Forced Response System Identification of Cavitation Instabilities in Rocket Engine Turbopumps 

Matthew C. Campbell

Advisor: Professor Spakovszky

Axial Inducers are often used in high performance rocket engines in order to reduce system weight and cost.  Inducers can operate at high rotational speeds and low inlet pressures, which can lead to cavitation instabilities.  The resulting dynamic behavior can lead to thrust oscillations and couple with the structures of the launch vehicle, leading to POGO instability (named after the POGO jumping stick) that can cause catastrophic failure.

 The current characterization of inducer cavitation dynamics, especially the so-called pump transfer matrices, is still limited as the only experimental transfer matrix data available dates back to the 1970s. There is a critical need for more in-depth experimental characterization of the cavitation dynamics and related damping in turbopump inducers so as to allow a more accurate and reliable assessment of POGO instability. A two-dimensional reduced order model for rotating cavitation was developed at the GTL and validated with experimental measurements in the MIT inducer at the Aerospace Corporation. The MIT inducer is representative of the SSME low pressure oxidizer pump inducer both in terms of cavitation performance and dynamic behavior. The goal of the project is to conduct forced response system identification experiments aimed at characterizing the cavitation dynamics. The modeling framework is used to guide the experimental setup where forcing is introduced via a piston pulser. The response is recorded via unsteady velocity and pressure measurements upstream and downstream of the inducer. Spectral analysis allows to estimate the inducer transfer functions and thus the system eigenvalues and damping.