Possible Origin
The Apollo program used a Saturn V to launch the first manned test of the complete Apollo spacecraft, consisting of the Apollo Command/Service Module and a separate Apollo Lunar Module landing craft in a low Earth orbit, on the Apollo 9 mission. However, the original plan for this test would have used two launches of the smaller Saturn IB Earth-orbital launch vehicle: the first would launch the crew in the Command/Service Module, followed shortly by a second, unmanned launch of the Lunar Module, with which the crew would rendezvous in orbit. Some people have speculated that tentative plans of an "Apollo 21" might have been made assuming availability of the extra Saturn V this would have left, although by the time NASA resumed planning manned missions using this numbering sequence (defined on April 24, 1967), the dual-Saturn IB plan had been replaced with the single Saturn V. There is no evidence NASA ever planned an eleventh Moon landing mission.
Read more about this topic: Apollo 21
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