Category X-15 EXTENDING THE FRONTIERS OF FLIGHT

Unfortunately, the reliability demonstrated during the PFRT program did not continue at Edwards. Early in the flight program, vibrations, premature chamber failures, pump seal leaks, and corrosion problems plagued operations. Potentially the most serious problem was a 1,600-cycle vibration. Fortunately, the natural frequencies of the engines dampened the vibration below 100 g. However, between 100 and 200 g, the vibration could be dampened or could become divergent, depending on a complex set of circumstances that could not be predicted in advance, and the vibration always diverged above 200 g.[80]

The vibrations caused a great deal of concern at Edwards. On 12 May 1960, as the program was trying to get ready for the first XLR99 flight, the Air Force called a meeting to discuss the problem. Although Reaction Motors had experienced only one vibration shutdown every 50 engine starts at Lake Denmark, personnel at Edwards reported that there had been eight malfunction shutdowns out of 17 attempted starts. The vibration began when the main-propellant valves opened for final chamber start, although the engines had not experienced vibrations during the igniter phase. Since the demonstrated rate of occurrence had jumped from 2% at Lake Denmark to 47% at Edwards, nobody could ignore the problem. Engineers discovered that the 1,600-cycle vibration corresponded to the engine-engine mount resonant frequency, and that Reaction Motors had not seen the vibration using the earlier non-flight-rated engine mounts at Lake Denmark. As a temporary expedient, Reaction Motors installed an accelerometer that shut the engine down when the vibration amplitude reached 120 g, a move the company believed would permit flight­testing to begin.-181

The engine (serial number 105) used at Edwards differed only slightly in configuration from those used at Lake Denmark; for example, it used an oxidizer-to-fuel ratio of 1.15:1 instead of 1.25:1. The desired operating ratio at altitude was 1.25:1, and this is what Reaction Motors had used during their tests. However, to simulate the 1.25:1 ratio on the ground, the engine had to run at 1.15:1 to compensate for atmospheric and propellant density differences at the lower altitude. Reaction Motors had tested this reduced oxidizer-to-fuel ratio only twice at Lake Denmark, and had not encountered vibrations either time. The company recommended a series of actions, including checking for purge gas leaks at the PSTS, changing the propellant ratio back to 1.25:1, and performing more engine test firings.-82

By the beginning of June 1960, the problem did not seem to be getting any better. The Air Force conducted two tests with 17 starts on engine 105 at Edwards, with two vibration shutdowns using the ground orifice (1.15:1 ratio). When engineers reinstalled the flight orifice (1.25:1 ratio), three of five starts resulted in vibration shutdowns. Reaction Motors conducted 18 starts on engine 104, and three of the four initial starts resulted in vibration shutdowns, but all restarts were successful.82

A series of minor changes made to engine 104 by Reaction Motors seemed to ease the problem, and between the middle of July and the middle of August 1960, the engine accumulated 25 starts at Edwards without any vibration-induced shutdowns. In fact, only a single malfunction shutdown of any type was experienced, which was attributed to a severe "throttle chop" that the turbopump governor could not keep up with. Other XLR99s had experienced similar problems, and Reaction Motors warned the pilots to move the throttle slowly to avoid the situation.-1841

The Propulsion System Test Stand was the unlikely name for a non-flight X-15 fuselage that was used to test rocket engines. At least two of the fuselages were manufactured, one for Reaction Motors and one for Edwards AFB. Here technicians install an XLR99 in the PSTS in preparation for a test. (NASA)

Still, as late as the meeting of the Technical Advisory Group on 9-10 November 1960, the vibration problem persisted and the Air Force launched an effort to solve the problem. This program used two engines (006 and 012) at Lake Denmark and completed a series of baseline tests by the end of November that showed a 30% incidence rate of vibration shutdowns with the flight orifices installed. Reaction Motors found that modifying the liquid-oxygen inlet substantially lowered the incident rate of vibration shutdowns. Since this modification did not seem to have any other noticeable effect on the engine, the Air Force adopted it as a temporary fix.[85]

Separately, Reaction Motors determined that o-ring deterioration at the casing joint caused fuel pump seal leaks. Replacing the o-ring was difficult because it took technicians two or three shifts to remove the turbine exhaust duct, stator blades, rotor, and inlet housing; just to remove the exhaust duct necessitated the removal and re-safety-wiring of 60 bolts. Thus, although the o – ring failure itself was not serious, since it simply resulted in a steam leak, the repair required removing the engine from the aircraft, performing a time-consuming engine disassembly, and revalidating the engine installation. This process directly contributed to early flight delays using the XLR99.[86]

Ironically, the corrosion problem appeared to be the result of the unusually long engine life. With a few exceptions, the materials used by Reaction Motors for the turbopump were compatible with the various propellants, but those in contact with the hydrogen peroxide were experiencing more corrosion than desired. There were also some instances of galvanic action between the magnesium pump case and steel parts with decomposed peroxide as an electrolyte. As one

researcher noted, "the only thing really compatible with peroxide is more peroxide." There were no obvious fixes, so the program lived with the problem.[87]

