Category X-15 EXTENDING THE FRONTIERS OF FLIGHT

The X-15 was breaking new ground when it came to structural materials, since it was obvious from the start that most of the wetted surface would be subjected to temperatures up to 1,200°F. Exotic materials made from the rare elements had not advanced sufficiently to permit quantity production of these expensive alloys, so the list of candidate materials was narrowed to corrosion resistant steels, titanium, and nickel-base alloys ("stainless steels"). The following table shows the strength properties of the candidate materials at room temperature; various aluminum alloys are included as a comparison. All properties are for bare sheet stock, except for the AM-355 bar stock. Materials marked with an asterisk were heat-treated.-1591

Although 6A1-4V titanium and AM-350 CRES had good strength efficiencies over a wide temperature range, both of the alloys tended to fall off rapidly above 800°F. Inconel X, on the other hand, had only a gradual drop in strength up to 1,200°F. Because of this stability, North American chose Inconel X for the outer skin for the entire airplane. Regular Inconel (as opposed to Inconel X) was not heat-treatable, but it could be welded and was used in locations where high strength was not of paramount importance or where final closeout welds were necessary following heat treatment of the surrounding structures. To accomplish this, Inconel lands were incorporated into Inconel X structures prior to final heat treatment, and access-hole cover plates made from

Inconel were welded to these lands.-601

North American used high-strength aluminum (2024-T4) to form the inner pressure shell of the cockpit and part of the instrumentation bay. As a relief from high thermal stresses, the company used titanium for the structure of the fuselage and wings. Originally, the company used two titanium alloys: 8-Mn, which was the highest strength alloy then available but was not recommended for welding, and 5A1-2.5Sn, which had acceptable strength and was weldable. Later, North American began using a high-strength and weldable alloy, 6A1-4V, in some areas.

To combat the high concentrated loads from the engine, most of the aft fuselage structure used titanium framing. The majority of the structure used fusion welding, although the company also used a limited amount of resistance welding. North American radiographically inspected all critical welds to ensure quality.-611

The material that presented the most problems was probably the 5Al-2.5Sn titanium, which proved to have inconsistent tensile properties that made it difficult to work with. It also exhibited low ductility and notch sensitivity, and had a poor surface condition. These problems existed in both rolled and extruded forms of the metal. The surface condition was the most important factor governing the formability of titanium, so North American had to remove all oxygen contamination, inclusions, and grind marks by machining, polishing, or chemically milling the metal prior to the final finishing. As a result, North American procured titanium extrusions for the X-15 with sufficient extra material in all dimensions to allow technicians to machine all surfaces prior to use.-621

The limited amount of stretch and shrink that was possible with a titanium extrusion during stretch wrapping presented a different problem when North American went to form the side fairing frames. Each frame was composed of four titanium 5Al-2.5Sn extrusions. One of the problems was that the inside flanges were located in areas that had small bend radii, and it was necessary to prevent compression failure. The small bend radii were "relieved" (some material was removed prior to bending), and a gusset was later welded in to fill the relieved area. The alternative would have been to reduce compression by increasing the pull on the forming machine, thus shifting the bend axis closer to the inboard edge. This, however, would have resulted in a tension failure on the outboard flange.-631

North American found that one of the more interesting aspects of titanium was that a formed part was prone to crack until the residual stresses resulting from the forming had been removed. This delayed cracking could occur within a few minutes, or it might not become evident until weeks later. In response, North American initiated a process that provided stress relief for all parts except "slightly" formed parts, such as skin panels, since they exhibited few problems.-641

Forming the seven different pressure vessel configurations in the X-15 presented its own problems. When compatibility with the contained fluid permitted, titanium was the first choice of material. North American used a 26-inch Cincinnati Hydroform for the hemispherical ends of the 14-inch cylindrical nitrogen tanks with little difficulty. The company also attempted to form the 16-inch hemispheres for the helium tanks on this machine, but the optimum blank size was greater than the maximum machine capacity of 26 inches. Using a smaller-than-optimum blank required excessive hold-down pressure that resulted in small surface cracks. The alternative was to "spin form" the hemispherical ends. Engineers heated the blanks to approximately 1,600°F and used an internally heated spinning chuck to shape the disc. Unfortunately, this resulted in a surface with significant oxygen contamination, so North American used thicker parts and machined them to the correct thickness to eliminate the contamination. Machining was also required to match the hemispheres for each end of the tank prior to welding.-651

Finding the correct material for the main propellant tanks, especially the liquid-oxygen tank, took some investigation. Most steel and common heavy structural alloys gain strength but lose ductility when operated at low temperatures, although Inconel proved to be relatively insensitive to this. The martensitic alloys, such as heat-treated 4130 low-alloy steel and AM-350 CRES precipitation-hardened corrosion-resistant steel, followed predictable curves that showed severe ductility loss as the temperature decreased below -100°F. A titanium alloy containing 5% aluminum and 2.5% tin handled the low temperatures well, but did not have the requisite strength at 1,200°F. North American finally decided to manufacture the primary barrels of the tanks from Inconel X.1661

Initially, engineers used AM-350 CRES, formed on a 7,000-ton hydraulic press using a deep-draw process, for the 32-inch hemispheres of the main propellant tanks. Excessive thinning occurred until the optimum pressure on the press draw ring was determined. Even then, North American encountered some difficulty due to uneven forces from the pressure pins used to secure the blanks, resulting in non-uniformity around the periphery of the hemisphere. The engineers subsequently decided to discard the CRES hemispheres and to remanufacture them from Inconel X.1671

