Determination of Aluminized Solid Propellant Admittances by the Impedance Tube Method

The adaptation of the impedance tube technique in the measurement of the admittances and response functions of aluminized solid propellants is described. These quantities are needed for combustion stability analysis of aluminized solid rocket motors. To determine the acoustic energy gains and losses in the impedance tube, the acoustic pressures (amplitude and phase) are measured in the tube during the quasisteady burn period of a test through a distance covering two standing wave pressure minima. The measured data are then input into a data reduction computer program that is based upon the solution of the impedance tube wave equations and is capable of the simultaneous determination of the propellant admittance and the acoustic energy losses in the gas phase. Results obtained in a series of tests conducted with UTP-3001 and UTP-19360 aluminized propellants are presented. The paper demonstrates that the data measured in tests conducted with aluminized propellants and, hence, the determined propellant properties are more accurate than data measured in tests with nonaluminized propellants. Excellent agreement is demonstrated between the predicted and measured impedance tube wave structures. Finally, the results obtained by this method are utilized to determine the true role of aluminized solid propellants in the damping of combustion instabilities inside the rocket motor.