Mission-Specific Pogo Stability Analysis with Correlated Pump Parameters

Parameters that characterize the perturbational pressure and flow at the inlet and outlet of a pump are established through pogo stability analyses for a launch vehicle. Ground tests of the launch vehicle's engine indicate the presence of unsteady pump cavitation, and some flights of the launch vehicle exhibit frequency, amplitude, and phase locking between axial structural acceleration and engine chamber pressure-a condition emblematic of propulsion-structure interaction, or pogo. Models developed for several missions of the subject launch vehicle are used to establish the ranges of the pump parameters that yield instability during the flight pogo occurrences and stability at other times. The resulting nominal values of normalized pump cavitation stiffness and mass flow gain for the launch vehicle's engine fall into the ranges 0.62-0.86 and 0.31-0.59, respectively. These ranges account for sensitivity with respect to dynamic pump gain and to structural damping for the axial modes of the coupled launch vehicle-payload system. It is shown that cavitation stiffness is generally the predominate pump parameter when the feedline hydraulic and axial structural modes are separated in frequency. However, if the frequencies of these modes are in close proximity, mass flow gain has a strong destablizing effect.