X-15 Research Results

Chapter 3

Developing a Concept

WHEREAS THE COMPLETE DEVELOPMENT of the first powered aircraft was carried out by two men, the complexities of a modern aircraft require a ponderous procedure to shepherd it from technical proposal through design and construction and to provide support during its flight program. For the X-15, this shepherding role has been provided by the Aeronautical Systems Division (formerly Wright Air Development Center) of the Air Force Systems Command. It has included research-and-development support not only for an airplane with revolutionary performance but also for the most powerful -and potentially most dangerous- powerplant ever developed for aircraft use. lt has encompassed new concepts for pilot protection, numerous first-time subsystems, modifications and support for two launch airplanes, and the eventual rebuilding of two of the original three X-15's. It will surely include other items as the program goes on.

A third partner joined the X-15 team when North American Aviation, Inc., won the design competition with other aircraft manufacturers. The proposal of the Los Angeles Division of NAA was chosen by joint Air Force-Navy-NACA agreement as "the one most suitable for research and potentially the simplest to make safe for the mission." The contract called for the construction of three aircraft, with the expectation that two would always be in readiness and undergoing modification or repair. Two craft would have been enough to handle the anticipated research workload, but if there had been only two, a mishap to one of them -always a strong possibility in exploratory flight research- would have seriously curtailed the program.

Although NACA's studies showed possible solutions for many of the major problems, it remained for one of America's crack design groups actually to solve them. And it is also true in any ambitious endeavor that the magnitude of a problem seldom becomes fully apparent until someone tries to solve it. The basic problem North American faced was that of building an airplane of new materials to explore flight conditions that were not precisely defined and for which incomplete aerodynamic information was available. Yet it would have to accomplish this on an abbreviated schedule, despite an appalling inadequacy of data.

The original design goals were Mach 6.6 and 250 000 feet, but there were no restrictions to prevent flights that might exceed those goals. The flight program would explore all of the corridor to the maximum practical speed, and it would investigate the space-equivalent region above the corridor. The reentry maneuver would compound many factors. Both airplane and pilot would be subjected to acceleration forces of six times gravity (6 G's). The pilot would be required to maintain precise control during this period, with both airplane and control system undergoing rapid, large changes in response. These nice generalities had to be translated into hard, cold criteria and design data.

The development of any aircraft requires many compromises, since a designer seldom has a complete answer for every problem. If there are unlimited funds available to attack problems, an airplane can represent a high degree of perfection. But this process is also time-consuming. For the X-15 design, compromises were all the more inevitable and the optimization difficult. The X-15 would still be on the drawing boards if construction had been delayed until an ideal solution to every problem had been found. A reconciliation of the differing viewpoints of the several partners in the program was also necessary. While all were agreed on the importance of the program, their diverse backgrounds gave each a different objective.

In spite of differences, the project rolled ahead. In all of this, the overriding consideration was the brief time schedule. There literally was no room for prolonged study or debate. While three years may seem at first to be ample time in which to produce a new airplane, it must be remembered that simpler aircraft than the X-15 normally require longer than that for construction. For a new flight regime and use of a new structural material, the X-15 schedule was most ambitious. One important help in meeting it, however, was the initial decision to explore new flight regimes in a progressive manner, so that complete solutions for every problem need not be found before the first flight. Not all could be put off until the flight program began, though.

The sum product of a year of study, a year of design, and a year of construction is an airplane that is a composite of theory, wind-tunnel experiments, practical experience, and intuition - none of which provides an exact answer. The X-15 represents an optimization, within limits, for heating, structure, propulsion, and stability. It is also a compromise, with many obvious and not so obvious departures from previous jet- or rocket-plane experience. The fuselage consists largely of two cylindrical tanks for rocket-engine propellants. To these were added a small compartment at the forward end, for the pilot and instrumentation, and another at the aft end, for the rocket engine. Large, bulbous fairings extend along the sides of the fuselage to house control cables, hydraulic lines, propellant lines, and wiring that has to be routed outside the tanks. The big fuselage-fairings combination has a decided effect on the total aerodynamic lift of the X-15. The airflow near these surfaces provides well over half of the total lifting force, particularly at hypersonic speeds. Thus, the small size of the wings reflects the relatively small percentage of lifting force they are required to provide. (They do most of their work during launch and landing.)

photo iew of the X-15 attached to the wing of a B-52
An X-15 is seen just prior to launch from a B-52 at 45 000 ft., 200 miles from home base. The X-15 pilot und B-52 launch-panel operator have completed their pre-launch check procedures. The chase plane in the distance is keeping a sharp eye on the X-15 during the checkout.

