First published in 2003 as a volume in the NASA "Monograph in Aerospace History" series. This study contains photographs and illustrations.

For a while, it seemed the series of experimental aircraft sponsored by the U. S. government had run its course. Between the late 1940s and the late 1970s, almost thirty designations had been allocated to aircraft meant to explore new flight regimes or untried technologies. Then, largely, it ended. But there was a resurgence in the mid- to late1990s, and as we enter the fourth year of the new millennia, the designations are up to X-50. Many have a misconception that X-vehicles have always explored the high-speed and high-altitude flight regimes-something popularized by Chuck Yeager in the original X-1 and the exploits of the twelve men that flew the X-15. Although these flight regimes have always been in the spotlight, many others have been explored by X-vehicles. The little Bensen X-25 never exceeded 85 mph, and others were limited to speeds of several hundred mph. There has been some criticism that the use of X designations has been corrupted somewhat by including what are essentially prototypes of future operational aircraft, especially the two JSF demonstrators. But this is not new-the X-11 and X-12 from the 1950s were going to be prototypes of the Atlas intercontinental ballistic missile, and the still-born Lockheed X-27 was always intended as a prototype of a production aircraft. So although this practice does not represent the best use of "X" designations, it is not without precedent.

This study represents a means of highlighting the myriad of technological developments that made possible the safe reentry and return from space and the landing on Earth. This story extends back at least to the work of Walter Hohmann and Eugen Sanger in Germany in the 1920s and involved numerous aerospace engineers at the National Advisory Committee for Aeronautics (NACA)/NASA Langley and the Lewis (now the John H. Glenn Research Center at Lewis Field) and Ames Research Centers. For example, researchers such as H. Julian Allen and Alfred J. Eggers, Jr., at Ames pioneered blunt-body reentry techniques and ablative thermal protection systems in the 1950s, while Francis M. Rogallo at Langley developed creative parasail concepts that informed the development of the recovery systems of numerous reentry vehicles. The chapters that follow relate in a chronological manner the way in which NASA has approached the challenge of reentering the atmosphere after a space mission and the technologies associated with safely dealing with the friction of this encounter and the methods used for landing safely on Earth.

For a while, it seemed the series of experimental aircraft sponsored by the U.S. government had run its course. Between the late 1940s and the late 1970s, almost thirty designations had been allocated to aircraft meant to explore new flight regimes or untried technologies. Then, largely, it ended. But there was a resurgence in the mid- to late- 1990s, and as we enter the fourth year of the new millennia, the designations are up to X-50.

First published in 2003 as a volume in the NASA "Monograph in Aerospace History" series. This study contains photographs and illustrations.

OVERVIEW: This 700+ page book is part of the NASA HISTORY SERIES. A history of the design and achievements of the high-speed, 1950s-era X-15 airplane. FROM THE FORWARD BY WILLIAM H. DANA: The X-15 was an airplane of accelerations. When an X-15 pilot looks back on his X-15 flights, it is the accelerations he remembers. The first of these sensations was the acceleration due to B-52 lift, which held the X-15 at launch altitude and prevented it from falling to Earth. When the X-15 pilot hit the launch switch, the B-52 lift was no longer accessible to the X-15. The X-15 fell at the acceleration due to Earth's gravity, which the pilot recognized as "free fall" or "zero g." Only when the pilot started the engine and put some "g" on the X-15 was this sensation of falling relieved. The next impression encountered on the X-15 flight came as the engine lit, just a few seconds after launch. A 33,000-pound airplane was accelerated by a 57,000-lbf engine, resulting in a chest-to-back acceleration of almost 2 g. Then, as the propellant burned away and the atmosphere thinned with increasing altitude, the chest-to-back acceleration increased and the drag caused by the atmosphere lessened. For a standard altitude mission (250,000 feet), the weight and thrust were closer to 15,000 pounds and 60,000-lbf at shutdown, resulting in almost 4-g chest-to-back acceleration. The human body is not stressed for 4 g chest to back, and by shutdown the boost was starting to get a little painful. Milt Thompson once observed that the X-15 was the only aircraft he had ever flown where he was glad when the engine quit. On a mission to high altitude (above 250,000 feet), the pilot did not regain any sensible air with which to execute a pullout until about 180,000 feet, and could not pull 1 g of lift until 130,000 feet. Flying a constant angle of attack on reentry, the pilot allowed g to build up to 5, and then maintained 5 g until the aircraft was level at about 80,000 feet. There was a deceleration from Mach 5 at 80,000 feet to about Mach 1 over the landing runway, and the pilot determined the magnitude of the deceleration by the use of speed brakes. This ended the high-g portion of the flight, except for one pilot who elected to start his traffic pattern at 50,000 feet and Mach 2, and flew a 360-degree overhead pattern from that starting point. Flight to high altitude represented about two-thirds of the 199 X-15 flights. Flights to high speed or high dynamic pressure accounted for the other third, and those flights remained well within the atmosphere for the entire mission. The pilot of a high-speed flight got a small taste of chest-to-back acceleration during the boost (thrust was still greater than drag, but not by such a large margin as on the high-altitude flights). The deceleration after burnout was a new sensation. This condition was high drag and zero thrust, and it had the pilot hanging in his shoulder straps, with perspiration dripping off the tip of his nose onto the inside of his face plate. Milt Thompson collected anecdotes about the X-15 that remain astonishing to this day. Milt noted that at Mach 5, a simple 20-degree heading change required 5 g of normal acceleration for 10 seconds. Milt also pointed out that on a speed flight, the (unmodified) X-15-1 accelerated from Mach 5 to Mach 6 in six seconds. These were eye-opening numbers at the time of the X-15 program. Those of us in the program at flight 190 thought that the X-15 would continue indefinitely. Then, on flight 191, Major Michael J. Adams experienced electrical irregularities that made the inertial flight instruments unreliable and may have disoriented him. In any case, at peak altitude (266,000 feet), the X-15 began a yaw to the right. It reentered the atmosphere, yawed crosswise to the flight path, and went into a high-speed spin. It eventually came out of the spin but broke up during the reentry, killing the pilot.

