# Effect of Structural Materials on Monopropellant Thruster Propulsion Performance in Micro Scale
## 5 Conclusions  
Excessive heat loss in micro-scale propulsion has been a long-standing issue, particularly for monopropellant-type thrusters with a propellant that needs to be decomposed via a temperature-dependent catalyst. In order to provide a reference point conducive to the design of micro-propulsion systems and appropriate micro-scale fabrication materials, a numerical analysis was conducted to compare propulsion performance, considering the MEMS fabrication processes and materials used in previous studies on micro-propulsion systems. The result of the simulation was analyzed with respect to the thermal diffusivity of each material and propellant weight percentages to represent varying propellant enthalpies. From temperature distributions across the microthruster structures of different materials, the heat transfer coefficient and thermal energy loss were investigated, and different propulsion performances were identified. The thrust and chamber pressure degraded as thermal diffusivity increased; however, the rate of degradation depended on the amount of thermal diffusivity. The result indicates that in a high thermal diffusivity range, a change in structural materials only marginally affects the variation in propulsion performance, while in a low thermal diffusivity range, it has the opposite effect. Among the MEMS fabrication materials considered, glass exhibited the most favorable propulsion performance and the lowest heat loss, while the temperature distribution on its structural surface recorded the largest temperature difference among the candidate materials with a more insulating effect. Due to the non-negligible thermal energy loss from a thruster in the micro scale, there were varying propellant flow characteristics through the micronozzle causing correspondingly different propulsion performances with respect to structural materials. With the different nozzle exit Mach numbers depending on materials, the result indicated that the nozzle expansion ratio in the micro scale should be designed considering the thermal characteristics  
Figure 15: Normalized amount of thrust generation and chamber pressure of the microthruster in relation to various structural materials and propellant weight percentages.  
of structural materials used in order to achieve a targeted nozzle exit Mach number. The amount of degradation of thrust produced has been reported between 4 and 7% depending on materials for the 50 mN class thruster considered for the simulation, which is expected to be a good reference conducive to microthruster design and fabrication with various materials in different scales. However, as a catalyst embedded in a monopropellant thruster has propellant decomposition efficiency affected by its working temperature, further degradation of propulsion performance is expected to occur in a microthruster, in addition to the structural material effect reported in this study.  
Conceptualization, J.H. and K.S.P.; methodology, J.H. and K.S.P.; software, K.S.P.; validation, J.H. and K.S.P.; formal analysis, J.H. and K.S.P.; investigation, J.H. and K.S.P.; resources, K.S.P.; data curation, J.H. and K.S.P.; writing--original draft preparation, J.H. and K.S.P.; writing--review and editing, J.H. and K.S.P.; visualization, J.H. and K.S.P.; supervision, J.H. and K.S.P.; project administration, J.H. and K.S.P.; funding acquisition, J.H. and K.S.P. All authors have read and agreed to the published version of the manuscript.  
This research was supported by United Arab Emirates University with Grant No. G00004007 (Fund Code No. 12N095) and Grant No. G00003678 (Fund Code No. 12N094).  
The data presented in this study are available in this article.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.