Launch vehicles with liquid rocket engines have feed lines through which propellants flow to the engine. To prevent feedback between structural responses and propellant pressure and flow oscillations, a compliant device called a pogo accumulator is typically installed in the propellant feed line. Even if a catastrophic interaction is thus averted, the fluid-induced structural responses may exceed those for important flight events such as liftoff and atmospheric buffeting. In that case, the fluid-induced excitation must be predicted in order to ensure adequate structural margins for the launch vehicle and space vehicle hardware. Venting of compliant gas in the pogo accumulator prior to engine shutdown is known to exacerbate the fluid-induced excitation. In particular, for the Atlas V launch vehicle, a 5–7 Hz fluid mode with large pressure gains at the aft end of the liquid oxygen feed line often excites structural modes just prior to engine cutoff. A methodology for the prediction of these structural responses is presented.

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