Inconel X proved to be remarkably easy to work with considering its hardness, although the engineers had to make severely formed parts in multiple stages, with annealing accomplished between each stage. Nevertheless, problems arose. One of the first concerned fabricating the large Inconel propellant tank hemispheres. The propellant tanks comprised a large portion of the fuselage and were composed of an outer cylindrical shell and an inner cylinder. Inconel X semi­torus hemispheres at each end of the tank joined these two parts. The hemispheres were formed in two segments, with the split located midway between the inner and outer cylinders. Technicians welded the inner torus segment to the inner cylinder, and the outer torus segments to the outer tank, before joining the two assemblies.1681

After initial attempts to spin the bulkheads from a single, heated Inconel X blank were unsuccessful, the technicians built up the cones by welding smaller pieces together, and performed a complete X-ray inspection of each weld. After the cones were formed to the approximate size, they went through several stages of spinning, with a full annealing process performed after each stage. The first spin blocks used for the hemispheres were made from hardwood, and cast iron was used for the final sizing. A problem developed when transverse cracks began to appear during the spinning of the hemispheres.1691

Both North American and the International Nickel Company investigated the cracks, but determined that the initial welds were nearly perfect and should not have contributed to the problem. Nevertheless, engineers tried different types of welding wire, and varied the speed, feed, and pressure of the spinning lathe, but the welds continued to crack. It was finally determined that the welds were—ironically—too good; they needed to be softer. North American developed a new process that resulted in slightly softer but still acceptable welds, and the cracking stopped.1701

Fabricating the X-15 gave North American engineers some of the first large-scale experience with the newest high-strength alloys of titanium and stainless steel. The main propellant tanks formed an integral part of the fuselage, and after a great deal of investigation, North American manufactured the barrels from Inconel X. The experience gained from building the X-15 provided lessons used during the construction of the Apollo capsules and space shuttle orbiters. (North American Aviation)

North American gained experience in manufacturing the propellant tanks and fuselage structure long before it manufactured the first flight airplane. The company constructed three partial fuselages as ground-test articles for the rocket engines. Reaction Motors at Lake Denmark received two of these, while the third went to the Rocket Engine Test Facility at Edwards. Although

not intended as "practice," they did allow the workers in Inglewood to gain a certain level of expertise on a less-critical assembly before building the real flight articles.-171!

Forming the ogive section of the forward fuselage also presented some problems for North American. The usual method to construct such a structure was to form four semicircular segments of skin and weld them together. However, due to the size of the structure and the need to maintain a precise outer mold line, the engineers decided that the most expedient production method was to make a cone and bulge-form it into the final shape in one operation. The initial cone was made from four pieces of Inconel X welded together and carefully inspected to ensure the quality of the welds. It was then placed in a bulge-form die and gas pressure was applied that forced the part to conform to the shape of the die. This process worked well, with one exception. For reasons that were never fully understood, one of the four pieces of Inconel X used for one cone had a tensile strength about 28,000 psi greater than the others. During formation this piece resisted stretching, causing the welds to distort and creating wrinkles. North American eventually discarded the piece and made another one using four different sheets on Inconel; that one worked fine.^

Both titanium and Inconel were hard metals, and the tools used to form and cut them tended to wear out faster than equivalent tools used in the production of steel or aluminum parts. In addition, it took considerably longer to cut or polish compared to other metals. For instance, it took approximately 15 times longer to machine Inconel X than aluminum. This did not lead to any particular problems during the manufacture of the X-15 (unlike some of the tool contamination issues faced by Lockheed on the Blackbird), but it did slow progress and force North American to rethink issues such as machining versus polishing.!73!

The windshield glass originally installed on the X-15 was soda-lime-tempered plate glass with a single outer pane and double inner panes. Engineers had based this choice on a predicted maximum temperature of 740°F. Data obtained on early flights indicated that the outer face would encounter temperatures near 1,000°F, with a differential temperature between panes of nearly 750°F. It was apparent that soda-lime glass would not withstand these temperatures. The engineers subsequently selected a newly developed alumino-silicate glass that had higher strength and better thermal properties as a replacement. The 0.375-inch-thick alumino-silicate outer pane withstood temperatures up to 1,500°F during one test. The next test subjected the glass to a surface temperature of 1,050°F with a temperature gradient from the outer to inner surface of 790°F without failure. In actuality, the thermal environment on the X-15 glass was more complicated, although slightly less severe. The outer surface could reach 800°F, while the inner surface could reach 550°F; however, the inner temperature lagged behind the outer temperature. During rapid heat build-up on high-speed missions, the maximum temperature differential reached 480°F at a time when the outer glass was only 570°F. At this point, both the outer and inner panes began to rise in temperature rapidly.774

Technicians at the Flight Research Center installed the alumino-silicate glass in the outer pane of all three X-15s, although they continued to use soda-lime plate glass for the inner panes until the end of the program. Corning Glass Company supplied all of the glass. The thermal qualification test was interesting. Corning heated an 8.4 by 28-inch panel of the glass to 550°F in a salt bath for 3 minutes, and then plunged it into room-temperature tap water. If it did not shatter, it passed the test.!75!

welding, and otherwise joining this material to make a practical machine." Storms described special techniques for contouring the skins that involved hot machining, cold machining, ovens, freezers, cutters, slicers, and rollers. For instance, one special tool fixture needed to control the contour during a heat-treating cycle of the wing skin weighed 4,300 pounds, while the skin it held weighed only 180 pounds. Despite the publicity normally associated with the use of Inconel X, Charlie Feltz remembered that titanium structures gave North American the most trouble. Fortunately, the use of titanium on the X-15 was relatively small, unlike what Lockheed was experiencing across town on the Blackbird.[76]

