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 using the same engines as a third stage. The C-1 would become the C-2 upon insertion of a new S-III stage with two new to thrust engines, keeping the S-IV and Centaur on top. The C-3 was a similar adaptation, inserting the S-II stage with four of the same 150-200,000 lbf thrust engines, keeping the S-III and S-IV stages of the C-2, but eliminating the Centaur. The first stage of the C-3 would also be increased to over by either replacing the four center H-1 engines with one F-1 engine, or uprating all eight H-1 engines.