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,
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
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
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
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
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
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
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.