Admittance measurements of solid propellants by an acoustic oscillator technique

A dynamic acoustic oscillator technique has been developed for measuring the response of a burning solid propellant surface The experimental arrangement essentially consists of a centrally vented cavity with propellant at one end and a phase-locked mechanical driver, which excites the cavity in a resonant mode, at the other end The bandwidth or Q of the resonance is determined by phase modulation of the mechanical driver and measurement of the resultant side-band amplitudes An independent method for measuring the decay constant of the cavity incorporated into this system involves momentarily shorting out the drive system and observing the free-decay or growth of the oscillations Although the apparatus was designed primarily for investigation of stable or marginally stable propellant systems, it can also be used with unstable propellant systems by introducing a 180° phase shift in the drive circuit whenever the amplitude exceeds a predetermined value Experiments are described in which lock-on to resonance was achieved and maintained for runs of several seconds duration The oscillation amplitude and Q are generally not constant, but change appreciably and iri egularly during a run indicating that the damping of the cavity is undergoing substantial variations