Origins of Centaur
 The first rocket stage to fly using liquid hydrogen and liquid oxygen as propellants was the Centaur stage on top of an Atlas intercontinental ballistic missile. Centaur was the brainchild of Krafft Ehricke. For nearly three decades, Ehricke had prepared himself for the space age; when it dawned with Sputnik, he was ready. Within a month, he proposed a hydrogen-oxygen stage for use with the Atlas missile. Ehricke was able to move rapidly because previous work on the Atlas missile and the ideas of others about hydrogen-oxygen upper stages had laid the groundwork.
Ehricke became a space enthusiast at the age of eleven when he was captivated by Fritz Lang's "Girl in the Moon," shown in Berlin in 1928. Advanced in mathematics and physics for his age, he appreciated the great technical detail that Hermann Oberth had provided to make the film realistic. Young Krafft became acquainted with Tsiolkovskiy's space rocket using hydrogen-oxygen, which he read about in Scherschevsky's Die Rakete fuer Fahrt und Flug. He also tackled Oberth's Wege zur Raumschiffahn in his early teens, but was slowed by the mathematics. Ehricke graduated from the Technical University in Berlin (aeronautical engineering) and took postgraduate courses at the Humboldt University in celestial mechanics and nuclear physics. He was conscripted into the army, served in a Panzer division on the  Russian front during World War II, but was recalled and reassigned to rocket development work at Peenemunde in June 1942. There he came under the strong influence of Walter Thiel, in charge of rocket engine development, who was killed in the first British air raid on Peenemunde in October 1943. Peenemunde, under Maj. Gen. Walter Dornberger and Wernher von Braun, his technical director, had a single purpose-the rapid development of specific weapons-and there was no official tolerance of work not directly related to the main goal. In spite of this, Thiel shared Ehricke's desire to look beyond the immediate future to greater possibilities. Thiel himself drew plans for testing rockets larger than any yet dreamed of-on the order of 5-14 meganewtons (1-3 million lb thrust). He wanted to use natural gorges in Bavaria as testing sites. He talked to Ehricke about resuming his own earlier experiments with liquid hydrogen in small rocket thrust chambers. The experiments of Heisenberg and Pohl with a nuclear reactor using heavy water excited Thiel. When he heard that Heisenberg was planning to operate a turbine with steam heated by the heavy water reactor, Thiel urged Ehricke to study the possibilities of using nuclear energy for propulsion. Ehricke considered several working fluids, but both he and Thiel favored hydrogen and believed it was a fuel with a future.11
As the war was ending, Ehricke helped move Peenemunde records into Bavaria, to keep them out of Russian hands. He made his way on foot to Berlin where he found his wife and went into hiding until the Western Allies moved in. He was located by the U.S. Army, given a six-month contract, and came to the United States to rejoin the von Braun team as part of the Paperclip operation.*
Ehricke and von Braun recalled another time they had considered hydrogen. In 1947, von Braun asked Ehricke to check a report by Richard B. Canright of the Jet Propulsion Laboratory on the relative importance of exhaust velocity and propellant density for rockets of the V-2 size and larger (pp. 47-48). It had caught von Braun's attention because he and two associates had written a paper the previous year which Canright had cited.12 Von Braun had found, under the assumptions of fixed tank volume and a relatively heavy structural mass, that propellants with the highest densities and reasonably high exhaust velocities had the greater ranges. Canright, on the other hand, found that for large rockets and his assumptions (which included a variable tank volume and relatively light structural mass), exhaust velocitv was decidedly more important than density. Canright's analysis showed hydrogen to be superior to other fuels when using the same oxidizer. Both Ehricke and von Braun, familiar with Oberth's case for using hydrogen-oxygen in upper stages of rockets (appendix A-2), agreed that hydrogen had a good potential for certain applications. Practical experience with liquid hydrogen in rockets at that time, however, was still very small and its handling problems large. The Army, for whom von Braun and Ehricke worked, wanted practical propellants that could be stored and handled safely in the field. This convinced von Braun to stick to well tested and denser propellants, but Ehricke felt less restrained and hydrogen's potential remained prominent in his thinking.
 In 1950, Ehricke moved with von Braun to Huntsville, Alabama, but grew restless with both the climate and von Braun's conservative engineering. He joined Walter Dornberger at Bell Aircraft in 1952. Bell, then busy developing the Agena upper-stage also proved to be unable to offer Ehricke the opportunity for a breakthrough he was looking for; and in 1954, when K. J. Bossart of Convair contacted him to work on the Atlas ICBM, he was ready to move. Soon after, the Air Force established the Ballistic Missile Division under Brig. Gen. Bernard Schriever to accelerate development of the Atlas. Schriever insisted on total dedication to the job at hand and Ehricke found himself again in the same atmosphere as at Huntsville and Peenemdnde. In Charlie Bossard, however, Ehricke found a man of kindred spirit who, like Walter Thiel at Peenerrifinde, encouraged him to think beyond the immediate task. To imaginative Ehricke, this attitude and the climate and atmosphere of southern California were heaven.
By 1956, Ehricke was conducting in-house studies of vehicles for orbiting satellites. He approached the Rocketdyne Division of North American Aviation to obtain preliminary design data on various rocket propulsion systems employing turbinedriven pumps. He did not have much luck in getting government interest for his proposals, although he was a passionate believer in space exploration and a very persuasive person. The first week in October 1957, he visited Maj. George Colchagoff at ARDC headquarters. Ehricke had gotten wind of the Suntan project and was hoping to gain support for launching a satellite. It was the austere period under Secretary of Defense Charles Wilson, when "space" was out of favor. Although Colchagoff was receptive and personally convinced of the value of spaceflight, the official position made it difficult for Ehricke to round up support. On the Monday following Sputnik I's flight, however, Ehricke found many who indicated they had always favored spaceflight and now felt free to talk to him.
Excited by the new atmosphere, Ehricke returned to San Diego and began to streamline his plans. A.G. Negro, a Rocketdyne applications engineer, visited Convair on 11 October 1957 and returned with a request for information on a small pressure-fed rocket engine using liquid hydrogen and liquid oxygen. It was to produce 31 kilonewtons (7000 lb of thrust) and be capable of restart at altitude. By the end of October, Negro established the design characteristics of the engine, including a combustion pressure of 4 atmospheres and exhaust velocity of 4030 meters per second.13
In December 1957, General Dynamics-Astronautics submitted a proposal to the Air Force entitled, "A Satellite and Space Development Plan." It was for a four-engine, pressure-fed hydrogen-oxygen stage with each engine developing 31-33 kilonewtons (7000-7500 lb of thrust). According to Ehricke, "The reason why we selected this engine system in teamwork with the Rocketclyne Division of N.A.A. was simply that we wanted to avoid the delay by what we thought would have to be a brand new pump development. We were, for security reasons, not aware of the Pratt & Whitney's pioneer work in this field."14
The Air Force did not, buy the specific General Dynamics-Astronautics proposal, but in the following months activities within the government clearly foreshadowed an emerging space program. Among these, General Dynamics-Astronautics and Pratt & Whitney were brought together by the Air Force and the Advanced Research Projects  Agency and in August 1958 were authorized to proceed with the development of the Centaur stage, the first to use liquid hydrogen and liquid oxygen.
* Ehricke wanted to work for the Americans, and he hid each time someone knocked on his door, waiting for the right caller. One day his wife answered the door and routinely said, "I don't know where he is." As she did so, she recognized the insignia of a U. S. Army officer and immediately began screaming, "He's here! He's here!" Interview, 26 Apr. 1974. Paperclip was the project for bringing German rocket experts to the United States.