Editing Apollo program (section)

====Uncrewed Saturn V and LM tests====
On April 24, 1967, Mueller published an official Apollo mission numbering scheme, using sequential numbers for all flights, crewed or uncrewed. The sequence would start with [[Apollo 4]] to cover the first three uncrewed flights while retiring the Apollo{{nbsp}}1 designation to honor the crew, per their widows' wishes.<ref name="missionNumbers">{{Cite web |title=Apollo 11 30th Anniversary: Manned Apollo Missions |publisher=NASA History Office |date=1999 |url=https://history.nasa.gov/ap11ann/missions.htm |access-date=March 3, 2011 |archive-url=https://web.archive.org/web/20110220232013/https://history.nasa.gov/ap11ann/missions.htm |archive-date=February 20, 2011 |url-status= live}}</ref><ref>[[#Ertel et al.|Ertel & al. 1978]], Part 1(H)</ref>

In September 1967, Mueller approved a [[List of Apollo missions#Alphabetical mission types|sequence of mission types]] which had to be successfully accomplished in order to achieve the crewed lunar landing. Each step had to be successfully accomplished before the next ones could be performed, and it was unknown how many tries of each mission would be necessary; therefore letters were used instead of numbers. The '''A''' missions were uncrewed Saturn V validation; '''B''' was uncrewed LM validation using the Saturn IB; '''C''' was crewed CSM Earth orbit validation using the Saturn IB; '''D''' was the first crewed CSM/LM flight (this replaced AS-258, using a single Saturn V launch); '''E''' would be a higher Earth orbit CSM/LM flight; '''F''' would be the first lunar mission, testing the LM in lunar orbit but without landing (a "dress rehearsal"); and '''G''' would be the first crewed landing. The list of types covered follow-on lunar exploration to include '''H''' lunar landings, '''I''' for lunar orbital survey missions, and '''J''' for extended-stay lunar landings.<ref name="3Q1967">[[#Ertel et al.|Ertel et al. 1978]], p. 157</ref>

The delay in the CSM caused by the fire enabled NASA to catch up on human-rating the LM and Saturn{{nbsp}}V. Apollo{{nbsp}}4 (AS-501) was the first uncrewed flight of the Saturn{{nbsp}}V, carrying a Block{{nbsp}}I CSM on November 9, 1967. The capability of the command module's heat shield to survive a trans-lunar reentry was demonstrated by using the service module engine to ram it into the atmosphere at higher than the usual Earth-orbital reentry speed.

[[Apollo 5]] (AS-204) was the first uncrewed test flight of the LM in Earth orbit, launched from pad 37 on January 22, 1968, by the Saturn IB that would have been used for Apollo 1. The LM engines were successfully test-fired and restarted, despite a computer programming error which cut short the first descent stage firing. The ascent engine was fired in abort mode, known as a "fire-in-the-hole" test, where it was lit simultaneously with jettison of the descent stage. Although Grumman wanted a second uncrewed test, George Low decided the next LM flight would be crewed.<ref>{{cite book |last=Low |first=George M. |author-link=George Low |editor-last=Cortright |editor-first=Edgar M |editor-link=Edgar Cortright |title=Apollo Expeditions to the Moon |url=http://www.hq.nasa.gov/pao/History/SP-350/cover.html |access-date=August 1, 2013 |date=1975 |publisher=Scientific and Technical Information Office, NASA |location=Washington, D.C. |oclc=1623434 |lccn=75600071 |id=NASA SP-350 |chapter=Testing and Retesting To Get Ready For flight |chapter-url=http://www.hq.nasa.gov/office/pao/History/SP-350/ch-4-6.html |archive-date=February 19, 2008 |archive-url=https://web.archive.org/web/20080219204538/https://history.nasa.gov/SP-350/ch-9-5.html |url-status=dead }}</ref>

This was followed on April 4, 1968, by [[Apollo 6]] (AS-502) which carried a CSM and a LM Test Article as ballast. The intent of this mission was to achieve trans-lunar injection, followed closely by a simulated direct-return abort, using the service module engine to achieve another high-speed reentry. The Saturn V experienced [[pogo oscillation]], a problem caused by non-steady engine combustion, which damaged fuel lines in the second and third stages. Two S-II engines shut down prematurely, but the remaining engines were able to compensate. The damage to the third stage 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{{nbsp}}6 and identification of satisfactory fixes to the Apollo{{nbsp}}6 problems, NASA declared the Saturn{{nbsp}}V ready to fly crew, canceling a third uncrewed test.{{sfn|Brooks|Grimwood|Swenson|1979|loc=Ch. 10.5: [http://www.hq.nasa.gov/office/pao/History/SP-4205/ch10-5.html "Apollo 6: Saturn V's Shaky Dress Rehearsal"]}}