Sound Barrier

During World War II, when pilots of new, high performance fighter aircraft flew their plane too fast, they would start to buffet and shake as the they approached the speed of sound. The speed of sound, known as Mach 1, is 742 mph at sea level, and slightly less as you increase in altitude. Flying slower than the speed of sound is called subsonic flight. Flying faster than the speed of sound is known as supersonic flight, and flying five times the speed of sound, or Mach 5 is called hypersonic flight.

As the planes got going, nearing the speed of sound, the air moving around the outside of the aircraft started to go faster than the speed of sound, which created shock waves as this air encountered slower moving air. These shock waves would build up on the wings and fuselage, and shake the aircraft, sometimes causing the pilot to lose control. It was widely believed that if an aircraft went faster than the speed of sound, that it would fall apart, as if hitting an invisible wall, the sound barrier.

A sonic boom is a thunder-like sound produced when an aircraft travels faster than the speed of sound. Air is a fluid and is pushed apart with great force as an aircraft traveling at supersonic speeds cuts through the air, forming a shock wave of compressed air, similar to the bow waves created by a boat as it cuts through the water.

The shock wave produced by an aircraft is in the shape of a cone with its vertex located at the nose of the aircraft and pointing in the direction of travel of the aircraft. The cone spreads out behind the aircraft and increases in diameter as distance behind the aircraft increases. The cone shaped shock wave moves along with the aircraft at the same speed as the aircraft. The shock wave creates a sonic boom at each point in space that it passes. The air pressure within the shock wave is usually only a few pounds per square foot greater than normal atmospheric pressure. This is about the same pressure difference experienced by a change in elevation of about 20 to 30 feet. This additional pressure above normal atmospheric pressure is called overpressure. If the overpressure were released slowly it would produce little sound. When this overpressure is quickly released during a very short time interval it generates a sonic boom.

Flow rates less than Mach 1 are said to be subsonic, while those greater are said to be supersonic. A body moving through a fluid at speeds less than the speed of sound in the fluid is preceded by a region of gradually varying density and pressure. At speeds greater than the speed of sound, such a gradual transition is not possible and a shock wave of nearly discontinuously changing pressure and density is formed. In the case of a supersonic aircraft or a bullet, this shockwave is a double walled cone that forms with the front and back of the object at its vertices (projections in between like wings and stabilizers are placed at the vertices of intermediate shockwaves). Shock waves can also form whenever a fluid is heated so rapidly that the leading edge of its expansion travels at or above the speed of sound in the fluid. Roughly spherical shockwaves form when bombs, fireworks, and other pyrotechnic devices explode. A bolt of lightning generates a twisted cylindrical shockwave centered on the bolt's path. The sound of a shockwave produced by a supersonic aircraft is called a sonic boom, while the sound of a shock wave produced by lightning is called thunder.

An obstacle was experienced by subsonic aircraft attempting to fly at or above the speed of sound. Drag increases sharply, lift falls off, and the aircraft becomes difficult to control. At subsonic speeds the pressure waves created by the aircraft as it flies through the air are able to move ahead of the aircraft; at supersonic speeds they cannot escape in a forward direction as the source is moving faster than the pressure waves themselves. Thus shock waves build up on the aircraft's wings and fuselage, creating an apparent barrier to supersonic flight.

The use of aerofoils as lift surfaces depends on Bernoulli's principle, according to which the total energy of a flowing fluid remains constant; thus, if the velocity of the fluid increases, its pressure decreases in proportion. An aerofoil is a wing so shaped that (at subsonic speeds) air is accelerated over its rounded leading edge and curved upper surface, causing a reduced pressure above it. A smaller reduction in air velocity on its underside causes a slightly increased pressure below it. The combination of these pressure differences provides the lift. The design of practical aircraft wings has to take into account a number of complex factors, including suitable streamlining to avoid turbulence in the airflow, stability over different angles of attack, the provision of suitable control surfaces (flaps, ailerons, etc.), and adequate strength and rigidity. At supersonic speeds these forces are somewhat altered and the aerofoil has to be more sweptback and more streamlined. At hypersonic speeds (i.e. in excess of five times the speed of sound) the aerodynamics changes again and blunter noses and even smaller wings are needed.