The premature failure of the thrust chambers was of more concern. To insulate the stainless-steel cooling tubes from the 5,000°F flame, Reaction Motors used a 0.005-inch-thick, flame-sprayed Nichrome®-881 undercoat with 0.010 inch of oxygen-acetylene flame-sprayed Rokide Z zirconia as an insulating, erosion-resistant top coating. In service, the Rokide coating began to spall or flake due to thermal cycling from the large number of engine starts, and from vibration effects from an unstable flame. For instance, by January 1961 about 50 square inches of Rokide coating had peeled off engine 108 at Edwards, including 14 inches during a single vibration shutdown. The loss of the coating exposed the cooling tubes to the heat and erosive effects of the flame, overheating the ammonia coolant within the tubes and reducing the amount of cooling available. The superheated ammonia vapors also attacked the stainless steel and formed a very brittle nitrided layer. At the same time, the combustion gases began to melt and erode the tube surface. As this condition continued, the effective thickness of the tube wall gradually decreased until it burst. Raw ammonia then leaked into the chamber, causing more hot spots and eventually the complete failure of the chamber.-1891

In January 1961 the X-15 Project Office and the Materials Central Division of the Aeronautical Systems Division at Wright Field initiated a study of methods to improve the chamber life of the XLR99. Two possible approaches were to attempt to improve the Rokide coating system, or to develop an improved coating. The Air Force contract with Reaction Motors already included an effort to improve the Rokide coating, but researchers expressed little faith that this would achieve any measurable results. This resulted in the Air Force initiating a program to develop an alternate coating. In the meantime, engineers at the NASA Flight Research Center (FRC) surveyed other rocket engine manufacturers to find out whether they had developed workable processes. Both Rocketdyne and Aerojet were doing extensive laboratory testing of ceramics applied with plasma – arc devices, but neither had put the process into production. Both companies indicated that their experience with flame-sprayed alumina and zirconia had been unsatisfactory. Instead, Rocketdyne was working on metal-ceramic graduated coatings, and Aerojet was investigating the use of refractory metal (molybdenum and tungsten) overcoats on top of ceramics.-1901

At the time, the Air Force already had a contract with the Plasmakote Corporation to study graduated coatings in general, and this contract was reoriented to solving the XLR99 problem specifically. A second contract, this one with the University of Dayton, was reoriented to provide realistic techniques for laboratory evaluations of the coatings.-1911

A graduated coating consisted of sprayed layers of metal and ceramic; the composition changed from 100% metal at the substrate to 100% ceramic at the top surface. This removed the traditionally weak, sensitive interface between the metal and ceramic layers. Researchers produced the coatings by spraying mixed powders with an arc-plasma jet and gradually changing the ratio of metal and ceramic powders, with most of the coatings using combinations of zirconia with Nichrome, molybdenum, or tungsten. The FRC recommended adopting the new technique immediately as a way to repair damaged chambers at Edwards. They noted that engine 101 had been patched using Rokide coating, but the engine would soon need to be repaired again since the coating was not lasting. The Air Force and NASA decided that the next patch on engine 101 would use the new process, and NASA built a special fixture at the FRC to allow the chamber of a fully assembled engine to be coated.-921

Before the new coating was applied, NASA tested an existing Rokide chamber for 5.5 minutes, and 25 square inches of Rokide coating was lost during the test. Engineers then applied a

graduated coating segmented into areas using several different top coats, including tantalum carbide, titanium carbide, titanium nitride, zirconia with 10% molybdenum, and zirconia with 1% nickel. This chamber ran for 5.75 minutes, and only 3 square inches of the new coatings were lost. However encouraging, the tests were of relatively short duration and researchers did not consider them conclusive. One thing that became apparent during the tests was that it would be extremely difficult or impossible to reclaim failed chambers if the coating wore thin or was lost, since the internal damage to the tube might be sufficient to cause it to fail with no visible damage.[93]

One of the most significant issues experienced by the XLR99 during the flight program was the premature failure of the thrust chambers. Researchers eventually traced this to the spalling or flaking of the Rokide Z zirconia coating that had been applied to the inside of the chamber as an insulator. Although improved coatings were eventually developed, the Flight Research Center also developed an in-house capability to recoat the chambers when necessary, resulting in a significant cost savings compared to sending the chambers back to Reaction Motors or procuring new chambers. (NASA)

The Technical Advisory Group met on 11-12 January 1961 at the Reaction Motors facility at Lake Denmark. All in attendance agreed that chamber durability needed to be increased, and supported the development of a quick-change orifice to simplify ground runs. The group also recommended that the X-15 Project Office initiate the procurement of six spare chambers and sufficient long – lead material to construct six more. It could not be determined whether these chambers were actually procured.-194

Some documentation indicates that the XLR99 was redesignated YLR99 on 29 December 1961, although nothing appears to have changed on the engines themselves. The original source documentation from the period is inconsistent in its use of XLR99 or YLR99; this history will use XLR99 throughout simply to avoid confusion.1951

