposal that might offer an unconventional but potentially superior solution. The Air Force also informed the prospective contractors that an engineering study only would be required for a modified aircraft in which an observer replaced the research instrumentation, per the stated Navy preference. A significant requirement was that the aircraft had to be capable of attaining a velocity of 6,600 fps and altitudes of 250,000 feet. Other clarifications included that the design would need to allocate 800 pounds, 40 cubic feet, and 2.25 kilowatts of power for research instrumentation. A requirement that would come back to haunt the procurement was that flight tests had to begin within 30 months of contract award.

Another major piece of government-furnished equipment was the all-attitude inertial system, called a "stable platform" at the time. Early on, researchers realized the performance of the research airplane required a new method to determine altitude, speed, and attitude information. The original Langley study, as well as each of the contractor proposals, had suggested the use of a stable platform. Unfortunately, such as system was not readily available.

A meeting held at Wright Field on 14-15 November 1955 implied that the WADC would furnish the stable platform. Arthur Vogeley, the NACA representative, assumed that the Air Force had already developed a suitable device since his report stated that a newly developed Bendix platform weighed only 28 pounds and occupied less than a cubic foot of volume. Others within the NACA and North American were not as certain. During a meeting with North American personnel, Walt Williams specifically asked who was responsible for the stable platform, and no answer was immediately forthcoming.-1145!

Researchers apparently did not discuss the requirements for a stable platform until 24 May 1956 during a meeting at Langley. In attendance were representatives from Eclipse-Pioneer (a division of Bendix), the NACA, North American, and the WADC. This group discussed the platform mentioned at the November 1955 meeting, and Eclipse-Pioneer acknowledged that it was only a conceptual design and not a forthcoming product. Nevertheless, the meeting attendees thought

that development of a suitable platform would take only 24 months. Since the platform provided research data in addition to flight data, the NACA agreed to charge 40 pounds of the estimated 65-pound weight against research instrumentation. There was no mention as to why the original 28-pound estimate had grown to 65 pounds.-146

Despite its early participation, Eclipse-Pioneer did not exhibit any further interest, so the Flight Control Laboratory asked the Sperry Gyroscope Company if it was interested. By August 1956, Sperry had prepared a preliminary proposal, and on 4 October the X-15 Project Office held a technical briefing for Sperry at Wright Field.147

On 26 December 1956, the Flight Control Laboratory began the process to procure eight inertial flight data systems (six "Type A" units for the X-15 and two "Type B" units for ground research). The laboratory recommended awarding the $1,030,000 contract to the Sperry Gyroscope Company.-1148!

For unexplained reasons, the Air Materiel Command did not take immediate action and did not release a formal request for proposal to Sperry until 6 February 1957. Two weeks later Sperry replied, and the Flight Control Laboratory approved the technical aspects of the proposal on 28 March. In the meantime, however, a controversy had developed over contracting details. The negotiations reached a deadlock on 11 April 1957 and the Air Materiel Command informed the X – 15 Project Office that it intended to find another contractor. The Flight Control Laboratory and X – 15 Project Office argued that Sperry was the only company that stood a chance of meeting the X – 15 flight schedule, but procurements were the domain of the Air Materiel Command and the warnings fell on deaf ears.-1149!

It was evident that the issue was rapidly exhausting the patience of all concerned. On 22 April 1957, the director of development at the WADC, Brigadier General Victor R. Haugen, informed the Air Materiel Command that Sperry was the only company capable of developing the stable platform within the schedule constraints of the X-15 program. Having a general officer intervene was apparently the answer, and a cost-plus-fixed-fee contract signed on 5 June 1957 provided $1,213,518.06 with an $85,000 fee.156

Because of the contracting delays, the expected December 1958 delivery of the initial Sperry unit would not support the first flight of the X-15. This was not a significant problem since the initial X-15 flights would be low and slow enough to use a standard NACA flight test boom to provide the data ultimately supplied by the stable platform and ball nose. In fact, the NACA would likely have used the flight test boom even if the other instruments had been available, since it provided a known, calibrated source for acquiring initial air data. Most experimental aircraft use similar booms during early testing.151

More disturbing, however, was that it quickly became apparent that the weight of the stable platform had been seriously underestimated. In May 1958, Sperry undertook a weight-reduction program that, unfortunately, was particularly unsuccessful. By August, Sperry was reporting that the weight was approximately twice the original specification.157

It was just the beginning of serious trouble. By June 1958, the estimated cost was up to $2,741,375 with a $105,000 fee. Less than a year later the cost reached $3,234,188.87 with an $119,888 fee, mostly due to efforts to reduce the weight of the stable platform.157

specification, the system weighed 185.25 pounds. An alternate shock mount that did not meet the requirements but was probably acceptable brought the weight down to 165.25 pounds. Interestingly, Sperry admitted it had known about the weight problem for some time, but did not explain why it had not brought the issue to the government’s attention at an earlier date.-1154

Sperry defended its actions by listing the changes it had made to eliminate excess weight. These included substituting aluminum for stainless steel in some locations, reducing the thickness of various covers, and reducing component weight wherever practical. The need to include power supplies not anticipated in the original proposal also increased the weight of the system. Finally, Sperry also concluded that the stable platform was lighter and more accurate than any competing system. Apparently, Sperry’s justification was satisfactory since the X-15 Project Office accepted that the system was going to remain overweight and took no further action on the subject.[155]

As finally delivered, the stable platform was an Earth-slaved, Schuler-tuned system aligned in azimuth to a guidance vector coincident with X-15 centerline. The unit provided attitude, velocity, and altitude to the pilot with reference to these coordinate systems. There were three major components to the stable platform: the stabilizer, computer, and displays. Together they weighed approximately 165 pounds, occupied about 3 cubic feet of volume, and required a peak electrical load of 600 W. The stabilizer used three self-balancing accelerometers and three single-degree – of-freedom gyroscopes. A four-gimbal system provided complete attitude freedom in all axes.

An analog computer computed velocity and position data, and applied the necessary acceleration corrections. The computer was shock-mounted and shaped to conform to the contours of the X – 15 instrumentation compartment. Gaseous nitrogen from the X-15 cooled the stabilizer and computer to counteract the internal heat generated by the units, and the extreme external temperatures. The system was "designed to operate over a limited portion of the Earth’s surface." Specifically, it could accept a launch point anywhere within a 275-mile-wide corridor extending 620 miles uprange and 205 miles downrange from Edwards AFB.[156]

Sperry shipped the first stabilizer and computer to Edwards in late January 1959, and the Air Force intended to use the NB-52 carrier aircraft as a test vehicle. This was delayed for unknown reasons, so the Air Force made a KC-97 that was already being used for similar purposes by the Convair B-58 program available to the X-15 project. The first flights in the KC-97 took place in late April, but were of limited value given the low speed of the piston-powered Stratocruiser. In June 1959, North American successfully installed the Sperry system in X-15-3 prior to its delivery to Edwards. By the end of May 1960, there were four complete stable platforms at Edwards: one in X-15-1, one in X-15-3, one spare, and one undergoing repair.-1157

As delivered, the stable platforms could provide the following data:[158]

However, Sperry had made several compromises during the development of the X-15 stable platform, either to meet schedule or reduce weight. The designers knew that 300 seconds after launch (i. e., as the airplane decelerated to land) the pressure instruments would be adequate for vehicle altitude and velocity data, and that a system capable of operating from carrier aircraft takeoff to X-15 landing would be too heavy and bulky for the X-15. The final design had a very limited operating duration. The pilot aligned the system just before the X-15 separated from the NB-52, and the stable platform provided just 300 seconds of velocity and altitude data, along with 20 minutes of attitude data. This limited operating duration provided some relief for the weight problem.-115^

As it turned out, the lighter shock mount developed by Sperry was not adequate for the X-15. It performed fine during the XLR11 flights, but vibration tests in October 1960 prior to the beginning of XLR99 tests showed that the mount would not withstand more than 1.5 g at 110 cycles. North American redesigned the mount, since by this time saving weight had become a non-issue for the most part; having a reliable airplane was worth more than the few miles per hour the weight cost.

