Non-linear analysis of space launch vehicle POGO oscillations

The feasibility and effects of including nonlinearities in open-loop analysis of launch vehicle POGO oscillations was examined. The specific test case considered was the oscillation of a Titan 2 missile. A non-linear analysis using describing functions to represent fluid flow nonlinearities and including nonlinear fuel and oxidizer pump characteristics was conducted. The non-linear analysis gave improved predictions of the time of POGO oscillation onset and stopping as compared with a linear analysis using linear differential equations. Estimates of the frequency and magnitude of the oscillations are provided. The correlation between observed and predicted oscillation frequency and magnitude improves as additional nonlinearities are included in the analysis. For the vehicle model used, the most significant sources of nonlinearity were the oxidizer feed lines and the propellant pumps.