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.