Over the course of the flight program, the stable platform was the subject of several other changes that greatly improved its reliability. Many of these were the result of suggestions from John Hursh at the MIT Instrumentation Laboratory and Dr. Allen Smith from Ames, both of whom spent a great deal of time at the Flight Research Center during late 1960 working on the problems. As an example of these changes, NASA changed all critical germanium transistor amplifiers to silicon during November 1960. NASA also made changes to operating procedures as well as to hardware. Initially, a gyroscope failure required that the entire stable platform be returned to Sperry for repair, taking the unit off flight status for three to six weeks. In response, the FRC developed an in-house repair capability that significantly shortened turnaround times. Even better, during late 1960 NASA substituted a higher-quality gyroscope manufactured by Minneapolis-Honeywell, which resulted in fewer failures.-1160

The X-15 was one of the first aircraft to require what is today called an inertial measurement unit, or stable-platform. Gyroscopes of the era were large, heavy, and consumed a considerable amount of power. This model shows the three interlocked rings required to determine position in three dimensions. (NASA)

In retrospect, the performance specifications established in 1956 were well beyond the state of the art with respect to available gyros, accelerometers, transistors, and circuit techniques.

However, the system as originally built was able to perform at levels that, although marginal or subpar compared to the original specification, still allowed the X-15 to realize its full performance capabilities. Compared to modern laser-ring-gyro and GPS-augmented systems, the X-15 stable platform was woefully inaccurate, but it routinely bettered its 70-fps error specification for velocity. Initially its altitude-measuring ability was somewhat substandard, averaging about 2,200 feet (rms) uncertainty. The requirement was 2,000 feet, but the system eventually improved and met its specification. Reliability was initially poor, but by mid-1961 the overall reliability was approaching the high 90th percentile, with the altimeter function proving to be the most unreliable. Unfortunately, this improved reliability proved to be short-lived.-1161

The initial operational experience with the stable platform showed that it had a large error potential that grew as time passed from the initial alignment due to drift and integration noise. The unit integrated velocities to provide distance (X, Y, and Z) and specifically altitude, which had even more error buildup with time. Early flight tests showed that the displayed velocities were marginal even after the 90-second engine burn, and that the altitude was undependable for determining peak altitude or reentry setup. Because of this, the flight planners and pilots began to consider two other sources for controlling the energy imparted to the airplane: 1) engine burn time, as measured by a stopwatch in the NASA-1 control room, and 2) radar-measured velocity,

[1621

as displayed in the control room.

For the first government flight (2-13-26) with the XLR99 engine, the flight planners decided to use radar velocity as the primary indication with a radio call to Bob White at the desired engine shutdown condition. After the successful flight, researchers calculated that the airplane had exceeded the intended speed by about half a Mach number. Further analysis showed that the radar velocity display in the control room incorporated considerable smoothing of the data to provide a readable output. This introduced a lag of 4 seconds between the actual speed and the displayed speed, thus accounting for the overshoot. For the next few flights, NASA-1 started a stopwatch in the control room at the indication of chamber pressure on the telemetry, and radioed the pilot when it was time to shut down the engine.-163

Using a stopwatch to measure powered flight time proved to be the simplest and yet most accurate method of controlling energy, so a stopwatch was installed in the cockpit of all three airplanes. A signal from the main propellant valves started and stopped the stopwatch so that it displayed the total burn time even after shutdown. The pilot could then assess whether he had more or less energy than planned, and evaluate his energy condition and best emergency lake in the event of a premature shutdown. Although the reliability of the stable platform increased considerably during the course of the program and was eventually operating within its design specifications, the pilots continued to use the stopwatch (with a backup stopwatch in the control room) for most flights. It was cheap and easy, and almost never failed.164

By 1963 an increasing number of stable platform failures began to occur—some because of design deficiencies, others simply due to component deterioration. This led to NASA placing a new set of restrictions on X-15 flights, keeping them below 160,000 feet. Progress by Sperry to resolve the issues was slow, so an analysis was undertaken at the FRC to determine what in-house efforts could be made to bolster system performance and improve reliability.163

Beginning in late 1963, the FRC began redesigning critical components to improve both accuracy and reliability. Eventually, NASA engineers redesigned some 60% of the subassemblies in the stable platform. Overall, the volume used by the accelerometers, accelerometer electronics, and power supplies was reduced over 50%, and an accompanying reduction in power and cooling requirements was also realized. Although some of the improvements resulted from correcting deficiencies in the original design, most were achieved because the state of the art had improved considerably in the four years since work had begun. NASA completed the initial redesign efforts on the accelerometer loops and power supplies during the summer of 1964, and the first flight of the new components was in X-15-2 on 14 August 1964 (2-33-56). Technicians subsequently installed the revised components in X-15-3 also. This system allowed NASA to cancel the 160,000-foot altitude restriction on the airplanes.

Although the initial performance of the revised components was a little erratic, the increase in accuracy was substantial. For instance, 400 seconds into the flight the original system would have a +8,000-foot error in altitude; the revised system generally had a -1,000-foot error. (In both cases the specification required a less than -5,000-foot error; nothing on the positive side was satisfactory.) Eventually the engineers tuned the erratic performance out of the system. By May 1966, components designed at the FRC had essentially replaced the entire Sperry stable platform, and the system was redesignated the "FRC-66 Analog Inertial System."166

and a set of pilot displays. This system was even smaller and required less power and cooling than the redesigned FRC-66 analog system. In addition, the X-20 IFDS could automatically erect itself and perform an alignment cycle on the ground while the NB-52 was taxiing, and completely eliminated the need for information from the N-1 compass and APN-81 Doppler radar on the NB – 52. This made it somewhat easier to pilot the carrier aircraft as the X-15 approached the launch position; the APN-81 took 90 seconds to stabilize after even a gentle turn, requiring the NB-52 pilot to think well ahead of the drop time. To improve accuracy, however, the IFDS altitude loop was synched to the NB-52 pressure altimeter until 1 minute before launch.-1167

The inertial measurement unit was a gyrostabilized, four-gimbaled platform that maintained local vertical orientation throughout the flight. The inner platform contained three pendulous accelerometers that formed an orthogonal triad. The coupler electronics unit contained the power supplies and interface equipment, and a dual-function digital computer performed all computations. NASA first checked out the digital system in X-15-1 on 15 October 1964 (1-50­79), with satisfactory results.1168