In June 1959, the $450,000 Rocket Engine Test Facility at Edwards AFB came on line to provide local testing of the XLR99, although it would be almost a year before an XLR99 was available to use in it. This test facility provided a capability for engine checkout and pilot and maintenance – crew familiarization, as well as limited development firings. There were two test areas with a large blockhouse between them that contained various monitoring equipment and provided safe shelter for the ground crew during engine runs. During the early portion of the program, Reaction Motors used one area to test uninstalled engines, while the Air Force fired engines installed in one of the X-15s in the other area. Several "pillboxes" were also located near each area that provided shelter for other ground crews so that they could observe the operation of the engine.78

In preparation for the X-15 program, the Air Force constructed the Rocket Engine Test Facility at Edwards AFB to provide local testing of the XLR99. There were two test areas, each capable of supporting an X-15 during engine tests. For most of the flight program, the XLR99 had to be fired prior to every flight attempt, leading several engineers to complain they were testing the engines to death. Later in the program an engine could fly a second flight if no anomalies had occurred on the first. (U. S. Air Force)

In December 1959, the Air Force formally approved the XLR99 for flight in the X-15. Reaction Motors delivered a ground-test engine to Edwards at the end of May 1960, and the first flight engine at the end of July. Initially, the Air Force procured 10 flight engines, along with six spare injector-chamber assemblies. Later, the Air Force procured one additional flight engine. However, in January 1961, shortly after the first XLR99 test flight, only four engines were available to the flight program while Reaction Motors was assembling four others for delivery later in 1961. Reaction Motors continued to use four engines for ground tests, including two flight engines. Three of these engines were involved in tests to isolate and eliminate vibrations at low power levels, while the fourth investigated extending the Rokide loss that was affecting the life of the thrust chamber.-1791

In addition to ground simulators and the centrifuge, pilots and researchers used aircraft to simulate various aspects of the X-15. For instance, the Lockheed F-104 Starfighter closely approximately the wing loading of an X-15 during landing, and with the right combination of extended landing gear, flaps, and speed brakes, the F-104 at idle thrust did an excellent job of simulating the X-15. For the first 50 or so flights, the pilots dedicated an entire F-104 mission to practicing landing procedures. As new pilots entered the program, they conducted similar practices. Throughout the program, pilots used the F-104s to establish geographic checkpoints and important altitudes around the landing pattern at all the possible landing lakes.-1541

Scott Crossfield and Al White conducted similar work very early in the program using the North American YF-100A equipped with an eight-foot drag chute. Combined with extended gear and speed brakes, the F-100 at idle thrust did an adequate job of simulating the X-15 during landing, although not quite as well as the F-104. The entire process was a bit trickier since it required the in-flight deployment and release of the drag chute.1551

As Al White later remembered, "With gear down, speed brake extended, at idle power, and that drag chute deployed, the airplane was comparable to the X-15 on approach. I would start at about 25,000 feet, pick a spot on the lakebed, and see how close I could come to touching down on that spot. With all the room on the lakebed, it was not necessary to hit a spot, but it is always nice to have that much margin for error. I flew this trainer as much as I could, in preparation for that day that never came." Not flying the X-15 was one of the few disappointments during White’s significant career.-*56

Much of the X-15 flight planning took place prior to the first manned space flight. Since no one had ever left the atmosphere and returned in a winged vehicle (or anything else), there had been concern that the rapidly changing stability and control characteristics in the X-15 as it reentered the atmosphere might pose an unusually demanding piloting task. To address this question, engineers in the Flight Research Department of the Cornell Aeronautical Laboratory conceived the idea of simulating this brief (about 60 seconds duration) but unfamiliar X-15 piloting task in a NT-33A that was owned by the Air Force but operated by Cornell as a variable-stability trainer.-*57*

The NT-33A already had been equipped with a larger internal volume F-94 nose section that contained a three-axis (pitch, roll, and yaw) variable-stability and control system for in-flight simulation purposes. To support the X-15 program, Cornell modified the front cockpit to superficially resemble the X-15, with a side-stick controller on the right-hand console for atmospheric flight control and another side-stick on the left-hand console simulating the ballistic controls. An "instructor" pilot sat in the back cockpit with a normal set of T-33 controls. Jack Beilman at Cornell designed a programmable, non-linear function generator that changed the gains of 32 sensed aerodynamic and rigid-body-motion feedback variables. It also changed the flight-control sensitivities continuously during the

simulated reentry so that the NT-33A stability and control characteristics would match the predicted X-15 characteristics.-158!

The flight plan had the NT-33A entering a shallow dive at about 17,000 feet altitude and then pulling up to a ballistic trajectory that produced about 60 seconds of 0 g-about the same as the initial part of the X-15 reentry. At the same time, the variable-stability system on the NT-33A changed the flight-control sensitivities to simulate going from the vacuum of space to the rapidly increasing dynamic pressure of the atmosphere. Since the normal aerodynamic controls of the X – 15 would be ineffective outside the atmosphere, the pilot used the ballistic controller to establish the correct reentry pitch attitude.-*56

In the NT-33A simulation the "ballistic controller" produced no physical response whatsoever—it only changed the displayed pitch attitude on the instrument panel. (At this point in the simulation, the NT-33A was at 0 g.) In order to maintain the fidelity of the simulation, the X-15 pilot in the front cockpit wore a hood and had no view of the outside world, since there would be little view of the real world in the X-15 at the simulated altitudes. This deception was necessary for the high – angle-of-attack deceleration at the end of the simulated reentry because although the front cockpit instrumentation indicated the pilot was flying an unbanked steep descent (in the X-15), he was actually flying a steep 5-g turn in the NT-33A. The simulator achieved this deception by gradually biasing the attitude indicator to a bank angle of 75 degrees while the X-15 pilot used the ballistic controller to maintain wings-level flight at the proper airspeed, angle of attack, and descent rate on his cockpit instruments. It was a carefully choreographed ballet between the "student" in the front seat and the safety pilot in the back who was trying to keep the NT-33 from becoming a smoking crater in the high desert.*68!

