# Effect of Structural Materials on Monopropellant Thruster Propulsion Performance in Micro Scale
## 4 Results and Discussion
### Internal Flow  
From the thrust equation, it is notable that the velocity and pressure at the nozzle exit affect thrust performance, and thus flow characteristics with respect to the structural materials and corresponding thermal diffusivity warrant further investigation. Mach number, which is a function of velocity and temperature, is largely affected by the temperature, and its characteristics differ from those of velocity. The contour of the Mach number, coupled with different structural materials, is illustrated in Figure 10. This has a sonic speed contour with a Mach number 1 and supersonic flows at the diverging section, with contours of Mach numbers 1.5 and 1.7. It was found that the location of the Mach number contours varied depending on the structural materials. For example, where Mach 1.7 contour was located for the adiabatic case was the most downstream, while the copper case had it at the most upstream, with other material cases being between them. This potentially indicates that depending on structural materials, different nozzle expansion ratios should be considered to meet a targeted Mach number at the nozzle exit and the corresponding amount of heat loss in the micro scale. In a low thermal diffusivity range, such as adiabatic, glass, and LTCC, there was a bigger distance between the Mach 1.7 contours compared to other cases. For example, as the thermal diffusivity increased, the contours converged to an extreme degree. Due to the structural material effect on the Mach number, the nozzle exit Mach number was the highest for the copper microthruster..  
Figure 8: Comparison of the average temperature on the outside surface and that inside the chamber inlet of the microthruster with materials of various levels of thermal diffusivity and different propellant enthalpies (85, 90, and 95 wt% hydrogen peroxide). In regard to the 85, 90, and 95 wt% hydrogen peroxide, the Mach 1.7 contours of the materials closely converged in the case of the former but were divergent in regard to the latter two, with increasing weight percentages. With regard to the nozzle exit Mach number, it was observed to be the highest in the case of the 85 wt% propellant, mainly due to the temperature differences, despite having the lowest nozzle exit velocity. All the Mach 1.7 contours shifted in the downstream direction when the propellant weight percentage increased from 85 to 95 wt%, resulting in the lowest Mach number in the case of the 95 wt% propellant and a reduced likelihood of further expansion through the diverging section. With regard to Mach number 1 contours, they did not vary noticeably in relation to the different propellant weight percentages. In addition, the location of the sonic speed contours did not vary with respect to thermal diffusivity, indicating that the variation in the structural materials marginally affected the sonic speed contour near the nozzle throat. The temperature distribution on the symmetry plane in the fluid domain is illustrated in Figure 11. Here, three simulations are compared: the adiabatic, the glass, and the copper microthrusters. In relation to the adiabatic thruster, heat loss in the chamber was demonstrably negligible, while the temperature was largely constant throughout the whole chamber. In regard to the glass and copper microthrusters, the temperature was constant at the chamber inlet, as given as a boundary condition. The temperature at the near-wall region decreased as the heat transfer occurred at the fluid-solid domain interface. The region with a temperature lower than the adiabatic temperature gradually developed as it moved downstream due to continuous heat loss. It was observed that the lowest temperature region inside the chamber was in the rectangular channel located where the chamber met the nozzle. This was attributable to the long retention of the fluid, with resulting heat loss. The low temperature of the fluid emanated from this region and further heat transfer through the nozzle wall, which reduced the temperature of the fluid even further in the downstream region.