The overall performance of the IFDS during its first 16 flight attempts was excellent, with only two failures. However, problems with the IFDS caused two attempted launches in a row (1-A-105 and 1-A-106) to abort during June 1966. After the first abort, technicians replaced a relay and fixed a loose wire, but the second flight attempt a week later ended the same way. Engineers from Autonetics (a division of North American), Honeywell, the FRC, and Wright Field began investigating the problem. The failures were determined to be the result of yet more wiring problems, all easily corrected.-168

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Because of limitations in both the gyroscopes and onboard computers, the X-15 stable platform could only function for a limited amount of time in a 275-mile-wide corridor extending 620 miles uprange and 205 miles downrange from Edwards AFB. Later modifications to the system were more reliable and versatile, and at the end of the program, two of the X-15s were using digital inertial flight data systems developed for the Air Force Dyna-Soar program. (NASA)

At the same time, the installation in X-15-3 was not going as well as it had in X-15-1. On 6 January 1965, representatives from Honeywell met with FRC personnel to discuss problems with the installation. There were four primary concerns: cooling and thermal conditions, space availability, cabling, and the interface to the MH-96 adaptive control system. This latter issue was surprising since the X-20 also used a version of the MH-96. Also discussed was the relative accuracy expected from the new system versus data from the ball nose. It was pointed out by the

Honeywell representative that at low velocities there would be a significant difference between the IFDS-computed angle of sideslip and that sensed by the ball nose, but at high velocities the difference should be small.[170]

By April 1965 the FRC had made little progress installing the system in X-15-3, and only X-15-1 was flying with the Honeywell inertial system. Fortunately, by this time the modified Sperry systems were proving to be reliable, and no substantial problems had been experienced by X-15- 2 or X-15-3 since December 1964. Engineers finally installed the Honeywell IFDS in X-15-3 during a weather-induced down period at the end of 1965.-1171

Although the Honeywell IFDS was considered an improvement over the modified Sperry stable platform, the FRC decided that the FRC-66 system was preferred for the Mach 8 flights in the modified X-15A-2, so that airplane never received an IFDS. By the end of 1965, engineers had modified one of the Sperry computers to have Mach 8 scaling coefficients in preparation for the X-15A-2 envelope-expansion program.-1172

The improvements did not stop there. Eventually the FRC modified X-15-3 to include an Ames – developed guidance system that was applicable to future aerospace vehicles. This system coupled the IFDS inertial system, MH-96 adaptive control system, and ball nose to an Alert digital computer to investigate boost guidance command techniques. The navigation functions continued to be performed by the inertial system while the Alert computer handled the research objectives, including providing new displays to the pilot. This program allowed the pilot to fly a velocity – altitude window during boost, a bounded corridor during hypersonic cruise, and a precise corridor during reentry. It was an advanced system, and one that Space Shuttle only duplicated in its waning years.-1173

Although the XLR99 proved to be a remarkably capable research engine given its relatively short development period and limited operational experience, proposals were made from time to time to replace it. Usually these revolved around the idea of using a derivative of the Aerojet LR91 engine. In October 1966, Aerojet-General submitted an unsolicited proposal to North American that detailed the use of the LR91-AJ-7 engine in the X-15. Aerojet probably intended the proposal to support the concept of using an LR91 in the delta-wing modification.12^

The LR91 powered the Titan II ICBM, the Titan II Gemini Launch Vehicle, and the Titan III family of space launch vehicles. Aerojet had delivered over 180 engines at the time of the proposal, and had run more than 1,400 engine tests. The engine was man-rated for the Gemini application and the Titan IIIM developed for the Manned Orbiting Laboratory (MOL). The LR91-AJ-7 developed 100,000 lbf at 250,000 feet using nitrogen tetroxide and Aerozine-50 propellants.12^

Aerojet believed that the engine offered several advantages for the X-15. The storable propellants provided a higher bulk density, allowing additional specific impulse to be stored in the same volume, although Aerojet suggested limiting the X-15 to 92 seconds of powered flight. The propellants also eliminated the liquid-oxygen top-off system in the NB-52s since they had a very low boil-off rate and would not have to be replenished in flight. An autogenous pressurization system provided tank pressurization gases from the engine in proportion to propellant consumption, eliminating the need for separate pressurization gases and their mechanical systems (regulators, valves, etc.).130-

Aerojet pointed out that since the engine was in large-scale (for a rocket engine) and continuous production, costs would be lower, and a continuous-improvement program was in place that could benefit the X-15 program. The major changes to the LR91 configuration for the X-15 included modifying it to operate in a horizontal attitude and strengthening the engine to allow it to be reusable. These changes (especially the one to allow horizontal operation) were not as straightforward as they might seem, and a simple description of them took several pages. The modifications to make the engine reusable also took several pages to describe. Nevertheless, Aerojet believed it could provide an engine quickly-beginning by July 1967 allowed the first X-15 flight in March 1969.131

The government did not take any action on this proposal or others made along similar lines. Although working with liquid oxygen and anhydrous ammonia presented some issues for the ground crews, it was decidedly simpler than dealing with the hypergolic propellants in the LR91. Moreover, nobody readily believed that the engine would be as reliable and reusable as the XLR99 without a major development effort, something the X-15 program could not afford. Although an additional 40,000 pounds of thrust would have more than restored the performance lost due to the continual weight gains on the X-15, in the final analysis it just was not worth the time and money. Maybe it would have been worth it for the delta wing; but then, perhaps not.

In addition to the NB-52s there were numerous chase and support aircraft, mostly provided by the Air Force. The number of chase aircraft differed depending on what the flight profile looked like. The program generally used three chase aircraft on the early low-speed X-15 flights, four on most research flights, and five for the very long-range flights. Of course, all things were variable and additional chase aircraft were not uncommon, particularly during the middle years of the program.

Chase-1 was the prelaunch chase, and was usually a North American F-100F Super Sabre during the early years and a Northrop T-38A Talon later, although NASA used a Douglas F5D Skyray on a couple of occasions. Al White frequently flew this chase during the North American flights, but an Air Force pilot generally flew the airplane once the government took over. Chase-1 took off with the NB-52 and flew formation during the climb-out and cruise to the launch lake. The chase pilot visually verified various parts of the X-15 checklist, such as control surface movements, propellant jettison, ballistic system checks, APU start, and engine priming. The use of the F-100 presented some problems at the beginning of the program because the aircraft could not maintain a low enough speed to fly formation with the NB-52 during a right-hand turn; however, the T-38 proved to be more satisfactory.

Chase-2 was the launch chase and provided assistance for the X-15 pilot in the event of an emergency landing at the launch lake. Chase-2 was usually a Lockheed F-104 Starfighter flown by either another X-15 pilot or a NASA test pilot. The F-100 and T-38 could not produce enough drag to fly the steep final approach used by the X-15, which largely dictated the use of the Starfighter for this role. Conversely, the F-104 could not cruise at 45,000 feet due to its high wing loading, which made it unsuitable as Chase-1. Chase-2 normally stayed below 35,000 feet until 3 minutes before launch, and then went into afterburner and climbed to 45,000 feet just before the X-15 dropped. The pilot trailed the NB-52 during launch and then tried to keep up with the X-15 as it left the launch lake area. It was a futile gesture, but it proved useful on the few occasions in which the X-15 engine failed soon after ignition.

