X-15 Research Results

Chapter 2

The First Hypersonic Airplane

THROUGH TWO HUNDRED YEARS of analysis and experiment, scientists and engineers have slowly accumulated a detailed picture of flight through our atmosphere. They know that at high speeds the dense layer of air close to the Earth's surface generates pressures that hinder an aircraft, while at high altitudes the air density is so low that extremely fast speeds are necessary to generate enough pressure to keep a plane flying. They designed airplanes as a compromise between these forces, and flight became confined to a corridor that is bounded by ever-increasing combinations of altitude and velocity.

As man pushed aircraft farther up this flight corridor, the problems began to multiply. New aerodynamic knowledge and new scientific disciplines had to be added to the world of airflow. The concept of the atmosphere as a single gaseous envelope gave way to one that recognized it as a series of layers, each with its own characteristics. Airflow, too, was found to have distinct regions and characteristics. At velocities less than 500 mph, it is tractable and easily defined. At higher speeds, its character undergoes marked change, sometimes producing abrupt discontinuities in aerodynamic pressures. Even before man's first flight, the noted German physicist Ernst Mach had shown that a major discontinuity occurs when the velocity of airflow around an object approaches the speed of sound in air (760 mph at sea-level pressure and temperature). Later work showed that the air pressures an airplane experiences vary with the ratio of velocity of airflow to speed of sound, and scientists adopted this ratio, called Mach number, as a measure of the flow conditions at high speeds.

The effect of flight to Mach 1 produces large changes in the air pressures that support, retard, twist, pitch, roll, and yaw an airplane. But man edged past this speed into the realm of supersonic flight, and by the time Mach 1.5 was attained, airplanes had undergone a vast transition in technology. Some men saw in this transition the basis for pushing much farther up the flight corridor. In the early 1950's, a few visionary men looked far up that corridor and became intrigued by a goal much closer than the theoretical limit at the speed of light. They saw that the corridor flared dramatically upward at orbital speed (Mach 24), leading out of the Earth's atmosphere into space, defining the start of a path to the Moon, Mars, and beyond.

But if their gaze was on orbital flight, their minds were on a torrent of new problems that had to be overcome to achieve it. The supersonic-flight region led into hypersonic flight - a fearsome region with a thermal barrier, which looked far more formidable than had the earlier, sonic barrier. This new barrier came from the friction of air as it flows around an aircraft. At Mach 10, that friction would make the air hot enough to melt the toughest steel. At Mach 20, the air temperature would reach an unbelievable 17 000° F. Thus aerodynamic heating was added to the growing list of new disciplines.

Other new problems came into view. Flight above the atmosphere would render aerodynamic controls useless, requiring another method of control. The pilot's response to the weightlessness of orbital flight was a controversial subject. Some expressed grave doubts that he could withstand prolonged periods of orbital flight. The reentry into the atmosphere from space would perhaps compound all of the problems of hypersonic flight and space flight. Yet these problems were academic unless powerplants an order of magnitude more muscular than were then available could be developed to propel an aircraft into space. Little wonder, therefore, that the pioneers envisioned a slow and tortuous route to reach their goal. They had yet to realize that manned orbital flight was possible in one big jump, through the wedding of large ballistic missiles and blunt reentry capsules.

The vision of these men, however, began to stimulate thought and focus interest within the aeronautical community on the prospects for orbital flight. Early studies showed that much could be learned about space flight without achieving orbital speeds. By zooming above the normal flight corridor at less than orbital speeds, one could study non-aerodynamic control and weightlessness. Reentry from such a maneuver would approximate reentry from space. Perhaps more significant was the fact that if a speed of Mach 8-10 could be achieved, aerodynamics would be over the hump of hypersonic flow, for air pressures show far less variation above this speed than below.

