Driving processes of finite-amplitude axial mode instability in solid-propellant rockets.

Abstract

In a study of unstable combustion, a wide range of propellant formulations containing different oxidizers and other ingredients has been prepared and tested in several scale rocket motors in which incipient instability in the axial mode was studied by injecting finite-amplitude pulses. The resulting finite-amplitude wave instability was studied using multistation probing with piezoelectric transducers. Ammonium perchlorate propellants, in a certain burning-rate/pressure spectrum, were found to be highly unstable. Propellants outside this spectrum, including those based on potassium perchlorate and ammonium nitrate, which burn significantly faster or slower, appeared completely stable in our scale motors. Two propellants possessing similar burning rates were formulated; the ammonium nitrate/ potassium perchlorate propellant was stable, whereas the ammonium nitrate/ammonium perchlorate propellant was very unstable. Instability appears to be associated with the energy release profile of burning ammonium perchlorate. Limited study of double-base propellants has shown that axial instability occurs only with relatively slow-burning compositions at low pressures where the fizz burning zone is thicker. The experimental data have been developed to provide information of significance to the rocket design engineer. Using the compositional information presented, propellants can be selected with enhanced assurance that they are stable to both transverse and axial instability.