By March 1962, technicians at the FRC had the necessary equipment and training to recoat the chambers as needed. The cost of the tooling had come to almost $10,000, but the cost to recoat a chamber was only about $2,000-much less than the cost of procuring a new chamber from Reaction Motors. The coating finally approved for use consisted of 30 mils of molybdenum primer in the throat and 10 mils elsewhere, followed by 6 mils of a graduated Nichrome-zirconia coating and then 6 mils of a zirconia topcoat. NASA used this coating process for the duration of the flight program with generally satisfactory results.1961

As is the case with almost any new technology, some things can never be fully understood. One of the harder things to grasp when dealing with complex mechanical devices is component matching (or mismatching), i. e., why some items will work in a particular assembly and other seemingly identical items will not. For example, during the initial checkout of engines 108 and 111 at Edwards, both engines exhibited excessive vibrations. NASA replaced the igniter in engine 108 with a spare that reduced the vibration to acceptable levels. The igniter that had been removed from 108 was then installed in 111 and its vibration was reduced to acceptable levels. Compatibility was not a particular problem, but scenarios such as this did point out some puzzling inconsistencies.-1971

The concept of using a large aircraft to carry a smaller one aloft was not necessarily new, but the X-1 program was the first research effort that made extensive use of the idea. The original series of X-planes used two modified Boeing B-29s and three Boeing B-50s as carrier aircraft. However, despite the fact that thousands of B-29s and B-50s had been built, by the end of 1950 maintenance personnel at Edwards were finding that it was difficult to obtain replacement parts, especially for the B-29s. The performance of the aircraft had proven adequate for the original X-1 aircraft, but as the research airplanes got heavier, the performance of even the more-powerful B – 50s became marginal. In addition, the ability to take off at high gross weights was limited in the heat that was typical of the high desert during the summer months. Obviously, the research programs needed to find a better solution.1761

B-36

Three of the four competitors had sized their X-15 concepts around the premise of using a Convair B-36 as the carrier aircraft (Douglas had chosen a B-50). Easily the largest piston – powered bomber to enter operational service, the B-36 could fly over 400 mph and some versions could climb well above 50,000 feet. Convair manufactured 385 of the giant bombers between June 1948 and August 1954. The B-36 would have carried the X-15 partially enclosed in its bomb bays, much like the X-1 and X-2 had been in earlier projects. This arrangement had several advantages, particularly that the pilot could move freely between the X-15 and B-36 during the cruise to the launch location. This was extremely advantageous if problems developed that required jettisoning the X-15 prior to launch. The B-36 was also a large aircraft with more than adequate room for a propellant top-off system (liquid oxygen and ammonia), power sources, communications equipment, breathing oxygen, and monitoring instruments and controls. Launch would have occurred at approximately Mach 0.6 at altitudes between 30,000 and 50,000 feet. At the first industry conference in 1956, engineers at North American anticipated that a B-36 would be modified beginning in the middle of 1957 and ready for flight tests in October 1958.[77]

During their proposal effort, North American evaluated four different schemes for loading the research airplane into the bomber, which were generally similar to those of the other bidders. Engineers quickly rejected the idea of using a pit (like the X-1 and operationally for the GRB – 36D/RF-84K FICON project) because of the potential "fire hazard and accumulation of fumes." Similarly, they eliminated a plan to jack up the carrier aircraft nose gear, because of "the jockeying necessary to position the research aircraft plus the precarious position of the B-36." The most complicated scheme involved physically removing the vertical stabilizer from the research airplane, sliding the X-15 under the bomber, and then reattaching the vertical once the airplane was in the bomb bay. The potential loss of structural integrity that would result from frequently removing the vertical eventually eliminated this option.-178

North American had originally selected a Convair B-36 very heavy bomber as the carrier aircraft for the X-15. However, just before modifications were to begin, NASA and the Air Force decided to replace the B-36 with a much newer Boeing B-52 Stratofortress. The B-52 was a good deal faster than the B-36, providing a better launch environment for the research airplane and reducing maintenance requirements for the ground crew. (North American Aviation)

Ramp loading, which was similar to another method used in the FICON project, became the chosen solution.-1791 Loading the X-15 into the carrier aircraft began with "running the B-36 main landing gear bogies up on permanent concrete ramps by use of commercially available electric cable hoists attached to the gear struts." The ground crew then towed the research airplane under the bomber and hoisted it into the bomb bays.1801

The X-15 was suspended from three points: one on either side of the aft fuselage attached to the rear wing spar, and a third on the centerline behind the canopy firmly supported by the structure of the forward liquid-oxygen tank bulkhead. The same types of cartridges used by tactical aircraft to jettison external fuel tanks were used to explosively separate the shackles.-1811

The only major structural modification made to the B-36 would be the removal of bulkhead no. 7, which separated bomb bays 2 and 3, along with some compensating structural stiffening.1871 The X-15 would occupy most of the three forward bomb bays. Since the B-36 used a single set of doors to cover the aft two bomb bays, shorter doors were necessary to cover only bay no. 4.1831 Interestingly, the remaining 16-foot doors covering the last bomb bay would still be functional. A small, fixed fairing replaced the doors that normally covered bomb bay nos. 1 and 2. North American proposed installing a 9-foot-diameter, 6.5-foot-long heated compartment in the front of bomb bay no. 1, equipped with its own entrance hatch on the bottom of the fuselage. The compartment could seat three crewmembers, and included oxygen and intercom connections. A 36-inch hatch opened into the bomb bay, and a catwalk on both sides of the bomb bay allowed access to the X-15 in flight. An aerodynamic fairing with a rubber-sealing strip ran the full length of the bomb-bay opening.1841