The initial investigative work was guided by extensive theoretical analysis and ground-facility experiments, but critical problems abounded and possible solutions were largely speculative. Theoretical methods approximated an airplane as a cone and cylinder, with wings composed of flat plates. While these theories agreed with some of the results of wind-tunnel experiments, there were many disagreements. There were doubts about the accuracy of wind-tunnel measurements, because of their extremely small scale. Although large hypersonic tunnels were being developed, an airplane had already flown faster than the top speed that could be duplicated in any wind tunnel big enough for reliable development-testing. Many of the pioneers became convinced that the best way to attack the many unknowns would be to meet them head-on-in full-scale flight research. They pressed for an airplane to make the first step into the hypersonic, space-equivalent, and reentry flight regimes, to lay the groundwork for following airplanes. A decisive influence was the fact that rapid progress was already being made on the development of powerful, liquid-fueled rocket engines, though they were not intended for airplanes.

Among the several visionary men of the era, the late Robert Woods, of Bell Aircraft Corp. (now Bell Aerospace Corp.), was outstanding. His efforts to "sell" manned space flight began in June, 1952, some five years before the Earth's first artificial satellite appeared. In a bold proposal, he urged the United States to "evaluate and analyze the basic problems of space flight . . . and endeavor to establish a concept of a suitable test vehicle." One important and, to Woods, fundamental part of his recommendation was that the (then) National Advisory Committee for Aeronautics should carry forward this project. NACA was a government organization (later forming the nucleus of the National Aeronautics and Space Administration) that had long been in the forefront of high-speed aeronautical research. Many of the foremost proponents of hypersonic flight were on its staff. NACA had also coordinated aeronautical technology among the military services, civil aviation, and aircraft industry, and was responsive to their respective needs. NACA was most active and eager for a bold step into hypersonic flight.

Basic Studies began in 1954

But at a time when the current struggle was to push aircraft speeds from Mach 1.5 to 2.0, two more years elapsed before a climate developed in which the urgency for hypersonic flight was backed up by resources of money and manpower. In March, 1954, NACA's Langley Aeronautical Laboratory, Ames Aeronautical Laboratory, and High Speed Flight Station began the studies that led to the X-15 program. This early work was the first to identify all major problems in detail and examine feasible solutions. Only then could the researchers decide how big their first step should be. They knew at once that Mach 8-10 was unobtainable. Materials and technology were not available for such speeds. But the work of the Langley Laboratory showed that Mach 6-7 was within reach, as well as an altitude of 250 000 feet, well above the conventional flight corridor. And, of course, even Mach 6 was a giant step. To attain this speed would require a rocket engine of 50 000-pounds thrust and a weight of propellants 1 1/2 times the weight of the basic airplane. These were difficult goals, but within the state of the art.

The major problems would be to achieve a configuration that was stable and controllable over the entire range of speed and altitude, and prevent it from being destroyed by aerodynamic heating. The stability-and-control problem appeared to be solvable, although a few innovations would be required. Most importantly, the Langley study pointed to a way through the thermal barrier. It showed that if the airplane were exposed to high-temperature airflow for only a brief period of time, its structure could be designed to absorb most of the heating, and temperatures could be restricted to a maximum of about 1200° F. This concept of a "heat sink" structure was based upon use of a new high-temperature nickel-chrome alloy, called Inconel X by its developer, the International Nickel Co. Inconel X would retain most of its strength at 1200° F, a temperature that would melt aluminum and render stainless steel useless. However, no manufacturer had ever made an aircraft of Inconel X.

The Langley study influenced the X-15 program also through its somewhat philosophical approach to the craft's development and method of operation. In the view of the Langley study team, any new airplane should be a flight-research tool to obtain a maximum amount of data for the development of following airplanes. The design, therefore, should not be optimized for a specific mission, but made as useful as possible for exploratory flight - a rather vague criterion. A tentative time limit of only three years was set for the design and construction, in order that flight data could be obtained as soon as possible. Such a tight schedule established the need for somewhat of a brute-force approach. The design must stay within the state of the art and avoid the use of unconventional techniques that would require long development time. Other Langley guidelines specified the use of proven techniques as far as possible, and "the simplest way to do the job." They emphasized that the airplane should not become encumbered with systems or components not essential to flight research. These requirements were tempered by knowledge that a three-year development schedule would leave little or no time to perfect systems and subsystems before first flight.