Chase-3 covered landings at the intermediate lakebeds and was usually an F-104 flown by either another X-15 pilot or an Air Force test pilot. Unlike Chases 1 and 2, which took off with the NB – 52, Chase-3 waited until 30 minutes before X-15 launch to take off so that it would have enough fuel to loiter for a while. On flight profiles that had multiple intermediate lakes, Chase-3 would orbit between them. In the event the X-15 had to make an emergency landing, the F-104 would attempt to join up to provide support for the X-15 pilot during final approach and touchdown.

For flights out of Smith Ranch there were two intermediate chases, usually called 3 and 4 (the Edwards chase became Chase-5 in these cases).

The Lockheed F-104 Starfighter was used as a chase airplane and to practice landing maneuvers. In addition to the F-104Ns owned by NASA, various F-104s from the Air Force Flight Test Center were used as needed. (NASA)

Chase-4 covered the Edwards landing area, usually with an Air Force pilot. Again, only an F-104 could keep up with the X-15 in the landing pattern. This chase took off at the same time as Chase-3 and orbited 30-40 miles uprange along the flight path. The pilot began accelerating on cue from NASA-1 in an attempt to intercept the X-15 at the maximum possible speed and altitude as the X-15 descended into the Edwards area. Usually the chase pilot took his cues from the vapor trail left as the X-15 pilot jettisoned his residual propellants, since the research airplane was too small and too dark to acquire visually until the chase pilot was right on top of it. Chase-4 would make a visual inspection of the X-15 as it descended and provide airspeed and altitude callouts to the X-15 pilot during the final approach, in addition to verifying that the ventral had successfully jettisoned and the landing gear extended.-1129

Ferrying men and supplies to the contingency landing sites and High Range stations kept the NASA Douglas R4D (C-47/DC-3) Skytrain busy. In addition, the Air Force used Lockheed C-130 Hercules to move fire trucks and other heavy equipment. The C-130s also carried rescue teams during flight operations to ensure help would arrive swiftly in the event of a major accident.

(NASA)

At times there were other chase aircraft, with a photo-chase or a "rover" being the most frequent. The photo-chase filmed the X-15, although Chase-1 was frequently a two-seater and carried a photographer in the back seat as well. Rover was usually another X-15 pilot who just felt like tagging along. All of the X-15 pilots flew chase aircraft, as did many AFFTC test pilots, and students and instructors from the test-pilot schools at Edwards. The chase pilots (particularly other X-15 pilots) tended to use first names for themselves and the X-15 pilot during radio chatter; alternately, they simply used "chase" (without a number) since there was seldom more than one chase aircraft in the vicinity.

A number of other aircraft provided various support functions. In particular, the program used the NASA Gooney Bird (R4D/DC-3) to ferry men and supplies to the uprange stations and to inspect the lakebeds as necessary. The Air Force used several Lockheed C-130 Hercules turboprops to transport fire engines and other material to the lakebeds and High Range stations for each flight. These aircraft often made several trips per day carrying men and equipment. During the actual flight one of them orbited midway down the flight corridor, usually with a flight surgeon and response team in case the X-15 had to make an emergency landing. The program took safety very seriously.

Piasecki H-21 Shawnee helicopters were also shuttled to the primary emergency landing lake in case of an emergency, and additional H-21s were located at Edwards. These provided a quick means of moving emergency personnel to an accident scene, surveying the runways, and evacuating the X-15 pilot if necessary. The H-21 pilots also knew how to disperse fumes from a damaged X-15 by hovering near the crashed airplane, and they used this technique on at least one occasion, probably saving the life of the X-15 pilot.

The engine situation was somewhat more complicated. Given that everybody now agreed that the General Electric A1 (Hermes) engine was unacceptable, the Power Plant Laboratory listed the Aerojet XLR73, Bell XLR81, North American NA-5400, and Reaction Motors XLR10 as engines the airframe contractors could use. The four engines were a diverse collection.-1321

The Aerojet XLR73-AJ-1 had a single thrust chamber that used white fuming nitric acid and jet fuel as propellants. As it then existed, the engine developed 10,000 lbf at sea level, but a new nozzle was available that raised that to 11,750 lbf. The engine was restartable in flight by electric ignition and was infinitely variable between 50% and 100% thrust. A cluster of several engines was necessary to provide the thrust needed for the new research airplane. At the time the Power Plant Laboratory recommended the engine, it had passed its preliminary flight rating qualification, with a first flight scheduled for April 1956.[33]

The development of the Bell XLR81-BA-1, usually called the Hustler engine, was part of Project MX-1964—the Convair B-58 Hustler. The B-58 was a supersonic bomber that carried its nuclear weapon in a large external pod, and the XLR81 was supposed to provide the pod with extra range after it was released from the bomber. The engine was a new design based on the engine used in the GAM-63 RASCAL missile. A single thrust chamber used red, fuming nitric acid and jet fuel to produce 11,500 lbf at sea level and 15,000 lbf at 70,000 feet. Sufficient thrust for the hypersonic research airplane would come from a cluster of at least three engines. The existing XLR81 was not throttleable or restartable in flight. Since ignition occurred after the B-58 dropped the weapons pod, the engine included a minimum number of safety components to save weight. At the time the Power Plant Laboratory recommended the engine, it had passed its preliminary flight rating qualification, with a first flight scheduled for January 1957.[34]

Although the Power Plant Laboratory included the engine on its list of candidates, and history papers often mention it, the NA-5400 apparently had little to offer the program. North American was using the effort as the basis for component development, with no plans to assemble a complete engine. If they had, it would only have developed 5,400 lbf at sea level (hence its company designation). The turbopump assembly was theoretically capable of supporting engines up to 15,000 lbf, and the power plant proposed for the new research airplane consisted of three separate engines arranged as a unit. The engine was restartable in flight using a catalyst ignition system. The propellants were hydrogen peroxide and jet fuel, with the turbopump driven by decomposed hydrogen peroxide.-1351

The Reaction Motors XLR10 Viking engine presented some interesting options, although Reaction Motors had already abandoned further development in favor of the more powerful XLR30 "Super Viking" derivative. As it existed, the XLR10 produced 20,000 lbf at sea level using liquid oxygen and alcohol propellants. The XLR30 then under development produced 50,000 lbf using liquid oxygen and anhydrous ammonia. The Power Plant Laboratory preferred to connect two XLR10 thrust chambers to a single XLR30 turbopump, believing this arrangement took better advantage of well-developed components and lowered the risk. The fact that the XLR10/XLR30 discussion used over two pages of the four-and-a-half-page engine report showed the laboratory’s enthusiasm. Interestingly, as designed, the engine was not throttleable or restartable in flight, nor was it man-rated.1361

In response to one contractor’s comment that three of the four engines appeared unsuitable because they lacked a throttling capability, the government indicated it would undertake any necessary modifications to the engine selected by the winning airframe contractor.-1371

Between the time of the airframe bidders’ conference and the 9 May submission deadline, Boeing, Chance-Vought, Grumman, and McDonnell notified the Air Force that they did not intend to submit formal proposals. This left Bell, Convair, Douglas, North American, Northrop, and Republic. It would seem that Bell and Douglas would have the best chances, given their history of developing X-planes. The Navy D-558-3 study would also appear to provide a large advantage to Douglas. On the other hand, although Convair, North American, and Republic had no particular experience in developing X-planes, they were in the process of either studying or developing high-speed combat aircraft or missiles. Northrop had little applicable experience of any sort, but had a long history of producing innovative designs.