For a while, it seemed the series of experimental aircraft sponsored by the U.S. government had run its course. Between the late 1940s and the late 1970s, almost thirty designations had been allocated to aircraft meant to explore new flight regimes or untried technologies. Then, largely, it ended. But there was a resurgence in the mid- to late1990s, and as we enter the fourth year of the new millennia, the designations are up to X-50. Many have a misconception that X-vehicles have always explored the high-speed and high-altitude flight regimes---something popularized by Chuck Yeager in the original X-1 and the exploits of the twelve men that flew the X-15. Although these flight regimes have always been in the spotlight, many others have been explored by X-vehicles. The little Bensen X-25 never exceeded 85 mph, and others were limited to speeds of several hundred mph. There has been some criticism that the use of X designations has been corrupted somewhat by including what are essentially prototypes of future operational aircraft, especially the two JSF demonstrators. But this is not new---the X-11 and X-12 from the 1950s were going to be prototypes of the Atlas intercontinental ballistic missile, and the still-born Lockheed X-27 was always intended as a prototype of a production aircraft. So although this practice does not represent the best use of "X" designations, it is not without precedent.

It is a beginning. Over forty-five years have elapsed since the X-15 was conceived; 40 since it first flew. And 31 since the program ended. Although it is usually heralded as the most productive flight research program ever undertaken, no serious history has been con-assembled to capture its design, development, operations, and lessons. This monograph is the first step towards that history. Not that a great deal has not previously been written about the X-15, because it has. But most of it has been limited to specific aspects of the program; pilot's stories, experiments, lessons-learned, etc. But with the exception of Robert S. Houston's history published by the Wright Air Development Center in 1958, and later included in the Air Force History Office's Hypersonic Revolution, no one has attempted to tell the entire story. And the WADC history is taken entirely from the Air Force perspective, with small mention of the other contributors. In 1954 the X-1 series had just broken Mach 2.5. The aircraft that would become the X-15 was being designed to attain Mach 6, and to fly at the edges of space. It would be accomplished without the use of digital computers, video teleconferencing, the internet, or email. It would, however, come at a terrible financial cost-over 30 times the original estimate. The X-15 would ultimately exceed all of its original performance goals. Instead of Mach 6 and 250,000 feet, the program would record Mach 6.7 and 354,200 feet. And compared against other research (and even operational) aircraft of the era, the X-15 was remarkably safe. Several pilots would get banged up; Jack McKay seriously so, although he would return from his injuries to fly 22 more X-15 flights.Tragically, Major Michael J. Adams would be killed on Flight 191, the only fatality of the program. Unfortunately due to the absence of a subsequent hypersonic mission, aeronautical applications of X-15 technology have been few. Given the major advances in materials and computer technology in the 30 years since the end of the flight research program, it is unlikely that many of the actual hardware lessons are still applicable. That being said, the lessons learned from hypersonic modeling, simulation, and the insight gained by being able to evaluate actual X-15 flight research against wind tunnel and predicted results, greatly expanded the confidence of researchers. This allowed the development of Space Shuttle to proceed much smoother than would otherwise have been possible. In space, however, the X-15 contributed to both Apollo and Space Shuttle. It is interesting to note that when the X-15 was conceived, there were many that believed its space-oriented aspects should be removed from the program since human space travel was postulated to be many decades in the future. Perhaps the major contribution was the final elimination of a spray-on ablator as a possible thermal protection system for Space Shuttle. This would likely have happened in any case as the ceramic tiles and metal shingles were further developed, but the operational problems encountered with the (admittedly brief) experience on X-15A-2 hastened the departure of the ablators.

" Access -- no single word better describes the primary concern of the exploration and development of space. Every participant in space activities -- civil, military, scientific, or commercial -- needs affordable, reliable, frequent, and flexible access to space. To Reach the High Frontier details the histories of the various space access vehicles developed in the United States since the birth of the space age in 1957. Each case study has been written by a specialist knowledgeable about the vehicle described and places each system in the larger context of the history of spaceflight. The technical challenge of reaching space with chemical rockets, the high costs associated with space launch, the long lead times necessary for scheduling flights, and the poor reliability of the rockets themselves show launch vehicles to be the space program's most difficult challenge.