The design of the structure to withstand hypersonic flight brought one of the prime purposes of the X-15 into sharp focus: to gain knowledge about heating and the hot structural concept. The structure could have been protected by insulation or cooling techniques that would have kept temperatures well below 1200 F. A basic feature of the X-15 concept, however, was that a hot structure would permit more to be learned about aerodynamic heating and elastic effects than one protected from high temperatures. Therefore, 1200 F became a goal rather than a limit.

The complicating factor was that loads and temperatures must be absorbed with a minimum of structural weight. Yet Inconel X weighs three times as much as aluminum, and any excess weight has a critical effect on the performance of a rocket airplane. Each 500 pounds cuts performance by 100 mph, and structural engineers strive to shave every ounce of extra material from the structure.

photo of the X-15 in flight
An instant or two after launch, the X-15 is seen roaring off on its own, with Inconel X skin glistening in the sun. At burnout, it will be accelerating at 4 G's, or 90 additional miles per hour every second.

This science, or art, had already advanced aluminum aircraft structures to a high level of load-carrying efficiency. There were always unpredictable, troublesome interactions, however, and structural designers usually relied upon laboratory tests to confirm each new design. Normal practice was to build one airplane that was statically loaded to the equivalent of anticipated flight loads, in order to evaluate its strength.

The X-15, however, would have to enter the high-load region without this time-honored test of its structure. The most severe stresses are encountered when the structure undergoes aerodynamic heating, and no static-test facility existed in which the X-15 could try out a realistic temperature environment. Therefore, a static-test airplane was not built, and no tests were made of actual structural components. But the structural design for the high-temperature condition wasn't left to analysis alone. An extensive testing program was conducted during the design to prove out the approaches being taken. Many tests were made of sections of the structure under high temperatures and thermal gradients. These helped define some of the difficulties and also improved static-test techniques.

NAA saved considerable weight through the use of titanium in parts of the internal structure not subject to high temperatures. Titanium, while usable to only about 800 F, weighs considerably less than Inconel X. The structural design was influenced to some extent by the requirements for processing and fabricating these materials. Inconel X soon stopped being just a laboratory curiosity. Production-manufacturing techniques were developed to form, machine, and heat-treat it. In many instances, an exhaustive development program was required just to establish the method for making a part. Thus much practical experience was gained in the design, fabrication, and testing of new materials.

Additional weight was saved on the X-15 by the use of a rather novel landing-gear arrangement. The main landing gear consists of two narrow skis, attached at the aft end of the fuselage and stowed externally along the side fairings during flight. When unlocked, the skis fall into the down position, with some help from airflow. A conventional dual-wheel nose gear is used. This gear is stowed internally to protect its rubber tires from aerodynamic heating.

Contrasting with the X-15's small wings are its relatively large and massive tail surfaces. These surfaces, like the fins on an arrow, stabilize the craft in its flight. But, unlike an arrow, which ideally never veers from its path, the X-15 must be able to change alignment with the airflow, to maneuver and turn. And it is a most difficult design compromise to achieve the proper balance between stability and control. The problem in this case was greatly complicated by the different aerodynamic-flow conditions encountered within the flight corridor, and by the changes between the angle of airflow and the pitch and yaw axes required to maneuver the airplane. Criteria had been developed to guide the design, but they were derived largely from empirical data. They required considerable extrapolation for X-15 flight conditions.

Although the extrapolation in speed was rather large, the largest was in the extreme angle between pitch axis and flight path required for pullout during reentry. This results is a compounding problem, because it becomes increasingly difficult to stabilize an airplane at high angles of airflow (angle of attack) at high speed.

photo of the X15 skid type landing gear
This photograph provides an unusually clear view of the X-15's unique main landing gear, and shows how much of the lower vertical tail is left after its bottom part has been jettisoned for landing. The photo also reveals the new, knife-sharp leading edge that has been given to the X-15-3 configuration's upper vertical tail in order to study heat transfer through 2500-deg. airflow.