X-Plane Projects

The U.S. rocket program hit a wall in the late 1940's due to a lack of understanding of supersonic physics. Bigger versions of the V-2 rocket disintegrated at high speed. To counter this problem, the National Advisory Committee for Aeronautics (NACA, the predecessor of NASA) developed the X-plane program.

Of the planned exploratory research in aerodynamics over the past half century, a good portion from the late 1940s through the 1960s was primarily for increasingly faster and higher-flying airplanes. So it was hardly surprising that the research for NACA's during that time focused on technology and advances to help make these goals possible. More surprising, perhaps, is the renewed emphasis on high and fast flight in recent years, although the latest focus is significantly different from the initial work. Today, aircraft such as the proposed High Speed Civil Transport (HSCT) must meet new criteria for fuel efficiency and environmental impact as well as speed and performance. In the early days, the goals were less complex, and the focus was on paving the way to supersonic flight and space.

The "X" designation, originally "XS" for eXperimental Supersonic, applied to a family of experimental aircraft not intended for production beyond a limited number built solely for flight research. The D-558 did not bear the "X" label but were clearly intended for the same purpose. The research techniques used in the X programs became the pattern for all subsequent X-craft projects. The NACA X-1 procedures and personnel also helped lay the foundation of America's space program in the 1960s. The X-1 project defined and solidified the post-war cooperative union between U.S. military needs, industrial capabilities, and research facilities. The flight data collected by the NACA in the X-1 tests then provided a basis for American aviation supremacy in the latter half of the 20th century.

X-Plane Summary


The X-1, a joint effort of the Army Air Forces, NACA, and the Bell Aircraft Corporation, was built to get answers about flight in the transonic region (approaching and immediately surpassing the speed of sound) that researchers were unable to get through conventional ground and wind tunnel tests. Aircraft design had progressed rapidly during World War II, but as high-performance fighters such as the Lockheed P-38 Lightning developed the capability of dive speeds approaching Mach 1, they began to encounter difficulties. Shock-wave, or "compressibility," effects could cause severe stability and control problems and had led to the in-flight break-up of numerous aircraft. Many people began to believe that supersonic flight was an impossibility.

Although numerous researchers across the country agreed on the need for such an aircraft, they did not all agree on its design. Stack and other NACA engineers, along with the U.S. Navy, favored a jet-powered plane, while the Army Air Forces (AAF) wanted to pursue a rocket-powered design. As a compromise, the researchers decided on a two-pronged approach to their research plane. The AAF and NACA teamed up with Bell Aircraft to build three models of the X-1 rocket aircraft, while the Navy and NACA worked with the Douglas Aircraft Company to create the D-558-1 jet-powered Skystreak. The Skystreak's performance would not be as great as the X-1 design, but a rocket-powered aircraft was seen as a much riskier proposition. The dual approach, therefore, was thought to provide a greater assurance of success in a transonic research program.

The X-1 was modeled after the shape of a bullet, which was the only shape that had been proven capable of stable transonic or supersonic flight. Its four-chamber, 6,000-pound thrust rocket engine would give it a mere 150 seconds of powered flight, which led to the decision to air-launch the aircraft from a specially modified Boeing B-29 Superfortress. In December 1945, only nine months after Bell Aircraft received an Army contract to build the plane, the first X-1 rolled out of the factory.

There were five versions of the Bell X-1 rocket-powered research aircraft that flew at the NACA High-Speed Flight Research Station, Edwards, California. The bullet-shaped X-1 aircraft were built by Bell Aircraft Corporation, Buffalo, N.Y. for the U.S. Army Air Forces (after 1947, U.S. Air Force) and the NACA.

The X-1 Program was originally designated the XS-1 for EXperimental Sonic. The X-1's mission was to investigate the transonic speed range (speeds from just below to just above the speed of sound) and, if possible, to break the "sound barrier." Three different X-1s were built and designated: X-1-1, X-1-2 (later modified to become the X-1E), and X-1-3. The basic X-1 aircraft were flown by a large number of different pilots from 1946 to 1951.

The X-1 Program not only proved that humans could go beyond the speed of sound, it reinforced the understanding that technological barriers could be overcome. The X-1s pioneered many structural and aerodynamic advances including extremely thin, yet extremely strong wing sections; supersonic fuselage configurations; control system requirements; powerplant compatibility; and cockpit environments. The X-1 aircraft were the first transonic-capable aircraft to use an all-moving stabilizer. The flights of the X-1s opened up a new era in aviation.

