Dynamic Response to Global Oscillation of Propulsion Systems with Cavitating Pumps

A time-domain model was developed to evaluate the dynamic response of pumping systems in the accelerating environment of rockets with a focus on cavitation. The model was first verified by comparing the results with measurements in ground-based tests of an LE-7A rocket engine. In these tests, various resonances occurred and levels of pump cavitation or incorporation of an accumulator altered them. The model results simulated the test data well, matching both the frequency and the amplitude. The test and model results also demonstrated the stability of the LE-7A propulsion system within nonaccelerating environments. Then, the model was used to examine the response of the propulsion system in accelerating frames; sinusoidal vehicle oscillations over a range of frequencies were explored. Under noncavitating conditions, the pressure amplitudes within the propulsion system did not substantially exceed the quasi-static acceleration head response rho ah. However, under cavitating conditions (sigma = 0.02), the same accelerations produced violent responses with pressure and flow amplitudes about 2 orders of magnitude greater than in noncavitating conditions. The obvious conclusion is that vehicle oscillations can cause substantial pressure and flow amplitudes, particularly when the pump is cavitating, even if the ground-based tests and the calculations in static frames indicate stable and well-behaved responses.