A phenomenon is encountered in which the vertical tail loses ability to stabilize the airplane and the nose tends to yaw. Indeed, the only previous airplanes that had been flown to Mach numbers above 2 - the X-1A and X-2 - had experienced such large decreases in stability that the pilots lost control (disastrously, in the case of the X-2) when they maneuvered the craft to angles of attack of only 5 or 6 degrees. Yet the reentry maneuver of the X-15 would normally require it to operate at an angle of attack of 20 to 25 degrees.

The initial solution, proposed by NACA, was found in the large, wedge-shaped upper-and-lower vertical-tail surfaces, which are nearly symmetrical about the aft fuselage. A wedge shape was used because it is more effective than the conventional tail as a stabilizing surface at hypersonic speeds. A vertical-tail area equal to 60 percent of the wing area was required to give the X-15 adequate directional stability. Even this was a compromise, though, for weight and different flight conditions. As an additional factor of safety, therefore, panels that could be extended outward, thus increasing the pressure and stabilizing forces, were incorporated in the vertical tails. These panels -another NACA proposal- also serve as speed brakes, and the pilot can use them at any time during flight. Both braking effect and stability can be varied through wide ranges by extension of the speed brakes and by variable deflection of the tail surfaces. The large size of the lower vertical tail required for adequate control at high angles of attack required provision for jettisoning a portion of it prior to landing, since it extends below the landing gear.

A disadvantage of the wedge shape is high drag, caused by airflow around its blunt aft end. This drag force, when added to the drag from the blunt aft ends of the side fairings and rocket-engine nozzle, equals the entire aerodynamic drag of an F-104 jet fighter.

Control for maneuvering flight is provided by partially rotatable horizontal-tail surfaces. Roll control is achieved by a unique mechanism that provides differential deflection of the left and right horizontal-tail surfaces. This somewhat unconventional control technique, called "rolling tail", was unproven at the time of the X-15 design. However, NAA had studied such control systems for several years, its studies including wind-tunnel experiments from subsonic to supersonic speeds. Pitch control is provided by deflecting the left and right horizontal tails symmetrically.

The combination of large control surfaces and high aerodynamic pressures forced designers to use hydraulic systems to actuate the surfaces. This type of power steering introduces its own characteristics into aircraft-control response, as well as making the airplane absolutely dependend upon the proper functioning of the hydraulic system. It did facilitate the incorporation of electronic controls, which were shown to be helpful to the pilot, especially during reentry. There had to be assurance, however, that a malfunction of any component during flight would not introduce unwanted control motions. Thus the design of the control system provided a safe alternative response in the event of any component failure as well as for normal operation. Many of the X-15's operating characteristics are similarly based upon fail-safe considerations.

Close up of the X15's  tail section The blunt aft ends of the X-15's side fairings, vertical tails, and the rocket-engine nozzle represent one of the many compromises that a hypersonic configuration demanded. Together they produce as much drag as an F-104 jet fighter.

A unique feature of the control system is the three control sticks in the cockpit. One is a conventional center stick, which controls the airplane in pitch and roll as it would in a jet fighter or a Piper Cub. The center stick is directly linked to one that is at the pilot's right side. The latter is operated by hand movement only, so the pilot's arm can remain fixed during high accelerations experienced during powered flight and reentry. This is an essential feature, which enables the pilot to maintain precise control for these conditions. The third control stick is located at the pilot's left, and is used to control the X-15 when it is above the atmosphere. This stick actuates reaction jets, which utilize man's oldest harnessed-energy form, steam. The X-15 uses a modern form of superheated steam, from the decomposition of hydrogen peroxide (H2O2). This concept was later adopted for the Mercury-capsule jet controls. The reaction thrust is produced by small rocket motors located in the nose, for pitch and yaw control, and within the wings, for roll control. While such a system was simple in principle, control by means of reaction jets was as novel in 1956, when it was introduced, as orbital rendezvous is today. The transition from aerodynamic control to jet control loomed as the most difficult problem for this vast, unexplored flight regime.

cockpit photo of the X15
The X-15's cockpit is quite like a jet fighter's, except for its unique arrangement of three control sticks. The one at left governs the jet reaction controls, in space-equivalent flight. The one at right is used in high-G flight and is mechanically linked to the conventional stick at center.

