posted 10-29-2013 11:14 PM            

A Saturn V third stage, S-IVB-503, exploded shortly before it was scheduled to be ignited in a January 20 test at SACTO. The explosion completely destroyed the stage at test stand Beta III. Post-accident investigation revealed that one of the eight ambient temperature helium storage spheres located on the engine thrust structure exploded because of weld weakness resulting from use of the wrong weld material.

Following the loss of the S-IVB-503 stage during testing on January 20, NASA officials amended identification numbers of subsequent S-IVB stages to fill the vacancy created. The S-IVB-504 became the S-IVB-503N, S-IVB-505 became S-IVB-504N, and S-IVB-506 became S-IVB-505N. A replacement stage using an old S-IVB-507 tankage became S-IVB-506, and S-IVB-507 and subsequent stages retained the old identification. (The "N" at the end of the stage identification stands for the word "New," a designation that became necessary after an earlier stage version exploded, necessitating the use of a substitute stage.)