Hazard
If the oscillation is left unchecked, failures can result. One case occurred in the middle J-2 engine of the second stage, S-II, of the Apollo 13 lunar mission. Fortunately in this case the engine shut down before the oscillations could cause damage to the vehicle. Later events in this mission overshadowed the pogo problem. Pogo was also the cause of some of the serious problems experienced by the unmanned Apollo 6 test flight in 1968. One of the Soviet Union's N1-L3 rocket test flights suffered pogo oscillations in the first stage on November 23, 1972. The launch vehicle reached initial engine cutoff, but exploded 107 seconds after liftoff and disintegrated. There are other cases during unmanned launches in the 50s and 60s where the pogo effect caused catastrophic launch failures.
However, modern vibration analysis methods can account for the pogo oscillation to test that it is far away from the vehicle's resonant frequencies. Suppression methods include damping mechanisms or bellows in propellant lines. The Space Shuttle Main Engines each have a damper in the LOX line, but not in the hydrogen fuel line.
Read more about this topic: Pogo Oscillation
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