### Space Mission Requirements
Thus, an optimal value must be chosen to achieve considerable overall performance while maintaining suitable system inert mass and components' design simplicity and cost; this is possible when considering the thruster's material choice which is highly coupled with the resulted adiabatic flame temperature. The \(I_{sp}\) (s) criterion was evaluated for the considered propellants by a knockout condition, that the considered propellants shall possess \(I_{sp}\geq 200\) s as expressed in requirement three in Table 6, all propellants fulfilling this criterion shall score equally the highest score. The second criterion is the volumetric specific impulse \(\rho I_{sp}\) (g s cm\({}^{-3}\)), generally, the higher propellant density shall occupy lower tank volume, thus a higher value is considered better, and the score is evaluated accordingly. The third criterion, the decomposition chamber temperature \(T_{c}\) (K), is one of the most important parameters in this trade-off study, as conceptualized earlier. The lowest decomposition chamber temperature value is considered the best among all considered propellants, and a weight factor of (\(\times 2\)) is imposed to emphasize the importance of this criterion. Freezing temperature \(T_{F}\) (\({}^{\circ}\)C), or service temperature as more accurately described, since some EILs undergo precipitation [28] or glass transition as in the case of AF-M315E [29], is the fourth criterion assessed in the trade-off study. A low freezing point is required for the propellant's storable and operational stability over a long time and is important to reduce tank heating power consumption. The last criterion is the vapor pressure \(P_{\text{up}}\); EIL green monopropellants are characterized by very low vapor pressure that allows for stable ground testing, storability, and transportability as well as in-space operability. Since this study focused on unconventional autogenous pressurization, the use of low vapor pressure propellants is crucial for the propulsion system's operational stability. Higher vapor pressures, to some extent, would definitely optimize the use of electric heating power for thruster feed and tank pressurization, however, in early development phases the lower vapor pressure is more appreciated. Table 7 presents the propellant trade-off criteria, methods of calculation and evaluation for each criterion as well as the value function considered. The \"Value Function\" is a tool to assist in scoring each propellant against the trade-off criteria. Two main types of value functions are used, namely \"The Higher the Better\" and \"The Lower the Better\", and another one is a knockout condition. The latter condition would discard any propellant with theoretical specific impulse <200 s, while the other two value functions will be graded on a 0-10 scale, with the minimum and the maximum values depending on each value function type, refer to Figure 3. \begin{table}
\begin{tabular}{c c} \hline \hline
**Requirement** & **Description** \\ \hline
1 & Use of Green propellant complying with ECHA–REACH directive articles. \\
2 & Use of monopropellants classified as EIL. \\
3 & EIL Green Monopropellants should have specific impulse performance of \(I_{sp}\geq 200\) s. \\
4 & Freezing temperature of the propellant shall be \(\leq-10\)\({}^{\circ}\)C. \\
5 & Propellant must be liquid within pressure range [0.1, 3] MPa and temperature range [\(-30\), \(+80\)]\({}^{\circ}\)C. \\
6 & Propellant shall possess Low Vapor Pressure, typically below 20 kPa at room temperature (LMP-103S is \(\sim\)14 kPa @ 25 \({}^{\circ}\)C [16]). \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Propellants’ trade-off study requirements.