During the mid-1950s, the Air Force contracted with the Rocketdyne Division of North American Aviation to study rocket engines larger than those in intercontinental ballistic missiles. This began with the E-1, about three times larger than an ICBM engine, but Rocketdyne believed that an engine with a thrust of 4.5 meganewtons (1 million lb)- over six times larger than an ICBM engine-was feasible. In late 1956, the Air Force's Scientific Advisory Board was even bolder and recommended studies of engines up to 22 meganewtons (5 million lb of thrust). The Air Force, however, believed that such a large thrust was best attained by clustering smaller engines. In mid-1958, the Air Force contracted with Rocketdyne for design studies of the F-1 engine, with a thrust of 4.5 meganewtons. Shortly thereafter, responsibility for developing a large engine was transferred to NASA; in October, NASA opened the competition to other contractors and indicated a preference for 6.7 meganewtons (1.5 million lb of thrust). Rocketdyne won the competition and a development contract was signed early in 1959.
It was the Army, however, which took the initiative in proposing large launch vehicles using E-1 and F-1 engines, beginning with studies in the mid-1950s. In late 1957, the Army missile development tearn, under the technical direction of Wernher von Braun, submitted a national integrated missile and space development program to the Department of Defense. Included was a vehicle with a thrust of 6.7 meganewtons. In early 1958, the National Advisory Committee for Aeronautics formed a vehicle working group as part of a space technology committee. The working group was headed by von Braun and included Abe Silverstein, soon to become the chief planner at the new civilian space agency. The NACA group modified and extended the Army's recommended vehicles and propulsion systems. The favored high-energy propellant combination in both the Army and NACA plans appeared to be hydrazine-fluorine, a choice influenced by an Air Force development contract with Bell Aircraft for a small engine using this combination. In August 1958, the Advanced Research Projects Agency, responsible for planning and coordinating military space missions, ordered the Army to devise a development and funding plan for a large launch vehicle with a first stage using a cluster of existing ICBM engines; this was later to become Saturn I. NASA's request for the transfer of both the large vehicle and the Army's development  team met with strong opposition; an agreement in December 1958 left the Army team intact but responsive to NASA needs.