Nozzle Damping in Solid Rocket Instabilities

Abstract

Present understanding regarding the damping provided by solid-propellant rocket motor nozzles, during axial instabilities, is reviewed. Expressions describing the various modes of wave energy losses to the nozzle and the nozzle decay coefficient are derived and their use in practice is discussed. Available theories for the prediction of the nozzle admittance and available methods for the experimental determination of nozzle admittances are evaluated. Experimental nozzle admittance data obtained by use of the impedance tube method, for two different solid-propellant rocket nozzles, is presented and discussed. An analysis of the experimental nozzle admittance data shows that 1) the admittances of short nozzles are independent of the frequency when the wavelength of the oscillation is much longer than the length of the nozzle convergent section, 2) the admittances of short nozzles are practically independent of the geometrical details of their convergent sections, and 3) the measured nozzle admittances are larger than those predicted by the short nozzle theory. The reported experimental data are used to derive expressions for the nozzle decay coefficients for cylindrical combustors experiencing axial instabilities. These expressions are compared with a corresponding expression derived in related experimental studies and good agreement is shown.