Characterization of composite propellant slurry for additive manufacturing of solid rocket motor grains

In recent years, additive manufacturing has increasingly become a crucial production process because of the many advantages it presents, such as lower costs, versatility, and fewer design restrictions. This last factor is crucial in the production of solid rocket motors as their traditional manufacture methods rely on casting, which locks the possible grain geometries to the shape of the mandrel; moreover, this process can lead to propellant fractures, separation from the liner and cracking. For this reason, many studies have tried to combine additive manufacturing with the production of solid propellants but at the moment an optimal solution has not yet been found. In addition, the polymerization process of solid rocket motors takes place in large thermostatic hoven and the cross-linking reactions require the use of toxic isocyanates. In the previous works of this project, an inert propellant formulation was developed with the aim of optimizing its additive manufacturing production and an innovative curing process using UV rays has been studied; polybutadiene was chosen as binder, and ammonium sulphate was used as an inert substitute for ammonium perchlorate. In this thesis the author tried to improve the previously developed home-made printer, leading to a better understanding of the used slurry behavior and the 3D-printing settings given through the slicing software. The printing process showed some problems because the mixture was found to be very sensitive to the applied extrusion pressure, so it is necessary to optimize several parameters, in particular the extrusion speed. Depending on the configuration of the system and the speed imparted, it was found that the material may incur a phase separation phenomenon in which the percentage of solid content near the output increases, leading to possible breakages. Due to the nature of its components, the slurry also presents a viscoelastic behavior, so several rheometry tests have been carried out using a flat-plates rheometer, allowing to identify the values of viscosity and elasticity varying the frequency, the percentage of salt, and the type of polybutadiene used.