Analysis of Pogo Stability

Abstract

Many liquid propellant rocket vehicles have experienced self-excited longitudinal vibration. The self-excitation or instability arises from interaction of the vehicle structure with the propulsion system. The early experiences with this phenomenon involved vibrations in the first longitudinal mode of the vehicle; the nickname ‘pogo’ was coined on the basis of a superficial similarity of the out-of-phase motion of the ends of the vehicle with the motion of the pogo stick (child’s jumping stick). Figure 1 illustrates a typical occurrence of instability during powered flight in terms of longitudinal acceleration at a point on the vehicle structure. Corresponding oscillations occur within the liquid propellant and in the combustion chamber. The oscillation begins spontaneously, intensifies, and then dies away — typically in a period of 10 to 40 s. The frequency of vibration tracks that of the first structural mode which increases as propellant depletes. Less often, pogo vibrations have occurred in higher modes of longitudinal vibration; also multiple periods of instability, each involving a different mode of vibration, have occurred during operation of a single vehicle stage.