Ultrasonic measurement of the pressure-coupled response function at low frequency for composite solid propellants

Transient phenomena associated with solid propellant combustion are examined by means of an oscillatory burner designed to create pressure oscillations through the use of a rotary nozzle. The oscillatory burner is designed to operate at pressures up to 20.7 MPa (3000 psi), with the target operating pressure achieved through pre-pressurization with an inert gas (helium) and manipulation of constant and variable-area nozzle diameters. A spherically focused 1 MHz ultrasound transducer is pulsed at 2.5 kHz through the bottom of the propellant to obtain the thickness-time profile of the propellant, which is then converted to a burning rate by taking the time derivative. Two piezoelectric pressure transducers simultaneously record the pressure-time history of each experiment. The burning rate and pressure are then used to calculate the pressure-coupled response of each test. The experiments presented here were motivated by three primary goals. The first was to investigate the effect of propellant composition on the response function. The second was to evaluate effect of mean pressure on the response of propellants and the thud was to investigate the response of a propellant withm a plateau region. Experiments completed for response function purposes targeted pressures of 2.0, 5.0 and 12.5 MPa at frequencies ranging from 20-200 Hz. Data taken for propellants with different additives (aluminum and a transition metal oxide) were compared with baseline propellants to determine the effects of these additives on the response function. The effect of mean pressure was evaluated through comparison of tests on one propellant performed at two different mean pressures (2.0 and 5.0 MPa). The response within a plateau region was investigated through low frequency tests that targeted the plateau of a composite propellant (12.5 MPa).