Pogo oscillation
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Pogo oscillation is the term for a potentially dangerous type of oscillation found in rocket engines. This oscillation results in variations of thrust from the engines, generally caused by variations in fuel flow rate, placing stress on the frame of the vehicle. Although the term is frequently written POGO, it is not an acronym, but in fact a reference to the bouncing of a pogo stick.
There are several causes of pogo oscillation. The main one is when a surge in engine pressure increases back pressure against the fuel coming into the engine, reducing engine pressure, causing more fuel to come in and increasing engine pressure again. If the cycle happens to match a resonant frequency of the rocket then dangerous oscillations can occur through positive feedback, which can in extreme cases tear the vehicle apart.
The most famous pogo oscillation was in the Saturn V S1-C Stage, caused by the cruciform thrust structure. This structure was an "X" of two I-beams, with an engine on each beam and the center engine at the intersection of the beams. The center of the cruciform was unsupported, so the central F-1 engine caused the structure to bend upwards. The "Pogo" oscillation occurred when this structure sprung back, lengthening the center engine's fuel line below (which was mounted down the center of the cruciform), temporarily reducing the fuel flow and thus reducing thrust. At the other end of the oscillation, the fuel line was compressed, increasing fuel flow - causing a sinusoidal thrust oscillation during the 1st stage ascent.
If the oscillation is left unchecked, failures can result. One case occurred in the middle engine of the second stage 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 problems experienced by Apollo 6. There are other cases during un-manned launches in the 50s and 60s where the Pogo 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 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.