During this period, representatives from the airframe contractors met with NACA personnel on numerous occasions and reviewed technical information on various aspects of the forthcoming research airplane. The NACA also provided data from tests in the Ames 10-by-14-inch and Langley 11-inch tunnels. Coordination on the NACA side became easier when Arthur W. Vogeley, an aeronautical research scientist from the Flight Research Division at Langley, became the NACA project engineer on 10 January 1955. Vogeley would act as a single point of contact for the NACA, with offices at both Langley and Wright Field.1381

On 17 January 1955, NACA representatives met with Wright Field personnel and were informed that the research airplane was identified as Air Force Project 1226, System 447L, and would be officially designated the X-15.1391 The Fighter Aircraft Division of the WADC managed the project since the requirements for the aircraft most closely resembled those for a contemporary jet fighter. In reality, except for some procurement and oversight functions, the division would have little to do because the X-15 Project Office and the Research Airplane Committee actually controlled most aspects of the project. The X-15 enjoyed a national priority of 1-B, with a category of A-1. The Air Force also announced that the WADC project engineer would be First Lieutenant (soon to be Captain) Chester E. McCollough, Jr. BuAer subsequently selected George A. Spangenberg1401 as the Navy project engineer.1411

Early in March the NACA issued a research authorization (A73L179) that would cover the agency’s work on Project 1226 during the design competition and evaluation. The contractors concentrated on preparing their proposals and frequently consulted with both the NACA and WADC. For instance, on 15 April John I. Cangelosi from Republic called John Becker to obtain information on the average recovery factors used for swept-wing heat transfer. Later that day Becker transmitted the answer to NACA Headquarters, which then forwarded it to each of the competing contractors on 26 April.1421

The Air Force and the NACA also were working on the procedures to evaluate the proposals.

During March the NACA Evaluation Group was created with Hartley Soule (research airplane project leader), Arthur Vogeley (executive secretary), John Becker (Langley), Harry J. Goett (Ames), John L. Sloop (Lewis), and Walt Williams (HSFS) as members.

In early February, ARDC Headquarters sent a letter to all parties emphasizing that the evaluation was a joint undertaking, and the ultimate selection needed to satisfy both the military and the NACA. The evaluation involved the X-15 Project Office, the WADC laboratories, and the NACA, while the Air Materiel Command and Navy played subordinate roles. The four evaluation areas were the capability of the contractor, the technical design, the airplane performance, and the cost.*43*

The Research Airplane Committee would begin evaluating the proposals when it met on 17 May at Wright Field. Slightly complicating matters, the Air Force raised the security classification on most X-15-related activities from Confidential to Secret. This restricted access to the evaluation material by some engineers and researchers, but mostly placed additional controls on the physical storage locations for the material.*441

The heating rates and low pressures encountered by the X-15 ruled out the use of traditional vane-type sensors to measure angle of attack (a) and sideslip (в). Based on a preliminary design completed by Langley in June 1956, NASA awarded a contract to the Nortronics Division of Northrop Aircraft Corporation for the detailed design and construction of a prototype and five production ball noses. The sensor and its supporting, sealing, and hydraulic-actuating mechanisms were an integral assembly mounted in the extreme nose of the X-15. The afterbody located behind the sphere contained the electronic amplifiers, power supplies, and control valves, with the electrical, hydraulic, and pneumatic connections between the sphere and the afterbody passing through a single supporting member. Rotary hydraulic actuators provided the required two degrees of freedom.-1174!

Officially called the "high-temperature flow-direction sensor," the device was 16.75 inches long with a base diameter of 13.75 inches. The total weight of the ball nose was 78 pounds, half of which was contributed by the thick Inconel X outer skins of the lip, cone, and sphere. In addition, 13 chromel-alumel thermocouples were located within the sphere to measure skin temperature during flight, and five other thermocouples measured selected internal temperatures. Nitrogen gas from the aircraft supply cooled the sensor. The ball nose was physically interchangeable with the standard NACA flight-test boom nose, and all connections to the sensor were made through couplings that automatically engaged when the ball nose (or boom) was mounted to the aircraft.-1173

The ball nose, or more officially, the high-temperature flow-direction sensor, was mounted on the nose of the airplane and provided angle of attack and angle of sideslip information to both the pilot and the research instrumentation. This elaborate mechanism was required since the pressure and temperature environment encountered by the X-15 ruled out more conventional vane-type sensors. (NASA)

The core of the ball nose consisted of a 6.5-inch-diameter Inconel X sphere mounted on the extreme tip of the X-15 nose. The sphere contained two pairs of 0.188-inch diameter orifices (one pair in the vertical plane (a orifices) and one pair in the horizontal plane (в orifices)), each 42 degrees from the stagnation point. Two functionally identical hydraulic servo systems, powered by the normal X-15 systems, rotated the sphere about the a and в axes to a position such that the impact pressures seen by all sensing orifices were equal. When this condition existed, the sphere was oriented directly into the relative wind. Two synchro transducers detected the position of the sphere with respect to the airframe, and this signal fed the various instruments in the cockpit and the recorders and telemetry system. Since the dynamic pressure during flight could vary between 1 psf and 2,500 psf, a major gain adjustment was required in the servo loop to maintain stability and accuracy. Measuring the pressure difference between the total-pressure port and one angle­sensing port provided a signal that adjusted the gain of the sphere-positioning loop. The ball nose could sense angles of attack from -10 to +40 degrees, and angles of sideslip within 20 degrees. The unit was capable of continuous operation at a skin temperature of 1,200°F. A 0.5- inch-diameter orifice located at the sphere stagnation point provided a total pressure source for the aircraft. Based on ground tests, the angular accuracy of the sensor was within 0.25 degree for dynamic pressures above 10 psf.^176

In early 1960 the FRC developed a simple technique for thermal testing the newly delivered ball noses: expose them to the afterburner exhaust from a North American F-100 Super Sabre. This seemed to work well until one of the noses suffered a warped forward lip during testing.

Engineers subsequently determined the engine was "operated longer than necessary," resulting in temperatures in excess of 2,400°F instead of the expected 1,900°F. Ultimately, the FRC tested the ball nose "many consecutive times" with "satisfactory results.”^1771-

The ball nose performed satisfactorily throughout the flight program, encountering only occasional minor maintenance problems. Late in the program, various parts began to wear out, however, and the need to replace some of them presented difficulties. For instance, the procurer of replacement dynamic-pressure transducers found that the original vendor was not interested in fabricating new parts, and no suitable alternate vendor could immediately be located. Eventually

NASA found a new vendor, but this illustrates that the "vanishing vendor" phenomenon frequently encountered during the early 21st century is not new.[178]

The sphere mounted on the extreme nose of the ball nose was machined from Inconel X to very precise tolerances. The X-15 was manufactured before the advent of modern computer – controlled milling machines, so such precise work was accomplished by human operators on traditional lathes and drill presses. The ball noses for the X-15A-2 were manufactured from TAZ – 8A cermet since the temperatures in the Mach 8 environment were even more severe. (NASA)

