Category X-15 EXTENDING THE FRONTIERS OF FLIGHT

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-

15.ШЛ

As conceived at the time of the rollout in 1958, the contractor flight program consisted of four phases. The first was called "B-52 and X-15 Lightweight Captive Flight Evaluation" and was intended to verify the mated flight characteristics with an unfueled X-15, operational procedures, jettison characteristics (using dye), systems (APU, hydraulic, heat and vent, and electrical) operations, B-52 communications and observation, and carrier aircraft performance at launch speed and altitude. The second phase was the "X-15 Lightweight Glide Flight Evaluation" and involved launching an unfueled X-15 on an unpowered glide flight to verify the launch procedure, low-speed handling characteristics, and landing procedures.-1521

The third phase was the "B-52 and X-15 Heavyweight Captive Flight Evaluation," which was intended to replicate the first series of tests with a fully loaded X-15, demonstrate topping off the liquid-oxygen system, and verify that a full propellant load could be jettisoned using actual propellants. The last phase involved the initial "X-15 Powered Flight Evaluation" using the interim XLR11 engines. The schedule showed the first captive flight on 31 January 1959, with the first glide flight on 9 February 1959 and the first powered flight on 2 April 1959. Somehow, it would not work that way.-1531

X-15-1 arrived at Edwards on 17 October 1958, trucked over the foothills from the North American Inglewood plant. The second airplane joined it in April 1959, and the third would arrive later. As Air Force historian Dr. Richard P. Hallion later observed, "In contrast to the relative secrecy that had attended flight tests with the XS-1 a decade before, the X-15 program offered the spectacle of pure theater." It was not that the X-15 program necessarily relished the limelight – it simply could not avoid it after Sputnik.-1541

Beginning in December 1958, North American conducted numerous ground runs with the APUs installed in X-15-1 at Edwards. The company intended this to build confidence in the units before the first flights, but it did not turn out that way. Bearings overheated, turbines seized, and valves and regulators failed, leaked, or did not regulate. The mechanics would remove the failed part, rebuild it, and try again. More failures followed. Scott Crossfield later described this period as "sleepless weeks of sheer agony." Harrison Storms eventually got together with the senior management at General Electric, who sent Russell E. "Robby" Robinson to Edwards to fix the problem. For instance, Robinson noted that invariably after one APU failed, the other would follow within a few minutes. The engineers finally deduced that a sympathetic vibration transferred through the shared mounting bulkhead caused the second one to fail. North American devised a new mounting system that separated the APUs onto two bulkheads.-1551

The North American pilot was Scott Crossfield, the person who arguably knew more about the airplane than any other individual did. After they performed various ground checks, technicians mated X-15-1 to the NB-52A and then conducted additional ground tests. All of this delayed the original schedule by about 60 days. When the day for the first flight arrived, Crossfield described it as a "carnival at dawn." Things were different during the 1950s. Crossfield and Charlie Feltz shared a room in the bachelor officer quarters (BOQ) at Edwards; there was no fancy hotel in town. They each dressed in a shirt and tie before driving to the flight line-nothing casual, even though Crossfield soon changed into a David Clark MC-2 full-pressure suit. When they got to the parking lot next to the NB-52 mating area, more than 50 cars were already waiting. The flight had been scheduled for 0700 hours. Based on his previous rocket-plane experience, Crossfield predicted they would take off no earlier than noon, and maybe as late as 1400.1561

Crossfield was pleasantly surprised. At 1000 hours on 10 March 1959, the mated pair took off on its scheduled captive-carry flight (retroactively called program flight number 1-C-1). It had a gross take-off weight of 258,000 pounds, and lifted off at 172 knots after a ground roll of 6,085 feet. During the 1 hour and 8 minute flight, Captains Charlie Bock and Jack Allavie found that the NB-52 was an excellent carrier for the X-15, as was expected from numerous wind-tunnel and simulator tests.-1571

Scott Cross field spent many hours in his David Clark full-pressure suit while the X-15-1 was prepared for its initial flights. Despite the daytime temperatures in the desert, Crossfield believed that his presence, ready to go, kept ground crew morale high. (NASA)

During the captive flight, Crossfield exercised the X-15 flight controls, and the recorders gathered airspeed data from the flight test boom to calibrate the instrumentation. Bock and Allavie found that the penalties imposed by the X-15 on the NB-52 flight characteristics were minimal, and flew the mated pair up to Mach 0.85 at 45,000 feet. Part of the test sequence was to make sure the David Clark full-pressure suit worked as advertised, although Crossfield had no doubts. This was a decidedly straightforward test. The suit should inflate as soon as the altitude in the cockpit went above 35,000 feet. As the mated pair passed 30,000 feet, Crossfield turned off the cabin pressurization system and opened the ram air door to equalize the internal pressure with the outside air. Once the airplanes climbed above 35,000 feet, Crossfield felt the suit begin to inflate, and "from that point on [his] movements were slightly constrained and slightly awkward." Still, Crossfield could reach all of the controls, including the hardest control in the cockpit to reach: the ram air door lever. Crossfield closed the door, and as the cockpit repressurized, the suit relaxed its grip. Pilots repeated this test on every X-15 flight until the end of the program. Near the end of the flight, Crossfield lowered the X-15 landing gear just to make sure it worked, even if it looked a little odd while still mated to the NB-52.[58]

The next step was to release the X-15 from the NB-52 to ascertain its gliding and landing characteristics. North American rescheduled the first glide flight for 1 April 1959, but aborted it when the X-15 radio failed. The NB-52A and X-15 spent 1 hour and 45 minutes airborne conducting further tests in the mated configuration. A combination of radio failure and APU problems caused a second abort on 10 April. Yet a third attempt aborted on 21 May 1959 when the X-15 stability augmentation system failed and a bearing in the no. 1 APU overheated after approximately 29 minutes of operation.-1591

The problems with the APU were the most disturbing. All of these flights encountered various valve malfunctions, leaks, and speed-control problems with the APUs, all of which would have been unacceptable during research flights. Tests conducted on the APU revealed that extremely high surge pressures were occurring at the pressure relief valve (actually a blowout plug) during the initial peroxide tank pressurization. The installation of an orifice in the helium pressurization line immediately downstream of the shut-off valve reduced the surges to acceptable levels. Engineers decided that other problems were unique to the captive-carry flights and deemed them of little consequence to the flight program since the operating scenario would be different. Still, reliability was marginal at best. The APUs underwent a constant set of minor improvements during the flight program, but continued to be a source of irritation until the end.*60

On 22 May, North American conducted the first ground run of the interim XLR11 engine using X – 15-2 at the Rocket Engine Test Facility. Scott Crossfield was in the cockpit for the successful test, clearing the way for the eventual first powered flight-if X-15-1 could ever make its unpowered flight. Another attempt at the glide flight on 5 June 1959 aborted even before the NB-52 left the ground, when Crossfield reported smoke in the cockpit. Investigation showed that a cockpit ventilation fan motor had overheated. The continuing problems with the first glide flight were beginning to take their toll, both physically and mentally, on all involved.*61

Because of the lessons learned on the aborted glide flights and during the XLR11 ground runs, engineers modified numerous pieces of equipment on the X-15. These included the APUs, their support brackets, the mounting bulkheads, and the bearings inside them. North American also improved the flight control system’s mechanical responsiveness. In addition, technicians accomplished a great deal of work on the various regulators and valves, particularly in the hydrogen-peroxide systems. Storms remembered, "[I]n the final analysis, the regulators and valves were the most troublesome hardware in the program insofar as reliability was concerned.’,[62]

Because so many unknowns still existed, the Air Force and NASA decided to conduct a thorough post-flight inspection of the entire airplane surface for each mission, and to monitor the ablator char depth and back-surface temperatures throughout the performance buildup. The ablator weighed 125 pounds more than planned and, taken with the expected increase in drag, the maximum speed of airplane was expected to barely exceed Mach 7.-333-

Pete Knight made the first flight (2-52-96) in the ablator-coated, ramjet-equipped X-15A-2 without the external tanks on 21 August 1967. The flight reached Mach 4.94 and the post-flight inspection showed that, in general, the ablator had held up well. The leading-edge details on the wings and horizontal stabilizers had uniform and minor charring along their lengths. A careful examination revealed only minor surface fissuring with all char intact, and good shape retention. The char layers were approximately 0.050 inch deep on the wing leading edge and 0.055 inch deep on the horizontal stabilizer-well within limits.-334-

The ablator details for the canopy and dorsal vertical stabilizer showed almost no thermal degradation, and local erosion and blistering of the wear layer were the only evidence of thermal exposure. The leading edge of the forward vane antenna suffered local erosion to a depth of 0.100 inch because of shock-wave impingement set up by an excessively thick ablator insert over the ram air door just forward of the antenna. The remainder of the leading-edge detail on the aft vane antenna showed "minor, if not insignificant degradation."!335-

The most severe damage during the flight was to the molded ablator detail on the leading edge of the modified ventral, which showed heavy charring along its entire length. "The increased amount of thermal degradation was directly attributable to the shock wave interactions from the pressure probes and the dummy ramjet assembly." Additional shock waves originated from the leading edges of the skids. The shock impingement had apparently completely eroded the lower portion of the detail, and very little char remained intact. However, it was difficult to ascertain how much of the erosion had taken place during the flight since sand impact at landing had caused similar, although much less significant, erosion during an earlier ablator test flight. Still, this should have been a warning to the program that something was wrong, but somehow everybody missed it.-336-

The primary ablative layer over the airplane experienced very little thermal degradation as the result of this Mach 5 flight. On the wings and horizontal stabilizers, only the areas immediately adjacent to the molded leading edges were degraded. These areas exhibited the normal random reticulation of the surface, there was no evidence of delaminating, and all material was intact. Some superficial blistering of the ablative layer was evident on the outboard lower left wing surface. Martin Marietta believed the blistering was most probably the result of an excessively thick wear-layer application. Not surprisingly, the speed brakes exhibited some wear, but there was no significant erosion or sign of delamination. The only questionable area of ablator performance was the left side of the dorsal rudder. A number of circular pieces of ablator were lost during the flight. A close examination of the area revealed that all separation had occurred at the spray layer interface, and significant additional delamination had occurred. The heavy wear layer, however, had held most of the material in place. The program had seen similar delamination during some of the earlier test flights and traced it to improper application of the material. It then changed the installation procedures to prevent reoccurrence.[337]

After the inspection, Martin Marietta set about repairing the ablator for the next flight. With the exception of the leading-edge details for the ventral stabilizer and the forward vane antenna, refurbishment was minimal. Only the wings and horizontal stabilizers had experienced any degree of charring, and technicians refurbished them by sanding away the friable layer. They did not attempt to remove all of the thermally affected material, and sanding continued only until they exposed resilient material. The canopy leading-edge detail required no refurbishment, but Martin lightly sanded its surface to remove the wear layer to minimize deposition on the windshield during the next flight.[338]

As it finally rolled out of the paint shop at the Flight Research Center, the MA-25S-covered X – 15A-2 was the polar opposite of what an X-15 normally looked like. Instead of the black Inconel X finish, the airplane had a protective layer of white Dow DC90-090. Actually, this was a relief to the pilots since the MA-35S has a natural pink color. (NASA)

Martin repaired local gouges in the main MA-25S ablator using a troweled repair mix and kitchen spatulas, eliminating most of the sanding usually required for the patches. Technicians sanded the speed brakes to apparent virgin material and resprayed the areas to the original thickness. They also completely stripped the left side of the dorsal rudder and reapplied the ablator from scratch using the revised procedure.-1339

Although the ablator around the nose of the airplane had experienced no significant thermal degradation, a malfunction of the ball nose necessitated its replacement, and the ablator was damaged in the process. Unfortunately, Martin had depleted the supply of ablator during the refurbishment process, forcing the use of a batch of MA-25S left over from a previous evaluation. For the final preparation the entire surface of the airplane was lightly sanded and a new coating of the replacement DC92-007 wear layer was applied.-349

In what was perhaps the first true "follow-on" experiment, in 1961 researchers used X-15-2 to test a coating material designed to reduce the infrared emissions of the North American B-70 Valkyrie bomber. One of the complaints frequently voiced against the B-70 was that it was a large target. Although the concept of "stealth" (a term not yet applied to the idea) was not far advanced in 1960, engineers at Lockheed and North American both understood that reducing the radar and infrared signatures of strategic aircraft would at least delay their detection by the enemy. For instance, Lockheed specifically intended the shape and materials of the A-12/SR-71 Blackbird to lower its radar signature. North American conducted several detailed studies into the infrared and radar signatures of the B-70 to provide a basis for reduction attempts.-68

During the very short YB-70 development period, the Air Force directed North American to investigate ways to reduce the probability that the B-70 would be detected. The company made preliminary investigations into applying various radar absorbing materials to the airframe, particularly the insides of the air intakes. However, most of the North American effort appears to have concentrated on reducing the infrared signature of the aircraft. Exhausting cool air around the J93 engines was one means of reducing the infrared signature of the B-70.

As part of its research, North American developed a "finish system" (i. e., paint) that provided a low emittance at wavelengths used by Soviet infrared detecting devices, and allowed most of the excess heat to be radiated from the surface in wavelengths that were not normally under surveillance. The finish used a low-emittance basecoat with an organic topcoat that was transparent to energy in the 1-6-micron range. The topcoat was strangely opaque and highly emissive at wavelengths between 6 and 15 microns. This finish was relatively invisible to infrared detecting equipment and still allowed the skin to radiate excess heat overboard to maintain its structural integrity.-^

was applied a 1-mil-thick mixture of 85% Ferro Enamaling no. AL-8 Frit and 15% Hommel no. 5933 Frit. The Type II basecoat was a mixture of 40% Hanovia silver resinate and 60% Hanovia L. B. coating no. 6593 applied 0.004 mil thick. The topcoat was a mixture of 74% 3M Kel-F no. 2140, 24% 3M Kel-F no. 601, and 2% Al2O3 applied 1 mil thick. Most probably, the topcoats would have been opaque silver instead of the white finish used on the two XB-70A prototypes.-1701

The finish system was somewhat difficult to apply to an aircraft as large as the B-70, but the engineers expected that further development would yield improvements in the process. The most difficult problem was that the underlying surface had to be highly polished prior to applying the basecoat. In addition, the basecoat of both finishes had to cure at 750°F, while the topcoat of the Type II finish had to cure at 1,000°F (creating almost a ceramic finish). Accelerated environmental tests indicated that the surface would prove durable on the stainless-steel sections of the B-70, but its long-term adhesion to titanium appeared to be weak. Both finishes were relatively immune to exposure to hydraulic fluid, fuels, oils, and other substances encountered during operational service.-1711

To obtain real-world flight experience, the Type I coating was applied to one panel on the vertical stabilizer of X-15-2 in March 1961 and flown on flight 2-13-26 by Bob White. Since Inconel X is a type of stainless steel, the test was relatively representative of the proposed B-70 installation.