Accordingly, a Cornell team headed by engineering test pilots Bob Harper and Nello Infanti arrived at Edwards in May 1960 to begin a series of flights in the NT-33A in order to provide reentry training for six X-15 pilots (Neil Armstrong, Jack McKay, Forrest Petersen, Bob Rushworth, Joe Walker, and Bob White). Each pilot was to receive six flights in the NT-33A that included a matrix of simulated Mach numbers, altitudes, and various control malfunctions (principally failed

dampers) both separately and simultaneously.1611 Infanti was the "instructor pilot" for each of the X-15 simulation flights in the NT-33A, and the rest of the Cornell team consisted of crew chief Howard Stevens, electronics technician Bud Stahl, and systems engineer Jack Beilman. As Beilman remembers:

During one of the flights, with Neil Armstrong in the front seat, we were simulating failed dampers at something like Mach 3.2 and 100,000 feet altitude. Neil had great difficulty with this simulated undamped X-15 configuration and lost control of the airplane repeatedly.

Nello had to recover from each one of these "lost-control" events using the controls in the back cockpit. [Infanti later recalled that some of these recoveries were "pretty sporty."] The ground crew was monitoring the test radio frequency as usual and followed these simulated flight control problems with great interest.

After landing, the NT-33A taxied to the ramp and Howard Stevens attached the ladder to the cockpits and climbed up to talk to Infanti about the airplane status. I climbed up the ladder front side to talk to Neil Armstrong. He handed me his helmet and knee-pad, got down from the cockpit and we talked about the flight and walked toward the operations building. As we arrived at the door Armstrong extended his right hand to grasp the door handle-but his hand still held the side-stick that he had broken during his last battle with the X-15 dampers-off simulation. I was unaware of any report of this incident during the flight and had not noticed the stick in Armstrong’s hand when he exited the cockpit. Addressing the matter for the first time, Armstrong said-without additional comment—"Here’s your stick!"

[It developed that Infanti had been aware of the broken side-stick after it happened because Armstrong had held it up over his head in the front cockpit for Nello to see.]

After the debriefing, we took the broken side-stick to the NASA workshop where Neil found the necessary metal tubing and repaired the stick while I mostly watched him work. The side-stick was reinstalled and ready for the first flight the next morning. Really good test pilots fix what they break!

In general, the pilots considered the NT-33 flights worthwhile, but there were some "obvious discrepancies or malfunctions" during the early flights. There were also a fair number of delays in the flights due to various system malfunctions caused by the high temperatures at Edwards. Eventually the Cornell crew corrected the malfunctions, but the X-15 pilots considered the first 10 flights unsatisfactory since they did not adequately simulate the X-15 flight profile. This was largely because the programmed trajectories required the NT-33 to fly close to its maximum capabilities: something that was not as easy as it sounds, especially in the heat over the high desert.-1621

The X-15 pilots considered the final six flights, flown during the first half of September 1960, reasonably satisfactory. In fact, the pilots discovered a novel control technique for the divergent closed-loop lateral-directional oscillation encountered at Mach 3.5 and 10 degrees angle of attack with the SAS off during these flights. By using the rudder in conjunction with the turn and bank indicator (which was, in effect, a yaw-rate meter) the pilot was able to damp the oscillations. With this technique, the ailerons were only a steady-state controller; in fact, any attempt to use the ailerons for control caused an immediate divergence. Researchers further investigated this technique on the North American fixed-base simulator with good results.1631

the X-15 flight profile somewhat more convincingly than the NT-33, making it possible to investigate new piloting techniques and control-law modifications without using an X-15. The most limiting factor was that the JF-100C was a single-seat aircraft, meaning that no safety pilot was available to lend assistance if things went wrong. To establish the X-15 flight characteristics on the JF-100C, technicians connected two portable analog computers to the airplane so that the combination became, essentially, a fixed-base simulator. One analog computer simulated the basic F-100C flight characteristics, and researchers manipulated the variable-stability gains until the motion traces matched those obtained from the North American X-15 simulator. Joe Walker and Bob White flew these pseudo fixed-base simulations until they were satisfied that the JF – 100C adequately represented the X-15.[64]

Much of the X-15 flight planning took place prior to the first manned space flight. There was concern that the rapidly changing stability and control characteristics in the X-15 as it reentered the atmosphere might pose an unusually demanding piloting task. To address this, the Cornell Aeronautical Laboratory developed a method of simulating this environment using an NT-33A operated by Cornell as a variable stability trainer. The simulations were hardly ideal, but provided much needed confidence to the original cadre of X-15 pilots. (U. S. Air Force)

The first actual flight of the JF-100C with the new mechanization was made on 24 March and was considered generally satisfactory. The major discrepancies were that the Dutch-roll and roll – subsidence modes appeared to be less stable than those of the actual X-15. Nevertheless, the JF – 100C was capable of performing some interesting simulations. For instance, six flights in late July 1961 simulated the X-15 at Mach 3.5, 84,000 feet, and 10 degrees angle of attack; later flights extended this to Mach 6 and angles of attack of 20 degrees. The aircraft returned to Ames on 11 March 1964 after making 104 flights for pilot checkout, variable-stability research, and X-15

[65]

support.

