# Evaluation of UV-Curable Solid Rocket Propellants' Properties for Advanced 3D Printing Technologies
## 1 Introduction
The sustainability of 3D printing technology has received positive evaluations from major aerospace companies, which recognize significant economic advantages due to the reduced materials and energy required to produce the same components. The ability to develop more complex geometries, reducing the structure weight and increasing the payload capacity, is one of the main benefits offered by 3D printing. Furthermore, the potential to reduce the production timeline is another crucial factor that contributes to the success of this technology [6]. Within the solid propellant sector, 3D printing has the potential to revolutionize grain production by enabling customized geometries that are otherwise unachievable through conventional casting techniques. However, the high solid content and small particle size required for optimal performance significantly increase the viscosity, complicating extrusion-based printing processes [7]. While 3D printing technologies, such as selective laser melting, have led to significant innovations in the fabrication of launcher structures, solid propellants continue to rely on traditional manufacturing methods. This is due to numerous challenges encountered during the extrusion process. The high percentage of solid content and small particle sizes necessary to achieve high propulsive performance have been observed to increase the viscosity of composite propellants, making extrusion even more complex [8]. Although propellants exhibit pseudoplastic behavior, the considerable pressures required to overcome viscous forces within the extruder lead to significant increases in the cost and need for safety systems [9; 10; 11]. Furthermore, under high applied pressures, the separation of the binder from the solid component may occur, resulting in inconsistent results [12]. The presence of curing agents in the mixture further increases the viscosity over time, complicating the process of printing a solid propellant. In recent years, a novel technique has emerged that leverages the thixotropic properties of propellants through the application of ultrasound [12]. This method effectively reduces the viscosity of the propellant near the nozzle, facilitating extrusion. However, it is important to note that this technique is not universally suitable for all types of curing agents. In some cases, it can lead to the rapid hardening of the propellant, potentially resulting in a blockage within the extruder [4; 12; 13]. While previous studies [14; 15; 16; 17] have successfully demonstrated the feasibility of LED-UV photocuring in solid propellant formulations, they have typically relied on conventional extrusion methods that may necessitate ultrasound to mitigate high viscosity. In contrast, the present study introduces an alternative extrusion method that utilizes temperature effects and the inherently low viscosity of specific curing agents to facilitate rapid curing and maintain the shape fidelity without ultrasound assistance. Building on the foundational findings of earlier work [18; 19; 20; 21], this work further explored the impact of varying UV wavelengths and preheating on the binder's polymerization behavior, providing critical insights into optimizing the rheological properties of the propellant. For this reason, rheological properties, including the pot-life and pseudoplasticity, were also studied. A prototype printer was developed to conduct printing tests, focusing on identifying and addressing critical challenges in the additive manufacturing of solid propellants. The findings of this research provide valuable insights into optimizing the printability and performance, with potential applications in advanced propulsion systems.