### Space Mission Requirements
The HNP family of green monopropellants ignite using catalytic decomposition. [PERSON] et al. 2017 [25] performed tests with newly developed proprietary catalysts and showed excellent response and stability compared to hydrazine, either in pulsed mode operation or continuous mode, with preheating temperatures starting from 200 \({}^{\circ}\)C for HNP221 and HNP225 monopropellants. HNP2 xx family performance chart represented in Figure 2 provides for comparison with hydrazine and state-of-the-art EILs, as well as highlighting the melting point of Inconel\({}^{\circledR}\) 625. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Propellant** & **Formulation** & \(I_{sp}\) **(s)** & \(\rho\) & \(\rho I_{sp}\) & \(T_{c}\) \\  & & & **(g cm\({}^{-3}\))** & **(g s cm\({}^{-3}\))** & **(K)** \\ \hline LMP-103S & \({}^{(1)}\) 63.0\% \({}^{(2)}\) 18.4\% \({}^{(6)}\) 18.6\% & 252 & 1.24 & 312.48 & 1903 \\ FLP-103 & \({}^{(1)}\) 63.4\% \({}^{(2)}\) 11.2\% \({}^{(5)}\) 25.4\% & 254 & 1.31 & 332.74 & 2033 \\ FLP-106 & \({}^{(1)}\) 64.6\% \({}^{(3)}\) 11.5\% \({}^{(5)}\) 23.9\% & 255 & 1.357 & 344.6 & 2087 \\ FLP-107 & \({}^{(1)}\) 65.4\% \({}^{(4)}\) 9.3\% \({}^{(5)}\) 25.3\% & 258 & 1.351 & 348.5 & 2142 \\ \hline \hline \end{tabular} \({}^{(1)}\) ADN. \({}^{(2)}\) Methanol. \({}^{(3)}\) MMF. \({}^{(4)}\) DMF. \({}^{(5)}\) Water. \({}^{(6)}\) Ammonia (aq. 25\% concentration). @ 20\({}^{\circ}\)C.  
\end{table} 
Table 3: ADN-based monopropellants properties [18; 22; 23] (ideal vacuum \(I_{sp}\) by [22] using NASA CEA @ 2.0 MPa chamber pressure, 50:1 expansion ratio assuming frozen condition [16]). Among the state-of-the-art green monopropellants surveyed above, four EILs were considered for a trade-off study, Table 5, either for their maturity or for their promising potential. During the process of nominating candidate propellants for the propulsion system design, the rocket performance characteristics of each propellant (such as the volumetric specific impulse) were not the main focus as the selection criteria. Significant attention was put on the propellants' thermochemical characteristics (i.e., the adiabatic flame temperature) since the lower adiabatic flame temperature would impact the thruster design simplicity as well as mass and costs reduction. The rest of the selection aspects such as operation pressure-temperature conditions, service temperature and vapor pressure were placed according to the typical requirements of the spacecraft propulsion systems under study. The characteristics of the proposed EIL green monopropellants for MIMPS-G500 mN propellant trade-off study and the propellants trade-off requirements are presented in Tables 5 and 6, respectively. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Propellant**} & **Theoretical** & **Density** & **Volumetric** & **Freezing** & **Vapor** \\  & **Vacuum** & \(\rho\) & **Temp.** & **Temp.** & **Pressure** \\  & **\(I_{sp}\) (s)** & **(g cm\({}^{-3}\))** & **(g s cm\({}^{-3}\))** & **\(T_{e}\) (K)** & **(kPa)** & \\ \hline \hline HNP209 & 260 & 1.32 & 343 & \(\sim\)1900 \\ HNP221 & 241 & 1.22 & 294 & 1394 \\ HNP225 & 213 & 1.16 & 245 & 990 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Performance and physical properties of HNP2 xx green monopropellants family [25] as cited in [27]. Figure 2: HNP2 xx green monopropellants family performance chart compared to other EILs. Adapted from [PERSON] and [PERSON] 2017 [25] with permission. Trade-off criteria in Table 7 were set to fulfill previously elaborated design goals and the rationale behind each criterion is described in the following. The first criterion is the specific impulse \(I_{sp}\) (s) which is by definition one of the most important performance parameters in the design and evaluation stages. Generally, \(I_{sp}\) increases with higher combustion temperatures and by burning propellants possessing lighter and molecularly simpler combustion products. As highlighted before, the higher the value of such parameter is not necessarily the better for the system performance.