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:
- A – Unmanned Saturn V and command and service module (CSM) development (Apollo 4, Apollo 6)
- B – Unmanned Apollo Lunar Module (LM) development (Apollo 5)
- C – Manned CSM evaluation in low Earth orbit (Apollo 7)
- D – Manned CSM and LM development in low Earth orbit (originally planned for Apollo 8; flown as Apollo 9)
- E – Manned CSM and LM operations, a simulated lunar mission in an elliptical medium Earth orbit with an apogee of 3,500 nautical miles (6,500 km); never flown
- F – Manned CSM and LM operations in lunar orbit, proving LOR
- G – First manned lunar landing
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 of Apollo 8, so this was flown as a lunar orbital mission using just the CSM (sometimes referred to as "a C prime mission") and the E mission was canceled.
Revised sequence (Aug 1968):
- C' – Manned CSM flight into lunar orbit (Apollo 8 flew as this revised "C-prime" mission)
- D – Manned CSM and LM development in low Earth orbit (Apollo 9)
- E – Cancelled (this would have been a step backward from Apollo 8 as flown)
- F – Manned CSM and LM operations in lunar orbit, a "dress rehearsal" for the first landing (Apollo 10)
- G – First manned lunar landing (Apollo 11)
- H – precision landings with up to two-day stays on the Moon, with two lunar extravehicular activities or "moonwalks" (Apollo 12, Apollo 13 (planned), Apollo 14)
- 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.
- Murray, Charles, and Catherine Bly Cox. Apollo: The Race to the Moon. New York: Simon and Schuster, 1989. pp. 315–16.
- "Part 2(D) – July through September 1967". The Apollo Spacecraft – A Chronology. Volume IV. NASA. 1975. Retrieved 2008-01-29.