plug-type hatch cover with a quick-release, outward opening door. NASA discontinued the crewed Block I program, using the BlockI spacecraft only for uncrewed SaturnV flights. Crew members would also exclusively wear modified, fire-resistant A7L Block II space suits, and would be designated by the Block II titles, regardless of whether a LM was present on the flight or not. 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 Apollo1 designation to honor the crew, per their widows' wishes. In September 1967, Mueller approved a 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. The delay in the CSM caused by the fire enabled NASA to catch up on human-rating the LM and SaturnV. Apollo4 (AS-501) was the first uncrewed flight of the SaturnV, carrying a BlockI 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