Wernher von Braun's team at the U.S. Army Ballistic Missile Agency (ABMA) started studying the problem in April 1957. They calculated that a rocket with the required performance would require a lower-stage booster with a thrust of about 1.5 million pound-force (6.7 MN) thrust at takeoff. As it happened, the Air Force had recently started work on just such an engine, eventually emerging as the F-1. But the F-1 would not be available in the time frame that the DoD was demanding and would be limited to about 1 million lbf in the short term anyway. Another possibility was a Rocketdyne engine, then known as the E-1, which provided about , four of which would reach the required thrust levels. This approach became the favorite and was paired with a first stage built from a cluster of nine tanks placed atop a thrust plate where the engines and plumbing would be attached. The design envisaged eight rocket tanks similar to the Redstone stage strapped around a central larger tank derived from a Jupiter rocket. The design and diameter similarities would enable the use of the same tooling and facilities used to produce the older tanks, speeding up the design and production phases of the new stage. Contrary to what was reported to the press at the time (and propagated commonly ever since), the tanks were not simply Redstone and Jupiter tanks, but much longer versions built anew at the same diameter.