The X-1 aircraft were almost 31 feet long and had a wingspan of 28 feet. The X-1 was built of conventional aluminum stressed-skin construction to extremely high structural standards. The X-1E was also 31 feet long but had a wingspan of only 22 feet, 10 inches. It was powered by a Reaction Motors, Inc., XLR-8-RM-5, four-chamber rocket engine. As did all X-1 rocket engines, the LR-8-RM-5 engine did not have throttle capability, but instead, depended on ignition of any one chamber or group of chambers to vary speed.

The first X-1 was air-launched unpowered from a Boeing B-29 Superfortress on Jan. 25, 1946. Powered flights began in December 1946. On Oct. 14, 1947, the X-1-1, piloted by Air Force Captain Charles "Chuck" Yeager, became the first aircraft to exceed the speed of sound, reaching about 700 miles per hour (Mach 1.06) and an altitude of 43,000 feet.

The X-1E was used to obtain in-flight data at twice the speed of sound, with particular emphasis placed on investigating the improvements achieved with the high-speed wing. These wings, made by Stanley Aircraft, were only 3 3/8-inches thick at the root and had 343 gauges installed in them to measure structural loads and aerodynamic heating. The X-1E used its rocket engine to power it up to a speed of 1,471 miles per hour (Mach 2.24) and to an altitude of 73,000 feet. Like the X-1 it was air-launched.

X-2 through X-5

The Douglas D-558-1 Skystreak and D-558-2 Skyrocket were, with the Bell XS-1, were the earliest transonic research aircraft built in this country to gather data so the aviation community could understand what was happening when aircraft approached the speed of sound. In the early 1940s, fighter (actually, in the terms of the time, pursuit) aircraft like the P-38 Lightning were approaching these speeds in dives and either could not get out of the dives before hitting the ground or were breaking apart from the effects of compressibility - increased density and disturbed airflow as the speed approached that of sound and created shock waves.

At this time, aerodynamicists lacked accurate wind - tunnel data for the speed range from roughly Mach 0.8 to 1.2 (respectively, 0.8 and 1.2 times the speed of sound, so named in honor of Austrian physicist Ernst Mach, who - already in the second half of the 19th century - had discussed the speed of a body moving through a gas and how it related to the speed of sound). To overcome the limited knowledge of what was happening at these transonic speeds, people in the aeronautics community - especially the NACA, the Army Air Forces (AAF - Air Force after 1947), and the Navy - agreed on the need for a research airplane with enough structural strength to withstand compressibility effects in this speed range. The AAF preferred a rocket - powered aircraft and funded the XS-1 (eXperimental Supersonic, later shortened to simply X), while the NACA and Navy preferred a more conservative design.

The flight research took place at the Muroc Army Air Field, with participation from a NACA contingent under Walter C. Williams that became the core of the later NASA Dryden Flight Research Center. While the D-558 with its jet engine was slower and less glamorous than the rocket-powered, air-launched XS-1, it flew for longer durations and thus gathered a lot of data more easily than its Bell counterpart. The D-558-2 was variously configured with jet and rocket engines, conventional takeoffs and air launchings. But the rocket-powered D-558-2 number 2 became the first aircraft to reach Mach 2.

All three of the Skyrockets had a height of 12 feet 8 inches, a length of 42 feet, and 35-degree swept wings with a span of 25 feet. Until configured for air launch, NACA 143 featured a Westinghouse J34-40 turbojet engine rated at 3,000 pounds of static thrust. It carried 260 gallons of aviation gasoline and weighed 10,572 pounds at take-off. NACA 144 (and NACA 143 after modifications in 1955) was powered by an LR-8-RM-6 rocket engine rated at 6,000 pounds of static thrust. Its propellants were 345 gallons of liquid oxygen and 378 gallons of diluted ethyl alcohol. In its launch configuration, it weighed 15,787 pounds. NACA 145 had both an LR-8-RM-5 rocket engine rated at 6,000 pounds of thrust and a Westinghouse J34-40 turbojet engine rated at 3,000 pounds of static thrust. It carried 170 gallons of liquid oxygen, 192 gallons of diluted ethyl alcohol, and 260 gallons of aviation gasoline for a launch weight of 15,266 pounds.