No observable physical changes occurred during the Mach 4.43 flight, during which the aircraft’s exterior reached 525°F, and the engineers made no attempt to measure the infrared qualities of the coating during this single flight. It was made simply to determine whether the coating would survive the aero-thermo environment, and appears to have been successful.-721

During the mid-1960s, a proposed delta-wing modification to X-15-3 might have kept the program flying until 1972 or 1973. Unlike many proposals, such as the "orbital X-15," the delta wing was a real project and was the subject of a great deal of research and engineering.

The delta-wing X-15 grew out of hypersonic cruise research vehicle studies conducted during the early 1960s. A "hypersonic cruise" vehicle would spend minutes or tens of minutes at hypersonic velocities, in contrast to the original X-15 that spent only a few tens of seconds at that velocity. The delta-wing X-15 configuration used the third airplane with the MH-96 and the basic modifications made to X-15A-2. Proponents of the concept, particularly John Becker at Langley, found the idea very attractive. Becker opined that "the highly swept delta wing has emerged from studies of the past decade as the form most likely to be utilized on future hypersonic flight vehicles in which high lift/drag ratio is a prime requirement i. e., hypersonic transports and military hypersonic cruise vehicles, and certain recoverable boost vehicles as well."[287]

Researchers held a meeting at the FRC on 9-10 December 1964 to determine exactly what research could be undertaken with a delta-wing hypersonic-cruise X-15. Attendees included researchers from Ames and Langley, and of course groups from the AFFTC and FRC. Not surprisingly, Ames and Langley did not necessarily agree on the exact nature of the research, and NASA Headquarters asked each center to submit a position paper outlining its preferences.

The Langley paper included an evaluation of possible aerodynamic and heat-transfer experiments, in-flight engine-inlet tests, and various structural recommendations that might be applicable to the delta-wing X-15. The entire paper was remarkably short-only 11 pages, or about the same size as the original 1954 paper that had been the genesis of the X-15 program.*288

John Becker and David E. Fetterman, Jr. led the Langley group that defined the aerodynamic and heat-transfer experiments primarily concerned with evaluating the differences between data gathered during full-scale flight tests and the results of wind-tunnel and analytical data. They noted that "[s]ince air-breathing hypersonic cruise vehicles will fly at such large Reynolds numbers that turbulent conditions will occur over the entire wing, it is obviously of great importance to establish the turbulent heat-transfer characteristics of delta wings. The lack of any rigorous theory to turbulent heat transfer places great emphasis on experimental determinations.

It has proved impossible, however, to achieve natural turbulent boundary layers on delta wing models in hypersonic ground facilities except over the rearward regions." Becker and Fetterman used similar logic to justify investigations into areas such as turbulent skin friction and Reynolds analogy, turbulent boundary-layer profile surveys, interference heating, and pressure distributions.-128^

The discussion of engine-inlet testing led to a rather surprising conclusion: "an inlet flight test program on the X-15 is not recommended." J. R. Henry believed that ground facilities such as the 20-inch hypersonic tunnel at Langley and the 3.5-foot hypersonic tunnel at Ames were more than adequate for research into inlet configurations up to Mach 8. Henry’s case was convincing, and despite the ongoing interest in the HRE experiment, Langley dropped engine-inlet testing from further consideration for the delta wing.[290]

Structural considerations were not so much possible experiments, but rather recommendations prepared by J. C. Robinson representing the views of the Structures Research Division at Langley. The primary recommendation was to manufacture the main wing structure from one of the nickel – or cobalt-base "superalloys" using simple construction methods such as "corrugated webs welded to machined cap members with a machined waffle plate skin between the cap members and welded or riveted to them." It was also recommended that the leading-edge material "should be refurbishable, fabricated of thoria-dispersed nickel or a refractory metal, radiation cooled, and should have expansion joints to accommodate differences in thermal expansion between it and the main structure."[291]

The Ames position paper submitted by J. Lloyd Jones was more extensive, consisting of 18 pages using much smaller typeface. The paper also levied some criticism of the existing X-15 program.[292]

Ames agreed with Langley that the primary area of research should be turbulent boundary layers "because of the difficulty of providing turbulent boundary layer flow on models in wind tunnel tests at hypersonic speeds." Ames noted that the existing X-15 had already validated wind-tunnel results up to Mach 5, and the Mach 8 data would be a natural extension. Ames also pointed out that the delta-wing X-15 "is more representative of hypersonic cruise aircraft configurations" and would therefore yield more useful data. Ames also believed that "knowledge that will accrue of the handling qualities of a delta winged vehicle representative of current concepts of airbreathing cruise configuration will certainly be of value."[293]

The researchers at Ames seemed to want to take the lead on the delta-wing configuration, much as Langley had done on the original X-15, and their list of potential research areas was a good deal longer. Besides turbulent-boundary-layer research, Ames wanted to look at skin-friction surveys, pressure distributions and local flow fields, boundary-layer-transition definitions, ablative studies, panel-flutter studies, and low-speed and hypersonic-handling qualities. To accompany these experiments, Ames believed that "an increased program of ground based research relating to hypersonic cruise aircraft technology should be initiated."[294]

Although Ames acknowledged that "the X-15 program to date unquestionably has been very successful" and had turned into "a research facility with which to conduct studies never envisioned at its inception," it believed the program was open to some criticism. "For example, measurement of heat transfer was one of the major research experiments conducted on the airplane, but design inflexibility resulted in the acquisition of heat transfer data in an environment which compromised their value to the extent that they cannot be fully explained nor understood."[295]

To overcome this fault on the new airplane, Ames indicated, "the key to the potential benefits…

for a delta wing X-15 program lies in the design of the airplane to accomplish well defined experimental tasks as primary objectives." To this end, Ames made several recommendations for the vehicle configuration:12961

1. Provision for removable test panels in the areas selected for measurements… to provide for heat transfer, boundary layer and structural tests on both experimental and prototype panels. Design of the test panels and supporting structure should be guided by the requirement to avoid localized heat sinks that withdraw heat away from the boundary layer in a non-uniform manner.

2. Provide a smooth primary test area along the bottom centerline of the airplane free of all protuberances. This consideration may well lead to the requirement for a low-wing configuration to insure a test region with a definable uniform flow field.

3. A removable fuselage nose section ahead of the wheel well with an instrument compartment and a removable tip. This feature would provide for replacement nose sections having different geometry.

4. An instrumentation bullet located at the wing-fin juncture may be advantageous.

Ames went on to describe an elaborate research program that began with wind-tunnel studies, followed by flights using X-15A-2, and finally tests with the delta-wing aircraft. The program would concentrate most of the theoretical and wind-tunnel work at Ames. Based on the scope of the work described, it probably would have been expensive, although the report did not include a cost estimate.

After reviewing the papers submitted by Ames and Langley, with additional input from the Air Force and FRC, NASA defined two primary objectives for the delta-wing X-15 program:12921

1. Aerodynamic research-The flight tests would provide realistic aerodynamic data under fully developed turbulent flow conditions to supplement ground-based research where such conditions cannot be achieved. Answers would be obtained to key questions relating to hypersonic aerodynamics of delta wings, large-scale behaviors of flap-type controls, tip-fin interference effects, and handling qualities of a configuration typical of present thinking for a future hypersonic air-breathing vehicle. Aerodynamic research on this vehicle would be unclouded by propulsion effects, inasmuch as most of the data would be taken under gliding conditions.

2. Structural research-The delta-wing proposal would permit the evaluation in a practical flight application of a hot radiation-cooled structure designed for repeated flights at temperatures between 1,500 degrees and 2,200 degrees Fahrenheit. It would also focus technical effort on a refurbishable, hot-wing leading edge design.

Paul Bikle concluded that "[i]n general, a delta-wing X-15 program could establish a baseline of confidence and technology from which decisions regarding the feasibility and design of advanced air-breathing vehicles could be realistically made. The proposed time for the delta X-15 fits well with that for an overall hypersonic research vehicle program and the cost does not appear to be unreasonable."12981

Things seemed to be progressing rapidly. In January 1965 the FRC drafted a statement of work for North American to conduct a detailed study of the delta-wing concept. This document indicated that "NASA is considering a hypersonic cruise vehicle research program which involves a modification of the basic X-15 configuration for study of various aerodynamic, structural, and flight control problems. The program will also include limited investigations of flight to altitudes extending to about 180,000 feet." Extreme high-altitude research was not a requirement.12991

NASA suggested that an existing "X-15 airplane would be modified to incorporate a representative slender hypersonic wing substituted for the present wing and horizontal tail. The wing structure would be designed for sustained hypersonic flight at a Mach number of 7, but would also be capable of flight to Mach number 8 for limited time periods. The basic X-15 fuselage structure, rocket engine, flight control and other systems would be retained with minimum modification, and the present B-52 launch system and high range facility would be utilized."13001

The work statement went on to indicate that the airplane should have a load limit of 5 g and a

2,0- psf design dynamic pressure. Potential "improvements" included relocating the wing to be flush with the bottom of the fuselage, increasing the dynamic pressure to 2,500 psi and the load factor to 7.33 g, including external propellant tank(s), and relocating the nose landing gear. Other possibilities included the addition of a permanent thermal-protection system (in lieu of ablative coatings) over the fuselage to prevent contamination of the wing with ablative products.-13011

The delta wing seemed to languish at the FRC for the remainder of the year while engineers put together a project development plan. By the end of the year, the FRC had released the second draft of the plan for internal review, providing more detail on how things might progress. The opening paragraph provided the justification for the program:13021

Three of the most probable uses for hypersonic airbreathing aircraft are transport over long ranges, military reconnaissance, and as maneuverable reusable first-stage boosters. There are currently no military or civilian requirements of over-riding importance for any one of these. Their potential, however, constitutes a clear justification to proceed with comprehensive programs to develop the required hypersonic technology.

Unfortunately, the justification also included a rationale for not supporting the program, given the budget crunch NASA was experiencing as it continued the Apollo program to the detriment of the aeronautics budget. Nevertheless, the FRC pressed on with detailed plans. The FRC considered the delta-wing project an extension of the X-15 program, and assumed that all existing agreements between the Air Force and NASA would continue. Researchers believed that the manpower requirements for the program could be satisfied "with the present complements of the Langley, Ames, and Flight Research Center."13031

Researchers at the FRC estimated the program would cost $29,750,000 spread between FY67 and FY73. Of this, the airframe contractor would receive $24,600,000 to build the flight vehicle, while the other $5,150,000 would be "in-house" expenses. The second year represented the largest annual expenditure: $14,500,000 to the contractor and a little over $1,000,000 in-house. The planners warned, however, that "if the military withdraws their operational support from the general X-15 program, this project would be responsible for additional expenses over the 4-1/2 year operational period. These expenses could amount to as much as 17 million dollars." The preliminary schedule showed a request for proposals in August 1966, a contract award in March 1967, modifications to X-15-3 between December 1967 and October 1968, and a first flight in January 1969. The 37-flight research program continued until December 19 72.13041

In the project plan, the FRC had expected the X-15 Project Office at Wright-Patterson AFB to handle the procurement of the modifications, although NASA would pay the bills. Although it would seem logical for North American to perform the modifications, several other contractors (Lockheed, Northrop, and Republic) had expressed interest in the program, so the FRC proposed to make it a competitive process.13051

The Air Force was not totally in favor of this since they saw their involvement with the X-15 winding down, and had little apparent interest in the delta-wing program. Despite negotiations between the Air Force and NASA at various levels, the X-15 Project Office declined to participate in the expected procurement of the delta-wing airplane; however, it did agree to transfer the X – 15-3 airframe to NASA for the modification at the end of its flight program.

After nearly two years of delay, on 13 May 1967 the delta-wing program had progressed far enough for the FRC to issue a request for proposals (PR-7-174) for a formal conceptual design study. The primary objectives of the study were to 1) develop a preliminary design for evaluating the modification of X-15-3 to a delta-wing configuration, and 2) formulate an accurate estimate of performance, weight, cost, and schedule for such modification. A secondary objective was to analyze alternate approaches, such as unsegmented leading edges; eliminate the use of ablatives; and incorporate a fly-by-wire control system, multiplane airfoils, symmetrical tip fins, and different propulsion systems.-1306

There appears have been only a single respondent to the request for proposals. North American submitted a two-volume, 500-page proposal containing detailed engineering concepts and cost data-and that was just the proposal to do the study! By this time, North American had already been testing the delta-wing X-15 in wind tunnels for over a year. The North American low-speed and hypersonic tunnels and the Langley 20-inch hypersonic tunnel had tested 1/15-scale and 1/50-scale models at Mach numbers between 0.2 and 6.9 and Reynolds numbers up to 10,000,000 (equivalent, based on model length).-307

The proposed North American delta-wing X-15-3 was not a simple conversion. The 603-square – foot delta-wing planform had a 76-degree leading-edge sweep, but the span was the same as that of the original X-15 to ensure that there would be no clearance issues with the NB-52 carrier aircraft. Elevons (30.8 square feet each) at the trailing edge provided longitudinal and roll control by deflecting up to 4.5 degrees up or 5.0 degrees down. The existing dorsal and ventral stabilizers provided directional stability with the addition of wing-tip fins, and the existing dorsal rudder provided directional control. Engineers could adjust the removable tip fins on the ground for cant and tow-in, which allowed them to change the relative levels of directional and lateral stability to investigate the handling qualities of the vehicle. The tip fins compensated for the blanking of the normal centerline vertical stabilizers by the fuselage at hypersonic speeds and large angles of attack.308

The position of the wing was the subject of a great deal of study since the HRE was expected to be carried on the ventral stabilizer (as the dummy was on X-15A-2), and the aircraft also had to be stable in flight without the 1,000-pound engine. It proved to be a difficult problem. The final answer before NASA terminated the program was to position the wing in the best location to compensate for the HRE. On flights without the engine, as much research equipment or ballast as possible would be located in a new aft experiment compartment. Likewise, the shape of the wing leading edge was of some concern, but North American noted that "the effect of leading edge radius on the low-speed aerodynamic characteristics of highly swept delta wings is not well understood." Engineers did not think the effect on the leading-edge shape at supersonic speeds would be significant, because positive pressure on the wing lower surface would produce most of the lift. Nevertheless, since the leading-edge shape significantly influenced the landing characteristics, North American investigated various configurations in its low-speed wind tunnel. The company had not found a satisfactory answer, and was awaiting further data from NASA wind-tunnel tests, when the program ended.309

