Pogo—the self-excited vibration of a liquid-fueled rocket—arises from the inherent interaction of a flexible rocket structure with a fluid propulsion system. An occurrence of pogo can be detected in measurements of longitudinal acceleration, seen as a "blossom" of vibration superimposed on the steady acceleration. The oscillations grow, reach a maximum, and then recede.
The pogo oscillation typically falls in the range of 5–60 hertz (cycles per second) and tracks the slowly changing frequency of the vehicle's vibration. Accelerations up to 17 g's (zero to peak) have been observed at the interface between the launch vehicle and payload and up to 34 g's at an engine. Such high accelerations cause stress that may exceed the vehicle's structural limits and lead to ultimate mission loss. Pogo is a particular concern for piloted vehicles because, depending on the frequency, oscillations as low as 0.25 g's in amplitude can impair an astronaut's ability to read instrument panels.
Typically, pogo occurs when the natural frequency of a propellant feed line, which is primarily controlled by cavitation bubbles caused by the operation of an engine pump, comes close to a readily excited natural frequency of longitudinal structural vibration. A close matching of the propellant and structural frequencies is typically avoided by installing an accumulator in the feed line. The accumulator contains a volume of gas that acts like a soft spring to reduce the propellant frequency to well below that of critical structural frequencies.