In 1957, when Russia launched the first satellite, the ability of the United States to respond depended on one small launch vehicle still under development, Vanguard, and modifications to ballistic missiles. The subsequent space race featured a rapid buildup of launch vehicle capability in this country during the 1960s, culminating with the giant Saturn V which launched the Apollo lunar expeditions beginning in 1968. A significant part of the increased launch capability resulted from technical decisions made in 1958 and 1959 to use liquid hydrogen in the upper stages of the Centaur and Saturn vehicles-and that story is not well known. The decision to use liquid hydrogen in developing the nation's largest launch vehicle was particularly bold, for many experienced engineers doubted the advisability of using a highly hazardous fuel associated with the Hindenburg disaster of 1937, a gas difficult to liquefy, a liquid so cold-close to absolute zero-that storage and handling are difficult, and so light-1/14 the density of water-that large tank volumes are required, with attendant problems of vehicle mass and drag. Hydrogen had been considered in astronautics and aeronautics several times before; but in each case, as the problems became better known, the attempt was abandoned, What was different in this case? Why was there so much confidence about hydrogen within the young space agency to warrant risking the success of the nation's manned spaceflight program? The decision, of course, turned out to be the right one. Subsequent advancements in the technologies of liquefying, storing, transporting, and using large quantities of liquid hydrogen made it just another flammable liquid that could be handled and used safely with reasonable caution. The key role that liquid hydrogen played in the success of the Centaur and Saturn launch vehicles has long interested the author. As a participant in research on hydrogen for rockets in the 1950s and a proponent for its use, the author understood the potential as well as the risks and in recent years wanted to investigate more fully the circumstances leading to the 1958 and 1959 decisions. In digging into the background for the decisions and the status of hydrogen technology that influenced those decisions, the question arose: how far back to investigate? The flammability of gaseous hydrogen has been known for centuries; its large heat content was measured in the 18th century; and it was liquefied by Dewar in 1898. Five years later, Tsiolkovskiy, the Russian rocket pioneer, proposed its use in a space rocket, as did Goddard in 1910. In the 1920s, Oberth correctly assessed the advantage of using hydrogen in the upper stages of space vehicles. None of these rocket pioneers experimented with hydrogen; other fuels appeared more attractive in the face of hydrogen's disadvantages, particularly its low density. One German experimenter, Walter Theil, tried to use liquid hydrogen in a small rocket engine a few years before World War II, but numerous leaks and higher priority tasks ended the experiments. The first systematic investigations of liquid hydrogen to propel aircraft and rockets began in the United States in 1945 and although earlier developments undoubtedly had an influence, where the author chose to start this book at that point. In describing the history of rocket technology, it is easy for an engineer-author to become immersed in the technical aspects that may be of little interest to some readers. The author tried to minimize mathematics, technical language, and other specialized details, but some are unavoidable if propulsion research is to be presented fairly and accurately. Adding to this problem has been the conversion of many familiar English units into the metric system. Those accustomed to thinking of rocket performance in terms of specific impulse will not find it here; instead, they will have to settle for its equivalent, exhaust velocity.