stages would be key. Every pound used in the stage or its fuel would mean that much less cargo, given any particular booster (first stage). Since it was the power-to-weight ratio that they needed, upper stages based on liquid hydrogen seemed to be the only way forward – the light weight of the fuel makes up for any difficulty handling it. The Saturn proposal had always included such a stage for orbital insertion, the Centaur, a hydrogen-burning stage derived from the Atlas ICBM. For the intermediate stages the designers had somewhat more flexibility. The Committee members outlined a number of possible solutions grouped into three different classes: class "A," class "B," and class "C." Common among all three classes, with the exception of the proposed C-3, was the new first stage consisting of a cluster of eight H-1 engines attached to the Jupiter/Redstone tank cluster, which would become the S-I stage, as well as the two engine Centaur upper stage. The class "A" designs were the low-risk solutions; von Braun's current design became the A-1, consisting of a Titan I second stage between the S-I first stage and Centaur third stage. The A-2 replaced the second stage of the A-1 with a cluster of Thor IRBMs. Though the class "A" vehicles would have had the earliest flight availability due to the utilization of existing hardware, they failed to meet the first two mission for the Saturn rocket. Additionally, the 120 inch upper stages posed a potential structural weakness, and the proposed 160 inch upgrade would limit growth potential, violating fourth request of the original directive. The single class "B" design considered by the committee, the B-1, consisted of a four stage design with the aforementioned S-I first stage and Centaur fourth stage. The second stage would be an all-new 220 inch LOX/RP-1 design using four of the H-1 engines used by the first stage, along with a new four-engine third stage derived from Centaur but with a 220 inch diameter. Though the B-1 vehicle met the mission requirements, it would have been too costly and taken too much time to develop the new second stage. The class "C" designs used liquid hydrogen in all upper stages. C-1 would consist of the existing S-I booster, a new Douglas Aircraft 220 inch S-IV stage powered by four upgraded versions of the Centaur engines with to thrust per engine, and a modified Centaur