There were many other new and peculiar conditions for the pilots to face. Altogether, they would be tackling the most demanding task ever encountered in piloted aircraft. Some of the control-system and physical characteristics were tailored to their capabilities to attain the desired airplane-pilot combination. While the pilot is an integral part of the concept, with maximum provision made for his safety, he needs to be able to escape from unforeseen hazardous conditions. The difficulty, in the case of the X-15, was that to create a system that would protect the pilot during escape anywhere within the flight corridor or above it would require a development program nearly as large as that of the airplane. lt would also require a prohibitive increase in airplane weight. The result was that an over-all escape capability was not provided. The airplane itself was regarded as the best protective device for the pilot at high speeds. At low speeds, he could use an ejection seat similar to that used in most military aircraft.

But "low speed" for the X-15 is 2000 mph, and to provide for escape over this much of the corridor required a state-of-the-art advance in escape systems. Extensive wind-tunnel and rocket-sled testing was necessary to achieve an aerodynamically stable ejection seat. Another major effort was required to provide protection for the pilot against windblast during ejection. Finally, the desired escape capability was provided by a combination of pressure suit and ejection seat.

Major Advance in Powerplant Needed

Aircraft speeds couldn't be pushed far up the flight corridor without major advances in powerplants. And the farther up the corridor one goes, the tougher the going gets. There's enough power in one engine of the trusty old DC-3 to pull a planeload of passengers along at 100 mph, but it isn't enough even to pump the propellants to the X-15 rocket engine. Although by 1955 the United States had eight years' experience with aircraft rocket engines, one of 50 000-pounds thrust was a big advance over any used for that purpose before. Missiles had provided the only previous experience with lange rocket engines. And the X-15 couldn't become a one-shot operation. Its engine would have to be an aircraft engine, capable of variable thrust over at least 50 percent of the thrust range and having other normal cockpit control features, such as restarting. A major problem was the threat of a launch-pad disaster with such a lange rocket engine and the enormous amount of fuel carried aboard the X-15. This potential danger had to be minimized not only to insure the safety of the X-15's pilot but that of the pilot and crew of the B-52 that would launch it. Thus, safety of operation became an overriding consideration for the X-15 engine.

drawing of the X15 rocket engine
A closeup of the X-15's remarkable XLR-99 rocket engine. Its 57 000-lb. maximum thrust is equivalent at burnout to 600 000 hp. The engine can be throttled from 40-percent to 100-percent thrust. Its propellants flow at the rate of 13 000 lb. per minute at maximum thrust, exhausting the entire 18 000-1b. fuel supply in 85 seconds. The engine's nozzle diameter is 39.3 in.; its over-all length, 82 in.; its weight, 1025 lb.

The problem was two-fold. The huge amount of fuel that was pumped through the engine meant that in the event of engine malfunction, a lot of unburned fuel could accumulate in a fraction of a second. At the same time, combustion difficulties were inherent in an engine in which burning takes place by mixing two liquids together, rather than a liquid and a gas, as in a jet engine or automobile engine. While initially it appeared that a missile engine could be adapted for the X-15, it soon became evident that none would meet the stringent safety requirements.

Subsequently, Reaction Motors, Inc. (now the Reaction Motors Division of the Thiokol Chemical Corp.), was selected to develop what became the XLR-99 rocket engine. lt was clear that this firm was undertaking the development of more than just a suitable engine composed of thrust chamber, pumps, and controls. The technical requirements contained a new specification that "any single malfunction in either engine or propulsion system should not create a condition which would be hazardous to the pilot."

Reaction Motors was well prepared for this task. lt had built many rocket engines for the X-1 and D-558-II research airplanes, and in some 384 flights it had never had a disastrous engine failure. As a result of this background, its engineers adopted a rigorous design philosophy that left its mark on every detail part in the propulsion system. While endeavoring to prevent malfunctions, they designed the engine so that the conditions following any malfunctions would be controlled before they became hazardous. They accomplished this by developing an igniter system that insures that all residual propellants are vaporized and burned in the combustion chamber. Another feature was a system that automatically monitors engine operation and senses component malfunctions. Whenever a malfunction occurs, the system shuts down the engine safely. For some controls, component redundancy was used to provide safety against a single malfunction. However, rather than parallel entire components, some unique designs were developed that utilize redundant paths within components.