As the modified X-15A-2 was being prepared for flight, however, there began a concern over whether the Inconel X sphere in the original ball noses could handle the additional heat generated at Mach 8. Researchers at NASA Lewis developed a TAZ-8A cermet that Rohr Corporation used to manufacture a new sphere specifically for the X-15A-2. This sphere was delivered in mid-1966, but did not initially pass its qualification test due to a faulty braze around the beta pressure port. Rohr subsequently repaired the sphere and it passed its qualification test. Interestingly, the FRC tested this new sphere (and the forward lip of the cone, which was also manufactured from TAZ – 8A) in much the same way as the original ball noses were qualified—this time in the afterburner exhaust of a General Electric J79 engine at 1,850°F. During November 1966, the FRC tested the new sphere, as well as a slightly modified housing necessary to accommodate the ablative coating on the fuselage, in the High-Temperature Loads Calibration Laboratory. NASA installed the new

The ball nose had to withstand pressures up to 2,500 psf and temperatures up to 1,200°F. NASA researchers developed a relatively straight-forward heating test using the afterburned exhaust of a jet engine on the ramp at the Flight Research Center. The original ball noses were tested using Pratt & WhitneyJ57 engines from North American F-100 Super Sabres, while the later X-15A-2 noses used General Electric J79 engines from Lockheed F-104 Starfighters. (NASA)

The ball nose only provided angle of attack, angle of sideslip, and total pressure; like all aircraft, the X-15 needed additional air data during the landing phase. North American had installed a total-head tube (also called the alternate probe) ahead of the canopy to provide the total pressure during subsonic flight, and static pressure ports were located on each side of the fuselage 1 inch above the aircraft waterline at station 50.[180]

A different pitot-static system was required for the X-15A-2 since the MA-25S ablator would cover the normal static locations. Engineers chose a vented compartment behind the canopy as the static source, and found it to be suitable during flight tests on the X-15-1. The standard dogleg pitot tube ahead of the canopy was replaced by an extendable pitot because the temperatures expected at Mach 8 would exceed the thermal limits of the standard tube. The retractable tube remained within the fuselage until the aircraft decelerated below Mach 2; the pilot then actuated a release mechanism and the tube extended into the airstream. This was very similar in concept to the system eventually installed on the space shuttle orbiters.-1181

In order to get flight-testing under way, North American completed the first two aircraft with interim Reaction Motors XLR11-RM-5 engines. Two XLR11s were installed in each aircraft, producing a total of 11,800 lbf at sea level. These engines were quite familiar to personnel working in the experimental rocket aircraft programs at Edwards, since the Bell X-1, Douglas D – 558-2, and Republic XF-91 all used the same powerplant (or its XLR8 Navy equivalent).-1132!

The basic XLR11 configuration was called G6000C4 by Reaction Motors and consisted of four thrust chambers producing 1,475 lbf each with a turbopump unit, valves, regulators, and controls mounted forward of the chambers. Other variants of the XLR8/XLR11 family used pressure-fed propellants instead of a turbopump. The four chambers were mounted on a support beam assembly that was the main structural member of the engine. A single turbopump provided the pressure to inject the liquid-oxygen and ethyl-alcohol-water propellants, while valves in the oxidizer and fuel lines controlled the flow of the propellants to the chambers. Each thrust chamber contained an igniter, and the pilot could ignite or shut down individual chambers in any sequence, allowing a measure of "thrust stepping." However, once the pilot shut down a chamber, he could not restart that chamber. Fuel circulated through passages in each exhaust nozzle and around each combustion chamber individually for cooling, and then into the firing chambers to be burned. Each engine weighed 345 pounds dry (including pumps) and was approximately 60 inches long, 36 inches high, and 24 inches wide. On paper each engine (including the turbopumps) cost about $80,000, although technicians at Edwards assembled all of the engines used in the X-15 program on site from components left over from earlier programs.-133!

It was surprisingly easy to install the XLR11 in the X-15, considering that the designers had not intended the aircraft to use the engine. Part of this was due to the mounting technique used for the XLR99: the engine was bolted onto a frame structure, which was then bolted into the engine compartment of the aircraft. A new frame was required to mount the two XLR11 engines, but the structural interface to the aircraft remained constant. However, the XLR11 used ethyl alcohol – water for fuel instead of the anhydrous ammonia used in the XLR99. This necessitated some modifications to the system, but none of them were major-fortunately, the two liquids had a

similar consistency and temperature. Surprisingly, no documentation describing the changes seems to have survived; however, as Scott Crossfield remembers:[134]

[S]ince the XLR11 engines were installed as two units including their own fuel pumps, the X – 15 needed only to supply the tank pressures to meet the pumps inlet pressure requirement and the engines didn’t know what airplane they were in. There were, of course, structural changes, i. e., engine mounting and I believe some ballast but nothing very complex. That is a relative statement. The difference in mixture would make the ideal fuel/lox load different but I don’t remember that was a significant problem.

forms, in the Bell X-1 series, Douglas D558-2, and Republic XF-91 programs at Edwards AFB. All of the engines used for the X-15 were made from leftover components from earlier programs. (NASA)

Charlie Feltz remembers that there were no modifications to the fuel tanks. North American had already built and sealed them by the time NASA decided to use the XLR11s. It was determined that both the metal and the sealant were compatible with alcohol, so there was no need to reopen the tanks. There were some minor changes to the plumbing and electrical systems to accommodate the new engines, along with cockpit modifications to provide the appropriate instrumentation and controls.-11351 Nevertheless, considering that North American had designed the airplane with no intention of installing anything but the XLR99, the changes were of little consequence and did not materially delay the program.-11361

In the final installation, the two engines were mounted on a single tubular-steel mounting frame attached to the airplane at three points. The mount canted the upper engine slightly nose-down and the lower engine in a slightly nose-up attitude so that their thrust vectors intersected at the airplane’s center of gravity.-11371

After the last XLR11 flight, NASA placed the remaining engines, spare parts, and special tools into long-term storage. Despite being almost 20 years old, the engines later found their way into the heavyweight lifting bodies.11381

The airframe proposals from Bell, Douglas, North American, and Republic arrived on 9 May 1955. Convair and Northrop evidently decided they had little to offer the competition. Two days later the various evaluation groups (the WADC, NACA, and Navy) received the technical data, and the results were due to the X-15 Project Office by 22 June.*451

In mid-May, Soule, as chair of the NACA evaluation group, sent the evaluation criteria to the NACA laboratories. The criteria included the technical and manufacturing competency of each contractor, the schedule and cost estimates, the design approach, and the research utility of each airplane. Each NACA laboratory had specific technical areas to evaluate. For instance, Ames and Langley were assigned to aerodynamics; Ames, the HSFS, and Langley to flight control; HSFS to crew provisions and carrier aircraft; and the HSFS and Lewis to the engine and propulsion system. Soule expected all the responses no later than 13 June, giving him time to reconcile the results before submitting a consolidated NACA position to the Air Force on 22 June. Later arrangements ensured that engine evaluations, also coordinated among the WADC, NACA, and Navy, would be available to the Research Airplane Committee on 12 July. The final evaluation would take place during a meeting at Wright Field on 25 July.*46*

Given the amount of effort that John Becker and the Langley team had put into their preliminary configuration, one might have thought that all of the contractors would use it as a starting point for their proposals. This was not necessarily the case. The Air Materiel Command had made it clear from the beginning that the Becker concept was "representative of possible solutions."

