Pogo - longitudinal vehicle oscillations

Being a huge, long, metal structure in a very dynamic environment, with large quantities of fluid moving inside at speed, the stack is prone to vibrations along its length. This lengthwise stretching and squeezing is known as "Pogo" from its similarity to the spring action of the "Pogo stick," a contemporary child's toy. The structure itself vibrates at particular frequencies which, as the fuel tanks empty, sweep through a substantial range. Other variables, such as vehicle and payload mass, and propellant characteristics, make it exceedingly difficult to predict the frequency and magnitude of these pogo oscillations.

Many launch vehicles suffer pogo problems during development and the Saturn V was never completely free of it. The S-II, in particular, exhibited severe pogo which plagued the early flights. One partial cure for this is to shut down the centre engine earlier than the others. (ap15fj)