The added complexities needed to achieve safe operation of the X-15 engine make it appear to be a "plumber's nightmare" when compared to other rocket engines of its era. And normally the penalty for complexity is reduced reliability in operation. But in-flight reliability has been 96 percent - a remarkable figure compared to that of missile engines of similar design. lt is obvious that safety of operation was not gained at the sacrifice of over-all reliability.

The engine burns a mixture of anhydrous ammonia (NH3) and liquefied oxygen (LO2). These propellants pose a few handling problems, because of the corrosive properties of ammonia and the low temperatures of liquid oxygen, which boils at -297 F. Since the propellant tanks are an integral part of the airplane structure, temperature extremes between structure close to the lox tank and surrounding structure have exerted a major influence on thermal stresses and structural design. The lox tank has a capacity of 1003 gallons; the ammonia tank, 1445 gallons. This gives a burning time of 85 seconds at full thrust. An important feature of the X-15's lox system is the need for replenishing it after takeoff, because of the large amount lost through boil-off during the climb to launch altitude aboard the B-52. This topping-off takes place continuously, under control of a B-52 crewman, from tanks within the B-52, which have a capacity 1 1/2 times that of the X-15's.

The X-15 also carries, besides engine propellants, vast quantities of hydrogen peroxide (H2O2), liquefied nitrogen (-310 F), gaseous nitrogen, and gaseous helium (-240 F), used to operate various subsystems. With such large amounts of super-cold liquids flowing within the airplane, its internal components need protection from freezing, not high temperature. This paradoxical situation in a hot airplane requires the use of many heating elements and insulation blankets. (Another paradoxical situation is that Inconel X, of which much of the plane is made, not only resists high heating but retains excellent material properties at temperatures as low as -300 F.)

Many unique systems and subsystems had to be developed to meet a host of new power requirements and functions. The auxiliary-power requirements, in particular, were severe, for not only is there large demand for hydraulic and electrical power but the aerodynamic controls will not function without hydraulic power. Therefore, dualization is used in critical components, from fuel tanks to hydraulic actuators. The hydraulic pump and electrical generator are driven by a 50 000-rpm, high-temperature steam turbine, which operates on hydrogen peroxide. The hydrogen-peroxide tanks supply the reaction-jet control system.

A second major subsystem is the air-conditioning unit, which protects the pilot and instrumentation from the effects of heating and also cools the auxiliary-power system. It operates from liquid nitrogen, and, in addition to cooling, pressurizes the cockpit, instrument compartment, pilot's suit, hydraulic reservoir, and canopy seal.

One of the most complex and vital subsystems is the payload, the research-instrument system. It was, of course, of utmost importance to bring back a record of the temperatures and the aerodynamic forces in this new environment, and the response of the structure to them. This required installing thermocouples and probes and tubing within the structure, as well as inserting them into the layer of airflow around it. That necessitated cutting holes in the wings and along the fuselage in locations that plagued the structural engineers, and the installation had to be done while the airplane was being built. By the time this work was completed, some 1400 pounds had been added to the airplane's weight. The research-instrument system was perhaps the only one in which such a large weight would be accepted, though reluctantly.

Throughout the design and construction, one goal for the X-15 was to make it as simple as possible, to use conventional design techniques, and to use proven components wherever possible. Even a cursory glance shows, however, that there is little conventional about the X-15 or its systems. The many new concepts were the products of necessity rather than desire. Newly conceived components together with new materials and new processes have made even simple systems become complex development projects. As a result, a rigorous product-improvement-and-development program is still underway five years after the first flight. Thus, from a 1956 aerodynamic design, a 1957 structural design, 1958 fabrication techniques, and a 1959-64 development-test program, the X-15 has evolved into an airplane in which updating and systems research have been important factors.

The prime objective of the X-15 program has remained flight research, however. By the time of the first flight, much had already been learned about hypersonic flow by focusing the talents of many men on X-15 problems. Many of the worries over flight above the atmosphere had been dispelled. Yet hypersonic, exo-atmospheric, and reentry-flight research was still a vague and obscure world. Were the problems imagined or real? And what of those problems that man cannot foresee? The X-15 team was shure of only one thing. The problems would come to light through probing the flight corridor, until all the interactions among aerodynamics, structure, stability, systems, and pilot control had been forced into view, and the adequacy or inadequacy of man's knowledge and capability revealed.

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