What major lesson learned from the Apollo 13 incident directly prompted significant revisions for later Apollo missions?
Significant redesigns for subsequent Apollo missions to prevent oxygen tank failure
The failure mechanism exposed by the destruction of the oxygen tank was taken extremely seriously by NASA engineers. The knowledge gained from understanding how ground handling issues combined with improper testing procedures led to necessary system overhauls. Specifically, the failure of that critical oxygen tank directly resulted in significant redesigns implemented across the hardware for all subsequent Apollo missions. These revisions were intended to ensure that the electrical and insulation vulnerabilities that caused the accident were eliminated, thereby making similar catastrophic failures extremely unlikely to recur.

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Apollo 13's 'successful failure' explained - YouTube