Transient burning behavior of phase-stabilized ammonium nitrate based airbag propellant

Understanding the burning rate of an automotive airbag propellant is extremely important. Airbag propellants need to produce gas on the order of milliseconds. The speed with which the airbag inflator needs to work ensures that the propellant experiences a large pressure transient during combustion. Therefore, one factor that needs to be considered with airbag propellant combustion is how the propellant burns under a rapidly changing chamber pressure. This paper presents the results from a study on the dynamic burning behavior of a phase-stabilized ammonium nitrate (PSAN) propellant during rapid pressure changes. First, the steady-state burning behavior of the propellant was investigated with an optical strand burner and it was found that the burn rate could be fit to Saint-Robert's law with the expression being r_b,SS = 1.393P ^0.873 for pressures up to 28 MPa and r_b,SS = 3.779P ^0.570 for higher pressures. Second, the dynamic burning behavior of the propellant was measured directly in an "O-frame" chamber that used real-time x-ray radiography (RTR) to measure the instantaneous regression behavior of the propellant. For high-pressurization rate tests (~2000 MPa/s), the dynamic burning rate was found to be almost twice the steady-state value. Finally, the dynamic burning data taken from pressure and RTR system measurements was fit to a correlation relating the dynamic burning rate to the instantaneous chamber pressure and pressurization rate. Almost all of the experimentally determined dynamic burn rates were found to be within ±15% of the correlated expression.