Editing Apollo 8 (section)

===Saturn V redesign===

The Saturn V rocket used by Apollo{{nbsp}}8 was designated AS-503, or the "03rd" model of the Saturn{{nbsp}}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 (spaceflight)|boilerplate]] command and service module. Apollo{{nbsp}}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.{{sfn|Bilstein|1996|pp=360–370}}

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.{{sfn|Bilstein|1996|pp=360–370}}
[[File:Ap8-KSC-68PC-147.jpg|thumb|upright=1.4|left|Apollo 8 atop Saturn{{nbsp}}V being rolled out to [[Kennedy Space Center Launch Complex 39|Pad 39A]] atop the [[crawler-transporter]]]]

Of equal importance was the failure of three engines during flight. Researchers quickly determined that a leaking hydrogen fuel line ruptured when exposed to vacuum, causing a loss of fuel pressure in engine two. When an automatic shutoff attempted to close the liquid hydrogen valve and shut down engine two, it had accidentally shut down engine three's liquid oxygen due to a miswired connection. As a result, engine three failed within one second of engine two's shutdown. Further investigation revealed the same problem for the third-stage engine—a faulty igniter line. The team modified the igniter lines and fuel conduits, hoping to avoid similar problems on future launches.{{sfn|Bilstein|1996|pp=360–370}}

The teams tested their solutions in August 1968 at the MSFC. A Saturn stage IC was equipped with shock-absorbing devices to demonstrate the team's solution to the problem of pogo oscillation, while a Saturn Stage II was retrofitted with modified fuel lines to demonstrate their resistance to leaks and ruptures in vacuum conditions. Once NASA administrators were convinced that the problems had been solved, they gave their approval for a crewed mission using AS-503.{{sfn|Bilstein|1996|pp=360–370}}

The Apollo 8 spacecraft was placed on top of the rocket on September 21, and the rocket made the slow {{convert|3|mi|adj=on}} journey to the launch pad atop one of NASA's two massive [[crawler-transporter]]s on October{{nbsp}}9.<ref name="satVillust">{{cite book|last=Akens |first=David S. |title=Saturn Illustrated Chronology |url=https://history.nasa.gov/MHR-5/cover.htm |archive-url=https://web.archive.org/web/20080307132427/https://history.nasa.gov/MHR-5/app_h.htm |archive-date=March 7, 2008 |access-date=February 1, 2008 |year=1971 |publisher=[[Marshall Space Flight Center]] |location=Huntsville, AL |id=MSFC MHR-5 |chapter=Appendix H—Saturn at the Cape |chapter-url=https://history.nasa.gov/MHR-5/app_h.htm |url-status=live}}</ref> Testing continued all through December until the day before launch, including various levels of readiness testing from December{{nbsp}}5 through 11. Final testing of modifications to address the problems of pogo oscillation, ruptured fuel lines, and bad igniter lines took place on December 18, three days before the scheduled launch.{{sfn|Bilstein|1996|pp=360–370}}