The Saturn V rocket used by Apollo 8 was designated AS-503, or the "03rd" model of the Saturn V ("5") Rocket to be used in the Apollo-Saturn ("AS") program. When it was erected in the Vehicle Assembly Building on December 20, 1967, it was thought that the rocket would be used for an uncrewed Earth-orbit test flight carrying a boilerplate command and service module. Apollo 6 had suffered several major problems during its April 1968 flight, including severe pogo oscillation during its first stage, two second-stage engine failures, and a third stage that failed to reignite in orbit. Without assurances that these problems had been rectified, NASA administrators could not justify risking a crewed mission until additional uncrewed test flights proved the Saturn V was ready.
Teams from the MSFC went to work on the problems. Of primary concern was the pogo oscillation, which would not only hamper engine performance, but could exert significant g-forces on a crew. A task force of contractors, NASA agency representatives, and MSFC researchers concluded that the engines vibrated at a frequency similar to the frequency at which the spacecraft itself vibrated, causing a resonance effect that induced oscillations in the rocket. A system that used helium gas to absorb some of these vibrations was installed.
