## 2 Research Status on Anhydrous Hydrazine Fuel
### Research Status on MMH Fuel
It undergoes \(\beta\)-bond breaking to produce the stable CH\({}_{2}\)=NH and many free H radicals. H is very reactive and reacts continuously with MMH. [PERSON] [74] reported the total reaction rate constant for H-atom removal by H on MMH at 298 K, 26.0 Torr. [PERSON] et al. [75] investigated the multichannel reaction mechanisms CH\({}_{3}\)NHNH\({}_{2}\) + H\(\rightarrow\)product (R1) and (CH\({}_{3}\))\({}_{2}\)NNH\({}_{2}\) + H\(\rightarrow\)product (R2) and studied their dynamic properties using a two-energy direct kinetic approach. The rate constants of the reactions were evaluated in the temperature range of 200 to 2000 K using a small curvature tunneling correction at the MCG3 MPWPW91//MPW1K/6-311G(d,p) level, using typical variational transition state theory. Theoretical and experimental studies have shown that substituting H with CH\({}_{3}\) groups significantly increases the reaction rate coefficient. [PERSON] et al. [19] performed a detailed analysis of this class of reactions and calculated to obtain their reaction rate constants, and the results showed that the abstraction energy barriers of the central amine H atom, the two terminal amine H atoms, and the methyl H atom were 4.16, 2.95, 5.98, and 8.50 kcal mol\({}^{-1}\), respectively. The rate coefficients in cm\({}^{3}\) molecule\({}^{-1}\) s\({}^{-1}\) for the temperature range of 290 to 2500 K were:  
\[\text{k}_{1}=9.63\times 10\text{T}^{-202.596}\text{ exp}(-154.2/\text{T}) \tag{9}\]  
\[\text{k}_{2}= 2.04\times 10\text{ T}^{-182.154}\text{ exp}(104.1/\text{T}) \tag{10}\]  
\[\text{k}_{3}= 1.13\times 10\text{ T}^{-202.866}\text{ exp}(-416.3/\text{T}) \tag{11}\]  
\[\text{k}_{4}= 2.41\times 10\text{T}^{-233.650}\text{ exp}(-870.5/\text{T}) \tag{12}\]The results indicate that the abstraction of terminal amine H atoms to form transCH3 NHNH radicals is the primary channel among the different abstraction channels. The theoretical H abstraction rate coefficient is 8.16 \(\times\) 10\({}^{-13}\) cm\({}^{3}\) molecule\({}^{-1}\) s\({}^{-1}\) at 298 K without adjustable parameters. Some important reaction rate constants are reported for removing H atoms from MMH by radicals (CH3, NH2) [76]. In 1995, [PERSON] et al. [77] developed a set of reaction mechanisms (43 substances, 204 reactions) containing H/C/N/O for simulating the combustion of dual-based propellants (nitrocellulose and nitroglycerine). Later, [PERSON] et al. [78] presented the gas-phase thermochemistry of more than 50 C/H/N species and developed the MMH/O2 [79] and MMH/NTO [80] systems for ignition and combustion. The primary reaction mechanism of the MMH/O2 system consists of 70 species and 373 equilibrium primitive reactions with good agreement between measured and calculated values of ignition delay times in dilute and dilute mixtures. The primary reaction mechanism of the MMH/NTO system consists of 403 equilibrium reactions between 82 species, with preliminary identification of important reactions for combustion above 1000 K.  
[PERSON] et al. [81] developed a reaction mechanism for the decomposition of nitromethane (CH3 NO2). [PERSON] et al. [82] published a reaction mechanism for MMH/FRNA containing 81 substances and 513 reactions, which is now the only more detailed and complete reaction mechanism for MMH/FRNA two-component propellants. In 2006, [PERSON] et al. [83] observed and analyzed the MMH/NTO self-ignition ignition process using a constant volume bomb device and a high-speed camera and obtained critical images of combustion, which are essential for preventing two-component propellant vapor mixing. [PERSON] et al. [84] investigated numerically with CFD simulations for a small liquid rocket engine's combined MMH/NTO propellant reaction mechanism for some practical applications. Table 3 summarizes some of the studies conducted on MMH. Critical Assessment of Existing Literature  
The reviewed studies provide valuable insights into MMH's thermal decomposition kinetics, reaction mechanisms, and combustion properties. Understanding the practical