A model-based thruster leakage monitor for the Cassini spacecraft

The Cassini spacecraft uses thrusters to maintain three-axis attitude control and to perform many other functions. If one of the thrusters leaks, the opposing thrusters will be fired excessively to maintain the commanded attitude. Obviously, the draining of hydrazine and the excessive firings of thrusters must be stopped quickly. A set of three model-based leakage detection monitors has been designed and implemented in the flight software to protect Cassini against the occurrence of this unlikely event. Test results indicate that this design can detect thruster leakage quickly and robustly. Once the leak is detected, the on-board fault protection logic will activate remedial actions which include the swapping of thruster branches.