During the mid-1960s, a proposed delta-wing modification to X-15-3 might have kept the program flying until 1972 or 1973. Unlike many proposals, such as the "orbital X-15," the delta wing was a real project and was the subject of a great deal of research and engineering. Despite endorsements from the Flight Research Center and John Becker at Langley, support remained lukewarm within the Air Force and NASA. The FRC was still evaluating the proposals for the delta­wing study on 15 November 1967 when the crash of the X-15-3 effectively ended all thought of such a modification. (NASA)

North American was also somewhat uncertain about the leading-edge material, mainly because of the expected 2,200°F temperatures encountered on the design mission. North American built a segmented leading edge made from columbium alloy that successfully passed tests at 2,400°F. This appeared satisfactory, at least for initial use. The company expected that no available material would prove satisfactory for the lower surface of the wing, and that some form of thermal protection system would have to be developed. Unsurprisingly, many of the North American ideas looked similar to concepts the company was investigating for the space shuttle. One of the most promising ideas was to use metallic heat shields supported by standoff clips with a layer of low – density insulation sandwiched between the shield and the wing skin. Only an area about 2 feet wide just behind the leading edge needed this type of protection since the airflow further aft smoothed out sufficiently to keep temperatures within the ability of alloys such as TD nickel to

Г3101

Originally, North American had envisioned using upswept wing tips to replace the directional stability lost by the removal of the ventral rudder from the delta-wing configuration. Although the X-15 program seldom used the ventral rudder, this was because most missions flew at high angles of attack, where the lower rudder was detrimental to stability. The delta-wing program, on the other hand, wanted to fly sustained high-speed cruise missions that would require little high – angle-of-attack work. The initial round of tests in the North American hypersonic tunnel revealed that the upswept wing tips were inadequate above Mach 6. Researchers tested various configurations in both the North American hypersonic tunnel and the 20-inch Langley hypersonic tunnel until they found a set of tip fins that extended both above and below the wing centerline to be adequate. Nevertheless, engineers decided to make it easy to replace the fins just in case the wind-tunnel tests proved to be inaccurate.13111

The large-angle-of-attack capability of the basic X-15s was no longer required since researchers did not intend the mission to go to high altitudes. Given that the maximum angle of attack envisioned for the new airplane was less than 15 degrees, engineers decided to use a fixed-flow direction sensor to sense the angle of attack and sideslip. A hemispherical nose with five pressure taps could provide an air-data computer with sufficient information to derive the necessary angles without the complexity and weight of the ball nose. Conceptually, this was identical to the fixed alpha nose flown during six of the last X-15-1 flights.13121

North American proposed to stretch the fuselage 10 feet to 62.43 feet overall, and to manufacture what was essentially a new fuselage from the cockpit rearward. The company would stretch the propellant tank section 91 inches and provide new mounting provisions for the delta wing. North American manufactured test specimens from Rene 41 and Inconel 718 to determine which would be the best material for this area; these tests were in progress when NASA canceled the program. The space between the liquid-oxygen and ammonia tanks accommodated the standard center-of – gravity instrumentation compartment, and North American added a new 29-inch-long compartment behind the fuel tank but ahead of the engine to hold fuel and gases for the HRE. Designers also wanted to replace the existing ogive forward fuselage (in front of the canopy) with a 20-degree, included-angle-cone section. This semi-monocoque structure would use Rene 41 outer skin and Inconel X (or Inconel 718) frames, and titanium would be used for the inner skin of the equipment compartment.13131

North American investigated several different powerplants for the airplane, with the leading challenger being an Aerojet YLR91-AJ-15 from the second stage of a Titan II ICBM. This engine used unsymmetrical dimethylhydrazine and nitrogen tetroxide as propellants, and had already been man-rated for the NASA Gemini program. When equipped with a 25:1 nozzle, this engine completed the reference mission without the use of external propellant tanks. In fact, at a launch weight of 52,485 pounds and a burnout weight of 18,985 pounds, the YLR91 would have allowed a maximum velocity of 8,745 fps, well in excess of the 7,600 fps required by NASA. The effect of carrying the 1,000-pound HRE would have reduced this by about 400 fps.13141

North American briefly investigated the idea of using a separate "sustainer" engine to provide thrust to overcome drag during hypersonic cruise. Although the integration issues involved with incorporating a second engine and its propellants into the airframe eventually convinced all concerned that it would be too difficult, the particular engines investigated show how a program could come full circle. One of the engines investigated was the Bell YLR81-BA-11, a variation of the one of the engines proposed to power the X-15 in 1954. North American also investigated several variants of the Reaction Motors LR11 family that had been used for the initial X-15 flights,

along with the Aerojet LR52 (AJ-10).-1315

The engine that was ultimately selected was a modified XLR99 that provided 83,000 Ibf at 100,000 feet and was throttleable down to 8,000 lbf for sustained cruise. This was the version of the XLR99 that used a single thrust chamber and nozzle, not the Reaction Motors concept that used a second, remotely located alternate chamber. The increased internal fuel would permit sustained flights at Mach 6.5 using the low-thrust "sustainer" capability of the modified XLR99 to overcome drag but not produce any acceleration. The addition of a single centerline external tank would allow Mach 8 flights.-316

The main landing gear would be a version of the gear developed for the X-15A-2, appropriately strengthened for the almost 19,000-pound normal landing weight of the delta-wing design. As on the X-15-A-2, North American proposed to use both short and long versions of the rear shock struts; the long ones would provide clearance for the HRE under the ventral, while flights that did not carry the HRE would use the short ones. The nose gear would be moved to the instrument compartment behind the pilot, and the recorders and other research instrumentation normally carried there would be moved to a new compartment in front of the pilot where the original nose gear well was.-1317!

Although it was not part of the delta-wing baseline, North American was investigating the use of a fly-by-wire control system on the delta-wing X-15. Engineers believed that this would reduce the overall system size, weight, and volume, and provide better overall performance. This system would have used an analog flight-control system, not a digital one. The MH-96 adaptive control system was capable of accepting electrical inputs that were equivalent to flying in a fly-by-wire mode, and Honeywell designed the MH-96 to interface to the fly-by-wire flight-control system in the Dyna-Soar. In X-15-3 these were paralleled with mechanical linkages, and the delta wing could eliminate these mechanical linkages altogether. Given the facts that a fly-by-wire system had never flown, and the delta-wing airplane was flying in a new performance envelope anyway, NASA was not supportive of this effort.316

The final delta-wing design spent a considerable amount of time in this NASA Langley wind tunnel before the program was cancelled. (NASA)

Although the delta-wing airplane was generally described as a modification of X-15-3, about the only structure from the original airplane that would remain would be the cockpit and the aft thrust structure. Most of the electronics (i. e., the inertial system and MH-96) would also remain.

However, at least by weight, the majority of the aircraft would be new. Since the North American study had not been completed when X-15-3 was lost, the company had not estimated the final cost, but the amount would probably have been substantial.

Although no formal contracting arrangement existed, North American pressed on with a great deal of research into the delta-wing configuration. By March 1967, wind-tunnel models had accumulated over 300 hours of testing, with a 1/50-scale model used for high-speed tests and a 1/15-scale model used for low-speed tests. According to a North American news release, "the four year research program has also enabled North American to check out the integrity of components using new super alloys that will be required at hypersonic speeds. Tanks, wing sections, and other components have been fabricated of such materials and put through exhaustive thermal and structural tests."[319]

Despite endorsements from the FRC and John Becker at Langley, support remained lukewarm within the Air Force and NASA. The FRC was still evaluating the proposals for the delta-wing study on 15 November 1967 when the crash of the X-15-3 effectively ended all thought of such a modification. Since the general concept depended upon the use of the electronic systems that were unique to X-15-3, most researchers did not consider it readily possible to convert one of the other airframes; besides, the accident effectively sealed the fate of the entire X-15 program.-132^

Forrest "Pete" Petersen flew the X-15 for 15 months from 23 September 1960 until 10 January 1962, making five flights. These included two flights with the XLR11 and three flights with the XLR99. He reached Mach 5.30, a maximum speed of 3,600 mph, and an altitude of 101,800 feet.

Forrest Silas Petersen was born on 16 May 1922 in Holdrege, Nebraska. After he graduated from the Naval Academy in June 1944, he reported to the destroyer USS Caperton (DD 650) and participated in campaigns in the Philippines, Formosa, and Okinawa. Petersen switched from the "black shoe" Navy to "brown shoes" when he graduated from flight training in 1947 and was assigned to VF-20A.

Petersen completed two years of study at the Naval Post Graduate School and received a bachelor of science degree in aerospace engineering. He continued his studies at Princeton University and received a master’s degree in engineering in 1953. In 1956 he attended the Naval Test Pilot School and remained as an instructor following graduation. The Navy assigned him to the X-15 program in August 1958, and he served with NASA until January 1962. He was a joint recipient of the 1961 Robert J. Collier Trophy presented by President John F. Kennedy at the White House in July 1962, and the NASA Distinguished Service Medal presented by Vice President Lyndon B. Johnson.

Petersen served as commanding officer of VF-154 prior to being assigned to the office of director, Division of Naval Reactors, Atomic Energy Commission, for nuclear power training. He reported to USS Enterprise in January 1964 and served as executive officer until April 1966. Petersen received the Bronze Star during Enterprise’s first combat tour in Vietnam. Afterward, he became an assistant to the director of naval program planning in the office of the chief of naval operations. In November 1967 he assumed command of USS Bexar (APA-237) and received the Navy Commendation Medal with Combat V. He later served as deputy chief of naval operations for air warfare, and commander of the Naval Air Systems Command. Vice Admiral Petersen retired from active duty in May 1980 and died of cancer in Omaha, Nebraska, on 8 December 1990.

The "High Altitude and High Speed Study" by the El Segundo Division of the Douglas Aircraft Company had been funded by the ONR as a follow-on to the D-558 research aircraft that loosely competed with the Air Force X-1 series. Duane N. Morris led the study under the direction of the chief of the Aerodynamic Section, Kermit E. Van Every. Although the concept is generally mentioned—briefly—in most histories of the X-15, what is almost always overlooked is how insightful it was regarding many of the challenges that would be experienced by the X-15 a few years later.129

By the spring of 1954, when the X-15 approval process began, Douglas had not accomplished a detailed design for a new airplane, but recognized many of the same problems as John Becker and the researchers at Langley. The Douglas engineers also examined peripheral subjects—carrier aircraft, landing locations, etc.—that the initial Langley studies did not address in any detail.121

One interesting aspect of the Douglas Model 671 was that the contractor and the Navy had agreed that the aircraft was to have two mission profiles: high speed and high altitude (with the emphasis on the latter). This was in distinct contrast to the ongoing Langley studies that eventually led to the X-15. Although the Becker team at Langley was interested in research outside the sensible atmosphere, there was a great deal of skepticism on the part of others in the NACA and the Air Force. Douglas did not have this problem—the ONR strongly supported potential high-altitude research.

Excepting the Langley work, the Douglas study was probably the first serious attempt to define a hypersonic research airplane. Most of the other companies investigating hypersonics were oriented toward producing operational vehicles, such as the ICBMs and BoMi. Because of this, they usually concentrated on a different set of problems, frequently at the expense of a basic understanding of the challenges of hypersonic flight. The introduction from the Douglas study provides a good background:11221

The purpose of the high altitude study…is to establish the feasibility of extending human flight boundaries to extreme altitudes, and to investigate the problems connected with the design of an airplane for such flights.

The project is partially a result of man’s eternal desire to go higher, faster, or further than he did last year. Of far more importance, however, is the experience gained in the design of aircraft for high-speed, high-altitude flight, the collection of basic information on the upper atmosphere, and the evaluation of human tolerance and adaptation to the conditions of flight at extreme altitudes and speeds.

The design of an airplane for such a purpose cannot be based on standard procedures, nor necessarily even on extrapolation of present research airplane designs. Most of the major problems are entirely new, such as carrying a pilot into regions of the atmosphere where the physiological dangers are completely unknown, and providing him with a safe return to Earth. The type of flight resembles those of hypersonic, long-range, guided missiles currently under study, with all of their complications plus the additional problems of carrying a man and landing in a proper manner.

The study consists of a first approach to the design of a high-altitude airplane. It attempts to outline most of the major problems and to indicate some tentative solutions. As with any preliminary investigation into an unknown regime, it is doubtful that adequate solutions have been presented to every problem of high-altitude flight, or even that all of the problems have been considered. It would certainly appear, however, that the major difficulties are not insurmountable.

The Model 671 was 41.25 feet long (47.00 feet with the pitot boom), spanned only 18 feet with 81 square feet of area, and had an all-up weight of 22,200 pounds. In many respects, it showed an obvious family lineage to the previous D-558s. The fuselage consisted of a set of integral propellant tanks, and dive brakes were located on each side aft, as in most contemporary fighters.

