X-15: The Technology

Four companies competed for the main contract, covering design and construc­tion of the X-15: Republic, Bell, Douglas, and North American. Each of them brought a substantial amount of hands-on experience with advanced aircraft. Republic, for example, had Alexander Kartveli as its chief designer. He was a highly imaginative and talented man whose XF-105 was nearly ready for first flight and whose XF-103 was in development. Republic had also built a rocket plane, the XF – 91- This was a jet fighter that incorporated the rocket engine of the X-l for an extra boost in combat. It did not go into production, but it flew in flight tests.

Still, Republic placed fourth in the competition. Its concept rated “unsatisfac­tory” as a craft for hypersonic research, for it had a thin outer fuselage skin that appeared likely to buckle when hot. The overall proposal rated no better than aver­age in a number of important areas, while achieving low scores in Propulsion System and Tanks, Engine Installation, Pilot’s Instruments, Auxiliary Power, and Landing Gear. In addition, the company itself was judged as no more than “marginal” in the key areas of Technical Qualifications, Management, and Resources. The latter included availability of in-house facilities and of an engineering staff not committed to other projects.53

Bell Aircraft, another contender, was the mother of research airplanes, having built the X-l series as well as the X-2. This firm therefore had direct experience both with advanced heat-resistant metals and with the practical issues of powering piloted aircraft using liquid-fuel rocket engines. It even had an in-house group that was building such engines. Bell also was the home of the designers Robert Woods and Walter Dornberger, with the latter having presided over the V-2.

Dornberger’s Bomi concept already was introducing the highly useful concept of hot structures. These used temperature-resistant alloys such as stainless steel. Wings might be covered with numerous small and very hot metal panels, resembling shin­gles, that would radiate heat away from the aircraft. Overheating would be particu­larly severe along the leading edges of wings; these could be water-cooled. Insulation could protect an internal structure that would withstand the stresses and forces of flight; active cooling could protect a pilot’s cockpit and instrument compartment. Becker described these approaches as “the first hypersonic aircraft hot structures concepts to be developed in realistic meaningful detail.”54

Even so, Bell ranked third. Historian Dennis Jenkins writes that within the pro­posal, “almost every innovation they proposed was hedged in such a manner as to make the reader doubt that it would work. The proposal itself seemed rather poorly organized and was internally inconsistent (i. e., weights and other figures frequently differed between sections).”55 Yet the difficulties ran deeper and centered on the specifics of its proposed hot structure.

Bell adopted the insulated-structure approach, with the primary structure being of aluminum, the most familiar of aircraft materials and the best understood. Cor­rugated panels of Inconel X, mounted atop the aluminum, were to provide insula­tion. Freely-suspended panels of this alloy, contracting and expanding with ease, were to serve as the outer skin.

Yet this concept was quite unsuitable for the X-15, both on its technical merits and as a tool for research. A major goal of the program was to study aircraft struc­tures at elevated temperatures, and this would not be possible with a primary struc­ture of cool aluminum. There were also more specific deficiencies, as when Bell’s thermal analysis assumed that the expanding panels of the outer shell would prevent leakage of hot air from the boundary layer. However, the evaluation made the flat statement, “leakage is highly probable.” Aluminum might not withstand the result­ing heating, with the loss of even one such panel leading perhaps to destructive heating. Indeed, the Bell insulated structure appeared so sensitive that it could be trusted to successfully complete only three of 13 reference flights.56

Another contender, Douglas Aircraft, had shared honors with Bell in building previous experimental aircraft. Its background included the X-3 and the Skyrocket, which meant that Douglas also had people who knew how to integrate a liquid rocket engine with an airplane. This company’s concept came in second.

Its design avoided reliance on insulated structures, calling instead for use of a heat sink. The material was to be a lightweight magnesium alloy that had excellent

The North American X-15. (NASA)

heat capacity. Indeed, its properties were so favorable that it would reach tempera­tures of only 600°F, while an Inconel X heat-sink airplane would go to 1,200°F.

