# Evaluation of UV-Curable Solid Rocket Propellants' Properties for Advanced 3D Printing Technologies
## 2 Materials and Devices
### Delta UV Curing System  
The mechanical properties of photopolymerizable solid propellants are closely related to the characteristics of the polymer, the photoinitiator, and the degree of conversion achieved by their mixture when exposed to radiation at a specific wavelength [31; 32; 33]. In the earlier steps of this research, the reactivity of the propellant to UVA radiation at a wavelength of 375 nm was evaluated [18], revealing significant conversion and improvements in mechanical properties. [PERSON] et al. [34] observed enhancements in the mechanical performance of UV-cured solid propellants exposed to various frequencies. Although the previous study by [PERSON] et al. did not disclose their specific formulation, the current research pursued a multifrequency analysis based on the absorption characteristics of the photoinitiator used here. In fact, BAPO exhibits two distinct absorption plateaus: one at UVA wavelengths and another in the low-UVB-high-UVC range. For this reason, a UVA-UVC illuminator (Figure 3) was designed to examine the degree of conversion of the dispersion consisting of PB, pentaerythitol tetrakis(3-mercaptoethanolpropionate) (PTTM), which  
Figure 2: Electronic scheme of the PID circuit. is crucial for the thiol-ene reaction, and bis-(2,4,6-trimethylbenzoyl) phenylphosphine oxide (BAPO). The core of the illuminator consists of a hexagonal metal plate, serving as a heat sink, on which three UVA LEDs (Luminus LT-4789, Led-Tech.de, Moers, Germany) and three UVC LEDs (Luminus LT-4783, Led-Tech.de, Moers, Germany) are mounted, with wavelengths of 375 and 285 nm, respectively. These wavelengths were chosen based on a trade-off between the market availability and the absorption spectrum of the photoinitiator, i.e., BAPO, which exhibits two peaks at 288 nm and 371 nm [35; 36]. Furthermore, the selected UVA LEDs are identical to those utilized in the final 3D printer prototype. The metal plate is connected to a delta kinematics system (Anycubic Kossel Delta Printer series, Anycubic Technology Co., Ltd., Shenzhen, China) via an additional hexagonal PLA plate. The movement of the delta mechanism is controlled by three stepper motors located under the print bed, which are connected to a BTT SKR 1.3 board. The LED circuit is segmented into two parallel subcircuits, with each containing LEDs configured in series. The LEDs are turned on and off using two manual switches integrated into the illuminator structure. The circuit is powered at 24 V and integrated with the BED pin outputs of the BTT SKR 1.3 board. The decision to place LEDs of the same category in series was based on the power limits of the board. To ensure safety and prevent exposure to UV radiation, the entire system is enclosed by walls, not depicted in Figure 3a, shielding the eyes of the user. The precise movements achievable with the delta kinematics allowed for the accurate characterization of the illuminator. On the left side of Figure 4, a MATLAB (R2024a) model illustrates the predicted light intensity distribution in the XY plane at various distances from the transmitters, taking into account the LED positions and reflections on the walls. Using the DeltaOhm HD2102.1 radiometer (Delta OHM, Padua, Italy), the UVA irradiance was measured; the results are presented in Figure 4 on the right. The distances analyzed for UVA ranged from a minimum of 17 to 44.5 mm, with 2.5 mm spacing between the measurement planes. According to their  
Figure 3: UVA-UVC illuminator assembly (**a**) and views of the LED platform (**b**,**c**). Not to scale. (**a**) (1) Mechanism moving LED platform; (2) print bed. (**b**) (1) UVA LEDs; (2) UVC LEDs; (3) metal plate (heat sink); (4) PLA plate (cinematism interface); (5) clamping system. (**c**) Layout of UVA and UVC LEDs. datasheet, the chosen UVC LEDs were expected to have an emission equal to one-tenth of that of UVA. Following the characterization of the illuminator, a distance was selected to achieve a homogeneous distribution of irradiance across the surface of the sample (with variations below 10%).