A conventional configuration was deliberately chosen for the study, and no benefits have yet been discovered for any unconventional arrangement. Actually, for the prime objective of attaining very high altitudes, the general shape of the airplane is relatively unimportant. Stability and control must be provided, and it must be possible to create sufficient lift for the pullout and for landing; but, in contrast to the usual airplane design, the reduction of drag is not a critical problem and high drag is to some extent beneficial. The planform of the wing is unimportant from an aerodynamic standpoint at the higher supersonic Mach numbers. Therefore, it was possible to select the planform based on weight and structure and landing conditions. These considerations led to the choice of an essentially unswept wing of moderate taper and aspect ratio.11231

The empennage of the Model 671 was completely conventional and looked much like that of the Mach 2 D-558-2 that preceded it. However, Douglas realized that the design of the stabilizers was one of the greater unknowns of the design. "The tail surfaces are of proper size for stability at the lower supersonic Mach numbers, but there is some question of their adequacy at very high supersonic speeds. Further experimental data in this speed range are necessary before modifications are attempted. In addition, it may be possible to accept a certain amount of instability with the proper automatic servo controls." Unlike the Becker group, Douglas did not have access to a hypersonic wind tunnel.[124]

Nevertheless, preliminary investigations at Douglas indicated that "extremely large tail surfaces, approaching the wing area in size, are required to provide complete stability at the maximum Mach number of about 7." Engineers investigated several methods to improve stability, with the most obvious being to increase the size of the vertical stabilizer. However, placing additional area above the fuselage might introduce lateral directional dynamic stability problems "due to an unfavorable inclination in the principle axis of inertia and the large aerodynamic rolling moment due to sideslip (the dihedral effect)." The preferred arrangement was to add a ventral stabilizer and keep the ventral and dorsal units as symmetrical as possible. However, Douglas recognized that a large ventral stabilizer would present difficulties in ground handling and during landing. The engineers proposed that the fin should be folded on the ground, unfold after takeoff, and then be jettisoned just before touchdown. Alternately, Douglas believed that some sort of autopilot could be devised that would allow the use of more conventional-sized control surfaces.11251

Douglas conducted an evaluation of available power plants, and reached much the same conclusions the X-15 program would eventually come to. The desired engine should produce about 50,000 lbf with a propellant consumption of about 200 pounds per second. The only powerplant that met the requirements was the Reaction Motors XLR30-RM-2 rocket engine, which used liquid oxygen and anhydrous ammonia propellants. The high (245 lbf-sec/lbm) specific impulse (thrust per fuel consumption) was desirable since it provided "a maximum amount of energy for a given quantity of propellant." The high density of the propellants allowed a smaller tank size for a given propellant weight, allowing a smaller airframe. However, the researchers worried that since the original application was a missile, it would be difficult to make the engine safe enough for a manned aircraft.-11261

Douglas had some interesting observations about drag and power-to-weight ratios:11271

The function of drag in the overall performance must be reconsidered. The effect of drag is practically negligible in the power-on ascending phase of flight (for a high altitude launch), because of the very large thrust to weight ratio. Throughout the vacuum trajectory, the aerodynamic shape of the airplane is completely unimportant. During the descending phase of flight, a large drag is very beneficial in aiding in the pullout, and the highest possible drag is desired within the limits of the pilot and the structure. In fact, during the pullout it has been assumed that drag brakes would be extended in order to decelerate as soon as possible. However, because of excessive decelerative forces acting upon the pilot, it is necessary to gradually retract the brakes as denser air is entered, until they are fully retracted in the later stages of flight.

For a given propulsion unit (i. e., fixed thrust and fuel consumption), the overall performance of the present design [Model 671] is much more dependent upon the ratio of fuel weight to gross weight that it is upon the minimum drag or the optimum lift-drag ratio. Even though the fuel is expended in approximately the first 75 seconds of flight (a relatively small fraction of the total flight time), the ultimate performance as measured by the maximum altitude is affected to a great extent by small changes in the fuel to gross weight ratio. As an example, an increase in fuel weight/gross weight from 0.65 to 0.70 results in an increase in peak altitude of about 35% for a typical vertical flight trajectory, other parameters remaining

constant.

To better understand the nature of the various propellants then available for rocket engines, engineers reviewed numerous reports by the Caltech Jet Propulsion Laboratory, the NACA, and RAND. Only two oxidizers—oxygen and either red fuming or white fuming nitric acid-seemed to offer any increase in performance. Douglas was seeking better propellants than the liquid oxygen and alcohol used in the Reaction Motors LR8, effectively ruling out nitric acid since it was less dense than oxygen. The available fuels were alcohol (CH3OH or C2H5OH), anhydrous ammonia (NH3), hydrazine (N2H4), and gasoline. Alcohol offered no improvement, and hydrazine was too expensive and too difficult to handle safely, narrowing the choice to anhydrous ammonia and gasoline. Interestingly, Douglas ruled out liquid hydrogen because "on the basis of density, hydrogen is seen to be a very poor fuel." It would be 20 years before the Centaur upper stage would prove them wrong.-1128!

The Douglas Model D-671 was a proposed follow-on to the successful D-558 series of research airplanes developed under Navy auspices and flown at the High-Speed Flight Station. Preliminary investigation showed the concept was capable of roughly the same performance as the eventual X-15, but the Navy declined further development of the Douglas concept when it joined the X-15 program in late 1954. (Douglas Aircraft Company)

An auxiliary power unit (APU) rated at about 8 horsepower was necessary to support the electrical requirements of the instruments, controls, and radio. Investigation showed that the lightest alternative would be a small turbine generator using hydrogen peroxide or ethylene oxide monopropellant. The Walter Kidde Company and American Machine and Foundry Company were developing units that could satisfy the requirements. Both companies claimed they could develop a 10-horsepower hydrogen peroxide unit that weighed about 56 pounds, including propellants for 30-horsepower-minutes. Given the trouble of the future X-15 APUs, perhaps North American should have better reviewed this part of the Douglas report.-129!

to obtain reasonable estimates." They continued that "it is unfortunate that the largest contributing factor to the high temperatures of reentry, the convective heating from the boundary layer, is the one about which there is the least knowledge." Nevertheless, they took some educated guesses.[130]

The expected average heat level approached 1,400°F, with peak temperatures above 3,300°F on the wing leading edges and nose. Douglas believed "it would be impossible to design a structure for this temperature [1,400°F] which satisfies both the stress and weight requirements…." To overcome this, engineers recommended the use of some as-yet-undeveloped "good insulating material" with a density of 20 pounds per cubic foot and an insulating value of 0.20 British Thermal Units (Btu) per pound. For the purposes of the study, Douglas used a C – 110M titanium – alloy structure and skin protected by an unspecified ablative coating. Water sprayed into stainless-steel sections of the wing leading edges and nose area allowed superheated steam to remove unwanted heat, keeping these areas below their melting points. Alternately, Douglas investigated injecting cool gas (bottled oxygen) into the boundary layer to provide cooling. The study noted, however, that "none of these systems have yet been proven by practical application." The designers protected only a few areas, such as the cockpit, with batt insulation since the study assumed no heat transfer to the interior of the aircraft.-1131

Not surprisingly, Douglas chose an air-launch configuration. What is interesting is that the launch parameters were Mach 0.75 at 40,000 feet—well beyond the capabilities of anything except the Boeing B-52, which was still in the early stages of testing. Douglas summarized the need for an air launch by noting that "[t]he performance is increased, but the prime reason for the high altitude launch is the added safety which 40,000 feet of altitude gives the pilot when he takes over under his own rocket power." Trade studies conducted by Douglas indicated that an increase in launch altitude from sea level to 40,000 feet would result in a 200,000-foot increment in maximum altitude on a typical high-altitude mission. Additional benefits of a higher launch altitude diminished rapidly above 40,000 feet since most of the initial improvement was due to decreasing air density.-132

interested in the high-altitude research and at one point estimated the D-671 could reach 1,000,000 feet altitude. Although Douglas only conducted a minimal amount of research into the concept before it was cancelled, they foresaw many of the issues that would ultimately confront the X-15 development effort. (Douglas Aircraft Company)

Engineers spent a great deal of time studying possible flight paths, but "no attempt has been made in the present study to determine an absolute optimum flight path, because of the large number of variables involved." The designers noted that the airframe and propulsion systems could theoretically support a maximum altitude in excess of 1,130,000 feet; however, based on a conservative pullout altitude of 30,000 feet, the vehicle was more realistically limited to 770,000 feet. The pullout altitude (and the limiting decelerations, which were really the issue) was "directly traceable to the single limiting factor of the presence of a human pilot." The 770,000-foot, 84- degree profile resulted in a 10-g pullout maneuver, about the then-known limit of human tolerance.-1133!

Some thought was given to using a "braking thrust," which would allow a small amount of propellant to be saved and used during reentry. Either a mechanical thrust reverser would be installed on the rocket engine, or the airplane would reenter tail-first. This technique would have allowed slightly higher flights by reducing the stresses imposed by the pullout maneuver, although less propellant would be available for the ascent. The designers did not pursue this concept since entering tail-first involved undesirable risks, and the mechanical complexity of a thrust reverser seemed unnecessary, at least initially.!134!

The theoretical maximum performance was 6,150 mph and 190,000 feet for the speed profile, and 5,200 mph and 1,130,000 feet for the altitude profile (but limited, as discussed above). Landings would be made at Edwards AFB because of its "long runways and considerable latitude in the choice of direction and position of touchdown." The study noted that there would be little opportunity to control either the range or the heading by any appreciable amount after engine burnout. "Since the airplane must land without power at a specified landing site, it is obvious that it must be aimed toward the landing site at launch." Douglas estimated that a misalignment of 5 degrees in azimuth at burnout would result in a lateral miss of over 45 miles.!133!

One of the concerns expressed by Douglas was that "rocket thrust will not be sufficiently reproducible from flight to flight, either in magnitude or in alignment." Engineers estimated a thrust misalignment of less than one-half of a degree could impart 500 pounds of side force on the aircraft, causing it to go significantly off course. Researchers investigated several possible solutions to thrust misalignment, including using a larger rudder, using the auxiliary reaction control system, installing movable vanes in the exhaust,!136! performing gas separation in the nozzle,-!137! and mounting the rocket engine on a gimbal. All of these methods contained various problems or unknowns that caused the engineers to reject them. Further consideration showed that thrust misalignment was largely a non-issue since early low-speed flights would uncover any deficiencies, allowing engineers to correct them prior to beginning high-speed flights.!138!

The estimated landing speed was 213 mph, with a stall speed of 177 mph. Engineers accepted this relatively high speed "given the experimental nature of the aircraft and the high skill level of the pilots that will be flying it." The study noted that the slower speeds were possible if high-lift leading-edge devices were used or the area of the wing was increased. However, the increased weight and/or the resulting complications in the leading-edge cooling system appeared to make these changes undesirable.!139!

The high-altitude profile would use "flywheels, gyroscopes, or small auxiliary jets" for directional control outside the atmosphere, with Douglas favoring hydrogen peroxide jets in the wing tips and at the rear of the fuselage. Flywheels were rejected because they were too complex (for a three- axis system), and gyroscopes were too heavy. Each of the hydrogen peroxide thrusters would generate about 100 lbf and use 1 pound of propellant per second of operation. The engineers arbitrarily assumed that a 25-pound supply of propellant was required since no data existed on potential usage during flight. A catalyst turned the liquid hydrogen peroxide to steam at 400-psi pressure.-114^

The projected performance of the airplane caused Douglas engineers to investigate escape capsules for the pilot: "Because of the high altitude and high speed performance of the aircraft, it is believed that all ordinary bailout procedures, such as escape chutes and ejection seats, are of no value to the pilot." At the time, Douglas believed that ejection seats were only "suitable up to a Mach number of approximately one at sea level, with somewhat higher speeds being safe at higher altitudes." Instead, the engineers decided to jettison the entire forward section of the fuselage, including the pilot’s compartment, much like the Bell X-2. The total weight penalty for the capsule was about 150 pounds. The study dismissed pressure suits, stating that "it is very doubtful that sufficient pressurization equipment could be carried by the pilot during…ejection… to sustain suit pressurization from the maximum altitude to a safety zone within the earth’s lower atmosphere." Douglas stated flatly that "an ejection seat or other ordinary bailout techniques will be inadequate in view of the problem of high speeds and high altitudes." Scott Crossfield would later disagree.-1141!

In order to withstand the reentry temperatures, the cockpit windscreen used two 0.5-inch layers of quartz with a 0.25-inch vented air gap between them. This would keep the inner windscreen below 200°F. A thin sheet of treated glass placed inside the inner quartz layer reduced ultraviolet and other harmful radiation. Although the potential dangers of radiation above the atmosphere were largely unknown, Douglas predicted that little harm would come from the short flights (a few minutes) envisioned for the D-558-3. However, "proper precautions to prevent any one pilot from making too many successive flights in a weeks or months time interval should be taken….’,[142]

One of the technical innovations of the eventual X-15 program was the "ball nose" that sensed the angle of attack and angle of sideslip during high-speed and high-altitude flight. The Douglas study foresaw the need for a new pitch and yaw sensor "capable of sensing exceedingly low forces or pressures, but capable of withstanding the maximum dynamic pressures encountered during the complete pullout." However, Douglas thought that "the instrument need not be precise, for it is only to serve as a guide for pointing the nose into the wind at heights where a pilot might otherwise lose all sense of orientation." Four possible solutions emerged:[143]

By the beginning of the X-15 program, the WADC Aero Medical Laboratory had only partly succeeded in developing a full-pressure suit, almost entirely with the David Clark design. This led to a certain amount of indecision regarding the type of garment needed for the X-15. However, North American proposed the use of a full-pressure suit as a means to protect the pilot during normal operations and emergency escape.

Despite the early state-of-development of full-pressure suits, Scott Crossfield was convinced they were necessary for the X-15. Crossfield also had great confidence in David Clark—both the company and the man. In fact, the detail specification of 2 March 1956 required North American to furnish just such a garment, and the company issued a specification for a full-pressure suit to the David Clark Company on 8 April 1956. Less than a month later, however, the X-15 Project Office, on advice from the Aero Medical Laboratory, advised North American to plan to use a partial-pressure suit. It was the beginning of a heated debate.-981

North American, and particularly Scott Crossfield, refused to yield, and during a meeting in Inglewood on 20-22 June 1956 the Air Force began to concede. David Clark demonstrated a full – pressure suit, developed for the Navy, during preliminary X-15 cockpit mockup inspection. Although the suit was far from perfected, the Aero Medical Laboratory believed that "the state-of – the-art of full pressure suits should permit the development of such a suit satisfactory for use in the X-15.""

During a meeting on 12 July 1956, representatives from the Air Force, Navy, and North American reviewed the status of full-pressure suit development, and the Aero Medical Laboratory committed to make the modifications necessary to support the X-15. The North American representative, Scott Crossfield, agreed that the Aero Medical Laboratory should provide the suit for the X-15. Crossfield insisted that the laboratory design the garment specifically for the X-15 and make every effort to provide an operational suit by late 1957 to support the first flight. The X-15 Project Office accepted responsibility for funding the development program. Crossfield could not legally change the suit from a contractor-furnished item to government-furnished equipment, but agreed to recommend that North American accept such a change. There was little doubt that Charlie Feltz would concur."0-

Although the 12 July agreement effectively settled the issue, the paperwork to make it official moved somewhat more slowly. The Air Force did not change the suit from contractor-furnished to government-furnished until 8 February 1957. At the same time, the Aero Medical Laboratory issued a contract to the David Clark Company for the development of a full-pressure suit specifically for the X-15.-1011

The first X-15 suit was the S794-3C, which incorporated all of the changes requested after a brief period of evaluating the first two "production" S794 suits. The complete suit with helmet, boots, and back kit weighed just 37 pounds. David Clark shipped this third suit to Inglewood for evaluation in the X-15 cockpit mockup from 7-13 October 1957. While at North American, the suit underwent pressure checks, X-15 cockpit compatibility evaluations, ventilation checks, and altitude-chamber runs. Unfortunately, the altitude-chamber runs proved pointless since the North American chamber only went to 40,000 feet and the suit controller had been set to pressurize above 40,000 feet.-102

The suit was then taken to the Aero Medical Laboratory for evaluation, and on 14 October was demonstrated in the Wright Field centrifuge during two 15-second runs at 7 g. The following day, 23 more centrifuge runs demonstrated the anti-g capability of the suit, which proved satisfactory. On 16 October, the suit underwent environmental testing at temperatures up to 165°F. The ventilation of the suit at these temperatures was unsatisfactory, but David Clark engineers understood the issue and the government did not consider it significant. Mobility tests were conducted in the centrifuge on 17 October at flight conditions up to 5 g with satisfactory results, and altitude chamber tests ended at 98,000 feet for 45 minutes. As a result of these evaluations, the Air Force requested numerous minor modifications for subsequent suits, but the Aero Medical Laboratory formally accepted the S794-3C on 12 November 1957.-103

The list of modifications required for the S794-4 suit took four pages, but they were mostly minor issues and did not represent a significant problem for the David Clark Company, although the resulting suit was almost 3 pounds heavier. Scott Crossfield demonstrated this suit during a cockpit inspection on 2 December 1957 when he put the suit on, inflated it to 3 psi, walked from one end of the room to the other (a distance of some 100 feet), and then entered the X-15 cockpit without assistance. Those in attendance were favorably impressed.-1104!