Becker agreed with this; he in no way thought that his was an optimal design, and the bidders were encouraged to look into other configurations they believed could meet the requirements.*47*

As it turned out, each of the four proposals represented a different approach to the problem, although to the casual observer they all appeared outwardly similar. This is exactly what the government had wanted—the industry’s best responses on building the new airplane. Two of the bidders selected the Bell XLR81 engine, and the other two chose the Reaction Motors XLR30. Despite this, all of the airplanes were of approximately the same size and general configuration. In the end, the government would have to evaluate these varied designs and determine which would most likely allow the desired flight research.

One of the unique items included in the X-15 design was a side-stick controller. Actually, the airplane included two side sticks: one on the right console for the aerodynamic controls, and one on the left console for the ballistic controls. The right and center controllers were linked mechanically and hydraulically to provide simultaneous movement of both sticks; however, the side stick required only one-third as much movement to obtain a given stabilizer motion.[182]

NASA had installed a similar side stick in one of the North American YF-107A aircraft to gain experience with the new controller. A review of early X-15 landing data (using the side-stick) revealed a "striking similarity" with landings made in the YF-107. Despite large differences in speed and L/D ratios, the variations in angle of attack, normal acceleration, pitching velocity, and horizontal stabilizer position exhibited the same tendencies for the pilot to over-control the airplane using the side stick. During the YF-107 program, several flights were generally required before a pilot became proficient at using the controller and could perform relatively smooth landings; the same was true of the X-15.[183]

Regarding the side-stick controller, Bob White commented that "the side aerodynamic control stick designed for the X-15 has received the usual critical analysis associated with a departure from the conventional." As pilots reported their experiences using the side stick, North American began making minor modifications to correct undesirable characteristics. In the end, the company found that most of the initial design features were satisfactory. The most frequent complaint was the location of the stick in relation to the pilot’s arm, since the stick had been located based on Scott Crossfield’s input, and other pilots differed in size and proportions. However, Crossfield was a strong proponent of the side stick and North American soon devised a way to adjust the stick into one of five different fore-aft locations prior to flight based on individual pilot preference. After this, the side stick gained favor rather quickly.-11841

The all-moving horizontal stabilizers deflected symmetrically for longitudinal control (elevators) and differentially for lateral control (ailerons). The rolling tail that had caused so much controversy within the government early in the program proved to be quite satisfactory in operation.

According to Bob White, "the pilot is not aware of what specific type of lateral control is allowing the roll motion. His only concern is in being able to get the aircraft response he calls for when

deflecting the control stick___ From experience to date [after 45 flights], the rolling tail has

provided a good rolling control for the X-15, and there have been no undesirable aircraft motions coupled in any axis because of lateral-control deflection."11881

Conventional rudder pedals actuated the movable portions of the dorsal and ventral vertical stabilizers. Just prior to the landing flare, the pilot would jettison the lower portion of the dorsal stabilizer to provide sufficient ground clearance; otherwise, the dorsal rudder would contact the ground before the landing skids. Speed brakes were located on each side of the fixed portion of the dorsal and ventral stabilizers. Irreversible hydraulic actuators actuated all of the aerodynamic control surfaces.11861

The aerodynamic controls were effective up to about 150,000 feet. Nevertheless, many X-15 pilots manually used the ballistic control system in addition to the aerodynamic controls above 100,000 feet, and the MH-96 on X-15-3 automatically began blending in the ballistic control system thrusters above 90,000 feet. As Neil Armstrong, who was a principle engineer on the MH- 96, commented, "a rule of thumb is that when dynamic pressure on control surfaces reduces to 50 psf, there should be a switchover from aerodynamic to reaction control." Despite some early concerns about controlling a vehicle above the sensible atmosphere, in practice it quickly became routine.11871

The Westinghouse-manufactured stability augmentation system (SAS) dampened the aerodynamic

controls in all three axes. The system consisted of three rate gyros, two pitch-roll servocylinders, one yaw servocylinder, and various electronics, displays, and controls. Essentially, the system included a channel for each axis that sensed the aircraft rate of change in pitch, roll, and yaw, and automatically provided signals to the respective servocylinders to move the horizontal and vertical stabilizers to oppose the airplane angular inputs. An additional interconnect damper, called "yar," provided a crossfeed of the yaw-rate signal to the roll damper. This interconnection was necessary for stability at high angles of attack, primarily because of the high roll input of the lower rudder. The yar interconnect was disabled when the lower rudder was removed during later flights. The authority of the SAS was equal to the pilot’s authority in pitch and yaw, and to twice the pilot’s authority in roll. The pilot could turn dampening on or off for each individual axis, and select the damping gain for each axis. Originally, the SAS gyro package was located in the instrument compartment behind the pilot. However, a vibration at high gains reported by Scott Crossfield during the first X-15 captive flight resulted in North American moving the gyros to the center of gravity compartment under the wings, thus removing the gyro from a point influenced by fuselage bending.-1188-

North American incorporated two side-stick controllers in the X-15 cockpit. The controller on the right console operated the aerodynamic flight control systems while the controller on the left operated the ballistic control system thrusters. The aerodynamic controller was mechanically linked to the conventional center stick. In X-15-3, the MH-96 adaptive flight control system automatically blended the ballistic thrusters in when needed, eliminating the need for the pilot to use the left side-controller. (NASA)

The SAS caused numerous pilot comments. During early flights below Mach 3.5, the dampers used moderate gains and the pilots quickly expressed a desire for "a stiffer aircraft," particularly in pitch and roll. North American subsequently increased the gain, resulting in generally favorable pilot opinions. It is interesting to note that at angles of attack above 8 degrees with low damper gain or with the roll damper off, pilots had great difficulty in controlling the lateral and directional motions to prevent divergence. This was primarily because of an adverse dihedral effect that was present above Mach 2.3. Although this was of some concern to the pilots, and the subject of a great deal of investigation by the researchers, the airplane exhibited acceptable handling characteristics as long as the dampers were functioning. In general, the airplane exhibited about the same handling qualities expected based on extensive simulations at Ames, and the pilots thought the damper-off handling was slightly better than the simulator predicted, but still considered the natural stability to be marginal.-1189

The SAS was unique for the time because it provided 10 pilot-selectable gain rates for each axis. However, the system experienced some annoying problems during development and early operations. During the first studies using the fixed-base simulator, the dampers sustained unwanted limit cycles (or continuous oscillations) from linkage lags and rate limiting. Pilots later observed the phenomenon in flight. The frequency of the limit cycle was about 3.2 cycles per second, resulting in changes in bank angle of about 1 degree. This limit cycle was not constant,

changed due to control input, and had a tendency to "beat." North American was unable to identify a way to eliminate the limit cycles, but modified the electronic filter to reduce its lag. This greatly lowered the amplitude of the limit cycles, and the pilots found the results acceptable.-190

The X-15 made extensive use of a stability augmentation system to dampen the aerodynamic controls in all three axes. The SAS was unique for the time since it provided ten pilot-selectable gain rates for each axis via rotary switches in the cockpit. Flight simulations showed that it would be nearly impossible for a pilot to control the X-15 in some flight regimes without the SAS. (NASA)

Although the modified filter greatly improved the issue with the limit cycles in roll, a new problem soon arose. It became apparent during ground tests that it was possible to excite and sustain a SAS-airplane vibration at 13 cycles per second with the modified filter. A breadboard of the modified filter was flown (flight 2-12-23) at higher damper gains, but Scott Crossfield failed to excite the vibration. During the rollout after landing, however, Crossfield encountered a severe vibration that required disabling the SAS. This experience led to the mistaken belief that the vibration could only occur on the ground. To prevent a recurrence, North American installed a switch that automatically lowered the gain whenever the pilot extended the landing gear.

