Apollo 13

The Apollo 13 mission was scheduled to explore the Fra Mauro formation, or Fra Mauro highlands, named after the 80-kilometer-diameter Fra Mauro crater, located within it. It is a widespread, hilly geological (or more properly, selenological) area covering large portions of the lunar surface around Mare Imbrium, and is thought to be composed of ejecta from the impact which formed the mare. With the failure of the mission, the flight to Fra Mauro was done on Apollo 14.

The flight's problems began during the liftoff with a lesser-known malfunction: during the second-stage burn, the center engine shut down two minutes early. The four outer engines were run for longer than planned, to compensate for this.[6] Engineers later discovered that this was due to dangerous pogo oscillations which might have torn the second stage apart; the engine was experiencing 68g vibrations at 16 hertz, flexing the thrust frame by 3 inches (76 mm). However, the oscillations caused a sensor to register excessively low average pressure, and the computer shut the engine down automatically.[7] Smaller pogo oscillations had been seen on previous Apollo missions (and had been recognized as a potential problem from the earliest unmanned Titan-Gemini flights), but on Apollo 13 they had been amplified by an unexpected interaction with the cavitation in the turbo-pumps.[8] Later missions included anti-pogo modifications, which had been under development since before Apollo 13. Those modifications solved the problem. They entailed (a) the addition of a helium gas reservoir in the center engine’s liquid oxygen line to dampen pressure oscillations, (b) an automatic cutoff for the center engine in case this failed, and (c) simplified propellant valves on all five second-stage engines.

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