Again, though, this concept missed the point. Managers wanted a vehicle that could cope successfully with temperatures of 1,200°F, to lay groundwork for opera­tional fighters that could fly well beyond Mach 3. In addition, the concept had virtually no margin for temperature overshoots. Its design limit of 600°F was right on the edge of a regime of which its alloy lost strength rapidly. At 680°F, its strength could fall off by 90 percent. With magnesium being flammable, there was danger of fire within the primary structure itself, with the evaluation noting that “only a small area raised to the ignition temperature would be sufficient to destroy the aircraft.”57

Then there was North American, the home of Navaho. That missile had not flown, but its detailed design was largely complete and specified titanium in hot areas. This meant that that company knew something about using advanced metals. The firm also had a particularly strong rocket-engine group, which split off during 1955 to form a new corporate division called Rocketdyne. Indeed, engines built by that association had already been selected for Atlas.58

North American became the winner. It paralleled the thinking at Douglas by independently proposing its own heat-sink structure, with the material being Inco­nel X. This concept showed close similarities to that of Becker’s feasibility study a year earlier. Still, this was not to say that the deck was stacked in favor of Beck­er’s approach. He and his colleagues had pursued conceptual design in a highly impromptu fashion. The preliminary-design groups within industry were far more experienced, and it had appeared entirely possible that these experts, applying their seasoned judgment, might come up with better ideas. This did not happen. Indeed, the Bell and Douglas concepts failed even to meet an acceptable definition of the new research airplane. By contrast, the winning concept from North American amounted to a particularly searching affirmation of the work of Becker’s group.59

How had Bell and Douglas missed the boat? The government had set forth per­formance requirements, which these companies both had met. In the words of the North American proposal, “the specification performance can be obtained with very moderate structural temperatures.” However, “the airplane has been designed to tolerate much more severe heating in order to provide a practical temperature band within which exploration can be conducted.”

In Jenkins’s words, “the Bell proposal…was terrible—you walked away not entirely sure that Bell had committed themselves to the project. The exact opposite was true of the North American proposal. From the opening page you knew that North American understood what was trying to be accomplished with the X-15 program and had attempted to design an airplane that would help accomplish the task—not just meet the performance specifications (which did not fully describe the intent of the program).”60 That intent was to build an aircraft that could accomplish research at 1,200°F and not merely meet speed and altitude goals.

The overall process of proposal evaluation cast the competing concepts in sharp relief, heightening deficiencies and emphasizing sources of potential difficulty. These proposals also received numerical scores, while another basis for comparison involved estimated program costs:

North American	81.5 percent	$56.1 million
Douglas Aircraft	80.1	36.4
Bell Aircraft	75.5	36.3
Republic Aviation	72.2	47.0

North Americans concept thus was far from perfect, while Republic’s repre­sented a serious effort. In addition, it was clear that the Air Force—which was to foot most of the bill—was willing to pay for what it would get. The X-15 program thus showed budgetary integrity, with the pertinent agencies avoiding the tempta­tion to do it on the cheap.61

On 30 September 1955, letters went out to North American as well as to the unsuccessful bidders, advising them of the outcome of the competition. With this, engineers now faced the challenge of building and flying the X-15 as a practical exercise in hypersonic technology. Accordingly, it broke new ground in such areas as metallurgy and fabrication, onboard instruments, reaction controls, pilot training, the pilots pressure suit, and flight simulation.62

Inconel X, a nickel alloy, showed good ductility when fully annealed and had some formability. When severely formed or shaped, though, it showed work-hardening, which made the metal brittle and prone to crack. Workers in the shop addressed this problem by forming some parts in stages, annealing the workpieces by heating them between each stage. Inconel X also was viewed as a weldable alloy, but some welds tended to crack, and this problem resisted solution for some time. The solution lay in making welds that were thicker than the parent material. After being ground flat, their surfaces were peened—bombarded with spherical shot—and rolled flush with the parent metal. After annealing, the welds often showed better crack resistance than the surrounding Inconel X.

A titanium alloy was specified for the internal structure of the wings. It proved difficult to weld, for it became brittle by reacting with oxygen and nitrogen in the air. It therefore was necessary to enclose welding fixtures within enclosures that could be purged with an inert gas such as helium and to use an oxygen-detecting device to determine the presence of air. With these precautions, it indeed proved possible to weld titanium while avoiding embrittlement.63

Greases and lubricants posed their own problems. Within the XT5, journal and antifriction bearings received some protection from heat and faced operat­ing temperatures no higher than 600°F. This nevertheless was considerably hotter than engineers were accustomed to accommodating. At North American, candidate lubricants underwent evaluation by direct tests in heated bearings. Good greases protected bearing shafts for 20,000 test cycles and more. Poor greases gave rise to severe wearing of shafts after as few as 350 cycles.64

In contrast to conventional aircraft, the X-15 was to fly out of the sensible atmo­sphere and then re-enter, with its nose high. It also was prone to yaw while in near­vacuum. Hence, it needed a specialized instrument to determine angles of attack and of sideslip. This took form as the “Q-ball,” built by the Nortronics Division of Northrop Aircraft. It fitted into the tip of the X-15’s nose, giving it the appearance of a greatly enlarged tip of a ballpoint pen.

