The Saturn IB stack consisted of two stages, the booster and an S-IVB stage on top, both of which needed to be fired in order to reach orbit. An S-IVB stage could be modified in a fashion similar to von Braun's original proposals, making a smaller but perfectly usable station. In this case, the equipment would be carried on top of the S-IVB in the location normally reserved for the Lunar Module, but the lack of a large access port meant it would have to remain there instead of being inserted into the tank. Considerable design work along these lines was carried out.