One of the tasks assigned to the JF-100C was investigating the effects of damper failure on the controllability of the X-15. Researchers had obtained the early wind-tunnel data on sideslip effects with the horizontal stabilizer at zero deflection, and used this data in the 1958 centrifuge program at Johnsville. Based on these data, reentries using an angle of attack of less than 15 degrees were possible even with the roll damper off. On the other hand, reentries at angles greater than 15 degrees (which were required for altitudes above 250,000 feet) with the roll damper off showed a distinct tendency to become uncontrollable because of a pilot-induced oscillation (PIO).[66]

As with a typical PIO, if the pilot released the control stick, the oscillations damped themselves. Nevertheless, researchers suspected that a large portion of the X-15 flight envelope was uncontrollable with the roll dampers off or failed. Investigations were initiated to find a way to alleviate the problem. The first method tried (perhaps because it would have been the easiest to implement) was pilot-display quickening. Sideslip and bank-angle presentations in the cockpit were quickened (i. e., presented with less delay) by including the yaw rate and roll rate, respectively. Researchers experimented with various quickening gains during investigation on the fixed-base simulator, but found no combination that significantly improved the pilot’s ability to handle the instability.-^

Shortly after the centrifuge program was completed, researchers conducted a wind-tunnel test to gather sideslip data with the horizontal stabilizer closer to the normal trim position (which was a large leading-edge-down deflection of -15 to -20 degrees). When researchers programmed the results of these tests into the fixed-base simulator at North American, it showed that the PIO boundary for reentry with the roll damper off had dropped from 15 degrees to only 8 degrees, adding new urgency to finding a solution.-681

To verify the magnitude of the problem in flight, several X-15 pilots explored the fringes of the expected uncontrollable region by setting the airplane up at the appropriate angle of attack and turning the roll and yaw dampers off. In each case, lateral motions began immediately. The pilots experimented with various combinations of angle of attack and control inputs in both the X-15 and the JF-100C to better define the problem.-691

Lawrence W. Taylor and Richard E. Day from the FRC, and Arthur F. Tweedie from North American independently investigated using the rolling tail to control sideslip angle during certain types of instability. An unconventional control technique, called "beta-dot," evolved from these investigations and showed considerable promise on the fixed-base simulator. This technique consisted of sharp lateral control inputs to the left as the nose swung left through zero sideslip (or vice versa to the right). The pilot kept his hands off the stick except when making the sharp lateral inputs, which eliminated the instability induced by inadvertent inputs associated with merely holding onto the center stick. However, when pilots used this technique in the JF-100C, it did not seem to work as well. Further investigations showed that it worked somewhat better in the X-15 when the pilot used the side-stick controller instead of the center stick.-701

It appeared that the beta-dot technique might allow reentries from high altitudes with the dampers failed, if anybody could figure out how to perform the maneuver successfully. As Bob Hoey, the flight planner who later discovered the ventral-off stability fix for the same problem, recalled, "the beta-dot technique is one of those things that is really difficult to explain. You could watch someone make 20 simulated reentries and still not understand what they were doing. The method was based on making a very sharp aileron pulse, timed exactly right, and totally foreign

to normal, intuitive piloting technique. Properly timed, this pulse would completely stop the rolling motion, although not necessarily at wings level. With a little finesse, you could herd the thing back to wings level flight, but, if at any time you reverted to a normal piloting technique, even for a second, you were in big trouble. Art Tweedie [who discovered this method] and Norm Cooper [a North American flight controls expert] could make successful simulator reentries with the dampers off while drinking a cup of coffee! This obviously became a big challenge for the rest of us." Hoey became pretty good at the technique himself, at least in the simulator.-171!

Dick Day later wrote that "Robert Hoey, lead Air Force engineer on the X-15 project, introduced the control technique to some of the X-15 pilots. Two pilots in particular, Major Robert White and Captain Joe Engle, became so adept at controlling ground and flight simulators that they considered the method would serve as a backup in case of roll damper failure. Fortunately, the beta-dot technique was not required because removing the ventral solved the dampers-off controllability problem. It is worth noting, however, that the complete beta-dot equation was later used in the yaw channel of the Space Shuttle control system to overcome unstable control coupling." It is another enduring legacy of the X-15 program.-721

All of the X-15 pilots trained using this technique, but the actual usefulness of the beta-dot maneuver was questionable. Furthermore, a lateral input in the wrong direction, which was conceivable considering other potential problems clamoring for the attention of the pilot, could be disastrous. One of the reasons the technique was so foreign to the pilot was that the aileron pulse had to be in the same direction as the roll, which is hardly intuitive for most pilots. Then the pilot had to remove the pulse just as the needle on the sideslip indicator hit the null mark. As Hoey remembers, "about half the pilots were dead-set against [the beta-dot maneuver] and essentially refused to consider it as an option. Others conquered the technique and actually became fairly proficient in its use on the fixed-base and in-flight simulations." Pilots flew the in-flight simulations using the NT-33 and JF-100C variable-stability airplanes, which somehow managed to survive the program.731

Researchers at Ames modified a North American JF-100C (53-1709) Super Sabre into a variable – stability trainer that could simulate the X-15 flight profile somewhat more convincingly than the NT-33, making it possible to investigate new piloting techniques and control-law modifications without using an X-15. The most limiting factor was that the JF-100C was a single-seat aircraft, meaning there was not a safety pilot to assist if things went wrong. (NASA)

