Unsteady solid propellant pressure combustion response using a piston burner

A small-scale solid propellant laboratory burner has been designed to operate under piston-driven oscillating conditions from 40 to 400 Hz and 300 to 900 kPa mean pressure, with 10% amplitudes. The device was used to investigate the burning rate response of a widedistribution, bimodal AP (2:200 μm)-HTPB (IPDI) composite propellant (MURI TKC#5b). Results show that mean pressure and exit temperature decrease with increasing frequency due to increased heat transfer associated with oscillatory gas motion. Propellant burning rate is calculated numerically from pressure, volume, and frequency data using mass and energy balances. A linear analytic solution for pressure and temperature frequency response is also obtained for the case of constant mass injection to characterize general system behavior. The dynamic burning rate response appears to be nearly linear at low frequencies (< 150 Hz) and increasingly non-linear at higher frequencies due to an increasing combustion response magnitude with frequency and nearly constant relative pressure amplitude. In the range of frequencies determined to be most linear (40 to 150 Hz), the magnitude of the pressure-coupled frequency response ranges from 2 to 7 ± 1, with burning rate lagging pressure by 50 to 75°. The in-phase (real) component of the response ranges from 1 ± 1 to 4 ± 2. The results demonstrate the potential for the piston burner to measure pressure frequency response at low frequencies and also to measure non-linear dynamic burn rates over a range of frequencies at high spectral resolution within a single test. The results also demonstrate that nonlinear response can appear as spectral structure in the apparent linear response when the data are interpreted linearly.