The three X-1s and the D-558 were, in a sense, the first generation of research aircraft planned by NACA and the military. The second generation was not far behind--in fact, follow-on aircraft were already in the planning stages before the X-1 even reached powered flight. configured the airplane for air-launch instead of ground take-off. The Army Air Forces and NACA also signed an agreement in February 1947 detailing a joint effort for additional research aircraft, designated the X-2, the X-3, the X-4 and the X-5.

The goals of this multi-aircraft flight research effort were twofold. The derivative versions of the X-1, as well as the X-2 and the D-558-2, were built to explore higher speeds and altitudes, both to help manufacturers build aircraft that could operate in that realm and to provide information useful for future space flight. The X-3, X-4, and X-5, as well as the delta-wing XF-92A, explored the behavior of various configurations in the transonic range.

The X-2 (Starbuster) The X-2 was a swept-wing, rocket-powered aircraft designed to fly faster than Mach 3 (three times the speed of sound). It was built for the U.S. Air Force by the Bell Aircraft Company, Buffalo, New York. These were constructed of K-monel (a copper and nickel alloy) for the fuselage and stainless steel for the swept wings and control surfaces. The aircraft had ejectable nose capsules instead of ejection seats because the development of ejection seats had not reached maturity at the time the X-2 was conceived. The X-2 ejection canopy was successfully tested using a German V-2 rocket. The X-2 used a skid-type landing gear to make room for more fuel. The airplane was air launched from a modified Boeing B-50 Superfortress Bomber.

The X-2 was, in a sense, a third generation research aircraft, designed to go further in investigating problems of aerodynamic heating as well as stability and control by operating at speeds of Mach 3 and at altitudes between 100,000 and 130,000 feet. To make the plane more heat-resistant, the X-2 was made of stainless steel and a nickel alloy. Its 15,000-pound-thrust Curtiss-Wright rocket engine also had more than twice the thrust of the X-1 family engine.

Unfortunately, the X-2's research career was destined to be short. The first X-2 exploded during Bell Aircraft's initial flight testing of the airplane. The explosion occurred while the X-2 was attached to its B-50 launch plane, resulting in the death of not only the X-2 pilot but one of the B-50 crew members as well. The second X-2 made its first Air Force powered flight in November 1955. Its performance was, in fact, impressive, and on its 12th powered flight, Air Force Captain Iven C. Kincheloe took it higher than anyone had ever flown. His flight to approximately 126,000 feet prompted Popular Science to dub Kincheloe "First of the Spacemen." Yet on its very next flight, the last Air Force flight before turning the plane over to NACA for its more thorough research program, tragedy struck. Captain Milburn G. Apt, flying his very first rocket flight, took the X-2 to a record speed of Mach 3.2, or 2,094 miles per hour. But as he turned back to the base, the X-2 went out of control and began spinning. The X-2 had been designed with a jettisonable nose, which was supposed to protect the pilot until he reached a speed slow enough for a normal bail out. But when Apt jettisoned the nose cone, the shock knocked him unconscious. He came to in time to jettison the canopy but was unable to bail out before the cockpit section crashed into the desert.

The accident ended the X-2 research program, but it did lead to a couple of changes in the X-15 program that followed. First, the idea of a jettisonable cockpit was abandoned in favor of an ejection seat. Second, a possible factor in the X-2 accident was thought to be Apt's cockpit instruments. Some researchers thought Apt might have believed he was going slower than he really was, leading him to initiate a turn sooner than he should have. As a result, the X-15 was equipped with a gyro-stabilized inertial navigation system (INS) and flight instrumentation that would give the pilot much more precise and accurate flight information.

The second and third generation rocket planes had produced some valuable information about flight at high speeds and altitudes. But it had come at a cost. So it was against a mixed background of triumphant records and tragic failures that the NACA flight research team at Dryden began working on the X-15--a program that aimed to achieve not only what the early rocket planes had left undone but also goals two or three times as high.