There were two other answers to the PIO problem at high angles of attack. The first was to make the stability augmentation system truly redundant, at least in the roll axis, by installing the alternate stability augmentation system (ASAS); however, this took almost a year to accomplish. Another answer-discovered by Dick Day and Bob Hoey using the simulator-proved to be remarkably easy, and unexpected: remove the ventral rudder. With the lower rudder on, a considerable portion of the reentry from an altitude mission would be within the uncontrollable region should a damper fail. However, a similar reentry with the lower rudder removed would not enter the predicted uncontrollable region at all. The downside was that the pilots faced significantly reduced flying qualities at low angles of attack without the rudder. Despite a few gripes from the pilots, everybody eventually agreed to remove the lower rudder for almost all of the high-altitude missions. Only a few missions of the X-15A-2 used the ventral rudder, which in this case provided an adequate stand-in for the eventual dummy ramjet. In all, the program would make 73 flights with the ventral rudder on and 126 with it off.1741

By the time of the 1961 industry conference, researchers had determined that the fixed-base simulator and the F-104 in-flight landing pattern simulator were the two most valuable training tools available to the program. The centrifuge and variable-stability aircraft contributed to the overall pilot experience level, but were not necessary for use on a flight-by-flight basis. This mostly explains why only the first group of pilots got the thrills of "riding the wheel" at Johnsville and flying the NT-33 trainer.-1751

After three months of investigations, the Becker group believed that the development of a Mach 7 research aircraft was feasible. Those at NACA Headquarters who followed the progress of their work, as well as the parallel work on hypersonic aircraft concepts at the other NACA laboratories, agreed. It was time to formally present the results to the NACA upper echelon and the Department of Defense.-11^

The preliminary specifications for the research airplane were surprisingly brief: only four pages of requirements, plus six additional pages of supporting data. As John Becker subsequently observed, "it was obviously impossible that the proposed aircraft be in any sense an optimum hypersonic configuration." Nevertheless, Langley believed the design would work. At the same time, a new sense of urgency was present: "As the need for the exploratory data is acute because of the rapid advance of the performance of service [military] aircraft, the minimum practical and reliable airplane is required in order that the development and construction time be kept to a minimum." In other versions of the requirements, this was even more specific: "It shall be possible to design and construct the airplane within 3 years." The researchers were nothing if not ambitious.11^

On 4 May 1954, Hugh Dryden sent a letter to Lieutenant General Donald L. Putt at Air Force Headquarters stating that the NACA wanted to initiate a new manned hypersonic research aircraft program. The letter suggested a meeting between the NACA, Air Force Headquarters, and the Air Force Scientific Advisory Board to discuss the project. Putt responded favorably and recommended inviting the Navy as well. The general also noted that "the Scientific Advisory Board has done some thinking in this area and has formally recommended that the Air Force initiate action on such a program." On 11 June 1954, Dryden sent letters to the Air Force and Navy inviting them to a meeting on 9 July 1954 at NACA Headquarters.117

Attendees included Clark Millikan, Ezra Kotcher from the WADC, and a variety of Air Force and Navy technical representatives. The Air Research and Development Command (ARDC) and Air Force Headquarters also sent policy representatives. During the meeting, Hartley Soule and Walt Williams reviewed the history of previous research airplanes. Hugh Dryden reported the reasons why the NACA believed a new research aircraft was desirable, and said the time had come to determine whether an agreement existed on the objectives and scope of such a project. Dryden emphasized the need for information on full-scale structural heating and on stability and control issues at high speeds and high altitudes. He also indicated that the NACA thought that actual flight-testing combined with theoretical studies and wind-tunnel experiments produced the best results. The Langley study became the starting point for further discussions since it was the most detailed available, with John Becker and John Duberg, who was substituting for Norris Dow, leading the discussions.-118

Those in attendance were in general agreement that a new project was feasible. However, Hugh Dryden, reflecting what John Becker described as "his natural conservatism," stated that the fact it was feasible to build such a research airplane did not necessarily make it worth building; he wanted further study before deciding. The Navy representative indicated that some "military objective" should be included in the program, but Clark Millikan stressed the need for a dedicated research airplane rather than any sort of tactical prototype. The group agreed the performance parameters discussed by the Langley study represented an adequate increment over existing research airplanes, and that a cooperative program would be more cost-effective and more likely to provide better research data at an earlier time. The meeting closed with an agreement that the military would continue studying the NACA proposal, and that Hugh Dryden would seek Department of Defense approval for the project.119

Unexpectedly, the Office of Naval Research (ONR) announced at the meeting that it had already contracted with the Douglas Aircraft Company to investigate a manned vehicle capable of achieving 1,000,000 feet altitude and very high speeds. The configuration evolved by Douglas "did not constitute a detailed design proposal," but was only a "first approach to the problem of a high-altitude high-speed research airplane." Representatives from the NACA agreed to meet with their ONR counterparts on 16 July to further discuss the Douglas study.

Pressure suits, more often called "space suits" by the public, are essentially taken for granted today. Fifty years ago they were still the stuff of science fiction. These suits serve several necessary purposes, with supplying the correct partial pressure of oxygen being the most obvious (although masks or full-face helmets can also accomplish this). The most important purpose, however, is to protect the pilot against the increasingly low atmospheric pressures encountered as altitude increases—pressures that reach essentially zero above about 250,000 feet. At high altitudes, the blood and water in the human body want to boil—not from heat, but from the pressure differential between the body and the environment.-1771

A distant precursor of the full-pressure suit was, arguably, the dry suits used by turn-of-the – century commercial salvage divers, complete with their ported brass helmets and valve fittings. In 1920, renowned London physiologist Dr. John Scott Haldane apparently was the first to suggest that a suit similar to the diver’s ensemble could protect an aviator at high altitudes. There appeared, however, to be little immediate need for such a suit. The normally aspirated piston – powered airplanes of the era were incapable of achieving altitudes much in excess of 20,000 feet, and the major concern at the time was simply keeping the pilot warm. However, the increasing use of supercharged aircraft engines during the late 1920s led to the first serious studies into pressure suits. Suddenly, aircraft could fly above 30,000 feet and the concern was no longer how to keep the aviator warm, but how to protect him from the reduced pressure.-1781