However, five flights later (2-14-28), Joe Walker encountered a 13-cps vibration during reentry from 169,600 feet. After the flight, Walker reported that the vibration was the most severe he had ever encountered (or ever wanted to). The shaking was triggered by pilot inputs at 130 psf dynamic pressure and continued until the damper gain was reduced and the dynamic pressure climbed above 1,000 psf. Fortunately, the amplitude of the shaking was constrained by the rate limits of the control surface actuators. North American and NASA began investigating the problem again.-11911

TOTAL HOURS

TOTAL FLIGHTS

Failures of the stability augmentation system contributed to the maintenance woes suffered by the X-15 early in the flight program, but oddly, most of the failures were on the ground; the system seldom failed in flight. Nevertheless, an auxiliary stability augmentation system was added to the first two airplanes as insurance against an SAS failure. The X-15-3 did not carry an SAS or ASAS since the mH-96 adaptive flight control system performed both functions. (NASA)

The problem was that the lightly damped horizontal stabilizers were excited at their natural frequency (13 cps) by pilot inputs to the control system. The gyro picked up this vibration and the dampers were able to sustain the vibration with input to the control surfaces. Engineers also found a second natural frequency for the stabilizers at 30 cps. North American subsequently installed notch filters in the SAS and pressure feedback valves in the control surface actuators, eliminating the vibrations.-192

The SAS proved to be unreliable in the beginning, but fortunately most failures occurred during ground testing. The program recorded only seven in-flight failures during the first 78 flights (defined as NB-52 takeoff to X-15 landing). Of these failures, one was an electronic module, three were malfunctioning cockpit gain switches, and three were broken wires in the X-15. Engineers ultimately traced all except the failed electronics module to human error.192

There was never any doubt that the X-15 flight program would take place at Edwards AFB, California. However, Edwards would play a key role as infrastructure was developed to support the X-15. The program was an involved undertaking, and the operational support required was extensive. Logistically, Edwards would become the linchpin of the entire effort.

MUROC TO EDWARDS

The Mojave Desert-called the "high desert" because of its altitude-is approximately 100 miles northeast of Los Angeles, just on the other side of the San Gabriel Mountains. First formed during the Pleistocene epoch, and featuring an extremely flat, smooth, and hard surface, Rogers Dry Lake is a playa, or pluvial lake, that spreads out over 44 square miles of the Mojave, making it the largest such geological formation in the world. Its parched clay and silt surface undergoes a cycle of renewal each year as desert winds sweep water from winter rains to smooth the lakebed out to an almost glass-like flatness.-Ш-

Lieutenant Colonel Henry H. "Hap" Arnold decided that Rogers Dry Lake would make a "natural aerodrome," and in September 1933 the Army Air Corps established the Muroc Bombing and Gunnery Range as a training site for squadrons based at March Field near Riverside, California. It continued to serve in that capacity until 23 July 1942, when it became the Muroc Army Air Field. During World War II the primary mission at Muroc was to provide final combat training for aircrews before their deployment overseas.-12-

Until the beginning of World War II, the Army Air Corps conducted the majority of its flight-testing at Wright Field, Ohio. However, the immense volume of testing created by the war was one of the factors that led to a search for a new location to test the first American jet fighter, the Bell XP- 59A Airacomet. The urgent need to complete the program immediately dictated a location with year-round flying weather. In addition, the risks inherent in the radical new technology used in the aircraft dictated an area with many contingency landing areas, and one that minimized the danger of crashing into a populated area. After examining a number of locations around the country, the Army Air Forces selected a site along the north shore of Rogers Dry Lake about six miles away from the training base at Muroc.-13-

When Bell test pilot Robert Stanley arrived at the base in August 1942, he found just three structures: an unfinished hangar, a wooden barrack, and a water tower. Things would begin to change quickly as more than 100 people arrived at the base to support the project. On 2 October 1942, Stanley made the first "official flight" of the XP-59A (it had actually lifted off for the first time on the previous day during high-speed taxi tests), introducing flight-testing to the high desert. Only five years later, on 14 October 1947, Captain Charles E. "Chuck" Yeager became the first man to exceed (barely) the speed of sound in level flight when he achieved Mach 1.06 (approximately 700 mph) at 42,000 feet in the Bell XS-1 research airplane. Muroc’s place in the history books was firmly established.[4]

However, with the arrival of the X-1, flight-testing at Muroc began to assume two distinct identities. The Air Force typically flew the research airplanes, such as the X-3, X-4, X-5, and XF – 92A, in conjunction with the NACA in a methodical fashion to answer largely theoretical questions. The bulk of the testing, however, focused on highly accelerated Air Force and contractor evaluations of prototype operational aircraft, and was often much less methodical as they tried to get new equipment to combat units as quickly as possible at the height of the Cold War.[5]

Not surprisingly, the rather informal approach to safety that prevailed during the late 1940s, and even into the 1950s, was one of the factors that contributed to a horrendous accident rate. There were, of course, a number of other factors. The corps of test pilots at Muroc remained small and commonly averaged more than 100 flying hours per month. They flew a wide variety of different types and models of aircraft, each with its own cockpit and instrument panel configuration. Chuck Yeager, for example, reportedly once flew 27 different types of airplanes in a single one-month period. The year 1948 was particularly tragic, with at least 13 fatalities recorded at or near the base. One such fatality was that of Captain Glen W. Edwards, who was killed in the crash of a Northrop YB-49 flying wing on 4 June 1948. In December 1949 the Air Force renamed the base in his honor, while other pilots have streets named after them.[6]

Edwards AFB, California, hosted the X-15 flight program. The "new" main base complex is located at the center left in this photo, with the NASA Flight Research Center being slightly above the main base on the edge of the lakebed. Rogers Dry Lake was the planned site for all X-15 landings, and 188 times, it worked out that way. Two would land at Cuddeback, one at Delamar, four at Mud, one at Rosamond, one at Silver, and one at Smith Ranch; the X-15-3 broke up in flight and did not land on its last flight. (U. S. Air Force)

On 25 June 1951, the government established the Air Force Flight Test Center (AFFTC) at Edwards, and a $120 million master plan was unveiled for construction at the base. Part of the appropriation paid to remove the Atcheson, Topeka, and Santa Fe railroad from the northern portion of Rogers Dry Lake and bought out the silt mines that had been located along the route. However, the major undertaking was to relocate the entire base two miles west of the original South Base location and construct a 15,000-foot concrete runway. With the increased number of flight test programs at the base, the natural surfaces of the Rogers and Rosamond dry lakebeds took on even greater importance as routine and emergency landing sites. The first AFFTC commander, Brigadier General Albert Boyd, later commented that the dry lakes were nothing less than "God’s gift to the U. S. Air Force." That same year, the USAF Test Pilot School moved from Wright Field to the high desert.[7]