The ball itself was cooled with liquid nitrogen to withstand air temperatures as high as 3,500°F. Orifices set within the ball, along yaw and pitch planes, measuring differential pressures. A servomechanism rotated the ball to equalize these pressures by pointing the balls forward tip directly into the onrushing airflow. With the direction of this flow thus established, the pilot could null out any sideslip. He also could raise the nose to a desired angle of attack. “The Q-ball is a go-no go item,” the test pilot Joseph Walker told Time magazine in 1961. “Only if she checks okay do we go.”65

To steer the aircraft while in flight, the X-15 mounted aerodynamic controls. These retained effectiveness at altitudes well below 100,000 feet. However, they lost

effectiveness between 90,000 and 100,000 feet. The X-15 therefore incorporated reac­tion controls, which were small thrusters fueled with hydrogen peroxide. Nose-mounted units controlled pitch and yaw. Other units, set near the wingtips, gave control of roll.

No other research airplane had ever flown with such thrust­ers, although the X-1B con­ducted early preliminary experi­ments and the X-2 came close to needing them in 1956. During a flight in September of that year, the test pilot Iven Kinche – loe took it to 126,200 feet. At that altitude, its aerodynamic controls were useless. Kincheloe

, . . , . . flew a ballistic arc, experiencing

Attitude control or a hypersonic airplane using aerody – г 1

namic controls and reaction controls. (U. S. Air Force) near-weightlessness for close to

a minute. His airplane banked to the left, but he did not try to counter this movement, for he knew that his X-2 could easily go into a deadly tumble.66

In developing reaction controls, an important topic for study involved determin­ing the airplane handling qualities that pilots preferred. Initial investigations used an analog computer as a flight simulator. The “airplane” was disturbed slightly; a man used a joystick to null out the disturbance, achieving zero roll, pitch, and yaw. These experiments showed that pilots wanted more control authority for roll than for pitch or yaw. For the latter, angular accelerations of 2.5 degrees per second squared were acceptable. For roll, the preferred control effectiveness was two to four times greater.

Flight test came next. The X-2 would have served splendidly for this purpose, but only two had been built, with both being lost in accidents. At NACA’s High – Speed Flight Station, investigators fell back on the X-lB, which was less capable but still useful. In preparation for its flights with reaction controls, the engineers built a simulator called the Iron Cross, which matched the dimensions and inertial characteristics of this research plane. A pilot, sitting well forward along the central arm, used a side-mounted control stick to actuate thrusters that used compressed
nitrogen. This simulator was mounted on a universal joint, which allowed it to move freely in yaw, pitch, and roll.

Reaction controls went into the X-1B late in 1957. The test pilot Neil Arm­strong, who walked on the Moon 12 years later, made three flights in this research plane before it was grounded in mid-1958 due to cracks in its fuel tank. Its peak altitude during these three flights was 55,000 feet, where its aerodynamic controls readily provided backup. The reaction controls then went into an F-104, which reached 80,000 feet and went on to see much use in training X-15 pilots. When the X-15 was in flight, these pilots had to transition from aerodynamic controls to reaction controls and back again. The complete system therefore provided overlap. It began blending in the reaction controls at approximately 85,000 feet, with most pilots switching to reaction controls exclusively by 100,000 feet.67

Since the war, with aircraft increasing in both speed and size, it had become increasingly impractical for a pilot to exert the physical strength to operate a plane’s ailerons and elevators merely by moving the control stick in the cockpit. Hydrauli­cally-boosted controls thus were in the forefront, resembling power steering in a car. The X-15 used such hydraulics, which greatly eased the workload on a test pilots muscles. These hydraulic systems also opened the way for stability augmentation systems of increasing sophistication.

Stability augmentation represented a new refinement of the autopilot. Conven­tional autopilots used gyroscopes to detect deviations from a plane’s straight and level course. These instruments then moved an airplane’s controls so as to null these deviations to zero. For high-performance jet fighters, the next step was stability augmentation. Such aircraft often were unstable in flight, tending to yaw or roll; indeed, designers sometimes enhanced this instability to make them more maneu­verable. Still, it was quite wearying for a pilot to have to cope with this. A stability augmentation system made life in the cockpit much easier.