During the early 1930s Mark E. Ridge determined that a suitably constructed pressurized suit would allow him to make a record-breaking altitude flight in an open balloon. His efforts to interest the United States military in this endeavor failed, and instead he contacted John Haldane in London for help. At the time, Haldane was working with Sir Robert Davis of Siebe, Gorman & Company to develop deep-sea diving suits. Together, Haldane and Davis constructed a hypobaric protection suit for Ridge. For a number of reasons, Ridge was never able to put the suit to actual use, although he tested it in a pressure chamber at simulated altitudes up to 90,000 feet.-1791

In 1934 famed aviator Wiley Post commissioned the B. F. Goodrich Company to manufacture a pressure suit of his own design. Unfortunately, the rubberized fabric suit did not work all that well. The basic design was modified by B. F. Goodrich engineer Russell Colley, and after some trial and error, Post was able to use it successfully on several record-breaking flights to altitudes of 50,000 feet.1801

While work on derivatives of the Ridge-Haldane-Davis suit continued in England, the U. S. Army Air Corps finally recognized, somewhat belatedly, the need for a pressurized protective garment for military aviators and started the classified MX-117 research program in 1939. This drew several companies into pressure-suit development, including B. F. Goodrich (with Russell Colley), Bell Aircraft, the Goodyear Rubber Company, the U. S. Rubber Company, and the National Carbon Company. From 1940 through 1943, engineers produced a number of designs that all featured transparent dome-like plastic helmets and airtight, rubberized fabric garments that greatly restricted mobility and range of motion when fully pressurized. The development of segmented, bellows-like joints at the knees, hips, and elbows improved mobility, but still resulted in an extremely clumsy and uncomfortable ensemble. The striking visual aspect of these suits resulted in their being called "tomato worm suits," after the distinctive tomato hornworm.[81]

By 1943 the Army Air Corps had largely lost interest in the concept of a full-pressure suit. The newest long-range bomber, the Boeing B-29 Superfortress, was pressurized and seemed less likely to require the suits than earlier aircraft. As Scott Crossfield later opined, "During World War II the armed services, absorbed with more vital matters, advanced the pressure suit not a whit."-82

After the war, Dr. James P. Henry of the University of Southern California began experimenting with a new concept in aircrew protection. The capstan-type partial-pressure suit operated by imposing mechanical pressure on the body directly, compressing the abdomen and limbs much like the anti-g suits then entering service. The compression was applied by inflatable bladders in the abdominal area and pneumatic tubes (capstans) running along the limbs. A tightly fitting, rubber-lined fabric hood that was fitted with a neck seal and a transparent visor fully enclosed the head.-83

In Worcester, Massachusetts, a small company named after its founder, David Clark, produced anti-g suits for the Air Force and experimental pressure suits for the Navy. Scott Crossfield described Clark as "one of the most interesting men I have ever met in the aviation world." Although Henry had approached the David Clark Company for assistance in developing his suit concept, contracts for anti-g suits between David Clark and the U. S. government made direct cooperation appear to be a conflict of interest. Instead, Clark sent materials and an experienced seamstress, Julia Greene, to help Henry continue his development in California. Just after the war, the Air Force asked Clark to observe a test of the Henry partial-pressure suit in the altitude chamber at Wright Field. Henry demonstrated the suit to a maximum altitude of 90,000 feet, and remained above 65,000 feet for more than 30 minutes; everybody was suitably impressed. The Air Force asked David Clark to produce the Henry design, and all parties soon reached an agreement that included Julia Greene returning to Worcester. David Clark produced the first suit for Jack Woolams, a Bell test pilot scheduled to fly the XS-1, and made additional suits for Chalmers "Slick" Goodlin and a little-known Air Force captain named Chuck Yeager.-84

These early partial-pressure suits did, in fact, work. On 25 August 1949, Major Frank K. "Pete" Everest was flying the first X-1 on an altitude flight when the canopy cracked and the cockpit depressurized. The laced partial-pressure suit automatically activated, squeezing Everest along the torso, arms, and legs, supporting his skin and keeping his blood from boiling. He landed, uncomfortable but unhurt. This was the first recorded use of a partial-pressure suit under emergency conditions.-1851

Continued improvements resulted in the T-1 suit, the first standardized partial-pressure suit used by the Air Force. The Air Force used the T-1 suit in a variety of aircraft, including the stripped-down "featherweight" versions of the Convair B-36 intercontinental bomber that frequently flew missions lasting in excess of 24 hours at altitudes above 50,000 feet. Unfortunately, the T-1 suit was not a particularly comfortable garment.-861

The discomfort of the so-called "Henry suit" was an unfortunate aspect of the fundamental design of partial-pressure suits. This was at least partially eliminated in the subsequent MC-1, MC-3, and MC-4 series (the MC-2 suit was an experimental full-pressure suit to be discussed later) by the placement and adjustment of panels during customized fitting. However, the suits did accomplish their main purpose: to protect the wearer from the effects of emergency decompression at altitude.-1871

Taking a different route, after the war the U. S. Navy began investigating the possibility of developing a full-pressure suit in cooperation with B. F. Goodrich and Russell Colley. This led to a progressive series of refinements of the basic design that resulted, in the early 1950s, in the first practical U. S. full-pressure suit. At the same time, the David Clark Company was also experimenting with full-pressure suits under Navy auspices. On 21 August 1953, Marine Corps Lieutenant Colonel Marion E. Carl took one of the D-558-2 aircraft to an unofficial record altitude of 83,235 feet while wearing a David Clark full-pressure suit.-1881

