In September 1967, Owen Maynard of the Manned Spacecraft Center in Houston, Texas proposed a series of Apollo missions that would lead up to a manned lunar landing. Seven mission types were outlined, each testing a specific set of components and tasks, and each previous step would need to be completed successfully before the next mission type could be undertaken. These were:
The planned sequence was changed when it became clear the first manned Lunar Module, LM-3, would not be ready in time for the December 1968 launch date of Apollo 8, so Apollo 8 was flown as a lunar orbital mission using just the CSM (sometimes referred to as "the C Prime mission") and the E mission was canceled.
F - Manned CSM and LM operations in lunar orbit, a "dress rehearsal" for the first landing (Apollo 10)
I - long duration CSM lunar orbital surveys using a Scientific Instrument Module mounted in an empty Service Module bay. These were incorporated into the J missions.
J - longer three-day stays using an Extended LM, with three LEVAs and a Lunar Roving Vehicle (Apollo 15, Apollo 16, Apollo 17). Apollo 18 to 20 would have been J missions. Apollo 15 was originally planned as an H mission but was promoted to J as the program was curtailed.