Such a system used rate gyros, which detected rates of movement in pitch, roll, and yaw at so many degrees per second. The instrument then responded to these rates, moving the controls somewhat like before to achieve a null. Each axis of this control had “gain,” defining the proportion or ratio between a sensed rate of angu­lar motion and an appropriate deflection of ailerons or other controls. Fixed-gain systems worked well; there also were variable-gain arrangements, with the pilot set­ting the value of gain within the cockpit. This addressed the fact that the airplane might need more gain in thin air at high altitude, to deflect these surfaces more strongly.68

The X-15 program built three of these aircraft. The first two used a stability aug­mentation system that incorporated variable gain, although in practice these aircraft flew well with constant values of gain, set in flight.69 The third replaced it with a more advanced arrangement that incorporated something new: adaptive gain. This

was a variable gain, which changed automatically in response to flight conditions. Within the Air Force, the Flight Control Laboratory at WADC had laid ground­work with a program dating to 1955- Adaptive-gain controls flew aboard F-94 and F-101 test aircraft. The X-15 system, the Minneapolis Honeywell MH-96, made its first flight in December 1961.70

How did it work? When a pilot moved the control stick, as when changing the pitch, the existing value of gain in the pitch channel caused the aircraft to respond at a certain rate, measured by a rate gyro. The system held a stored value of the optimum pitch rate, which reflected preferred handling qualities. The adaptive-gain control compared the measured and desired rates and used the difference to deter­mine a new value for the gain. Responding rapidly, this system enabled the airplane to maintain nearly constant control characteristics over the entire flight envelope.71

The MH-96 made it possible to introduce the X-15’s blended aerodynamic and reaction controls on the same control stick. This blending occurred automatically in response to the changing gains. When the gains in all three channels—roll, pitch, and yaw—reached 80 percent of maximum, thereby indicating an imminent loss of effectiveness in the aerodynamic controls, the system switched to reaction controls. During re-entry, with the airplane entering the sensible atmosphere, the system returned to aerodynamic control when all the gains dropped to 60 percent.72

The X-15 flight-control system thus stood three steps removed from the conven­tional stick-and-cable installations of World War II. It used hydraulically-boosted controls; it incorporated automatic stability augmentation; and with the MH-96, it introduced adaptive gain. Fly-by-wire systems lay ahead and represented the next steps, with such systems being built both in analog and digital versions.

Analog fly-by-wire systems exist within the F-16A and other aircraft. A digital system, as in the space shuttle, uses a computer that receives data both from the pilot and from the outside world. The pilot provides input by moving a stick or sidearm controller. These movements do not directly actuate the ailerons or rudder, as in days of old. Instead, they generate signals that tell a computer the nature of the desired maneuver. The computer then calculates a gain by applying control laws, which take account of the planes speed and altitude, as measured by onboard instruments. The computer then sends commands down a wire to hydraulic actua­tors co-mounted with the controls to move or deflect these surfaces so as to comply with the pilot’s wishes.73

The MH-96 fell short of such arrangements in two respects. It was analog, not digital, and it was a control system, not a computer. Like other systems executing automatic control, the MH-96 could measure an observed quantity such as pitch rate, compare it to a desired value, and drive the difference to zero. But the MH-96 was wholly incapable of implementing a control law, programmed as an algebraic expression that required values of airspeed and altitude. Hence, while the X-15 with

MH-96 stood three steps removed from the fighters of the recent war, it was two steps removed from the digital fly-by-wire control of the shuttle.

The X-l 5 also used flight simulators. These served both for pilot training and for development of onboard systems, including the reaction controls and the MH-96. The most important flight simulator was built by North American. It replicated the X-l5 cockpit and included actual hydraulic and control-system hardware. Three analog computers implemented equations of motion that governed translation and rotation of the X-l 5 about all three axes, transforming pilot inputs into instrument displays.74

Flight simulators dated to the war. The famous Link Trainer introduced over half a million neophytes to their cockpits. The firm of Link Aviation added analog computers in 1949, within a trainer that simulated flight in a jet fighter.75 In 1955, when the X-l 5 program began, it was not at all customary to use flight simulators to support aircraft design and development. But program managers turned to such simulators because they offered effective means to study new issues in cockpit dis­plays, control systems, and aircraft handling qualities.