The design philosophy was also influenced by the fact that new aerodynamic regimes were to be explored in a carefully regulated, progressive manner, thus gradually exposing the airplane and pilot to any critical condition for which complete data might have been impossible to obtain during the speeded-up design period. Significantly, early plans were for the flight program to be conducted by NACA's High Speed Flight Station (now NASA's Flight Research Center) at Edwards, California, which at that time functioned as a part of the Langley Laboratory at Hampton, Virginia, though separated from it by some 2300 miles. This close tie brought into the program at the very beginning the viewpoints of the research pilots who would fly the X-15.

An important figure in the over-all coordination was H. A. Soulé, of the Langley Laboratory, who had directed NACA's part in the research-airplane program since 1944. He and his chief associates would steer the X-15 program through the conceptual studies and the design and construction phases with one goal - to develop a satisfactory airplane in the shortest practical time. This meant severe pruning of a multitude of proposed engineering studies, every one of which could be justified in the cause of optimization, but which together could lead to fatal over-engineering in the effort to achieve an ideal aircraft. It also meant stern attention to the progress of selected studies. Mr. Soulé's task was complicated by the fact that the interests of other government organizations would have to be served at the same time, since NACA's resources were too meager to enable it to undertake such an ambitious program alone.

By the fall of 1954, a technical proposal and operational plan had been formulated and presented to several government-industry advisory groups on aviation. NACA proposed that the new program should be an extension of the existing, cooperative Air Force-Navy-NACA research-airplane program. This joint program, which dates from 1944, had resulted in the well-known first flight to Mach 1, by the X-1 rocket airplane; the first flight to Mach 2, by the D-558-II rocket airplane; and the first flight to Mach 3, by the X-2 rocket airplane. Less well-known are 355 other rocket-air-plane flights and more than 200 jet-airplane flights made under this program. These were flights that in 1947 helped lay bare some of the problems of transonic flight, at speeds now commonplace for jet transports. These flights also laid the technical and managerial foundations for the X-15 program, and led to its immediate and full support by the United States Air Force, Navy, and Department of Defense.

Because of the magnitude of the new research-airplane program, a formal Memorandum of Understanding was drawn up among the Air Force, Navy, and NACA, setting the basic guidelines upon which the program operates to this day. A distinctive feature of the memorandum is that it is not just a definition of the lines of authority and control. Rather, it lays out a fundamental pattern of cooperation among government agencies that continues as a basic feature of the X-15 program, and has had no small effect on the successful pursuit of the research. In essence, it states briefly that each partner agrees to carry out the task it is best qualified for.

The Memorandum of Understanding may also be the only place where the true purpose of the X-15 program is spelled out. This is contained in a specific provision for disseminating the results of the program to the U.S. aircraft industry. It adds that the program is a matter of national urgency.

This urgency was already obvious. In less than 10 months from the time NACA initiated the study to determine if hypersonic flight was feasible, a detailed program had been submitted to the aircraft industry, and several firms were already making preliminary design studies for flight to Mach 6-7. This rapid progress, perhaps more than any other factor, tells of the invisible pressure that had resulted from the stimulus of the strong individuals who pioneered the X-15. A national program to develop the world's first hypersonic airplane was underway.

photo of the X-1 test plane
Noted predecessors of the X-15 in the cooperative research-airplane program of the Air Force, Navy, and NACA, dating from 1944, were the X-1 (above), which made the world's first supersonic flight, and the X-2 (below), which first flew to Mach 3.

photo of the X-2 on the ground

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