One of the more interesting suggestions concerning the carrier aircraft was that "a bank of powerful lights be turned on several minutes prior to launching so that the pilot [of the research airplane] will not be blinded by the sudden glare of daylight during launching."1851

The B-36 was equipped with a 1,000-gallon liquid-oxygen tank and a 100-gallon ammonia tank to top off the research airplane’s propellants. This was surprising because Bell and Douglas, as well as Reaction Motors, believed the rate of ammonia boil-off was so slow that no topping-off would be required. Suspended in the bomb bay above the X-15, the tanks allowed the propellants to be gravity-fed into the airplane. A nitrogen bottle pressurized and purged the tanks, and lines running outside the fuselage to the former tail turret allowed the carrier aircraft to jettison and vent the rocket propellants.1861

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

The 9 July 1954 meeting at NACA Headquarters and the resulting release of the Langley study served to announce the seriousness of the hypersonic research airplane effort. Accordingly, many government agencies and aircraft manufacturers sent representatives to Langley to examine the project in detail. On 16 July three representatives from the Air Research and Development Command (ARDC)-the Air Force organization that would be responsible for the development of the airplane-visited John Becker to acquaint themselves with the NACA presentation and lay the groundwork for a larger meeting of NACA and ARDC personnel.-11!

Independently of any eventual joint program, approval for the first formal NACA research authorization was granted on 21 July 1954. This covered tests of an 8-inch model of the Langley configuration in the 11-inch hypersonic tunnel to obtain six-component, low-angle-of-attack and five-component, variable-angle-of-attack (to about 50 degrees) data up to Mach 6.86.-12 Research authorizations were the formal paperwork that approved the expenditure of funds or resources on a research project. At the time, it was not unusual-or worthy of comment-for the NACA laboratories to conduct research without approval from higher headquarters or specific funding. This type of oversight would come much later.

During late July, Richard V. Rhode from NACA Headquarters visited Robert R. Gilruth to discuss the proposed use of Inconel X in the new airplane. Rhode indicated that Inconel was "too critical a material" for structural use, and the program should select other materials more representative of those that would be in general use in the future. Rhode later put this in writing, although Langley appears to have ignored the suggestion. This harkened back to the original decision that the research airplane was not meant to represent any possible production configuration (aerodynamically or structurally), but instead was to be optimized for its research role.-13!

The overall configuration of the airplane conceived by NACA Langley in 1954 bears a strong resemblance to the eventual X-15. This configuration was used as a basis for the aerodynamic and thermodynamic analyses that took place prior to the contract award to North American Aviation. This drawing accompanied the invitation-to-bid letters during the airframe competition, although it was listed as a "suggested means" of complying with the requirements. (NASA)

On 29 July, Robert J. Woods and Krafft A. Ehricke from Bell Aircraft visited Langley as part of the continuing exchange of data with the industry. On 9 August, the Wright Air Development Center (WADC) sent representatives from the Power Plant Laboratory to discuss rocket engines, in particular the Hermes A1 that Langley had tentatively identified for use in the new research airplane. The WADC representatives went away unimpressed with the selection. The next day Duane Morris and Kermit Van Every from Douglas visited Langley to exchange details of their Model 671 (D-558-3) study with the Becker team, providing a useful flow of information between the two groups that had conducted the most research into the problem to date.^4-

The Power Plant Laboratory emphasized that the proposed Hermes engine was not a man-rated design, but concluded that no existing engine fully satisfied the NACA requirements. In addition, since the Hermes was a missile engine, it could only operate successfully once or twice, and it appeared difficult to incorporate the ability to throttle or restart during flight. As alternatives to the Hermes, the laboratory investigated several other engines, but suggested postponing the engine selection until the propulsion requirements were better defined.^5-

The Hermes engine idea did not die easily, however. As late as 6 December 1954, K. W. Mattison,^6 a sales engineer from the guided-missile department of the General Electric Company, visited John Becker, Max Faget, and Harley Soule at Langley to discuss using the A1 engine in the new airplane. Mattison was interested in the status of the project (already approved by that time), the engine requirements, and the likely schedule. He explained that although the Hermes engines were intended for missile use, he was certain that design changes would increase the "confidence level" for using them in a manned aircraft. He was not sure, however, that General Electric would be interested in the idea.[7]

The development of an escape system had been the subject of debate since the beginning of the X-15 program. North American’s decision to use a combination of an ejection seat and a full – pressure suit was a compromise based largely on the ejection seat being lighter than the other alternatives. It was also heavily lobbied for by Scott Crossfield.

