cylindrical cargo module would then be inserted hydraulically into the tank through this opening, sealed, and then the tank would be repressurized to form a large living space. Power would be provided by solar cells lining the outside of the S-II. During the 1960s, as the Apollo mission transitioned from development to launch, a number of groups inside NASA were studying the post-Apollo era. Many ideas for continuing use of the existing Saturn hardware were proposed, and some of these were collected under the name "Apollo X", which became the Apollo Applications Program (AAP). By the time AAP started to receive funding, the Saturn V lines were planned to shut down after producing just enough Saturn Vs for the Moon missions alone. However, during the same period of time, on-orbit testing of the Apollo systems was proceeding much better than expected, and a number of proposed shake-down missions were no longer required. This left a small number of Saturn IB launchers available for use. 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. When the later Apollo missions were canceled (18 through 20), a supply of Saturn Vs became available. By this time, so much work had been done on the S-IV-derived system that they decided to continue along those lines instead. The Saturn V delivered enough impulse in its first two stages to place the complete, ground-built station in orbit, and the design was flown as the Skylab "dry workshop". Another project involving the Apollo-derived wet workshop was the proposed Manned Venus Flyby. Shuttle-derived concepts Several similar conversions of the Space Shuttle's external tank (ET) were also studied. During launch the ET accelerated to about 98% of orbital speed before being dropped and deliberately spun in order to increase its drag. A number of proposals suggested keeping the ET attached to the Shuttle all the