A study of low frequency combustion instability in rocket engine preburners using a heterogeneous stirred tank reactor model

During the shutdown of the space shuttle main engine, oxygen flow is shut off from the fuel preburner and helium is used to push the residual oxygen (between the oxidizer valve and the preburner combustion chamber) into the combustion chamber. During this process a low frequency combustion instability, or chug, occurs. This chug has resulted in damage to the engine's augmented spark igniter due to backflow of the contents of the preburner combustion chamber into the oxidizer feed system.

To determine possible causes and fixes for the chug, the fuel preburner was modelled as a heterogeneous stirred tank combustion chamber, a variable mass flow rate oxidizer feed system, a constant mass flow rate fuel feed system and an exit turbine. Within the combustion chamber gases were assumed perfectly mixed. To account for liquid in the combustion chamber, a uniform droplet distribution was assumed to exist in the chamber, with the mean droplet diameter determined from an empirical relation. A computer program was written to integrate the resulting differential equations.

Because chamber contents were assumed perfectly mixed, the fuel preburner model erroneously predicted that combustion would not take place during shutdown. The combustion rate model was modified to assume that all liquid oxygen that vaporized instantaneously combusted with fuel. Using this combustion model, the effect of engine parameters on chamber pressure oscillations during the SSME shutdown was calculated. These studies showed that decreasing the pressure downstream of the preburner's exit turbine, decreasing the fuel temperature, increasing the helium temperature and decreasing the length of the line connecting the helium storage tank to the helium check valve all had adverse effects on engine stability.