Development and Characterization of a Novel Additive Manufacturing Technology Capable of Printing Propellants with High Solids Loadings

The performance of solid rocket motors and missiles depend on the configuration of the solid propellant grain, which is burned to create thrust. Due to the limited casting processes that can allow one to change the shape of a solid propellant grain (and therefore the thrust) to a certain degree, there is a need to explore techniques such as additive manufacturing to expand the feasible design space of propellant grains. Using a technique called Vibration Assisted Printing (VAP), it recently became possible to use additive manufacturing to 3D print high performance propellant and other viscous materials at a fine resolution (0.6 mm) compared to other commercially available techniques. The VAP process uses vibration at the syringe tip to enable fast flow of the material without high reservoir pressures (which can lead to binder dewetting from particles and safety concerns for confined energetic materials) or significant formulation modifications (i.e. low volume loading or shear thinning agents). 

The objectives of this Ph.D. dissertation work were to: 1) additively manufacture high viscosity ammonium perchlorate composite propellant formulations (72-76 vol.%) without significant defects, 2) experimentally analyze 3D printed propellant microstructure and high pressure combustion properties, 3) develop and characterize a photopolymer that is compatible with energetic materials (including mixtures with opaque fuels such as aluminum particles) that can enable rapid, complete curing, 4) investigate the high pressure combustion of 3D printed functionally graded propellants made of different formulations, 5) develop a fundamental understanding of VAP process parameters, and 6) develop a fundamental understanding of the combustion dynamics of functionally graded propellant. The completed research objectives provide a foundation for constructing defect free, complex, functionally graded structures out of viscous materials. Although the focus of this work was primarily on solid propellant, the processes and methods applied in this thesis can be used to 3D print other energetic materials (i.e. propellants, explosives, and pyrotechnics) as well as inert composite mixtures (i.e. particulate or whisker ceramics/ablatives) into new configurations that could potentially improve important performance metrics (i.e. increase burning surface area in propellants, lower erosion rate in nozzles, or lower shrinkage in post-processed ceramics.