Observation of aluminum interaction with the binder melt layer using high-speed synchrotron-based phase contrast imaging

Particle interactions with the binder melt layer are a major factor in the combustion efficiency and stability of the propellant in a solid rocket motor. Metal particles tend to agglomerate on the surface of burning solid propellants, inhibiting the combustion process. Therefore, reduction of molten aluminum agglomeration in a solid rocket motor is vital to the improvement of solid rocket propulsion system performance. In other work, quenched samples have been used to study the impact of how metal particles alter the flow properties of the molten propellant surface. In-situ optical measurements have also been attempted for these particle-condensed phase interactions, but with little success due to the opacity and strong gradients within the flame. This study expanded on previous work using synchrotron–based x-rays to directly image these aluminum agglomerates as they interacted with the binder. X-ray absorption and phase contrast in the images allowed for the direct measurement of the particle size in combusting propellants in-situ at typical rocket pressures. Particle size distributions were collected in an optically accessible combustion vessel over a pressure range of 1.4- to 6.9-MPa (200- to 1000-psig). This investigation measured the aggregate sizes ranging from 300-to 650-µm at a 6.9 MPa chamber pressure. Interestingly, the lowest binder melting temperature produced the longest aluminum chaining at 1.1 mm and the highest binder melting temperature produced the second longest chain at 524 µm long. The study observed aluminum interactions with the binder melt layer on the propellant surface that may be a contributing factor in a plateau propellant. Additionally, data pointed to possible relationships between binder viscosity, burning rate, aluminum clusters, and chain formation in a solid propellant. The study concluded with collecting data on the molten aluminum volumetric changes in a propellant environment, thus defining a starting particle diameter for particle regression rates. Collecting the regression rates also provided information on the evaporation rate of aluminum, which burned in the propellant flame. All of these data are critical for understanding the combustion of aluminum-based solid rocket propellants and outlines knowledge gaps that need additional data.