The Aero Medical Laboratory had recommended an escape capsule, as prescribed by existing Air Force regulations, as early as 8 February 1955. However, the laboratory admitted that an escape capsule would require a long development period and would probably be unacceptably heavy. The laboratory’s alternative was an ejection seat with limb restraints used together with a full – pressure suit. Meetings held during October and November 1955 resulted in a direction to North American to develop an ejection seat that would incorporate head and limb restraints. The Air Force also told North American to document the rationale for adopting such a system.!1281

Privately, Scott Crossfield had already decided he did not like capsule designs. Part of this came from experience with the Douglas D-558-2 program. According to Crossfield, "We had a capsule nose on the Skyrocket but knew from the wind-tunnel data that if you separated the nose from the fuselage, the g-force would be so great it could kill you. I made up my mind I would never use the Skyrocket capsule. I would ride the ship down and bail out." Later events with a similar system on the X-2 would prove this fear correct.!1291

The North American analysis of potential accidents that could cause the pilot to abandon the X – 15 produced some surprising results. Despite the high-altitude and high-speed nature of the mission profiles, North American determined that 98% of potential accidents were likely to occur at dynamic pressures below 1,500 psi, Mach numbers below 4.0, and altitudes less than 120,000 feet. Using these as criteria, North American investigated four potential escape systems: fuselage – type capsules, cockpit capsules, encapsulated seats, and open ejection seats. The comparison included such factors as cockpit mobility, escape potential, mechanical reliability, post-separation performance, and airframe compatibility. This effort took some 7,000 man-hours to complete.

The results showed that an open ejection seat imposed the fewest performance penalties on the aircraft and took the least time to develop. The estimates from North American showed that a satisfactory escape capsule would add 9,000 pounds to the 31,000-pound airplane. Just as importantly, North American—and Scott Crossfield, who would be making the first flights in the airplane-believed the ejection seat offered a better alternative in the event of an emergency, mainly due to its relative mechanical simplicity.-1130!

Despite the report, the Air Force was not completely convinced. During a meeting at Wright Field on 2-3 May 1956, the laboratory again emphasized the perceived limitations of ejection seats. Primarily due to the efforts of Scott Crossfield, the Air Force finally agreed that "the X-15 was probably its own best capsule." The meeting also resulted in another action for North American, once again, to document its rationale for selecting the stable-seat and full-pressure suit combination.-131!

North American held the first formal cockpit inspection in July 1956 at its facility in Inglewood. This inspection featured a fully equipped cockpit mockup, complete with instruments, control sticks, and an ejection seat. The seat was a custom design that featured a new type of pilot restraint harness and small stabilizers to "weather-vane" it into the wind blast and prevent fatal tumbling or oscillation. A solid rocket motor provided about 3,000 lbf to ensure that the seat would clear the X-15. Despite Air Force policy to the contrary, nobody raised any objections about the seat during the inspection. By default, it became part of the official design.-132!

By November 1956, North American had tested a 0.10-scale isolated pilot-seat model of its design in the Naval Supersonic Laboratory wind tunnel at the Massachusetts Institute of Technology (MIT). Although the seat seemed to stabilize randomly in different orientations, the results were generally encouraging. In itself, this did not represent a serious problem, although all participants wanted to understand the dynamics involved. North American conducted additional tests in the Southern California Co-Operative Wind Tunnel in Pasadena to develop the final stabilization system configuration and determine the influence of the forward fuselage without the cockpit canopy.-133!

The debate over the X-15 ejection seat intensified on 27 September 1956 when Captain Milburn G. Apt was killed in the X-2. However, the accident also weakened the case for an escape capsule. The X-2 used a semi-encapsulated system whereby the entire nose of the aircraft, including the cockpit, was blown free of the main fuselage in an emergency. Unfortunately, Bell engineers had expected the pilot to be able to unbuckle his seat straps and manually bail out of the capsule after it separated, something Apt was unable to do. It demonstrated that an encapsulated system was not necessarily the best solution, but then neither was an ejection seat. Almost by definition, piloting X-planes was—and would remain—a dangerous occupation.134

During early 1958, researchers began testing the X-15 ejection seat on the rocket sleds at Edwards, with the preliminary runs concluding on 22 April. The series got off to a good start, with the first test seat ejected at 230 knots and the parachute successfully opening at 120 feet, lowering the anthropomorphic dummy gently to the ground. The dummy was equipped with telemetry that relayed data from rate gyros, accelerometers, and pressure transducers. The second test, this one at 620 knots and a dynamic pressure of 1,130 psf, also went well. The third test, under similar conditions, was again satisfactory. However, during the fourth run the shock­wave generator catapult exploded at Mach 1.26 and 2,192 psf. The accident damaged the seat, suit, and anthropomorphic dummy beyond repair. Engineers fired another seat during a static test on 24 April, but the post-ejection operation failed because of a mechanical problem in the initiation hardware. During the second static test on 14 May 1958, the parachute and parachute lines became tangled with the seat. In all, the test series provided mixed results. North American made several minor modifications in preparation for a second series of tests scheduled for June.-1135!

The high cost of the rocket-sled runs, coupled with the damaged seat hardware, was quickly exceeding the budget for the escape-system tests. Because of this, the X-15 Project Office decided to conduct only two tests, at 125 psf and 1,500 psf. Despite the earlier difficulties, Air Force and North American engineers believed these two tests could adequately demonstrate seat reliability.-1136!

