ADDITIVE MANUFACTURING OF VISCOUS MATERIALS: DEVELOPMENT AND CHARACTERIZATION OF 3D PRINTED ENERGETIC STRUCTURES
The performance of solid rocket motors (SRMs) is extremely dependent on propellant formulation, operating pressure, and initial grain geometry. Traditionally, propellant grains are cast into molds, but it is difficult to remove the grains without damage if the geometry is too complex. Cracks or voids in propellant can lead to erratic burning that can break the grain apart and/or potentially overpressurize the motor. Not only is this dangerous, but the payload could be destroyed or lost. Some geometries (i.e. internal voids or intricate structures) cannot be cast and there is no consistent nor economical way to functionally grade grains made of multiple propellant formulations at fines scales (~ mm) without the risk of delamination between layers or the use of adhesives, which significantly lower performance. If one could manufacture grains in such a way, then one would have more control and flexibility over the design and performance of a SRM. However, new manufacturing techniques are required to enable innovation of new propellant grains and new analysis techniques are necessary to understand the driving forces behind the combustion of non-traditionally manufactured propellant.
Additive manufacturing (AM) has been used in many industries to enable rapid prototyping and the construction of complex hierarchal structures. AM of propellant is an emerging research area, but it is still in its infancy since there are some large challenges to overcome. Namely, high performance propellant requires a minimum solids loading in order to combust properly and this translates into mixtures with high viscosities that are difficult to 3D print. In addition, it is important to be able to manufacture realistic propellant formulations into grains that do not deform and can be precisely functionally graded without the presence of defects from the printing process. The research presented in this dissertation identifies the effect of a specific AM process called Vibration Assisted Printing (VAP) on the combustion of propellant, as well as the development of binders that enable UV-curing to improve the final resolution of 3D printed structures. In addition, the combustion dynamics of additively manufactured layered propellant is studied with computational and experimental methods. The work presented in this dissertation lays the foundation for progress in the developing research area of additively manufactured energetic materials.
The appendices of this dissertation presents some additional data that could also be useful for researchers. A more detailed description of the methods necessary to support the VAP process, additional viscosity measurements and micro-CT images of propellant, the combustion of Al/PVDF filament in windowed propellant at pressure, and microexplosions of propellant with an Al/Zr additive are all provided in this section.