On 16 December 1957, David Clark took the S794-4 suit to Wright Field for further evaluation, and then to NADC Johnsville for centrifuge testing on 17-18 December. These centrifuge tests were much more realistic than the limited evaluations conducted at Wright Field on the previous suit, and included complete simulated X-15 flights. After some minor modifications, the Aero Medical Laboratory formally accepted the suit on 20 February 1958.-105-

The S794-5 suit, the first true "production MC-2," incorporated 34 changes. The Air Force sent the completed suit to Wright Field on 17 April 1958, and then to Edwards for flight evaluations. Personnel at Edwards had modified the back cockpits of a T-33 and F – 104B to accommodate the suit for the tests. The first flight in the T-33 on 12 May 1958 resulted in several complaints, primarily citing a lack of ventilation because no high-pressure air source was available. Initial concerns about a lack of mobility eased after the third flight as the pilot became more familiar with the suit. The suit seemed to offer adequate anti-g protection up to the 5-g limit of the T-33. Tests in the F-104B proved to be more comfortable, primarily because high-pressure air was available for suit ventilation, but also because the cockpit was somewhat larger, improving mobility even further. The pilots suggested various improvements (many concerning the helmet

and gloves) after these flights, but overall the comments were favorable. The suit accumulated

[1061

8.25 hours of flight time during the tests.

The Aero Medical Laboratory advised the X-15 Project Office on 10 April 1958 that David Clark would deliver the first suit for Scott Crossfield on 1 June 1958. The laboratory cautioned, however, that the X-15 project would receive only four suits under the current contract. The laboratory would receive other full-pressure suits for service testing in operational aircraft, but these were not compatible with the X-15 cockpit. If additional suits were required, the X-15 Project Office would need to provide the Aero Medical Laboratory with additional funds.-107

Given the lack of funds for additional suits, the X-15 Project Office investigated the feasibility of using a seat kit instead of the back kit used on the first four suits. This would allow the use of suits designed for service testing, and allow X-15 pilots to use the suits in operational aircraft. The benefits of using a common suit would have been substantial, but by May 1958 it was too late since the X-15 design was too far along to change. Although the X-15 Project Office continued to pursue the idea, the X-15 suit remained different from similar suits intended for operational aircraft. The X-15 Project Office subsequently found funds for two more suits.108

On 3 May 1958, the configuration of the suit to be delivered to Crossfield was frozen during a meeting in Worchester among representatives of the Air Force, David Clark, and North American. The decision was somewhat premature since the suit configuration was still in question during a meeting three months later at Wright Field. This indecision had already resulted in a two-month delay, and the need for further tests was apparent.109

The X-15 Project Office advised the newly assigned chief of the Aero Medical Laboratory, Colonel John P. Stapp, that the suit delays might postpone the entire X-15 program. To maintain the schedule, the X-15 project needed to receive Crossfield’s suit by 1 January 1959, a second suit by 15 February, and the remaining four suits by 15 May. Simultaneously, the X-15 Project Office informed Stapp of the growing controversy concerning the use of a face seal (actually a separate oral-nasal mask inside the pressurized helmet) instead of the neck seal preferred by the Aero Medical Laboratory.119

North American believed the pilot should be able to open the faceplate on his helmet, using the face seal as an oxygen mask. The Aero Medical Laboratory disagreed. Since the engineers had long since agreed to pressurize the X-15 cockpit with nitrogen to avoid risks associated with fire, a neck seal meant that the pilot could never open his faceplate under any conditions. North American and the NACA had already ruled out pressurizing the cockpit with oxygen, for safety reasons. Eventually, the program adopted a neck seal for the MC-2 suit, although development of the face seal continued for the highly successful A/P22S-2 suit that came later.111

Crossfield finally received his MC-2 pressure suit on 17 December 1958. In a report dated 30 January 1959, the X-15 Project Office attributed much of the credit for the successful development of the full-pressure suit to Crossfield.117

David Clark tailored the resulting MC-2 suits for the individual pilots. Each suit consisted of a ventilation suit, upper and lower rubber garments, and upper and lower restraint garments. The ventilation suit also included a porous wool insulation garment. The edges of the upper and lower rubber garments were folded together three times to form a seal at the waist. The lower half of the rubber garment incorporated an anti-g suit that was similar in design to standard Air Force- issue suits and provided protection up to about 7 g. The X-15 provided gaseous nitrogen to pressurize the portion of the suit below the rubber neck seal. The suit accommodated in-flight medical monitoring of the pilot.117

The outer garment was not actually required for altitude protection. An aluminized reflective outer garment contained the seat restraint, shoulder harness, and parachute attachments; protected the pressure suit during routine use; and served as a sacrificial garment during high-speed ejection.

It also provided a small measure of additional insulation against extreme temperature. This was the first of the silver "space suits" that found an enthusiastic reception on television and at the movies.[114]

The X-15 supplied the modified MA-3 helmet with 100% oxygen for breathing, and the same source inflated the anti-g bladders within the suit during accelerated flight. The total oxygen supply was 192 cubic inches, supplied by two 1,800-psi bottles located beneath the X-15 ejection seat during free flight. The NB-52 carrier aircraft supplied the oxygen during ground operations, taxiing, and captive flight. A rotary valve located on the ejection seat selected which oxygen source (NB-52 or X-15 seat) to use. The suit-helmet regulator automatically delivered the correct oxygen pressure for the ambient altitude until the absolute pressure fell below 3.5 psi (equivalent to 35,000 feet), and the suit pressure then stabilized at 3.5 psi absolute. Expired air vented into the lower nitrogen-filled garment through two one-way neck seal valves and then into the aircraft cockpit through a suit pressure-control valve. During ejection the nitrogen gas supply to the suit below the helmet was stopped (since the nitrogen source was on the X-15), and the suit and helmet were automatically pressurized for the ambient altitude by the emergency oxygen supply located in the backpack.-1115

Here Scott Crossfield sits in a thermal-vacuum chamber during tests of a prototype XMC-2 (S794-3C) suit. These tests used temperatures as high as 165°F and the initial suits suffered from inadequate ventilation at high temperatures. Production versions of this suit were used for 36 early X-15 flights, and in a number of other high-altitude Air Force aircraft. (Boeing)

Despite the fact that it worked reasonably well, the pilots did not particularly like the MC-2 suit. It was cumbersome to wear, restricted movement, and allowed limited peripheral vision. It was also mechanically complex and required a considerable amount of maintenance. Nevertheless, there was only one serious deficiency noted in the suit: the oxygen line between the helmet and the helmet pressure regulator (mounted in the back kit) caused a delay in oxygen flow such that the pilot could reverse the helmet-suit differential pressure by taking a quick, deep breath. Since the helmet pressure was supposed to be greater than the suit pressure to prevent nitrogen from leaking into the breathing space, this pressure reversal was less than ideal, but no easy solution was available.-116-

Improved Girdles for the Masses

Fortunately, development did not stop there, and the first of the improved A/P22S-2 (David Clark Model S1023) full-pressure suits arrived at Edwards on 27 July 1959. The development by the David Clark Company of a new method to integrate a pressure-sealing zipper made it possible to incorporate all of the layers of the MC-2 suit into a one-piece garment, significantly simplifying handling and maintenance. A separate aluminized-nylon outer garment protected the suit and provided mounting locations for the restraint and parachute harness. A face seal that was more comfortable and more robust replaced the neck seal, which had proven relatively delicate and subject to frequent damage. A modified helmet mounted the oxygen pressure regulator inside the helmet, eliminating the undesirable time delay in oxygen flow. This time David Clark mounted the suit pressure regulator in the suit to eliminate some of the plumbing.-1117-

The consensus among X-15 pilots was that the A/P22S-2 represented a huge improvement over the earlier MC-2. However, it would take another year before the Aero Medical Laboratory delivered fully qualified versions of the suit to the X-15 program. By July 1960, the A/P22S-2 pressure suits started arriving at Edwards and familiarization flights in the JTF-102A began later in the year, along with additional X-15 cockpit mockup evaluations and simulator runs. North American also subjected the first suit to wind-tunnel tests in the company facility in El Segundo.-118-

Joe Walker made the initial attempt at using the A/P22S-2 in the X-15 on 21 March 1961; unfortunately, telemetry problems forced Walker to abort the flight (2-A-27). Nine days later Walker made the first flight (2-14-28) in the A/P22S-2. Walker reported that the new suit represented an improvement in comfort and vision over the MC-2. By the end of 1961, the A/P – 22S-2 had a combined total of 730 hours in support of X-15 operations; these included 18 X-15 flights, 171 flight hours in the JTF-102A, and 554 hours of ground time.-119-

The A/P22S-2 was clearly superior to the earlier MC-2, particularly from the pilot’s perspective. The improvements included the following:-120-

1. Increased visual area—The double curvature faceplate in the A/P22S-2, together with the use of a face seal in place of the MC-2 neck seal, allowed the face to move forward in the helmet so that the pilot had a lateral vision field of approximately 200 degrees. This was an increase of approximately 40 degrees over the single contoured lens in the MC-2 helmet, with an additional increase of 20 percent in the vertical field of view.

2. Ease of donning—The MC-2 was put on in two sections: the lower rubberized garment and its restraining coverall, and the upper rubberized garment and its restraining coverall. This was a rather tedious process and depended on folding the rubber top and bottom sections of the suit together to retain pressure. The A/P22S-2 was a one-piece garment with a pressure-sealing zipper that ran around the back portion of the suit and was zippered closed in one operation. It took approximately 30 minutes to properly don an MC-2; only 5 minutes for the newer suit.

3. Removable gloves—In the MC-2 the gloves were a fixed portion of the upper rubberized garment. The A/P22S-2 had removable gloves that contributed to general comfort and ease of donning. This also prevented excessive moisture from building up during suit checkout and X-15 preflight inspections, and made it easier for the pilot to remove the pressure suit by himself if that should become necessary. Another advantage was that a punctured glove could be changed without having to change the entire suit.

The A/P22S-2 also featured a new system of biomedical electrical connectors installed through a pressure seal in the suit, avoiding the snap-pad arrangement used in the MC-2 suit. The snap pads had proven to be unsatisfactory for continued use, since after several operations the snaps either separated or failed to make good contact because of metal fatigue. This resulted in the loss of biomedical data during the flight. In the new suit, biomedical data were acquired through what was essentially a continuous electrical lead from the pilot’s body to the seat interface.-1211

The number of details required to develop a satisfactory operational pressure suit was amazing. Initially the A/P22S-2 suit used an electrically heated stretched acrylic visor procured from the Sierracin Corporation. The visors were heated for much the same reason a car windshield is: to prevent fogging from obscuring vision. Unfortunately, on the early visors the electrical coating was applied to only one side of the acrylic and the coating was not particularly durable, requiring extraordinary care during handling. Polishing would not remove scratches, so the Air Force had to replace the scratched visors. David Clark solved this with the introduction of a laminated heated visor in which the electrical coating was sandwiched between two layers of acrylic. This required a new development effort since nobody had laminated a double-curvature lens, although a Los Angeles company called Protection Incorporated had done some preliminary work on the idea at its own expense. The David Clark Company supplied laminated visors with later models of the A/P22S-2 suit.1221

Initially, the MC-2 suit used visors heated at 3 W per square inch, but the conductive film overly restricted vision. The Air Force gradually reduced the requirement to 1 W in an attempt to find the best compromise between heating the visor and allowing unimpeded vision. Tests in the cold chamber at the Aerospace Medical Center during late January 1961 established that the 1-W visors were sufficient for their expected use.1231

Another requirement came from an unusual source. Researchers evaluating the effects of the high-altitude free fall during Captain Joseph Kittinger’s record balloon jump realized that the X – 15 pilot would need to be able to see after ejecting from the airplane. This involved adding a battery to the seat to provide electrical current for visor heating during ejection.-1124!

Like the MC-2 before them, the A/P22S-2 suits were custom made for each X-15 pilot, necessitating several trips to Worcester. It is interesting to note that although the X-15 pilots were still somewhat critical of the lack of mobility afforded by the full-pressure suits (particularly later pilots who had not experienced the MC-2); this was only true on the ground. When the suits occasionally inflated for brief periods during flight, an abundance of adrenaline allowed the pilot to easily overcome the resistance of the suit. At most, it rated a slight mention in the post-flight report.

As good as it was, the A/P22S-2 was not perfect, and David Clark modified the suit based on initial X-15 flight experience. The principle modifications included rotating the glove rings to provide greater mobility of the hands; improved manufacturing, inspection, and assembly techniques for the helmet ring to lower the torque required to connect the helmet to the suit, and the installation of a redundant (pressure-sealing) restraint zipper to lower the leak rate of the suit. Other changes included the installation of a double face seal to improve comfort and minimize leakage between the face seal and suit, and modifications to the tailoring of the Link – Net restraint garment around the shoulders to improve comfort and mobility. David Clark also solved a weak point involving the stitching in the leather glove by including a nylon liner that

Г1251

The MC-2 suit led to the David Clark Company A/P22S suit that became the standard military and NASA high-altitude suit. The A/P22S and its variants have had a long career, and were used by SR-71 and U-2 pilots, as well as space shuttle astronauts. Here, NASA test pilot Joseph A. Walker

stands in front of an X-15 after a flight. (NASA)

Ultimately, only 36 X-15 flights used the MC-2 suit; the remainder used the newer A/P22S-2. Variants of the A/P22S-2 would become the standard operational full-pressure suit across all Air Force programs.