The Navy’s adventures in full-pressure suit development took some intriguing turns, and Scott Crossfield covers them well in his autobiography. The Navy ended up concentrating on the Goodrich designs. One of these was the Model H, an early developmental suit that the Navy considered unacceptable for operational use but showed a great deal of promise. Consequently, in a perfect example of interservice rivalry, the Air Force and Navy began separate development efforts—both based on the Model H—to perfect an operational full-pressure suit. By the early 1960s the Navy had progressed through a series of developmental models to the Mark IV, Model 3, Type 1, a production suit that Navy aircrews wore on high-altitude flights for several years.-1891

Air Force experience at high altitudes in the B-36 confirmed the need for a full-pressure suit to replace the partial-pressure suits used by the bomber crews. In response, the Air Force drafted a requirement for a suit to provide a minimum of 12 hours of protection above 55,000 feet. The goal was to construct a "fully mobile suit" that would weigh less than 30 pounds, operate with an internal pressure of 5 psi, and provide the user with sufficient oxygen partial pressure for breathing, adequate counterpressure over the body, and suitable ventilation.1901

Whatever the political nuances involved, in 1955 the Air Force issued a request for proposals for a full-pressure suit. Several contracts were awarded and the two leading designs were designated the XMC-2-ILC (International Latex Corporation) and the XMC-2-DC (David Clark Company). The ILC approach resulted in an unwieldy garment that used convoluted metal joints and metal bearing rings, and had limited mobility under pressure; it was known, however, to provide the required pressure protection. Unfortunately, the joint bearings produced painful pressure points on the body and were hazardous during bailout or ejection—hardly an ideal solution.1911

On the other hand, the David Clark suit featured a major breakthrough in suit design with the use of a new "distorted-angle fabric," called Link-Net, to control inflation and enhance range of motion. This eliminated the need for the tomato-worm bellows at the limb joints. David Clark had been developing this same basic suit with the Navy before that service opted to go with the Goodrich design. The Air Force selected the David Clark suit for further development.1921

The new Link-Net fabric was the result of an intensive effort by the company to develop a new partial-pressure suit fabric using both Navy and company money. Originally, David Clark had constructed several torso mockups using different unsupported sheet-rubber materials, but quickly discarded these when it became evident that a rupture in the material could cause the entire suit to collapse. The company began looking for a supported-rubber material that would meet the sealing requirements but would not collapse when punctured. Ultimately, David Clark selected a neoprene-coated nylon. A puncture in this material would result in a small leak, but not a sudden expulsion of gas.1931

The enormous advantages offered by the Link-Net fabric were hard to grasp. Coupled with advances in regulators and other mechanical pieces, David Clark could now produce a workable full-pressure suit that weighed about 35 pounds. Previously, during the early X-15 proposal effort, North American had estimated a suit would weigh 110 pounds.[94]

Further tests showed that two layers of nylon marquisette arranged with opposite bias provided the maximum strength in high-stress areas. This improved Link-Net material consisted of a series of parallel cords that looped each other at frequent intervals. The loops were interlocked but not connected so that the cords could slide over each other and feed from one section of the suit to another to allow the suit to deform easily as the pilot moved. The main characteristic required of the Link-Net was the lowest possible resistance to bending and twisting, but the elasticity had to be minimal since the suit could not increase appreciably in volume while under pressure. The use of a relatively non-elastic cord in the construction of Link-Net made it possible to satisfy these seemingly contradictory requirements. Clark chose nylon for the Link-Net because of its high tensile strength, low weight, and low bulk ratio.-195

The X-15 provided the first impetus to develop a workable full-pressure suit, and Scott Crossfield and Dr. David M. Clark were instrumental in the effort. The first X-15 full-pressure suit, the XMC – 2 (S794-3C) was demonstrated by Scott Crossfield in the human centrifuge at the Aero Medical Laboratory on 14 October 1957. Two 15-second runs were made at 7 g, and the following day an additional 23 tests were conducted to demonstrate the anti-g capability of the suit. (U. S. Air Force)

The first prototype David Clark Model S794 suit provided a learning experience for the company. For instance, the initial anti-g bladders were fabricated using neoprene-coated nylon, but failed during testing. New bladders incorporated a nylon-oxford restraint cover, and these passed the pressure tests. Materials evaluated for the gloves included leather/nylon, leather/nylon/Link-Net, and all leather. Eventually, the company found the best combination was leather covering the hand, a stainless-steel palm restrainer stitched inside nylon tape supported by nylon tape around the back, Link-Net from the wrist up to the top zipper, and a black cabretta top seam. However, pilots quickly found that gloves constructed in the straight position made it impossible to hold an object, such as a control stick, for more than 15-20 minutes while the glove was pressurized. When the company used a natural semi-closed position to construct the glove, the pilots could hold an object for up to 2 hours without serious discomfort. Perhaps the most surprising material used in the prototype suit was the kangaroo leather for the boots, which turned out to be soft and comfortable as well as sufficiently durable."

The construction of two "production" full-pressure suits (S794-1 and S794-2) followed. These suits were an improvement in terms of production and mobility but were, in reality, still prototypes. One of the major changes was extending the use of Link-Net material further from the joints to increase the amount of "draw" and provide additional mobility. Eventually David Clark concluded that the entire suit should use Link-Net. David Clark delivered these two suits to the Aero Medical Laboratory at Wright Field for testing and evaluation, and used the lessons learned to construct the first X-15 suit for Scott Crossfield."

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