<Li> CSM power down, LM power up: 05: 23 UTC (58: 10 Ground Elapsed Time) </Li> <P> April 15, 1970, 00: 21: 00 UTC; 137 nmi (253.7 km) </P> <P> April 17, 1970, 18: 07: 41 UTC (142: 54: 47 Ground Elapsed Time). Crew was on board the USS Iwo Jima 45 minutes later . </P> <P> The mission was launched at the planned time, 02: 13: 00 PM EST (19: 13: 00 UTC) on April 11 . An anomaly occurred when the second - stage, center (inboard) engine shut down about two minutes early . The four outboard engines and the third - stage engine burned longer to compensate, and the vehicle achieved very close to the planned circular 100 nautical miles (190 km) parking orbit, followed by a normal translunar injection about two hours later . The engine shutdown was determined to be caused by severe pogo oscillations measured at a strength of 68 g and a frequency of 16 hertz, flexing the thrust frame by 3 inches (76 mm). The vehicle's guidance system shut the engine down in response to sensed thrust chamber pressure fluctuations . Pogo oscillations had been seen on previous Titan rockets, and also on the Saturn V during Apollo 6, but on Apollo 13, they were amplified by an unexpected interaction with turbopump cavitation . Later missions implemented anti-pogo modifications that had been under development . These included addition of a helium - gas reservoir to the center engine liquid oxygen line to damp pressure oscillations, an automatic cutoff as a backup, and simplification of the propellant valves of all five second - stage engines . </P>

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