The Air Force conducted the test at 125 psf on 4 June 1958, and the results appeared to be satisfactory. Three successful tests took place during June, but the fourth test, on 3 July, revealed serious stability problems. North American discontinued further tests until it could determine a cause for the failures. A detailed analysis revealed that the seat would need several major modifications.-137!

The Air Force conducted the first test of the revised North American seat on 21 November 1958, but several of the sled rockets failed to ignite and reduced the desired 1,500-psf pressure to about 800 psf. Two tests during December also suffered from the failure of sled rockets. The only test conducted during January failed when the right-hand boom and fin failed to deploy. The leg restraints also failed during the test, but North American believed an instability caused by the boom malfunction caused this. The parachute failed to open until just before the test dummy hit the ground, causing significant damage to the dummy.138!

The ejection seat for the X-15 was a remarkable engineering achievement, and was the most sophisticated ejection seat yet developed at the time of the first X-15 flight. Still, it was much simpler than an encapsulated ejection system would have been. (U. S. Air Force)

The schedule was getting tight since the X-15 was nearly ready to begin captive-carry flights. On 12 January, the Aircraft Laboratory verbally approved the seat for the initial captive and glide flights between Mach 0.377 and Mach 0.720 at dynamic pressures between 195 and 715 psf. The X-15 Project Office considered this satisfactory given the inability of the NB-52 to go much faster.^

As developed by North American, the ejection seat contained provisions to restrain the pilot’s arms and legs to keep them from flailing in the airstream after leaving the aircraft, and also booms and canards to stabilize the seat during separation. After the seat left the aircraft, the pilot unbuckled and jumped from the seat, coming down on his own parachute. (North American Aviation)

Because of the unsuccessful January test, North American carefully rechecked and strengthened the booms and pressure-tested the seat’s gas system. The Air Force conducted the final sled-test on 3 March 1959 at Mach 1.15 and 1,600 psf—conditions somewhat in excess of requirements. Despite the failure of the leg manacles, the test was the most successful to date. North American proposed additional tests and a parachute program in April 1959, but the X-15 Project Office was happy with the results of the tests already run and declined. The X-15 finally had an ejection seat.^1401

The pilot used a backpack-type parachute after he separated from the seat. However, because of the design of the pressure suit, seat, and cockpit, neither the Air Force nor North American considered the standard quarter-deployment bag and 28-foot-diameter C-9 parachute acceptable. Instead, North American produced a special 24-foot-diameter chute and "skirt bag" specifically for the X-15. The company extensively tested this combination on a whirltower to verify the design of the skirt bag, the optimum pilot-parachute bridle length, and the effect of having the seat headrest permanently attached to the pilot chute. The tests in early 1958 included opening speeds up to 300 knots, and subsequent free-fall tests with an anthropomorphic dummy released from a Fairchild C-119 Flying Boxcar over the National Parachute Range in El Centro, California. During the initial tests, the C-119 released the dummy in a head-down attitude at 125

knots and 1,200 feet. These tests were unsuccessful because the pilot chute deployed into a low – pressure zone in the wake of the dummy and was not capable of pulling the main chute from the pack. North American extended the bridle length to 70 inches, allowing the pilot chute to escape the low-pressure area, and subsequent tests were successful.-141

Initially North American used the 24-foot diameter chute because it was the largest they could easily accommodate in the backpack and the engineers thought it would open more quickly, allowing safe ejection at lower altitudes. However, several flight surgeons had concerns that it would allow too high a descent rate for the pilot, and urged the certification of a larger parachute for use on the X-15. During October 1960, North American tested a repackaged 28-foot – diameter parachute at the National Parachute Range. These tests were successful and indicated no significant difference in opening time between the smaller and larger chutes. It became policy that each pilot could select whichever size parachute he wished to use. Most continued to use the 24- foot chute because the reduced thickness of the backpack made it more comfortable to sit on in the cockpit.142

In June 1965, NASA authorized North American to purchase five new 28-foot parachutes to replace the 24-foot units that had reached their 7-year service limit. The new chutes had a disconnect device that allowed the pilots to release one-half of the shroud lines during descent. They were less comfortable because they were thicker than the original parachutes, but as personnel at Edwards discarded the smaller units, they became standard.142

Despite the confidence Scott Crossfield and the North American engineers had in the ejection seat, apparently it was not universal. Pete Knight once commented, "They tell me that the seat is good for Mach 4 and 120,000 feet. I take it with a grain of salt, but I think the safest place to be is inside the airplane until we get to a more reasonable environment…. If you had to, as a last resort certainty you would take the chance, but I think most of the pilots have felt that we…would stay with [the airplane] as long as possible." At least everybody agreed that the cockpit was a safe place. Crossfield demonstrated that when the X-15-3 exploded on the ground while he was testing the XLR99 engine.-144-

The X-15 ejection seat, like all other seats of the era, was tested on the rocket sled track at Edwards AFB, California. The sled test results were mixed, with many failures of both the sled and the seat for various reasons, but ultimately the Air Force, NASA, and North American were satisfied that the seat would work as advertised. (U. S. Air Force)