The Douglas X-3, known as the Stiletto, was built to investigate the design of an aircraft suitable for sustained supersonic speeds. The X-3 was intended for sustained flight research above Mach 2, but was hampered by use of underpowered Westinghouse J34 turbojet engines which could not power the aircraft past Mach 1 in level flight. The highly instrumented X-3 was able to give engineers their first detailed data and analysis of the dynamics, and therefore the cause, of the inertial coupling problem. As a result, NACA advised North American Aviation to extend the wingspan and increase the vertical tail surface of the F-100 design. The modifications turned the F-100A into a highly effective supersonic fighter, and the knowledge gained through the X-3 flights and the F-100 experience has been applied in one form or another to virtually every supersonic fighter built since then.

The X-3 had, perhaps, the most highly refined supersonic airframe of its day as well as other important advances including one of the first machined structures. It included the first use of titanium in major airframe components. Its long fuselage gave the Stiletto a high-fineness ratio and a low-aspect ratio (the ratio of the wings span to its chord; in other words, it was short and stubby). Despite this refined configuration, the maximum speed it attained was Mach 1.21, during a dive. The general consensus was that the aircraft was sluggish and extremely underpowered. The X-3 also demonstrated coupling instability during abrupt rolling maneuvers, which could cause it to go wildly out of control, as happened on a flight on Oct. 27, 1954, with National Advisory Committee for Aeronautics (NACA) pilot Joe Walker at the controls.

The Northrop X-4, Bantam, was a single-place, swept-wing, semi-tailless airplane designed and built to investigate that configuration at transonic speeds (defined as speeds just below and just above the speed of sound, but in this case, the testing was done primarily at just below the speed of sound). The hope of some aerodynamicists was that eliminating the horizontal tail would also do away with stability problems at transonic speeds resulting from the interaction of supersonic shock waves from the wings and the horizontal stabilizers.

The X-4, for example, was a semi-tailless design similar to the D.H. 108 Swallow that had broken apart while trying to reach supersonic flight in 1946. The X-4 was a twin jet, swept wing aircraft built by Northrop, which had also designed a "flying wing" bomber prototype for the Air Force. Not surprisingly, the X-4, which had a vertical but no horizontal stabilizer, used the flying wing's concept of a combination elevator/aileron called an "elevon" to control its pitch and roll.

The X-4 was something of a maintenance nightmare, but it did accomplish some useful research. For one thing, flights using the X-4's large speed brakes were able to gather data about the flight characteristics of an aircraft with a low lift/drag ratio that helped the X-15 research program. The airplane also made it clear to designers that the X-4 configuration, which was modeled after not only the Swallow but also the Messerschmidt Me-163 rocket plane, was totally unsuitable for transonic or supersonic flight. Like the Swallow, the X-4 experienced severe oscillations about all three axes as it approached Mach 0.9. Increasing the thickness of the elevon trailing edges helped somewhat, but the problem could not be completely alleviated. Nevertheless, the X-4 supported General Jimmy Doolittle's assertion that "in the business of learning how to fly faster, higher, and farther, it is sometimes very important to learn what won't work."

X-15 and the Edge of Space

An unofficial motto of flight research in the 1940s and 1950s was "higher and faster." By the late 1950s the last frontier of that goal was hypersonic flight (Mach 5+) to the edge of space. It would require a huge leap in aeronautical technology, life support systems and flight planning. The North American X-15 rocket plane was built to meet that challenge. It was designed to fly at speeds up to Mach 6, and altitudes up to 250,000 ft. The aircraft went on to reach a maximum speed of Mach 6.7 and a maximum altitude of 354,200 ft. Looking at it another way, Mach 6 is about one mile per second, and flight above 265,000 ft qualifies an Air Force pilot for astronaut wings.

The X-15 was a rocket-powered aircraft 50 ft long with a wingspan of 22 ft. It was a missile-shaped vehicle with an unusual wedge-shaped vertical tail, thin stubby wings, and unique side fairings that extended along the side of the fuselage. The X-15 weighed about 14,000 lb empty and approximately 34,000 lb at launch. The XLR-99 rocket engine, manufactured by Thiokol Chemical Corp., was pilot controlled and was capable of developing 57,000 lb of thrust.

North American was also forging new ground with the X-15 airframe. The structure of the X-15 had to withstand forces up to 7 Gs, and the friction generated by its high speed was expected to create temperatures on the airframe as high as 1,200 degrees Fahrenheit. That was beyond the tolerance of any aircraft material used up until that time, including stainless steel. So North American built the X-15 out of a new, heat-resistant nickel alloy called Inconel X. The X-15 also incorporated rocket engine-powered reaction controls and was outfitted with 1,300 pounds of instrumentation, including no fewer than 1,100 sensors.

