As for why the first flight in the Apollo series was Apollo 4: The Grissom crew’s flight was A.S.-204 (fourth Apollo flight on a Saturn II). After the fire, at the widows’ request, “Apollo 1” was reserved for the flight that never took place. Then Low suggested retroactively naming the three unmanned Apollo/Saturn II flights Apollo 1A, Apollo 2, and Apollo 3, respectively. While that was being considered, A.S.-501 was named Apollo 4; subsequently, NASA headquarters decided not to rename the earlier flights after all.]