Of the 11 XLR99 flight engines that were produced during 1958-1960 to support the flight program, one (s/n 105) was destroyed in the 1959 ground accident and another (111) was destroyed in the 1967 crash of the X-15-3. During September 1975, researchers at Edwards conducted an inventory of existing engines and engine spares in anticipation that the engine might possibly be used in a future flight program. Seven flight-rated and one ground-test engine remained at Edwards, but the Air Force had already scrapped the others or given them to museums. Although the engineers thought most piece-parts were available from various sources, three high-cost spares (thrust chamber/injector assemblies, turbopump cases, and igniters) were in short supply.-1114

because of cracks in the tubing or injector spud. Six pump cases ($12,000 each) had been replaced during the X-15 flight program, mainly due to corrosion, and there were eight cases available for future use. Only 10 igniters ($4,000) were available, but the flight program had used 17, mainly due to detonation at shutdown-a condition that Reaction Motors had largely corrected.-1115!

In addition to the possibility of using existing engines in another program, several proposals had been made for augmented or improved versions of the XLR99 to support various projects. The first serious effort was to support the hypersonic research engine (HRE) experiment on the X – 15A-2. On 30 October 1963, Douglas E. Wall, the project manager for airborne hypersonic research at the Aeronautical System Branch at the FRC, wrote to James E. Love, the NASA X-15 program manager, advising him that the X-15A-2 would likely fall far short of the performance requirements for the HRE program.-116!

The region of interest for supersonic combustion testing was from 7,000 to 8,000 fps at dynamic pressures between 1,000 and 2,000 psf. Although Wall cautioned that he could not ascertain the extent of the performance shortfall until after preliminary flight tests, at the time it looked like the X-15A-2 would fall approximately 1,000 fps short. At a meeting held at Wright-Patterson on 25 September 1963, researchers recommended that the X-15 Project Office fund an upgrade to the XLR99, and the AFFTC and FRC representatives proposed three different modifications. The first was the use of an extended nozzle to increase performance at the mid-altitudes (^100,000 feet) for the expected ramjet experiments. The other modifications included a modified injector assembly and the use of a hydrazine fuel additive. Researchers expected that these modifications would take between 12 and 14 months to develop and implement. The X-15 Project Office agreed to look into the matter; however, there appears to be no record indicating that any action was taken.117!

Nevertheless, Reaction Motors did conduct several studies during 1964-1965 on possible improvements to the XLR99. At least one of these investigated the use of axisymmetric and two­dimensional nozzles, and another studied possible improvements to the thrust chamber. Reaction Motors engineers also kept up with the published reports from other rocket-engine manufacturers to see if any of their developments might be applicable to the XLR99.118!

The FRC already had some experience with increasing rocket-engine performance by using nozzle extensions on the Douglas D-558-2. These extensions were small, radiation-cooled members that permitted the rocket exhaust gases to attain higher exit velocities by expanding within the nozzle to ambient pressures. Because of their small size, the extensions had no serious aerodynamic effect or structural design implications. It appeared to researchers at the FRC that a lightweight, radiation-cooled nozzle extension could provide a desirable performance increase for the X-15A-2. The researchers admitted, however, that it would be more difficult to design such a nozzle for the XLR99 than for the XLR11 because of the former’s larger size and more severe operating environment. The size issue loomed largest because there was a possibility of adverse aerodynamic interference with the afterbody flow.119

In order to evaluate this potential, researchers ran a series of wind-tunnel tests that used several different nozzle extension designs. The tests were quite extensive and included various speed brake and horizontal stabilizer positions, ventral stabilizer shapes, and ramjet installations. Tests were conducted over free-stream Mach numbers from 2.3 to 8.0 using the Unitary Plan Tunnel at Langley (Mach numbers up to 4.63) and the von Karman Gas Dynamics Facility Tunnel B at the Air Force Arnold Engineering Development Center (AEDC) at Mach numbers 6.04 and 8.01. To withstand the high Mach numbers, researchers modified the 1/15-scale model to withstand temperatures of 900°F for up to 30 minutes.-1120!

The tests included nozzle extensions of various exit diameters and lengths representing expansion ratios of 22.1:1 to 33.6:1, along with various aerodynamic shrouds to reduce interference effects. In all, researchers investigated nine candidate nozzles, and the tests indicated that none of the nozzle extensions had any appreciable affect on overall drag or static margin, although the 22.2:1 nozzle was most suitable. The use of this nozzle increased the burnout velocity by 400 fps with no other changes to the airplane or engine.-121!

During January 1966, researchers at Langley ran more tests on the 1/15-scale model of the X – 15A-2 in the 4 by 4-foot unitary tunnel. These obtained data on various XLR99 nozzle extensions, including ones with area ratios of 11.2:1, 28.8:1, and 33.6:1 at Mach numbers up to 4.63. The X015 models used in the wind tunnels included various other modifications, including a redesigned aft fuselage boat-tail meant to smooth over the larger engine nozzle. All of the nozzle extensions actually improved the base drag coefficients over the basic configuration, and all exhibited less drag than the boat-tail configurations. Despite the seemingly minor cost of the nozzle modifications, neither the Air Force nor NASA took any action to produce any hardware or perform actual engine or flight tests.122!

