Origin
In general, pogo oscillation occurs 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. Flexing of fuel pipes can also induce fluctuations in fuel pressure. If the cycle happens to match a resonance frequency of the rocket then dangerous oscillations can occur through positive feedback, which can, in extreme cases, tear the vehicle apart.
Another situation in which pogo oscillation will occur is when the engine is moving upward with fluctuating speed. Owing to inertia, if the speed of the vehicle suddenly increases, the fuel inside the fuel tank tends to 'fall behind' and is forced into the turbopump, a situation somewhat similar to the slosh of liquid inside a tanker. This creates excess pressure to the turbopump and causes unintended excessive fuel to be delivered. This in turn creates excessive thrust and causes the vehicle to accelerate which leads to further increase in turbopump pressure and an unintended increase in fuel delivery. This can set up a vicious circle, and can result in structural failure in the vehicle.
The most famous pogo oscillation was in the Saturn V first stage, S-IC, caused by the cruciform thrust structure. This structure was an "X" of two I-beams, with an engine on the end of 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 sprang back, lengthening the center engine's fuel line bellow (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 first stage ascent.
Read more about this topic: Pogo Oscillation
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