Apollo 15 to Apollo 12, and since Apollo 11's backup crew had been designated for Apollo 13 (which did not land on the Moon), they were the first Apollo crew for a successful lunar landing mission to have backed up another such mission. Having had the opportunity to familiarize themselves with the Apollo spacecraft, they could devote more of their training time as prime crew for Apollo 15 to geology and sampling techniques. The astronauts at NASA who were scientists, such as geologist Harrison Schmitt, pushed though the early Apollo missions for a greater place for science as part of the mission,
Apollo program engine was more severe, preventing it from restarting for trans-lunar injection. Mission controllers were able to use the service-module engine to essentially repeat the flight profile of Apollo 4. Based on the good performance of Apollo 6 and identification of satisfactory fixes to the Apollo 6 problems, NASA declared the Saturn V ready to fly men, cancelling a third unmanned test. Apollo 7, launched from LC-34 on October 11, 1968, was the C mission, crewed by Schirra, Eisele and Cunningham. It was an 11-day Earth-orbital flight which tested the CSM systems. Apollo 8 was planned to be the D mission