The Saturn V (AS-504) used on Apollo9 was the fourth to be flown, the second to carry astronauts to space, and the first to bear a lunar module. Although similar in configuration to the SaturnV 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. Of the weight reduction in the second stage, about half came from a 16 percent reduction in the thickness of the tank side walls. Spacecraft, equipment and call signs Apollo9 used CSM-104, the third block II CSM to be flown with astronauts aboard. Apollo 8, lacking a lunar module, did not have docking equipment; Apollo9 flew the probe-and-drogue assembly used for docking along with other equipment added near the forward hatch of the CM; this allowed for rigid docking of the two craft, and for internal transfer between CM and LM. Had the switch in missions between Apollo8 and9 not occurred, the Earth-orbit mission would have flown CSM-103, which flew on Apollo 8. The Earth-orbit mission was originally supposed to use LM-2 as its lunar module, but the crew found numerous flaws in it, many associated with it being the first flight-ready lunar module off Grumman's production line. The delay occasioned by the switch in missions allowed LM-3 to be available, a machine the crew found far superior. Neither LM-2 nor LM-3 could have been sent to the Moon as both were too heavy; Grumman's weight reduction program for the LMs only became fully effective with LM-5, designated for Apollo 11. Small cracks in LM-3's aluminum alloy structure due to stresses such as the insertion of a rivet proved an ongoing issue; Grumman's engineers continued working to fix them until the LM had to be mounted on the SaturnV in December 1968, where it was housed inside the Spacecraft-Lunar Module Adapter, numbered as SLA-11A. LM-2 never flew in space and is in the National Air and Space Museum. The Apollo astronauts were provided with early versions of the Sony Walkman, portable