Editing Apollo 9 (section)

=== Launch vehicle ===
The [[Saturn V]] (AS-504) used on Apollo{{nbsp}}9 was the fourth to be flown, the second to carry astronauts to space,{{sfn|Press Kit|p=8}} and the first to bear a lunar module. Although similar in configuration to the Saturn{{nbsp}}V used on Apollo 8, several changes were made. The inner core of the F-1 engine chamber in the first (S-IC) stage was removed, thus saving weight and allowing for a slight increase in [[specific impulse]]. Weight was also saved by replacing the skins of the [[liquid oxygen]] tanks with lighter ones, and by providing lighter versions of other components. Efficiency was increased in the S-II second stage with uprated J-2 engines, and through a closed-loop propellant utilization system rather than Apollo 8's open-loop system.{{sfn|Mission Report|pp=A-59, inside back cover}} Of the {{convert|3250|lb}} weight reduction in the second stage, about half came from a 16 percent reduction in the thickness of the tank side walls.{{sfn|''Science News'' 1969-03-22b|p=283}}