Flight simulation showed its value quite early. An initial X-l5 design proved excessively unstable and difficult to control. The cure lay in stability augmentation. A 1956 paper stated that this had “heretofore been considered somewhat of a luxury for high-speed aircraft,” but now “has been demonstrated as almost a necessity,” in all three axes, to ensure “consistent and successful entries” into the atmosphere.76

The North American simulator, which was transferred to the NACA Flight Research Center, became critical in training X-l 5 pilots as they prepared to execute specific planned flights. A particular mission might take little more than 10 min­utes, from ignition of the main engine to touchdown on the lakebed, but a test pilot could easily spend 10 hours making practice runs in this facility. Training began with repeated trials of the normal flight profile, with the pilot in the simulator cock­pit and a ground controller close at hand. The pilot was welcome to recommend changes, which often went into the flight plan. Next came rehearsals of off-design missions: too much thrust from the main engine, too high a pitch angle when leav­ing the stratosphere.

Much time was spent practicing for emergencies. The X-l 5 had an inertial refer­ence unit that used analog circuitry to display attitude, altitude, velocity, and rate of climb. Pilots dealt with simulated failures in this unit, attempting to complete the normal mission or, at least, execute a safe return. Similar exercises addressed failures in the stability augmentation system. When the flight plan raised issues of possible flight instability, tests in the simulator used highly pessimistic assumptions concern­ing stability of the vehicle. Other simulated missions introduced in-flight failures of the radio or Q-ball. Premature engine shutdowns imposed a requirement for safe landing on an alternate lakebed, which was available for emergency use.77

The simulations indeed were realistic in their cockpit displays, but they left out an essential feature: the g-loads, produced both by rocket thrust and by deceleration during re-entry. In addition, a failure of the stability augmentation system, during re-entry, could allow the airplane to oscillate in pitch or yaw. This would change its drag characteristics, imposing a substantial cyclical force.

To address such issues, investigators installed a flight simulator within the gon­dola of a centrifuge at the Naval Air Development Center in Johnsville, Pennsylva­nia. The gondola could rotate on two axes while the centrifuge as a whole was turn­ing. It not only produced g-forces, but its g-forces increased during the simulated rocket burn. The centrifuge imposed such forces anew during reentry, while adding a cyclical component to give the effect of a yaw or pitch oscillation.78

Not all test pilots rode the centrifuge. William “Pete” Knight, who stood among the best, was one who did not. His training, coupled with his personal coolness and skill, enabled him to cope even with an extreme emergency. In 1967, during a planned flight to 250,000 feet, an X-l5 experienced a complete electrical failure while climbing through 107,000 feet at Mach 4. This failure brought the shutdown of both auxiliary power units and hence of both hydraulic systems. Knight, the pilot, succeeded in restarting one of these units, which restored hydraulic power. He still had zero electrical power, but with his hydraulics, he now had both his aerody­namic and reaction controls. He rode his plane to a peak of 173,000 feet, re-entered the atmosphere, made a 180-degree turn, and glided to a safe landing on Mud Lake near Tonopah, Nevada.79

During such flights, as well as during some exercises in the centrifuge, pilots wore a pressure suit. Earlier models had already been good enough to allow the test pilot Marion Carl to reach 83,235 feet in the Douglas Skyrocket in 1953. Still, some of those versions left much to be desired. Time magazine, in 1952, discussed an Air Force model that allowed a pilot to breathe, but “with difficulty. His hands, not fully pressurized, swell up with blue venous blood. His throat is another trouble spot; the medicos have not yet learned how to pressurize a throat without strangling its owner.”80

The David G. Clark Company, a leading supplier of pressure suits for Air Force flight crews, developed a greatly improved model for the X-l5. Such suits tended to become rigid and hard to bend when inflated. This is also true of a child’s long balloon, with an internal pressure that only slightly exceeds that of the atmosphere. The X-l 5 suit was to hold five pounds per square inch of pressure, or 720 pounds per square foot. The X-l 5 cockpit had its own counterbalancing pressure, but it could (and did) depressurize at high altitude. In such an event, the suit was to pro­tect the test pilot rather than leave him immobile.

The solution used an innovative fabric that contracted in circumference while it stretched in length. With proper attention to the balance between these two effects,

the suit maintained a constant volume when pressurized, enhancing a pilot’s free­dom of movement. Gloves and boots were detachable and zipped to this fabric. The helmet was joined to the suit with a freely-swiveling ring that gave full mobility to the head. Oxygen flowed into the helmet; exhalant passed through valves in a neck seal and pressurized the suit. Becker later described it as “the first practical full-pres­sure suit for pilot protection in space.”81

Thus accoutered, protected for flight in near-vacuum, X-15 test pilots rode their rockets as they approached the edge of space and challenged the hypersonic frontier. They returned with results galore for project scientists—and for the nation.