Pogo oscillation

Pogo oscillation is a self-excited vibration in liquid propellant rocket engines caused by combustion instability. The unstable combustion results in variations of engine thrust, causing variations of acceleration on the vehicle's flexible structure, which in turn cause variations in propellant pressure and flow rate, closing the self-excitation cycle. The name is a metaphor comparing the longitudinal vibration to the bouncing of a pogo stick. Pogo oscillation places stress on the frame of the vehicle which, in severe cases is dangerous.

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 longitudinally 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.

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