Low-Order Models for Low-Frequency Combustion Instability in Hybrid Rocket Engines

A low-order model for a hybrid rocket engine is proposed to investigate low-frequency combustion instabilities. The present work is based on a one-way coupling between a one-dimensional thermal model to evaluate the unsteady regression rate and a zero-dimensional model which imposes mass conservation inside the rocket chamber. In this way, it is possible to estimate the unsteady pressure level inside the rocket chamber and predict the amplitude of the oscillations. Two different approaches are compared. In the first approach, the Oxidizer-Fuel (OF) ratio is directly computed from the ratio between the fuel and oxidizer mass flow rates. In this way the OF ratio strongly depends on the regression rate oscillations and, as a consequence, the pressure shows large fluctuations. An alternative approach has been investigated by computing the OF ratio from the fuel and oxidizer istantaneous densities. The two approaches are compared on a representative test case.