<p>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.</p> <p>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. </p> <p>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. </p>

<div>The new technique of Vibration-Assisted 3D Printing (VAP) offers significant potential for leveraging the geometric flexibility of additive manufacturing (AM) into the realm of solid energetics. The first part of this work compares the print capabilities of a custom-made VAP printer to those of an established commercial direct-write printer using a polymer clay. Characterization tests were conducted and a variety of other shapes were printed comparing the two methods in their turning quality, feature resolution, unsupported overhang angle, negative space feature construction, and less-than-fully-dense self-supported 3D lattices. The porosity and regularity of the printed lattices were characterized using X-ray microtomography (MicroCT) scans. The quality of the shapes was compared using statistical methods and a MATLAB edge-finding code. The results show that the VAP printer can manufacture parts of superior resolution than the commercial printer, due to its ability to extrude highly viscous material through a smaller nozzle diameter. The VAP print speeds were also found to be as high as twenty times higher than those of the direct write printer.</div><div>Following up on this work, a second study explored the possibility of modifying grain geometry through variation of printed infill design using an ammonium perchlorate composite propellant (APCP). In the propellant formulation, a polymer that cures under ultra-violet (UV) light was used instead of the more common hydroxyl-terminated polybutadiene (HTPB). Although this formulation is a less-effective fuel than HTPB, its use enables layer-by-layer curing for improved structural strength during printing. Using VAP, cylindrical propellant charges were prepared using a gyroidal infill design with a range of internal porosities (infill amounts). Some additional propellant grains were prepared with both vertical and concentric layering of different infill amounts. These grains were then burned beginning at atmospheric pressure in a constant-volume Parr cell to measure the resulting pressure output. Analysis of the pressure trace data shows that a less-dense infill increases the maximum pressurization rate, due to the presence of small voids spaced roughly uniformly throughout the grain that increase the burning surface area. We show that additive manufacturing-based propellant grain modification can be used to tailor the pressure-time trace through adjustment of the number and size of small voids. Specifically, this study shows that, using a graded functional geometry, the duration of gas generation can be controlled. This work represents a preliminary effort to explore the possibilities to propellant</div><div>12</div><div>manufacture offered by additive manufacturing and to begin to address the challenges inherent in making it practical.</div>

<p dir="ltr">Additive manufacturing (AM) of gun propellants is an emerging and promising field which addresses the limitations of conventional manufacturing techniques. Overall, this thesis is a body of work which serves to bridge the gap between fundamental research and application of additively manufactured gun propellants.</p>

<p>Additive manufacturing of gun propellants is an emerging and promising field which addresses the limitations of conventional manufacturing techniques. Gun propellants are manufactured using wetted extrusion, which uses volatile solvents and dies of limited and constant geometries. On the other hand, additive techniques are faced with the challenges of maintaining the gun propellant’s energetic content as well as its structural integrity during high pressure combustion. The work presented in this thesis demonstrates the feasibility of producing functioning gun propellant grains using vibration-assisted 3D printing, a novel method which has been shown to extrude extremely viscous materials such as clays and propellant pastes. At first, the technique is compared to screw-driven additive methods which have been used in printing gun propellant pastes with slightly lower energetic content. In chapter two, diethylene glycol dinitrate (DEGDN), a highly energetic plasticizer, was investigated due to its potential to replace nitroglycerin in double base propellants with high nitroglycerin content. A novel isoconversional method was applied to analyze its decomposition kinetics. The ignition and lifetime values of diethylene glycol dinitrate were obtained using the new isoconversional method, in order to assess the safety of using the plasticizer in a modified double base propellant. In chapter three, a modified double base propellant (M8D) containing DEGDN was additively manufactured using VAP. The printed strands had little to no porosity, and their density was nearly equal to the theoretical maximum density of the mixture. The strands were burned at high pressures in a Crawford bomb and the burning was visualized using high speed cameras. The burning rate equation as a function of the M8D propellant as a function of pressure was obtained. Overall, this work shows that VAP is capable of printing highly energetic gun propellants with low solvent content, low porosity, with high printing speeds, and which have consistent burning characteristics at high pressures. </p>