## 2 System Analysis and Design Methodology
### Requirements and Design Considerations  
The propulsion system designated MIMPS-G is aimed to be a primary propulsion system that enables high-thrust impulsive maneuvers. MIMPS-G operates on green monopropellants classified as Energetic Ionic Liquids (EILs). The research interest and focus were oriented toward studying HAN-based and ADN-based propellants, and a special focus was given to low decomposition chamber temperature formulations. As ASCENT (formerly AF-M315E) is considered one of the most mature and widely used and proposed green monopropellants, other alternatives were sought to allow variation in propellant sources and performance improvement. During the mechanical design of the propulsion system, emphasis was made on system modularity and expandability, where the former will allow to easily orient components within a spacecraft with different standard CubeSat sizes and make the best use of allowable space. The latter, namely the expandability, is a unique design criterion that will further impact CubeSat utilization and clustering of COTS propulsion systems. Along with modularity, the expandability property will give the ability to increase propellant tanks and even thrust levels on a \"plug-and-play\" basis. Recently, researchers in the field of small satellites are seeking rapid prototyping and low-cost manufacturability [25] by employing additive manufacturing techniques. Metal 3D printing nowadays utilizes exotic space materials such as Inconel-625\({}^{\text{\textregistered}}\) and Ti-6 Al-4V  
Figure 5: Preliminary design flow chart. (Ti64). This manufacturing technique will help in reducing the parts number in a given design and thus overall part mass, as well as reducing prototyping and manufacturing processes lead-time. Design requirements, refer to Figure 4, imply having a thrust level of 0.5 N, gravimetric specific impulse \(\geq\)200 s, and total impulse \(\geq\)500 N s which is almost the lowest value available in the market in this class of commercial propulsion systems. Choosing an 0.5 N thruster was the maximum possible value from a single thruster to be integrated into a 1U unit size due to size and dimensions constraints. Moreover, when comparing 0.1 to 0.5 N thrusters, the higher value is considered better when employing impulsive maneuvers which translates to lower burn time. In addition, the high thrust becomes the dominant term between the external forces acting on the satellite such as gravitational forces and significant drag forces in very low orbits, (Section 9.1.1 in [36]); all these reasons contribute to better efficiency of high-thrust propulsion systems. Another point that favors the 0.5 N thruster is \"clustering\"; in the case of using a lightweight 3U satellite, 0.1 and 0.5 N thrust levels would not have a significant impact on performance, but in the case of clustering several propulsion modules to a larger size spacecraft (such as Figures A4-A6 (Appendix A)), every available newton of thrust will contribute significantly to the maneuver efficiency. Regarding the specific impulse value to be \(\geq\)200 s, two points have introduced this value, first one is the specific impulse value of hydrazine (i.e., \(\sim\)236 s theoretical vacuum) and the other is the value associated with high concentration hydrogen peroxide (e.g., HTP 98% \(\approx\) 186 s); the previous implies that the value of the used green propellant should be at least 200 s to outperform hydrogen peroxide as green monopropellant and still maintain a relatively high performance if considering the classic toxic hydrazine. Concerning the value of the gravimetric specific impulse mentioned, as widely interpreted in design literature, the higher \(I_{sp}\) is considered better, but this is not always the correct interpretation since it mostly comes usually at cost of higher combustion temperatures, and thus higher weight materials used in thruster's development and thermal management.