During the 1950s, development of high-energy rockets centered around the choices of hydrazine or ammonia with fluorine, and liquid hydrogen with fluorine or oxygen. Pratt & Whitney, a latecomer torocketengine development, began to study hydrogenfueled rocket engines in early 1956-the same time that the company began development of a hydrogen-fuelled jet engine in the Suntan project. Despite a growing....
 ....interest in high-energy rockets and a specific recommendation in late 1956 by the Air Force's Scientific Advisory Board to develop high-energy rocket engines, little was done until a series of stimuli about a year later: the faltering of the Suntan project, Russian satellite accomplishments, growing awareness of the military advantages of satellites, and the emerging role of the civilian space agency.
The first high-energy stage resulted from the efforts of Krafft Ehricke of General Dynamics-Astronautics. In December 1957, he proposed that the Air Force develop a hydrogen-oxygen upper stage for the Atlas using an engine designed by the Rocketdyne Division of North American Aviation. In the spring of 1958, the Air Force Suntan team proposed a hydrogen-fueled rocket engine using the Suntan experience of Pratt & Whitney and available facilities. Meanwhile, Abe Silverstein, a proponent for using liquid hydrogen in aircraft and rockets who was directly involved in such research since the early 1950s, was brought to Washington to head civilian space planning and projects. In the summer of 1958, spurred by the Air Force hydrogen engine proposal, military space needs, and Silverstein's planning activities, the Advanced Research and Projects Agency ordered Pratt & Whitney to develop a hydrogen-oxygen engine and General Dynamics-Astronautics to develop an upper stage for Atlas using the engine. This was to become the Centaur stage, 'Intended to serve both military and civilian space needs. Despite some military objections, management control of Centaur was transferred to NASA in mid-1959. Silverstein's interest in hydrogen, aided by continued experiments at NASA Lewis laboratory, was a key factor in later decisions involving upper stages for the Saturn launch vehicle.