Post X-15

The X-15 was not the only program that required a pressure suit, although it was certainly the most public at the time. The basic MC-2 suit underwent a number of one-off "dash" modifications for use in various high-performance aircraft testing programs. Many of the movies and still photographs of the early 1960s show test pilots dressed in the ubiquitous aluminized fabric – covered David Clark MC-2 full-pressure suits.

The A/P22S-2 suit evolved into a series of variants designated the A/P22S-4, A/P22S-6, and A/P22S-6A (David Clark models S1024, S1024A, and S1024B, respectively) for use in most high – altitude Air Force aircraft, including the SR-71. Regardless of the success of the A/P22S-2 suit and its modifications for Air Force use, the cooperation between the Navy and Russell Colley at Goodrich continued. The Navy full-pressure suits included the bulky Mark I (1956); a lighter, slightly reconfigured Mark II; an even lighter Mark III (some versions with a gold lame outer layer) with an improved internal ventilation system; and three models of the final Mark IV, which went into production in 1958 as the standard Navy high-altitude suit.-1126!

The original Mercury space suits were reworked Mark IV suits that NASA designated XN-1 through XN-4, but the engineers usually referred to them as the "quick-fix" suits. The A/P22S-2 formed the basis for the Gemini suits, and ILC returned to the fray to produce the EVA suits used for Apollo. In March 1972, the Air Force became the lead service (the Life Support Special Project Office (LSPRO)) for the development, acquisition, and logistics support efforts involving pressure suits for the Department of Defense. This resulted in the Navy agreeing to give up the Mark IV full-pressure suit and adopt versions of the A/P22S-4/6. Today, the standard high-altitude, full- pressure suits used for atmospheric flight operations (including U-2 missions), as well as those used during space shuttle ascent and reentry, are manufactured by the David Clark Company.-1127!

After the first 50 flights with the XLR99 engine, researchers at the FRC took a step back and reflected on the problems they had experienced. Excepting the single incident on the ground that gave Scott Crossfield his wild ride at the Rocket Engine Test Facility, the engine had proved to be remarkably safe during operation. Although there had been a multitude of problems, large and small, the program described itself as "engine safe."1981

One of the major factors in successful engine operation in the X-15 after launch was the amount of checkout the engine went through on the ground beforehand. This had its drawbacks, however, since "operating cycles on the hardware for ground assurance checks take a relatively large portion of the hardware life," according to C. Wayne Ottinger and James F. Maher. Illustrating this is the fact that 350 ground runs, including 100 with the XLR99 installed in the X-15, had been necessary to achieve the first 50 flights. For the first dozen flights, the FRC conducted a test of the engine installed in the X-15 before each mission. After the 12th flight, a flight attempt could follow a successful flight without a test firing-a process that saved 18 ground runs during the next 38 missions.1991

Between the conclusion of the PFRT and May 1963, 90 modifications were made to the engine configuration. In order to meet the safety criteria imposed by the Air Force, Reaction Motors used the "single-malfunction" concept, i. e., it designed the engine so that no single malfunction would result in a hazardous condition. The company used a dual-malfunction concept with regard to structural failure, meaning that if one member failed, another would carry its load. The PFRT series of tests convincingly demonstrated these capabilities, since 47 different malfunctions resulted in a safe shutdown.11001

Despite all of the effort that went into developing a restartable engine, this capability was not used during the first 50 flights, except for four flights on which it was used to start an engine that had failed on the first attempt. However, another feature proved to be a welcome addition: the ability to operate the pump and both igniter stages while the research airplane was attached

to the carrier aircraft. This allowed verification of over 90% of the moving components in the engine before the research airplane was dropped.-1401

When the engines first arrived at Edwards, several components (particularly leaking pumps and malfunctioning hydrogen-peroxide metering valves) accounted for an abnormally high percentage of the flight delays. Relaxing the operating requirements regarding certain pump leaks and limiting the duration of the pump run time did as much to reduce pump delays as did the ultimate fixes themselves. NASA also noted that "excessive time lag in obtaining approval for correction" and "excessive time required to develop the correction and complete flight hardware incorporation of fixes after approval" were significant contributors to the delays caused by the XLR99.[102]

The control box was the heart of the engine and was responsible for the control and sequencing of the engine. This was not a computer by the modern definition of the term, but rather a mechanical sequencer with some electronic components. The major problem experienced by this device during the first 50 flights was the failure of pressure switches due to ammonia corrosion of the silver contacts-echoes of the original warnings on the effects of ammonia exposure. Reaction Motors finally eliminated this problem by switching to gold contacts. In addition, there were random wiring discrepancies, servo amplifier failures, and timer failures.-103

During the latter part of 1962, several in-flight oxidizer depletion shutdowns resulted in second – stage igniter damage because reduced liquid-oxygen injector pressure allowed the reverse flow of ammonia into the oxidizer inlet. The subsequent minor explosion either bulged the igniter inlet manifold or blew the face off the second-stage igniter. Reaction Motors installed an auxiliary purge system to correct the problem. In addition, several sensing-line detonations had defied correction throughout the summer of 1963. These occurred in the second-stage chamber sense line during any thrust decrease when unburned combustible gas from the previous increasing pressure cycle entered the sense line. Interestingly, engineers initially attributed this problem to a lubricant used in the main propellant valve. They believed that the "liquid-oxygen safe" lubricant was impact-sensitive and responsible for the second-stage igniter explosions. Although further investigation later proved this theory incorrect, analysis of the lubricant revealed that some batches were out of specification on impact sensitivity.-1104!

The hydrogen-peroxide system that powered the turbopump experienced several problems, including erratic metering valve operation, catalyst-bed deterioration, seal failures, and corrosion. Engineers corrected the metering valve problem by increasing the clearance around the valve. The substitution of electrolytically produced hydrogen peroxide for organically produced product solved the catalyst-bed deterioration, although it technically violated the engine qualification since the PFRT had been run with electrolytically produced hydrogen peroxide. The development of improved gaskets and seals relieved the seal failures and solved most of the corrosion problems. The turbopump itself suffered only minor problems, mainly steam and propellant leaks. The lowering of specifications governing the allowable leakage rate provided the most progress in working with the problem.-105

The oxidizer system also created some headaches, even though it was largely a copy of the original XLR30 system. The major problems were propellant valve leakage and the need for a quick-change orifice. Improved lip and shaft seals initially helped control the leakage, and eventually Reaction Motors introduced a redesigned valve that eliminated the problem. Prior to the incorporation of the quick-change orifice, it was necessary to remove the engine from the aircraft in order to change the oxidizer-to-fuel ratio. Engineers changed the ratio based on the proposed altitude for the next flight to maximize the performance of the engine. Once Reaction Motors incorporated the quick-change modification, engineers at Edwards could insert different-sized probes into the orifice while the engine was in the aircraft. This eliminated the need to conduct a ground run after reinstalling the engine. Tailoring the oxidizer-to-fuel ratio actually allowed the engine to produce slightly over 61,000 lbf at some altitudes.-105

Although nearly everybody considered the XLR99 a good research airplane engine, the engine was far from perfect. Milt Thompson observed that "the LR99 was amazingly reliable if we got it lit, and if we did not move the throttle while it was running." Joe Vensel, the director of FRC flight operations echoed the advice: "[I]f you get the engine lit, leave it alone, don’t screw with it." This is perhaps overstating the case, but not by much. During the early part of the flight program, the XLR99 had a remarkably poor record of starting when the pilot wanted. Part of the problem was that the early flight rules said to start the engine at minimum throttle (50% for the very early engines, and 30% for the later ones). The engine simply did not like to start at those throttle settings. After the program decided to start the engine at 100% throttle, things got much better.107-

Still, even after the engine lit, it did not particularly like to throttle. As a result, Joe Vensel directed the pilots not to throttle the engine until after the X-15 had sufficient energy to make it back to Edwards. Milt Thompson talked him into changing his mind for one flight (3-29-48) in order to accommodate a research request, and Thompson ended up on Cuddeback Lake when the engine quit as he throttled back 42 seconds after launch. After that, the restriction was rigorously enforced: no throttle movement until the airplane could glide back to Edwards. Although the lower throttle limit on later engines was 30%, the program decided not to go below 40% because of the persistent vibration problem. The pilots also learned to move the throttle slowly to minimize the chances of the engine quitting. It mostly worked, and flight planner Bob Hoey does not remember any significant problems occurring later in the program.-1108!

During the flight program, eight in-flight propulsion problems resulted in emergency landings. These included one due to no ignition, one because the engine hung at 35% thrust, one shutdown when the throttle was retarded, two due to low fuel-line pressures, one turbopump-case failure, one ruptured fuel tank, and one due to a perceived lack of fuel flow from the external tanks on X – 15A-2. Overall, it was not a bad record for a state-of-the-art engine over the course of 199 flights.

Although 11 flight engines were manufactured, only eight were available to the flight program. One (s/n 105) was lost in the ground explosion that seriously damaged the X-15-3 before the XLR99 had even flown, and two other flight engines were dedicated to the ground-test program. Making 199 flights on eight engines was an outstanding achievement.

+ Lost in X-15-3.

Data courtesy of Robert G. Hoey.

As was done for most components on the X-15, all XLR99 maintenance was performed at Edwards using a local, depot-level maintenance approach. With few exceptions, the engines ran for a brief period in the PSTS before NASA installed them in one of the X-15s or stored them for future use. Since the X-15 maintenance philosophy was to provide sufficient spare engines and maintenance personnel to ensure 100% flight engine availability, it was normal to have a backlog of engines in flight-ready storage (essentially spares). The engine activity was divided into three categories: 1) installed in an X-15, 2) active maintenance, and 3) flight-ready storage. Early in the program, NASA conducted one or more ground engine runs (leak checks) after installing the engine in the airplane and before every flight. This requirement for an aircraft engine run between flights was relaxed later in the program, assuming there were no engine problems on the previous flight.1102*

Milton O. Thompson had more than his fair share of experience with the XLR99, and enjoyed sharing it during discussions with various groups after the X-15 program ended. One of his favorite stories concerned the emergency landing he had to make on Flight 3-29-48 when the XLR99 quit as he throttled back 42 seconds after launch. (NASA)

The staff of the AFFTC Rocket Engine Maintenance Shop from 1961 to 1968 in support of the XLR99 averaged about 37 people. Interestingly, in 1965 these technicians made about $4 per hour on average. This shop was responsible for all maintenance of all uninstalled XLR99s; the FRC handled minor repairs of installed engines. Every 30 operating minutes, on a test stand or in the airplane, each XLR99 had to undergo a "30-minute" inspection that took just over two weeks to complete. The Air Force overhauled the engines when needed, a process that took just over a month. Recoating the thrust chamber, done by the FRC, took a few days.-1110

Unlike many rocket engines of that era, the XLR99 was equipped with a malfunction-detection and automatic-shutdown system. For most engines, reliability is based on the number of start attempts. However, since one of the primary features of the XLR99 was its ability to restart in flight, its total reliability was defined as the number of successful engine operations per flight attempt, regardless of the number of start attempts. The resulting X-15 data and point estimates of reliability were as follows:[111

XLR99/X-15 flight attempts^112 169 Successful engine operations 165 Successful first-start attempts 159 Overall reliability 97.6%

First-start reliability 94.0%

Over the course of the X-15 program, the flight engines accrued a total of 550.53 minutes of run time, plus an undetermined amount on ground-test engines. A total of 1,016 engine starts were recorded for the flyable engines (dedicated ground-test engines incurred many more). Although there were numerous automatic shutdowns, there were no catastrophic engine failures. The safety of the XLR-99 engine (defined as the probability of non-catastrophic engine operation) may be conservatively estimated by dividing the number of successful starts (1,016) by the number of starts plus one (1,017) (assuming the next start to be catastrophic for the worst case). The resulting estimate of the probability of non-catastrophic engine operation is approximately

0.99902ДШ

In retrospect, the engine still casts a favorable impression. The XLR99 pushed the state of the art further than any engine of its era, yet there were no catastrophic engine failures in flight or on the ground. There were, however, many minor design and manufacturing deficiencies, particularly with the Rokide coating on the thrust chamber. Surprisingly, the primary source of problems on most large rocket engines-the turbopump-proved to be remarkably robust and trouble free.

In early 1957, just as North American was preparing to begin modifications on the B-36, the X – 15 Joint Operations Committee began considering replacements for the B-36 for various reasons. There were some concerns that the research airplane would not be as stable as desired during launch because of the relatively slow speed of the B-36. Another reason was that as the weight of the X-15 and its subsystems grew, the Air Force and NACA began to look for ways to recover some of the lost performance; a faster carrier would compensate somewhat for the increased X – 15 weight. Perhaps most vocally, personnel at Edwards believed that the 10-engine B-36 would quickly become a maintenance nightmare since the Air Force was already phasing it out of the inventory. A lack of spare parts and depot maintenance capabilities for the B-29 and B-50 carrier aircraft had already delayed the X-1 and X-2 programs on several occasions.1871

A survey by North American identified the Boeing B-52 Stratofortress, Convair B-58 Hustler, and

Boeing KC-135 Stratotanker as possible B-36 replacements. It is interesting to note that Douglas had apparently chosen the B-52 for their model 671 study four years earlier.-88

The supersonic B-58 was attractive from a performance perspective, but looked less attractive from the maintenance and availability standpoint. Nevertheless, on 22 January 1957, future X-15 pilot Neil Armstrong traveled to the Convair plant in Fort Worth to discuss the possibility of using a B-58 to launch the research airplane. The first problem was that the 22-foot wingspan and 18- foot tail-span of the X-15 both intersected the plane of the rearward-retracting main gear on the B-58. This would have necessitated moving the entire X-15 forward of the desired location. Convair engineers believed that this might be possible, but it would require designing a new nose gear for the B-58 since the X-15 would block the normal nose gear. Another possibility was to beef up the X-15 nose gear and use it while the pair was on the ground. The inboard engine nacelles on the B-58 would likely need to be "toed" outward or simply moved further out on the wing, and either would have necessitated major structural changes. Engineers would need to design a way to fold the X-15 vertical stabilizer because they could not make room for it within the B-58 fuselage without severing a main wing spar. The design of the B-58 included a weapons/fuel pod that weighed 30,000 pounds, only slightly less than the X-15. However, the baseline mission included using the fuel in the pod prior to dropping the pod, and the maximum drop weight was only 16,000 pounds. This would necessitate a new series of tests to validate that a heavier object would separate cleanly, especially at supersonic speeds. However unfortunately, the B-58 was obviously not going to work.-89

