# Evaluation of UV-Curable Solid Rocket Propellants' Properties for Advanced 3D Printing Technologies
## 1 Introduction  
Composite solid propellants are energetic materials that can sustain combustion once ignited by an external energy source. These propellants consist of a binder that is generally a polymer. Within this binder, oxidizer particles, typically ammonium perchlorate (AP), and other particles, usually micrometric aluminum powders, are dispersed. Curing agents, such as isocyanates, are employed to harden the propellant. The binder is crucial for maintaining the structural integrity of the propellant, ensuring that the oxidizer and fuel particles are uniformly distributed. This uniform distribution is essential for consistent combustion and performance. Additionally, additives such as plasticizers, stabilizers, and burn rate modifiers are incorporated to customize the propellant's characteristics for specific applications. Ammonium perchlorate acts as an oxidizer to maintain fuel combustion. Micrometric aluminum powders are used to enhance performance, releasing a substantial amount of energy upon combustion. In general, solid composite propellants are complex formulations designed to optimize the ballistic performance, structural integrity, and stability. The meticulous selection and combination of binders, oxidizers, fuels, and additives are critical to achieve optimal performance. Propulsion systems based on solid propellants are characterized by a wide range of thrust levels and applications. The advantages of these propellants include their high density and the absence of a need for feed systems. This last feature simplifies the construction of the propulsion system compared to their liquid counterpart, resulting in economic benefits and enhanced reliability [1]. The regression of the grain surface at a specific rate, also known as the regression rate, enables rapid gas generation inside the combustion chamber. The total burning surface area of the grain determines the mass flow produced, a critical parameter for the chamber pressure and, consequently, the generated thrust. Depending on the initial grain shape, the burning surface can increase, remain constant, or decrease, leading to a similar trend in the thrust law [2]. A key challenge in solid propellant manufacturing is closely related to this behavior. During traditional cast-and-cure manufacturing, a mandrel is submerged within the propellant to maintain the desired shape. However, the mandrel extraction process at the end of curing imposes limitations on the achievable geometries, thereby restricting the attainable thrust laws. Furthermore, the use of isocyanates, which are highly toxic, requires the implementation of robust safety systems [3]. To address these issues, UV light-sensitive propellants have been developed. The typical curing agents used in these propellants have a lower degree of toxicity compared to isocyanates. In addition, advancements in this technology have enabled the application of additive manufacturing methods for the production of solid propellants. In contrast, recent research has found that the use of isocyanates in HTPB propellants, even if rapidly curing, is incompatible with 3D printing, as the propellant does not cure fast enough to preserve its shape [4]. Therefore, the use of additives capable of polymerizing the propellant in seconds when exposed to UV radiation has effectively solved this problem. In recent years, 3D printing technology has made significant improvements. Consequently, more in-depth studies have been conducted on its compatibility with solid propellant production. The first 3D print was carried out in 1984 by [PERSON] of 3-D Systems Corp. using a prototype printer called the Stereolithography Apparatus. Initially, the technology was economically unfeasible and did not meet market needs. However, in the subsequent decades, costs have drastically decreased, making 3D printers a viable option for a wide range of industrial sectors [5].