Half-way through 1967, NASA had a plan for Apollo. The agency had 15 Saturn V rockets for lunar missions and a plan to methodically break down the tricky task of landing on and exploring the moon's surface.
Each mission was assigned a letter from A to J that designated its type. Two Saturn Vs were tested on the unmanned Apollos 4 and 6, making up the A missions. The unmanned flight test of the Lunar Module (LM) on Apollo 5 was the one B mission. Apollo 7's shakedown cruise of the Command and Service Module (CSM) was the C mission. Apollo 8, which threw the sequence out of order by going to the Moon with just a CSM, was dubbed the C-prime mission; this made the high-Earth orbit CSM test E mission unnecessary. Apollo 9 tested the CSM and LM in Earth orbit on the D mission, and Apollo 10's lunar landing dress rehearsal was the F mission. The first landing on Apollo 11 was the G mission.
After Apollo 11, NASA had nine Saturn V's still available for its extended lunar missions - enough to get the agency through its planned Apollo 20. The remaining rockets were assigned to missions NASA hoped to launch roughly every four months between Just 1969 and July 1972. Under the original plan, Apollos 12 through 15 were classified as H missions. There were precision landing that would stay on the Moon for two days with crews that would perform two EVAs - Moonwalks.
I missions, long duration missions with orbital surveys done from an instrument module aboard the orbiting CSM, were cancelled and merged into the J missions that used extended LMs to increase the surface stay to three days and had crews perform three EVAs. Apollos 16 though 20 were classified as J missions.
But things changed in 1970. NASA started looked forward to its post-Apollo goals. Namely to the Skylab program, which needed a Saturn V to launch the actual dry workshop that would be constructed on the ground; crews would follow riding the smaller Saturn 1B into orbit. Apollo 20 was cancelled to free up one Saturn V and the final three moon landings were pushed back to 1973 and 1974 to give Skylab a window to fly.
The near loss of Apollo 13 in April 1970 saw the mission's landing site and surface activities were reassigned to Apollo 14. NASA also canceled its fourth H and J-missions - what would have been Apollos 15 and 19. Apollo 15 was upgraded to a J mission and Apollos 18 and 19 were flat out cancelled. Apollo 17 became the end of the Apollo program.
It was bad, but things could have been worse. In August 1971, President Nixon proposed canceling Apollos 16 and 17 - his reason, at least publicly, was that Apollo 15 had been such a success why not move forward with Skylab and the Space Shuttle right away? Nixon's advisors persuaded him to let Apollo run its course.
Had the Apollo program gone all the way though to Apollo 20, crews might have explored the Marius Hills volcanic domes as well as the Copernicus, Tycho, and Censorinus craters - these were all planned landing areas under NASA original plan.
As for the crews, some patient astronauts would have finally left the Earth. Normal crew rotation, which saw a backup crew serve as prime crew three missions later, sheds some light on who would have flown when. Apollo 18 would have seen Dick Gordon, Apollo 12′s command module pilot (CMP), fly to the Moon again as commander along with Vance Brand as CMP and Jack Schmitt as Lunar Module Pilot (LMP). Apollo 13′s LMP Fred Haise would have commanded Apollo 19 with Bill Pogue as CMP and Gerry Carr as LMP. And Apollo 20 would have Pete Conrad in command of his second lunar mission with Paul Weitz as CMP and Jack Lousma as LMP.
But this might not have actually come to pass. Conrad, already having commanded a lunar mission, might have been passed over for Apollo 14′s CMP Stu Roosa. Jack Schmitt, of course, was bumped up from Apollo 18 to 17. He replaced Joe Engle who had served as Apollo 14's backup LMP, meaning Engle might have had his chance on 18.
Even though these missions never flew, there was one Apollo mission after 17. In 1975, NASA launched the Apollo half of the Apollo-Soyuz Test Project, which was internally designated Apollo 18. Still, this Earth-orbital mission was far from another trip to the moon.
Image: Astronaut Harrison H. Schmitt, Apollo 17 lunar module pilot, stands near the deployed United States flag on the lunar surface during extravehicular activity (EVA) of NASA's final lunar landing mission in the Apollo series. Credit: NASA