Launch Incident
The mission began with a little-known smaller incident: during the second-stage boost, the center (inboard) engine shut down two minutes early. The four outboard engines burned longer to compensate, and the vehicle continued to a successful orbit. The shutdown was determined to be due to dangerous pogo oscillations that threatened to tear the second stage apart. The engine experienced 68 g vibrations at 16 hertz, flexing the thrust frame by 3 inches (76 mm). The engine shutdown was triggered by sensed thrust chamber pressure fluctuations. Smaller pogo oscillations had been seen on previous Titan and Saturn flights (notably 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 dampen pressure oscillations, an automatic cutoff as a backup, and simplification of the propellant valves of all five second-stage engines.
Read more about this topic: Apollo 13
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