# Effect of Structural Materials on Monopropellant Thruster Propulsion Performance in Micro Scale
## 4 Results and Discussion
### Chamber Pressure and Thrust
The discrepancy was expected from the fact that a smaller nozzle throat area with a higher chamber design pressure than required was considered in the thruster design process of the previous study, which was to take into account the potential MEMS fabrication error, particularly through photosensitive glass wet etching process for the micronozzle and polishing process affecting the profile layer thickness and nozzle height. By comparing the simulation cases, it was found that regardless of propellant weight percentages, there was a dramatic degradation in the amount of thrust produced with respect to thermal diffusivity, particularly in the range of low thermal diffusivity up to 3.7 mm\({}^{2}\)/s, while the variation in the relatively large thermal diffusivity range from around 3.7 mm\({}^{2}\)/s to 105.8 mm\({}^{2}\)/s appeared to be marginal. Theamount of thrust degradation in the thermal diffusivity range of up to 3.7 mm\({}^{2}\)/s was 5.6, 6.0, and 6.4%, with the adiabatic thrust as a reference point for 85, 90, and 95 wt% hydrogen peroxide, respectively. Further degradation with respect to thermal diffusivity, for example up to 105.8 mm\({}^{2}\)/s, was 6.1, 6.5, and 6.9% in the case of the adiabatic, for 85, 90, and 95 wt% H\({}_{2}\)O\({}_{2}\), representing marginal thrust degradation of only about 0.5% in regard to all the propellants, for a thermal diffusivity range from 3.7 mm\({}^{2}\)/s to 105.8 mm\({}^{2}\)/s. Here, it was established that the variation in thrust was sensitive to the variation in thermal diffusivity in the low thermal diffusivity range, where there was greater variation in heat loss corresponding to the variation in thermal diffusivity. When thermal diffusivity was sufficiently high, the heat was conducted rapidly, and the structural material soon reached its thermal equilibrium state, resulting in a minor variation in thrust with respect to the variation in thermal diffusivity. Figure 14 describes the amount of thrust produced from the microthruster with respect to microthruster fabrication materials and propellant weight percentages. As discussed earlier, it was found that there is a larger variation in thrust in the case of lower-thermal-conductivity materials, such as glass and LTCC. These materials are characterized by a relatively larger variation in thrust, and the performance is sensitive to such materials, while highly conductive materials, such as silicon and copper, do not yield a significant difference between them in terms of thrust. The varying performances of the microthrusters composed of different materials and propellant weight percentages are summarized in Table 5.  
Figure 12: Chamber pressure of microthrusters with different thermal diffusivity materials and different propellant weight percentages (mass flow rate of 7 \(\times\) 10\({}^{-5}\) kg/s). Figure 14: Estimated amount of thrust produced by microthrusters with different materials. Figure 13: Thrust generation of microthrusters with different thermal diffusivity materials and different propellant weight percentages. In order to further investigate the sensitivity of the structural material, the thrust of the simulated microthruster and the chamber pressure were compared with the adiabatic as a reference, and the normalized thrust and chamber pressure are shown in Figure 15. Among the material types compatible with fabrication processes in regard to microthrusters, glass, exhibiting an approximately 4% degradation in thrust performance in relation to the simulated microthruster, registered the least degradation, while copper, with 7% degradation, exhibited the greatest amount of degradation among the materials. In terms of chamber pressure, there was evidence of pressure degradation depending on the materials considered due to thermal energy loss.