Modeling of Layered Ammonium Perchlorate Composite Propellant with Different Burning Rates

Several approaches exist to alter the burning behavior of ammonium perchlorate (AP) composite propellant, such as including catalysts in the formulation or casting perforated grains. However, a functionally graded composite propellant, where various regions have a different burning rate, could result in a higher surface area and effectively a higher burning rate. The combustion of layered AP propellant is a fundamental area of study, but has not been explored in literature due to challenges in constructing controlled, thin layers of propellant that adhere well to each other. However, additive manufacturing is making such functionally graded propellants possible to create, so better understanding of how layered composite propellants burn is important. The purpose of this study is to investigate the combustion of thin propellant layers (~mm) using the RocFire combustion code. Various layered propellant configurations were explored. The slow, outer layers consist of 60-130 µm AP/HTPB propellant while the fast, inner layers were 1) 20 µm, 2) 90 µm, or 3) bimodal 90 µm /60-130 µm AP with a 1:1 coarse to fine ratio propellants. Each layer thickness is 0.54 mm and there are 3 layers per configuration. All simulated propellants are constructed with a solids loading of 75 wt.% (58.6 vol.%) and are simulated at 10.34 MPa. The simulations show that layers regressing perpendicular to the burning front exhibited an accelerating burning rate as the burning surface area developed. Layers oriented parallel to the burning front exhibited dynamic burning rate transitions that were directionally dependent. Specifically, we observed more deviation when transitioning from a fast burning layer to a slow layer, than the inverse. The driving forces of these transitions are still not well understood, but several explanations are presented in this work. These results are relevant to compositional variation that may occur in cast propellants and for informing the design of additively manufactured layered propellant in the near future.