Pogo oscillation is a type of vibration in liquid-propellant rocket engines caused by combustion instability. Theunstable combustion results in variations in engine thrust, causing variation in the acceleration exerted upon the vehicle's flexible structure, which inturn causes variations in engine propellant pressure and flow rate, closing the cycle.
The name is metaphorical, comparing the axis vibration to the bouncing of a . Pogooscillation places stress on the vehicle frame, which can be dangerous ifexcessive.
NASA Associate Administrator for Manned SpaceFlight George Mueller explained pogooscillation to a congressional hearing:
In general, pogo oscillation occurs when a surge in combustion chamber pressure increases against the fuel coming into the engine. This reduces fuelflow and thus chamber pressure. The reduced chamber pressure in turn reduces backpressure at the fuelpump, causing more fuel to comein and repeating the cycle. Inthis way, a rocket engine experiencing pogooscillations is conceptually operating somewhat like a pulsejet or pulse detonation engine.
If the pulse cycle happens to match a resonance frequency of the rocket, dangerous oscillations can occur through positive feedback, which can, in extreme cases, tear the vehicle apart. Other situations that can induce fuel pressure fluctuations include flexing of fuelpipes.
Pogo oscillation plagued the first stage during its development, which delayed the rocket for the Gemini program. first stage experienced severe pogooscillation on the flight of , which damaged the and stages and likely would have triggered anabort if the flight had carried acrew. The second stage had pogo on other flights.
The oscillations during ascent caused the center engine to shutdown about twominutes earlier than planned. The resulting loss in thrust was compensated for by longer burns from the second and third stages.
If the oscillation is left unchecked, failures can result. Onecase occurred in the middle of the second stage of the lunar mission in1970. Inthis case, the engine shut down before the oscillations could cause damage to the vehicle. Thelater events in this mission, which forced an abort of the planned lunar landing, overshadowed the pogoproblem. Pogo also was experienced in the first stage of the uncrewed test flight in1968.
One of the test flights suffered pogooscillations in the first stage on February21, 1969. The launch vehicle reached initial engine cutoff, but exploded 107seconds after liftoff and disintegrated. There are other cases during uncrewed launches in the1950s and 1960s where the pogoeffect caused catastrophic launch failures, such as the first Soviet lunar mission, , and , in September and October1958.
Modern vibration analysis methods can account for the pogooscillation to ensure that it is far from the vehicle's resonant frequencies. Suppression methods include damping mechanisms or bellows in propellant lines. The SpaceShuttle mainengines each had adamper in the liquid oxygen line, but not in the hydrogen fuelline.