The broad flight plan of the X-15, inside and outside the atmosphere, created a real challenge for the X-15's designers. Just as an example, the broad speed range of the X-15 led them to put three control sticks in the cockpit. A conventional center stick was used at slower speeds, and a right-hand side stick was used for high-G maneuvering when it was critical not to over-control the plane. A left-hand side stick operated the reaction controls when the aircraft was outside the Earth's denser atmosphere.

The X-15 research aircraft was developed to provide in-flight information and data on aerodynamics, structures, flight controls, and the physiological aspects of high-speed, high-altitude flight. A follow-on program used the aircraft as a testbed to carry various scientific experiments beyond the Earth's atmosphere on a repeated basis.

For flight in the dense air of the usable atmosphere, the X-15 used conventional aerodynamic controls such as rudders on the vertical stabilizers to control yaw and movable horizontal stabilizers to control pitch when moving in synchronization or roll when moved differentially. For flight in the thin air outside of the appreciable Earth's atmosphere, the X-15 used a reaction control system. Hydrogen peroxide thrust rockets located on the nose of the aircraft provided pitch and yaw control. Those on the wings controlled roll.

Because of the large fuel consumption, the X-15 was air launched from a B-52 aircraft at 45,000 ft and a speed of about 500 mph. Depending on the mission, the rocket engine provided thrust for the first 80 to 120 sec of flight to accelerate to anywhere between Mach 2 and Mach 6 while climbing as high as 350,000 feet, execute a successful hypersonic reentry through Earth's atmosphere. The remainder of the normal 10 to 11 min. flight was powerless and ended with a glide back to a 200-miles-per-hour, unpowered landing on a dry lakebed.

Tracking an aircraft traveling 6,600 feet per second was also a new challenge for NASA and the Air Force. A special flight corridor, known as the "High Range," was created for the X-15 flights. It measured 485 miles long and 50 miles wide and stretched from Wendover, Utah, to Edwards Air Force Base. In addition, radar tracking and telemetry sites capable of receiving 600,000 pieces of information a minute were set up at Beatty and Ely, Nevada, as well as at Edwards, to provide continuous coverage. The route was also structured to follow a string of dry lakes from the Wendover launch point back to Edwards so the X-15 pilots would always have an emergency landing field within reach. Generally, one of two types of X-15 flight profiles was used; a high-altitude flight plan that called for the pilot to maintain a steep rate of climb, or a speed profile that called for the pilot to push over and maintain a level altitude.

The X-15 was flown over a period of nearly 10 years, from June 1959 to Oct. 1968, and set the world's unofficial speed and altitude records of 4,520 mph (Mach 6.7) and 354,200 ft in a program to investigate all aspects of piloted hypersonic flight. As with any experimental program there were several incidents and one fatality. Air Force pilot Mike Adams, on a 1967 flight that reached Mach 5.2 and an altitude of 266,000 feet, was distracted by a malfunctioning experiment and apparently misread a cockpit instrument, causing him to slip the X-15 sideways as it was approaching reentry to Earth's atmosphere. At that speed and altitude there is little margin for error, and the X-15 went out of control and broke apart. The death of Adams was a tremendous blow to the X-15 project team, and some people who worked on the program attribute the end of the program a year later in part to that tragic accident.

The main research goals of the X-15 were to investigate aerodynamic forces, heating, stability and control (including reaction controls), reentry characteristics, and human physiology at extremely high speeds and altitudes. Accomplishing this research was particularly difficult, not only because it required flying far beyond any condition or speed anyone had attempted before, but also because it required operating an aircraft throughout an incredibly wide envelope. Actually, the X-15 proved a whole lot more than that. In fact, it has been described as one of the most successful flight research programs ever conducted. In almost ten years and 199 flights, it produced no fewer than 750 research papers and reports on a broad range of aeronautics and aerospace topics and made more than two dozen significant contributions to future flight both within and outside the Earth's atmosphere.

The research that produced these monumental results fell into three major categories: exploring the upper boundaries of flight speeds and altitudes, filling in the area within those boundaries with additional information, and doing "piggyback" experiments that used the X-15's speed and altitude capabilities to conduct research unrelated to the X-15 itself.

