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.