Engineers are, by nature, generally cautious people. We add design factors and contingency factors and factors of safety into our design to account for any variance between design and construction, or for errors in our assumptions. Flight engineers tend to be even more cautious since they generally will have human lives at stake.
It was this cautious approach to testing the various stages of the Saturn V rocket that put the Apollo goal — reaching the moon before the end of the 1960s — at risk. In order to meet that goal, when George Mueller took over as NASA’s Associate Administrator for Manned Space Flight in September 1963 he declared that the Saturn V testing would go from a very incremental approach to an all-up approach. A full stack of stages should be tested as soon as stages are ready for testing.
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