The landing-gear configuration on the KC-135 and B-52 precluded carrying the X-15 under the fuselage, as had been the practice in all earlier research programs. Although the performance and availability of the KC-135 made it attractive, nobody could figure out where to carry the research airplane since the Stratotanker had a low-mounted wing and relatively short landing gear. Engineers quickly dropped the KC-135 from consideration.-1901

The B-52 also offered an excellent performance increment over the B-36, and since the Boeing bomber was still in production, the availability of spare parts and support should not become an issue. There was a large space on the wing between the fuselage and inboard engine nacelle that could be adapted to carry a pylon, and investigations were already under way to install similar pylons on later B-52s to carry air-to-surface missiles. In May 1957, NASA directed North American to perform an initial feasibility study on using the B-52 as an X-15 carrier. The study lasted several weeks and the results were favorable. At a meeting on 18-19 June 1957, the program officially adopted the B-52 as a carrier aircraft. Representatives from the FRC discussed concerns about maintenance and availability issues, and NASA recommended procuring two carrier aircraft to ensure that the flight program would proceed smoothly. The Air Force subsequently authorized North American to modify two B-52s in lieu of the single B-36.-91

The North American investigations showed that the X-15, as designed, would fit under the wing between the fuselage and inboard engine pylon at an 18% semi-span location. The wing structure in this location was capable of supporting up to 50,000 pounds, so the 31,275-pound research airplane did not represent a problem. Nevertheless, this was not the ideal solution. The X-15 pilot would have to be in the research airplane prior to takeoff, and the large weight transition when the B-52 released the X-15 would present some interesting control challenges.-921

Lawrence P. Greene, the North American chief aerodynamicist wrote, "One item which caused considerable concern in the early evaluation was the fact that in this installation, the pilot could not enter the airplane in flight as had been possible in the B-36. This limitation was of concern from both the fatigue and safety aspects; however, the time from take-off of the B-52 to launching the X-15 is about 1.5 hours, and considerable effort has been expended in plans for making the pilot comfortable during this time. In the event of an emergency, the configuration permits the pilot to eject safely while the X-15 and B-52 are still connected.’4931

Further analysis and wind-tunnel tests indicated that the potential problems were solvable, and that the increase in speed and altitude capabilities was desirable. Researchers conducted additional wind-tunnel tests of a 1/40-scale model in the Langley 7 by 10-foot tunnel and the University of Washington wind tunnel to explore possible flutter problems, but did not discover any critical issues. Researchers installed six-component strain-gage balances in both the B-52 and X-15 models, and the B-52 model had additional strain gages and a pressure gage located in the horizontal stabilizer to obtain measurements of possible tail buffet created by the X-15 installation.-1941

Initially the X-15 was to be carried under the left wing of the B-52. It was moved to the right wing to "permit easier servicing of the X-15 when installed on the B-52," although exactly what was easier to service was not described. Researchers had conducted most of the wind-tunnel tests with models of the X-15 under the left wing. However, since both aircraft were largely symmetrical, researchers decided that the test results were equally as valid for the right-wing configuration. The initial design also had an anti-buffet fairing that partially shielded the pylon from the airflow, but wind-tunnel tests showed that the fairing did not significantly help anything, and the engineers subsequently deleted it.1951

Originally, the Air Force indicated that it could make the two prototype B-52s (the XB-52 and YB – 52) available to the X-15 program. Personnel at Edwards feared that the use of these two non­standard aircraft would result in the same maintenance and parts availability problems they were attempting to avoid. By August 1957 the Strategic Air Command agreed to make an early- production B-52A available, and the Air Force subsequently assigned serial number 52-003 to the program in October 1957. In May 1958 the Air Force also assigned an early RB-52B (52-008) to the X-15 program. Both aircraft had been involved in isolating problems with the B-52 defensive fire control system, and Boeing delivered each aircraft to North American after the completion of their test programs.-1961

On 29 November 1957 the B-52A arrived at Air Force Plant 42 in Palmdale, California, after a flight from the Boeing plant in Seattle. North American placed the aircraft into storage pending modifications. On 4 February 1958, technicians moved the aircraft to the North American hangar and began modifying it to support the X-15 program. The aircraft, now designated NB-52A, flew to Edwards on 14 November 1958 and was subsequently named "The High and Mighty One." The RB-52B arrived in Palmdale for similar modifications on 5 January 1959, and, as an NB-52B, flew to Edwards on 8 June 1959; the airplane briefly wore the name "The Challenger.’4271

The Air Force initially contributed the third production B-52A (serial number 52-003) to the X-15 program. This airplane had been used in initial B-52 testing at Boeing in Seattle, and came to Edwards when its testing duties were completed. The airplane was modified by North American to support carrying and launching the X-15. The aircraft, now designated NB-52A, flew to Edwards on 14 November 1958 and was subsequently named The High and Mighty One. (NASA)

The major modifications to the two NB-52s included the following:^981

1. The no. 3 right main wing fuel cell was removed to allow the installation of pylon tie fittings and supports in the front and rear wing spars.

2. The inboard flap mechanism on both wings was disconnected, and the flaps were bolted to the flap tracks. A cutout through the right inboard flap provided clearance for the X-15 vertical stabilizer.

3. A pylon was installed between the right inboard engine nacelle and the fuselage. The pylon contained a primary hydraulic and a secondary, pneumatic-release mechanism for the research airplane.

4. Changes to the NB-52 avionics included the addition of an AN/APN-81 Doppler radar system to provide ground-speed and drift-angle information to the stable platform in the X-15, an auxiliary UHF communications system to provide additional communications channels, and a change in the AN/AIC-10 interphone system to provide an AUX UHF position.

5. The fuselage static ports were removed from the right side of the NB-52 to allow installation of the forward television camera. The airspeed system was recalibrated to use only the left static ports. This worked surprisingly well, even during sideslip maneuvers, with "no measurable difference" noted.

6. Two television cameras were installed in streamline fairings on the right side of the NB-52. The rear camera pointed generally forward and was equipped with the zoom lens to allow the launch operator to focus on areas of interest on the rear of the X-15. The forward camera used a fixed-length lens pointed outward and slightly rearward to allow a view of the X-15 forward fuselage. Two monitors were located at the launch operator position, and either could show the view from either camera. Four floodlights and three 16-mm motion picture

cameras were also installed. Two of these were Millikan DBM-5 high-speed units located in a window on the right side of the fuselage at station 374 and in an astrodome at station 1217. The third was an Urban GSAP gun camera mounted in the pylon pointed downward to show X-15 separation.

7. The NB-52 forward-body fuel cell was removed to provide space for inspecting and maintaining various fluid and gas lines installed in the wing. The mid-body fuel cell was removed and the fuselage area above the bomb bay was reworked to provide space for 15 nitrogen and nine helium storage cylinders. Early during the flight program, a separate liquid-nitrogen supply was added to the pylon to cool the stable platform on the X-15.

8. Two stainless-steel liquid-oxygen tanks (a 1,000-gallon "climb" tank and a 500-gallon "cruise" tank) were installed in the bomb bay. The tanks were not jettisonable, although the contents could be vented through a streamlined jettison line protruding from the forward left side of the bomb bay. Liquid oxygen would be sucked into the right rear landing gear well if the doors were opened while liquids were being jettisoned; this was procedurally restricted.

9. A launch operator station replaced the normal eCm compartment located on the upper rear flight deck. After the first flew flights with X-15-1, an astrodome-type viewing window was added to the NB-52 above the forward television camera in case the video system failed, and a duplicate set of controls for the liquid-oxygen top-off system were located above the window to allow the launch operator to top off the X-15 while looking out the window. A defrosting system was provided for the window, and two steel straps across the window provided safety for the launch operator in case the window blew out.

10. Changes to the NB-52 flight deck included the addition of a master launch panel on the lower left side of the main instrument panel, launch-indicating lights in the pilot’s direct field of vision, a normal launch switch on the left console, and an emergency launch handle below and to the left of the master launch panel. Changes were also made to the B-52 fuel control panel in both aircraft to reflect the removal of the fuel cells and eliminate the external tank position.

11. Breathing oxygen was made available to the NB-52 crewmembers at all times. In addition, oxygen was tapped from the NB-52 oxygen system to supply the X-15 research pilot with breathing oxygen until flight release.

12. A high-speed wheel, tire, and braking system was installed on the NB-52 because the original landing gear was only rated to 174 knots. The new system incorporated an adequate margin for no-flap takeoffs and landings at heavy weights, and was rated to 218 knots.

13. All military systems, including the tail turret and defensive fire-control system, were removed. The modifications to the rear fuselage to delete the tail turret differed between the two aircraft. The ability to carry the reconnaissance pod on the RB-52B was also deleted.

14. Later in the flight program, additional instrumentation was added to the launch operator position to allow monitoring of the MH-96 adaptive flight control system and X-20 inertial flight data system. A "stable platform control and monitoring unit" was also added to the NB-52B to allow the launch operator to monitor and control the stable platform during captive-carries of the pod-mounted system used for post-maintenance validation.

These changes differed somewhat from those initially proposed for the NB-52. For instance, the original design had a pressurized compartment in the bomb bay for an observer. When North American deleted this from the design, engineers moved the liquid-oxygen top-off tank there instead. The launch operator position was moved from the left side of the aircraft to the right side to permit "continuous observation of the research vehicle" after the X-15 itself was moved to the right side. This also allowed the launch operator to remain in his ejection seat for the entire launch process (previously he had to stand up occasionally to visually check the X-15).[99]

The change from a B-36 to a B-52 did not come cheaply. Although the basic aircraft was provided

at no charge to the program, North American submitted a bill for an additional $2,130,929.06 for the modification of the first B-52. The second airplane cost somewhat less since it did not require wind-tunnel testing and the basic engineering was already complete.

The Air Force named Captain Edward C. Gahl as the project pilot for the NB-52 carrier aircraft in 1957. Gahl was well up to the task. He was a graduate of the Experimental Test Pilot School and had been involved in flight-testing the B-52 and KC-135 prior to joining the carrier program. Unfortunately, Gahl perished in a mid-air collision on 16 June 1958, long before the NB-52A had completed its modifications. Captain Charles C. Bock, Jr., replaced him as the chief carrier pilot.™

After the modifications to the NB-52A were completed, engineers from the Air Force, Boeing, NASA, and North American conducted a ground vibration test on the pylon using the X-15-1. The tests built on data already accumulated by Boeing-Wichita while the B-52F was being integrated with the North American GAM-77 Hound Dog missile.-11011 Technicians constructed a structural steel frame to make the NB-52 wing as rigid as possible, effectively preventing any movement by the NB-52 wing, pylon, horizontal stabilizer, or fuselage. The X-15 was excited by electromagnetic shakers and sensors mounted on the X-15 fuselage, wing, horizontal stabilizer, and vertical stabilizers measured the amplitude of motion for various frequencies. Researchers used these data to determine the natural vibration frequencies of the pylon to verify data obtained from a series of flutter model tests of the NB-52/X-15 combination conducted by Boeing in a low-speed wind tunnel. The results from these two tests demonstrated that the flutter speed of the NB-52 when carrying the X-15 was well above the required launch conditions.-11021

However, there was some concern about the jet exhaust from engine nos. 5 and 6 of the NB-52 impinging on the X-15 empennage. Specifically, the engineers worried that the engine acoustics would detrimentally affect the X-15’s structural fatigue life. To mitigate this concern, at least initially, the engineers decided the NB-52 pilots would restrict engine nos. 5 and 6 to 50% thrust while carrying the X-15. The engineers and pilots believed this was an acceptable compromise between protecting the X-15 and the need to provide adequate power and control of the NB-52 during takeoff. At 50% power on these two engines, the tip of the X-15 horizontal stabilizer was exposed to 158 decibels and the sides of the vertical stabilizers were exposed to 144 decibels; at 100% power each value was about 10 decibels greater.-103

Although it appeared feasible to operate the carrier aircraft engines at reduced power, it was not desirable, so North American began redesigning some parts of the X-15 to increase their fatigue life. The modifications to the vertical stabilizers consisted of increasing the rivet diameter, using dimpled-skin construction instead of countersunk rivets, and increasing the gage of the corrugated ribs along the edge where they flanged over to attach to the cap strip. The horizontal stabilizer used larger rivets and dimpled construction.104

To verify the effectiveness of the modifications, researchers conducted several acoustic tests to establish the structural fatigue life of both the original and modified aft X-15 structures. A static ground test was run on a simulated X-15 empennage to determine the sound levels beneath the pylon (the hastily-constructed structure could not be attached to the pylon) with the B-52 engines operating at 85% rpm (equivalent to 50% thrust). Both the original and modified test panels withstood 20 hours of operation with no failure. Subsequent analysis indicated that the original panels would be adequate for operation at 50% power, and the new panels would allow operation at 100% power. North American decided to retrofit all three X-15s with the new structure, which would take several months.-103

Following completion of these tests, Captain Bock and Captain John E. "Jack" Allavie tested the NB-52A along with launch panel operator, William "Bill" Berkowitz from North American. To eliminate possible interference with the X-15, the engineers decided to bolt the inboard flaps in the closed position, meaning that the NB-52 pilots would have to fly the airplane without flaps. Therefore, the pilots dedicated the initial flights to developing techniques for no-flap operations and measuring various performance parameters of the modified NB-52. The takeoffs were conducted using 50% power on engine nos. 5 and 6 since it appeared that initial flights would be restricted to this power setting until all three X-15s were modified. The NB-52 also accomplished qualitative stability tests over the speed and altitude ranges anticipated for the X-15 program.-11061

There was very little no-flap, takeoff-and-landing experience with the B-52 available to draw on, so Bock and Allavie conducted the initial tests using predicted information and recommendations from Boeing personnel. Engineers based the anticipated takeoff speeds and distances on a lift coefficient of 0.75, meaning that the NB-52 had to be rotated about the aft main gear to an attitude that would produce the correct amount of lift. This was contrary to normal B-52 takeoffs where all four main gear lift at the same time. The pilots also realized that the 10% chord elevator used on the B-52 would have limited authority and that the horizontal-stabilizer trim setting would be important if reasonable takeoff distances were to be attained.-1071

The flight tests involved a fair amount of trial and error. For instance, on the first test at a gross weight of 315,000 pounds (the maximum predicted weight for an actual X-15 flight), Bock set the stabilizer trim 0.5 degrees more than the normal recommended trim of 0 degrees. The pilots ran engine nos. 5 and 6 at 50% power, and fuel loading simulated the weight (but not the drag) of the X-15 on the right wing. The predicted takeoff distance was 10,500 feet at a speed of 176 knots. However, the NB-52 would not rotate, even with the control columns pulled all the way back.

