## 2 Research Status on Anhydrous Hydrazine Fuel
### Research Status on MMH Fuel  
MMH is an essential compound in engineering and science due to its potential applications in propulsion systems and other industrial processes. Understanding MMH's decomposition kinetics, reaction mechanisms, and combustion properties is crucial for optimizing performance, ensuring safety, and improving efficiency in these applications. Methylhydrazine (CH\({}_{3}\)NHNH\({}_{2}\), MMH) is formed when a methyl group replaces an H atom on N\({}_{2}\)H\({}_{4}\). In the 1960s, [PERSON] et al. [33] investigated the primary reaction rate constants for uniform bond breaking of N-N bonds in N\({}_{2}\)H\({}_{4}\) and MMH [60]. [PERSON] and [PERSON] [25] studied N\({}_{2}\)H\({}_{4}\), MMH, and UDMH decomposition reactions at atmospheric pressure and 750-1000 K in adiabatic conditions. The decreasing order of magnitude of the decomposition rate was UDMH > MMH > N\({}_{2}\)H\({}_{4}\). [PERSON] et al. [61] conducted a pyrolysis study of 1-3% MMH/argon mixtures in 38.4 mm excitation tubes in the temperature range of 1040 to 1370 K and pressure range of 1.4 to 4.5 atm. A simplified kinetic model and mechanistic diagram were also presented to interpret the experimental data. The results show that MMH can decompose exothermally at 70 to 80 mo1% argon dilution. The complete kinetic model can simulate this explosive decomposition reaction, showing that pure gaseous MMH is much less sensitive to explosive bombardment than hydrazine. For the theoretical calculation of the pyrolysis reaction kinetics of MMH, [PERSON] and [PERSON] [62] investigated the kinetics of the thermal decomposition reaction of MMH using the quantum Rice-Ramsperger-Kassel (QRRK) theory and pressure drop master equation analysis. The potential barrier was calculated at the B3 LYP/6-311G(d,p) level, and the enthalpies of production, \(\Delta\)H'\({}_{f}\) (298.15 K) of the parent MMH (CH\({}_{3}\)NHNH\({}_{2}\)) and its corresponding radicals CH\({}_{3}\)NNH\({}_{2}\), CH\({}_{3}\)NHNH, and CH\({}_{2}\)NHNH\({}_{2}\) were determined to be 21.6, 48.5, 51.1, and 62.8 kcal/mol, respectively, by isocratic reaction analysis and various ab-initio calculation methods. The kinetic analysis undertaken at the CBS-QB3 level and simulating MMH's homogeneous decomposition under atmospheric pressure conditions informed that the N-N and C-N bond-breaking reactions are the main pathways. Subsequently, [PERSON] et al. [63] further updated the mechanism with experimental data on MMH excitation tube pyrolysis at 900-1370 K, 1.4-4.5 atm. They used the model to predict the decomposition curves and half-life decomposition times of seven sets of MMH/Ar mixtures at different temperatures and pressures for the excitation tube experiments. Good agreement was observed for dilute MMH/Ar mixtures below 940 K and above 1150 K. Model predictions further indicated that the overall MMH decomposition rate is consistent with first-order kinetics and that N-N bond splitting is the most sensitive reaction pathway to simulate homogeneous decomposition of MMH at high pressures. The model predicts longer ignition delays for high-concentration MMH/Ar mixtures. However, predictions deviate from experimental data with a latency of about 100 \(\upmu\)s in the temperature range of 1030 to 1090 K.  
[PERSON] et al. [64] made the first measurements of the time histories of NH\({}_{2}\) and NH\({}_{3}\) during MMH pyrolysis after reflecting excitation waves in an excitation tube using an improved method of laser absorption techniques, with MMH at a concentration of \(\sim\)1% in Ar, in the 941 to 1252 K temperature range, and pressures close to 2 atm. NH\({}_{3}\) and MMH were measured by direct absorption with CO\({}_{2}\) laser lines at 9.22 and 10.22 \(\upmu\)m, respectively. Based on NH\({}_{2}\) and NH\({}_{3}\), two critical reactions in the MMH pyrolysis system can be measured, as shown in Equations (7) and (8).