The X-15 program also produced a tremendous amount of information about hypersonic and exoatmospheric flight. Perhaps most importantly, it demonstrated that a high-performance reusable vehicle could be successfully flown by a pilot outside Earth's atmosphere, brought through reentry, and returned to an unpowered landing. In the process, the X-15 gave researchers a much clearer picture of the combined stress of aerodynamic loads and heating in a hypersonic, high-dynamic-pressure environment.

In addition, the X-15 led to the development of numerous technologies blunt-ended, wedge-shaped tail was found to solve directional stability problems at hypersonic speeds. The X-15 also led to the development of the first practical full-pressure suit for protecting a pilot in space and to a high-speed ejection seat. It successfully tested a "Q-ball" nose-cone air-data sensor, an inertial flight data system capable of functioning in a highly dynamic pressure environment, and the first application of energy management techniques.

The X-15 pilots also successfully demonstrated the use of reaction controls outside the Earth's atmosphere. Reaction controls were small rocket-powered jets placed strategically in the aircraft's wingtips and nose that could be fired to control the plane even when thin air rendered its aerodynamic flight controls useless. The idea grew out of the stability problems experienced with the X-1A at high altitude and were initially researched using one of Dryden's F-104s, but reaction controls were a critical technology for not only the X-15, but also the Mercury capsule, the Apollo Lunar Landing Module, and every piloted craft to ever fly in space. The Mercury capsule also used a variation of the X-15's controls, including the side-stick controller, on its orbital missions.

The X-15 flights also revealed an interesting physiological phenomenon that indicated just how difficult the pilots' job was and provided a baseline for monitoring the health of future astronauts. The heart rate of the X-15 pilots (and, in fact, the astronauts that followed) during their missions ranged between 145 and 180 beats a minute instead of a more typical 70-80. Aeromedical researchers found that the high pulse rates were not due to the physical stress of the pilots' environment, but to the psychological keyed-up, highly-focused state the missions required of them.

The third phase of the X-15 program yielded many other valuable contributions, including measurements of the sky brightness and atmospheric density, data from micrometeorites collected in special wing-tip pods, and an opportunity to explore Earth-resources photography. The X-15 also tested a number of prototype systems that were subsequently used in the Apollo program. For example, the aircraft tested the insulation later used on the Apollo program's Saturn booster rockets, and the X-15 pilots tested horizon-measuring instrumentation that aided development of navigation equipment for the Apollo capsule.

The X-15 program made many accomplishments, some of which include:

  1. First use of a full-pressure suit for spaceflight.
  2. First use of reaction controls for maneuvering in space.
  3. First use of a flight control system that automatically blended aerodynamic and reaction controls.
  4. Development of thermal protection for hypersonic reentry.
  5. Development of the first large, restartable, and throttleable rocket engine.
  6. Development of inertial flight data systems capable of functioning in a high-dynamic-pressure and space environment.
  7. Demonstration of a pilot's ability to operate in "micro-gravity".
  8. Demonstration of the first piloted reentry-to-landing from space.
  9. Acquisition of hypersonic acoustic measurements, which influenced structural design criteria for Mercury capsule.
  10. Verification of the validity of hypersonic wind tunnel data, which were later used in the design of the Space Shuttle.
Some of the biggest benefits reaped by the space program from the X-15 and other rocket aircraft efforts, however, did not come from tangible pieces of hardware or technology but from the intangible assets of people and experience. Since the Mercury spacecraft was being developed during the early stages of the X-15 research program, the aircraft had a somewhat limited impact on the design of the Mercury capsule. But the success of the X-15 flights provided the Mercury program managers with a level of confidence that was tremendously valuable. Furthermore, a number of the people at Dryden who had been involved with the rocket-powered X-planes and the X-15 went on to assume key leadership positions in the space program. Walt Williams, for example, became the operations director of the Project Mercury and Gemini Programs. And NACA research pilot Neil Armstrong, who had evaluated the use of reaction controls with both the F-104 and the X-15, went on to apply his knowledge to the Apollo program, hand-flying the Lunar Landing Module to the first landing on the moon in July 1969.

Topical Questions:

  • Why does the speed of sound have anything to do with space travel?
  • What were the lessons learned from the X-plane program?
  • What was the X-15, what was the goal of the program?
  • What were the achievements of the X-15 program with respect to space flight?