After the airplane passed the 10,000-foot marker on the runway, the pilots went to full power on engine nos. 5 and 6, and the aircraft broke ground at 12,650 feet at 195 knots. Engineers later calculated the actual lift coefficient for this takeoff at 0.639. During a normal B-52 takeoff with the flaps down, all four main gear leave the ground simultaneously and the lift coefficient is approximately 0.55.1081

Subsequent takeoff tests established that a trim setting of 2 degrees nose up was the optimum setting (this represented one-half of the available trim). This setting produced reasonable takeoff distances and a rapid but controllable rotation just prior to liftoff, with the pilot holding the column all the way back. The maximum lift coefficients were later determined to be approximately

0.71.11091

Landings also proved challenging. Again, the airplane needed higher than normal lift coefficients during landing in order to produce reasonable touchdown speeds and landing distances. Unlike the traditional B-52 landing on all four main gear at once, the NB-52s landed on their two aft main gear. The problem was that the designers had not intended the B-52 to do this. Very little control could be achieved as the aircraft rotated to a level attitude, and the forward main gear usually hit with a noticeable impact. Accelerometers installed in the pylon after the initial landing tests measured impact loads of 1.5-1.8 g. The engineers considered these annoying but acceptable.-11101

After the front main gear touched down, the pilots fully extended the NB-52 air brakes and the drag chute deployed at 140 knots. When landing at heavier weights, such as when returning with the X-15 still attached, the pilots used moderate braking. When these techniques were used with a 300,000-pound airplane, the touchdown speed was 172 knots and the landing roll took 10,800 feet. At 250,000 pounds, touchdown occurred at 154 knots and light braking used only 9,300

feet of runway. The importance of the drag chute was telling: one landing at 267,000 pounds with a failed drag chute required over 12,000 feet to stop even with heavy braking, and resulted in one brake being severely warped, necessitating its replacement.111

The NB-52 pilots now felt confident that they could control their airplane with the X-15 attached, so the first captive flight was attempted. The right wing sat on its outrigger wheel during the initial takeoff roll in order to keep spoiler extension and the associated drag at a minimum. The engineers did not expect the additional drag of the X-15 to result in any serious degradation of low-speed performance; however, there existed some concerns about the possible impingement of the X-15 wake on the right horizontal stabilizer of the NB-52.1121

Despite the concerns about exhaust impingement from engine nos. 5 and 6, the X-15 program had not taken a firm stand on what power levels to use. Bock and Allavie therefore decided to use full power on all eight engines for the flight on 10 March 1959. The takeoff gross weight was 258,000 pounds and the center of gravity was located at 26.5% mean aerodynamic chord (MAC). The actual takeoff distance was 6,085 feet and liftoff occurred at 172 knots. The lift coefficient developed on this takeoff was 0.66 since the pilots did not attempt to achieve maximum performance. Bock just wanted to demonstrate that the mated pair would actually fly as predicted, which it did for 1 hour and 8 minutes. The second flight (which was supposed to result in an X – 15 glide flight, but did not due to a radio failure) produced largely similar results. On the third flight (another unsuccessful attempt at a glide flight) engine nos. 5 and 6 were set to 50% thrust until an indicated airspeed of 130 knots was reached, and then they were advanced to full power. This procedure extended the takeoff distance to 7,100 feet at the same gross weight and similar atmospheric conditions.-1113!

Following takeoff, engine nos. 5 and 6 were set to 50% thrust at 5,000 feet altitude and the mated pair continued to climb using a circular pattern around Rogers Dry Lake. This kept Scott Crossfield in the X-15 within gliding distance of a suitable lake in the event of a possible emergency jettison. The NB-52 pilots flew all of these early tests to an altitude of 45,000 feet and Mach 0.85, which was pretty much the maximum performance of the mated pair. Bock and Allavie flew simulated launch patterns and practiced emergency and aborted launch procedures, and Crossfield accomplished X-15 propellant jettison tests using a water-alcohol mixture that included red dye. Before each flight, technicians covered the underside of the right horizontal stabilizer of the NB-52 with a powdery substance so that the impingement would be easy to identify.-1141

Since the X-15 horizontal and vertical stabilizers used for these initial carry flights were the original design, the engineers decided to inspect them after the third flight. The inspection revealed several structural failures in the upper vertical stabilizer. For the most part, the corrugated ribs had failed where they flanged over to attach to the cap strip, but the most extensive failure was an 18-inch separation of the rib from the flange on the side away from the NB-52 engines. Subsequent investigation showed that the failures were largely a result of a previously unsuspected source: the turbulent airflow created by the X-15 pylon and the B-52 wing cutout. Researchers made pressure measurements to determine the exact environment around the wing cutout. Fortunately, the subsequent analysis indicated an acceptable fatigue life for the modified X-15 structures, even though the engineers had not factored this particular environment into the design. After this round of tests and analysis was completed, the pilots made most subsequent takeoffs with all eight B-52 engines operating at 100% power.-1151

at Edwards during the summer were conducted in the early morning in any case, and if the takeoff roll was computed to be too long, one of the lakebeds could always be used (although this only happened once during actual flight operations). The NB-52B eliminated this particular deficiency. Unlike the A-model, the NB-52B was quipped with water injection for its engines. Bock and Allavie tested the NB-52B using water injection on just the outer four engines, and on all engines except nos. 5 and 6, with promising results. Bock noted that the use of water injection "appreciably increases take-off performance and is considered mandatory for take-off from the paved runway at a weight of 300,000 pounds when the ambient temperature exceeds 90 degrees Fahrenheit.’,[116]

Takeoffs were initially made using runway 04 at Edwards because that runway had several miles of lakebed overrun available. This allowed the pilots to fly a better pattern during climb-out, but more importantly, it avoided the use of heavy braking in case of an aborted takeoff. Engineers considered the use of maximum braking "undesirable" because of potential damage to the X-15 if one of the NB-52 tires failed. The other direction, runway 22, has a road at the end of it instead of lakebed.-1117!

Pilots found the lateral and directional control systems of the carrier aircraft capable of trimming out the unbalance of the NB-52/X-15 combination. Most of the pilots noted that lateral control became sensitive above Mach 0.8, but believed that launches were possible up to Mach 0.85 with no particular problems. The evaluations did not reveal any buffeting in level flight. It was possible to induce a minor airframe buffet in maneuvering flight at 1.6 g (80% of the pylon load limit), but only at speeds well below the normal operating range. It was discovered that the specific range deterioration of the NB-52 was about 7% with an empty pylon; with the X-15 attached, the specific range decreased by approximately 16%. Given that researchers never planned to launch the X-15 from a distance of more than 500 miles, and the B-52 was an intercontinental bomber, nobody considered this decrease in range significant. Nevertheless, a nonstop flight in May 1962 demonstrated that the pair could fly 1,625 miles from Edwards to Eglin AFB, Florida.118!

The Air Force also provided the second production RB-52B (the fifth B-model) to the X-15 program. The RB-52B (52-008) arrived in Palmdale for similar modifications on 5 January 1959, and as an NB-52B, flew to Edwards on 8 June 1959; the airplane briefly wore the name The Challenger. The NB-52B went on to a long career at the Flight Research Center before being retired in 2005. (U. S. Air Force)

The engineers and pilots predicted that launching the X-15 would result in an instantaneous rearward shift of the NB-52 center of gravity, coupled with a tendency for the carrier aircraft to roll to the left. The X-15 glide flight (i. e., with no fuel) was expected to result in a 4.5% shift in the center of gravity, while full-fuel flights would result in a 9% shift (which rose to about 12% on the later X-15A-2 flights). Engineers calculated that the rolling tendency and pitch-up were well within the capabilities of the NB-52 to counter, and in fact actual operations revealed no particular problems. Under "normal" conditions, the center of gravity actually shifted approximately 7% and required a 40-pound push force on the control column to compensate, but the resulting pulse usually dampened in one cycle.[119]

Some other minor problems were discovered during the NB-52 flight tests. For instance, the aft alternator cooling air duct on the right-wing leading edge and the air ducts on the right side of the NB-52 fuselage ingested hydrogen peroxide residue during pre-launch operation of the X-15 nose ballistic control system. Engineers did not consider the residue hazardous since it was composed primarily of water. Interestingly, while the X-15 was attached to the NB-52, operation of the X-15 ballistic control system had no noticeable effect on the bomber. Operation of the X – 15 aerodynamic flight control also had no appreciable effect on the NB-52; however, a slight airframe buffet was noted when the X-15 speed brakes were extended. A flap extension on the X-15 caused a small nose-down trim change, and extension of the X-15 main landing skids was not even apparent in the bomber. Initially, extension of the X-15 nose gear resulted in a "thump" that was felt and heard in the NB-52, but later changes to the X-15 extension mechanism eliminated the event.-1120

On the other side of the equation, the NB-52 had some effects on the X-15. For instance, the NB – 52 fuselage and wing created noticeable upwash and sidewash on the X-15. Because of the NB – 52 wing sweep, the right wing of the X-15 was nearer to the B-52 wing leading edge and, consequently, flow over the X-15 right wing was deflected downward more than over its left wing. This difference in effective angle of attack of the right and left wings resulted in a right rolling moment. There were also some concerns that the X-15 might strike the carrier aircraft during separation. Because there was only two feet of clearance between the X-15 dorsal stabilizer and the cutout in the NB-52 wing, the X-15 could potentially strike the cutout if the X-15 bank angle exceeded 20 degrees before the airplane dropped below the NB-52 fuselage level (about 2.5 feet vertically). It was decided that all X-15 controls should be in the neutral position when the airplane was dropped, allowing the automatic dampers to take care of correcting the attitude. The first few X-15 launches experimented with the settings needed for the dampers to do this, but Scott Crossfield soon developed a consistent set of settings.-121

Scott Crossfield unexpectedly demonstrated the effects of not using the dampers on the third flight (2-3-6) when the roll damper failed at launch. The X-15 rolling velocity increased rapidly to a peak value of 47 degrees per second and a peak bank angle of 40 degrees. The X-15 dorsal stabilizer dropped below the NB-52 wing cutout within 0.5 second, with the tail barely clearing the cutout. Crossfield finally managed to get the X-15’s wings level about 7 seconds after launch.121

The most obvious modification was a large pylon under the right wing to carry the X-15. This was in contrast to all earlier X-planes, which had been carried partially submerged in the bomb bay of the carrier aircraft, something that was not possible given the B-52 configuration. The pylon worked satisfactorily and allowed the NB-52s to carry other research airplanes, such as the lifting bodies, later in their careers. (NASA)

The damper generally applied a left-aileron input of 6-8 degrees, reducing the peak right-roll velocity to about 25 degrees per second. The pilot could do the same if the damper failed. Aileron inputs of only 2 degrees, however, resulted in peak roll velocities in excess of 50 degrees per second, with corresponding bank angles of over 40 degrees. This risked a tail strike during launch. As the X-15 cleared the NB-52 flow field, it tended to roll left, so the damper and/or pilot had to be prepared to correct this sudden opposite movement. It took approximately 0.8 second for the X-15 to drop 10 feet below the NB-52.-1123

Another modification to the two NB-52s was a notch in the right wing to accommodate the X-15 vertical stabilizer. Because there was only 2 feet of clearance between the X-15 dorsal stabilizer and the cutout in the NB-52 wing, the X-15 could potentially strike the cutout if the X-15 bank angle exceeded 20 degrees before the airplane dropped below the NB-52 fuselage level (about

2.5 feet vertically). Fortunately, this was never an issue during the flight program. (U. S. Air Force)

The first few seconds were quite a ride, at least during the first time for each pilot. However, it quickly became routine. Bob White described it as "what might be expected and, after the very first experience, is of no concern to the pilot as normal 1.0-g flight is regained within 2 seconds. The rolloff at launch stops as the X-15 emerges from the B-52 flow field. Since the bank-angle change is small, it is easily and quickly corrected. Launch has been made by using either the center or side aerodynamic control stick with equal satisfaction in both cases."[124]

During initial planning, the engineers set the X-15 launch parameters at Mach 0.78 and 38,000 feet. However, before the first flight, North American decided to raise the launch altitude to 40,000 feet to provide additional performance and increased safety margins. During early launches from 40,000 feet, the X-15 generally needed about 3,000 feet to recover before beginning its climb. After the first few flights, researchers decided to increase the launch parameters yet again, this time to Mach 0.80 and 45,000 feet, just below the previously determined buffet boundary for the NB-52/X-15 combination. Interestingly, when researchers raised the launch altitude to 45,000 feet, the research airplane needed between 4,000 and 9,000 feet to recover, negating much of the value of the higher launch altitude.-1125

Although simplistic by modern standards, preparation of the X-15 for flight was still a complicated procedure involving many people and pieces of ground – support equipment. These drawings show the relative placement of tank trucks and other equipment during the loading of liquid oxygen and anhydrous ammonia prior to flight. (NASA)

In June 1960 the Air Force installed an AN/APN-41 radar transponder in the NB-52A that allowed the High Range to track the carrier aircraft more accurately. This beacon was similar to the one installed in the X-15. The problem had been that the B-52 fuselage was often located between the X-15 beacon and the radar site before launch and acted as an effective shield. Installing a beacon on the B-52 avoided the problem. A series of test flights that made simulated launches from Silver Lake (the NB-52 did not carry the X-15 for the tests) showed that using the beacon to position the B-52 resulted in a more accurate launch location than had previously been attained. This provided an extra margin of safety should the X-15 pilot have to make an emergency landing, and also allowed flight profiles to be repeated more accurately, helping post-flight analysis. The NB-52B received a similar beacon during July 1960. Flight 1-9-17 on 4 August 1960 was the first flight to use the new beacon.-1126

In June 1965 the FRC estimated that the full-up weight of the X-15A-2 with a real ramjet and fuel had grown to 56,000 pounds. This was more than 1,000 pounds greater than the most recent analysis showed the NB-52 wing/pylon could safely tolerate. In January and February 1966 the Air Force modified the NB-52A to increase the allowable pylon weight to 65,000 pounds, allowing for the heaviest expected X-15A-2 flight with some reserve for gusts or other contingencies. The modifications consisted primarily of installing doublers and additional fasteners on various parts of the wing and pylon structure. Although the modifications allowed the NB-52 to carry the X – 15A-2 safely, performance suffered. For instance, the maximum launch altitude was 1,500 feet lower and the maximum launch speed was restricted to about Mach 0.8 when the research airplane carried the external tanks and ramjet. The Air force installed the same modifications on the NB-52B during its next major maintenance period.-1^27