In early 1967, Reaction Motors began another investigation of an improved nozzle for the XLR99 designed to increase thrust at high altitudes. The Air Force issued a work order for the study as an extension of the XLR99 engineering support contract, but did not record the exact reason for the study. The new nozzle had an expansion ratio of 22.5:1 instead of the 9.8:1 used on the existing XLR99s, resulting in an increase in vacuum thrust and vacuum-specific impulse of approximately 7% at a chamber pressure of 600 psi. Two percent of that improvement was the result of using a contoured nozzle instead of the 20-degree conical nozzle used on the original 9.8:1 extension.123

During the investigations of the new nozzle, all other parts of the engine remained unchanged, so it would have been easily possible to retrofit existing engines. The new engine produced a specific impulse of 298-lbf-sec/lbm and a thrust of 63,378-lbf in a vacuum. The new engine could be operated at sea level without flow separation, although its performance was somewhat below the standard XLR99 at low altitudes. The recommended nozzle design was an overturned bell nozzle composed of tangent circular arcs with a length and end diameter roughly equivalent to the normal 20-degree conical nozzle. The nozzle was designed with an exit angle of approximately 5 degrees rather than zero. This is because the last few degrees of wall-turning only added weight, since friction losses canceled out the theoretical thrust gain. Again, no further action resulted from the study.123

Perhaps the most ambitious upgrade was the one proposed to support the delta wing X-15 concept. One of the desired missions for the delta-wing airplane was a sustained 1-g Mach 7 cruise capability, and Reaction Motors sought a way to allow the XLR99 to act as a "sustainer" engine producing 8,000-10,000 lbf for several minutes at a time. The company investigated two different possibilities to provide the sustainer capability. The first used the existing XLR99 chamber to provide the same 57,000-lbf thrust and a separate, remotely located chamber to provide additional thrust during main engine operation and sustainer thrust during cruise. This was conceptually similar to the system used on the Atlas ICBM and the ill-fated Curtiss-Wright XLR25 in the Bell X-2. The second idea was to modify the existing chamber to both provide increased thrust and allow the sustainer function, and to use the previously investigated 22.5:1 expansion ratio nozzle. This second concept was similar to what the 1963 meeting at Wright – Patterson had recommended to fix the X-15A-2 performance shortfall. Reaction Motors estimated that it would take two years to develop and test the modified engine.-1125!

Surprisingly, Reaction Motors preferred using a separate sustainer chamber since it presented less risk and required less development time. Throttling the main chamber produced between 26,000 and 62,000 lbf, and the remote chamber produced between 8,000 and 21,000 lbf. This would have provided an engine capable of infinite throttling between 8,000 and 83,000 lbf. The Air Force disagreed with the risk assessment and considered the problem of integrating a second thrust chamber and nozzle into the X-15 too great, so the delta-wing program selected the single-chamber design despite the longer development time required.-126!

The major constraint imposed in considering the maximum thrust available from modifications to the XLR99 was the number of changes that had to be made to the turbopump. Unlike some other components of the XLR99, the turbopumps had been relatively trouble-free during development and operation. However, because of this lack of problems, nobody was thoroughly familiar with the pumps and their operation. To address this, Reaction Motors brought the original turbopump engineer, Haakon Pedersen, out of retirement. Pedersen proposed relatively modest changes to the turbopump that could provide a 40% increase in pumping capacity. The solution was deceptively simple: speed up the pump. This increased speed was not expected to "generate difficulties with the seals, bearings, or critical speed" or to "affect cavitation adversely." Pedersen did caution that he based these predictions on his own intuition since Reaction Motors had never tested the turbopumps at greater than 100% power. The increased speed, however, required a new turbine because the existing one could not accommodate the 72.5% increase in hydrogen – peroxide flow.122!

There is no record that Reaction Motors ever accomplished any testing on the modified XLR99 or its components. Given that NASA terminated the delta-wing X-15 project early in its development, it is likely that Reaction Motors never modified any hardware.

During the course of the X-15 program, various drawings and artist concepts were released that showed the research airplane-particularly the proposed delta-wing version-carried by a North American XB-70 bomber. The use of this Mach 3+ capable aircraft would have greatly extended the performance envelope of the X-15. However, given the theoretical uncertainties of launching an object from the back of a larger aircraft traveling at Mach 3, it is unlikely that the Air Force or NASA ever seriously considered this concept. After the fatal crash on 30 July 1966 of a Lockheed M-21 Blackbird while launching a D-21 drone from a similar configuration, it became even more unlikely. Nevertheless, sometime during 1966 North American conducted a study (logically called "XB-70/X-15"); unfortunately, however, no copy could be found in any archive, so its contents and conclusions are unknown.-1228

The use of the Mach 3+ capable XB-70A as a carrier aircraft would have greatly extended the performance envelope of the X-15. However, given the theoretical uncertainties of launching an object from the back of a larger aircraft traveling at Mach 3, coupled with the fact that only two Valkyries were manufactured, it is unlikely that the Air Force or NASA ever seriously considered this concept. (North American Aviation)