## 2 Research Status on Anhydrous Hydrazine Fuel
### Hydrazine Combinations with Other Species
\[\text{N}_{2}\text{H}_{4}+\text{NO}_{2}=\text{N}_{2}\text{H}_{3}+\text{HONO} \tag{2}\]  
\[\text{N}_{2}\text{H}_{3}+\text{NO}_{2}=\text{N}_{2}\text{H}_{3}\text{NO}_{2}= \text{N}_{2}\text{H}_{2}+\text{HONO} \tag{3}\]  
\[\text{N}_{2}\text{H}_{2}+\text{NO}_{2}=\text{NNH}+\text{HONO} \tag{4}\]  
\[\text{NNH}+\text{NO}_{2}=\text{N}_{2}+\text{HONO} \tag{5}\]  
The rate constants of these reactions were also estimated based on transition state theory and single molecule rate theory. The proposed mechanism can predict the low-temperature ignition of N\({}_{2}\)H\({}_{4}\)/NO\({}_{2}\) mixtures. These reactions release much heat, especially in Equation (5) which produces N\({}_{2}\). The temperature increase accelerates Equation (2) with a small activation potential. After that, [PERSON] et al. [52] extensively explored the extensive reaction pathways between N\({}_{2}\)H\({}_{4}\) and NO\({}_{2}\) through quantum chemical calculations and developed a kinetic reaction mechanism for the spontaneous ignition of N\({}_{2}\)H\({}_{4}\)/NO\({}_{2}\) gas mixtures. As shown in Equation (6), the mechanism shows that ignition proceeds mainly through this reaction, explaining the spontaneous ignition of the N\({}_{2}\)H\({}_{4}\)/NTO system at temperatures below \(-\)173 \({}^{\circ}\)C.  
\[\mathrm{N}_{2}\mathrm{H}_{\mathrm{m}}+\mathrm{NO}_{2}\rightarrow\mathrm{N}_{2 }\mathrm{H}_{\mathrm{m}-1}+\mathrm{HONO}/\mathrm{HNO}_{2} \tag{6}\]  
where m = 1 to 4. [PERSON] et al. [53] studied the reaction of N\({}_{2}\)H\({}_{4}\) with N\({}_{2}\)O\({}_{4}\) isomers at the level of G2M(CC3)//B3 LYP/6311++G(3 df, 2p). The results show that the reactions of N\({}_{2}\)H\({}_{4}\) with sym-N\({}_{2}\)O\({}_{4}\) (D2h), cis-ONONONONO\({}_{2}\) (Cs), and NO\({}_{2}\) need to overcome energy barriers of 14.2, 10.6, and 7.6 kcal/mol, respectively [51]. In contrast, N\({}_{2}\)H\({}_{4}\) can react spontaneously with trans-ONONONONO\({}_{2}\) (Cs) and cis-ONONONONO\({}_{2}\) (C\({}_{1}\)) to form HONO\({}_{2}\) + H\({}_{2}\)NN(H)NO with an approximate kinetic gas rate constant above 250 K of 4 \(\times\) 10\({}^{-10}\) cm\({}^{3}\) molecule\({}^{-1}\) s\({}^{-1}\). The resulting H\({}_{2}\)NN(H)NO can be rapidly cleaved to generate reactive N\({}_{2}\)H\({}_{3}\) radicals with a rate constant greater than 1 \(\times\) 10\({}^{7}\) s\({}^{-1}\) above 1000 K, sufficient to initiate spontaneous combustion chain reactions with NO\({}_{2}\) and N\({}_{2}\)O\({}_{4}\)[51]. [PERSON] et al. [54] further investigated the kinetics and mechanism of the spontaneous combustion reactions of N\({}_{2}\)O\({}_{4}\) with N\({}_{2}\)H\({}_{4}\) and CH\({}_{3}\)NHNH\({}_{2}\) using ab initio molecular orbital theory based on the UCCSD(T) method with 6-311+G(3 df, 2p) as the basis set. They illustrated that the spontaneous ignition reactions of N\({}_{2}\)O\({}_{4}\) with N\({}_{2}\)H\({}_{4}\) and CH\({}_{3}\)NHNH\({}_{2}\) to produce N\({}_{2}\)H\({}_{3}\)NO + HNO\({}_{3}\) and CH\({}_{3}\) NHN(H)NO/CH\({}_{3}\)N(NO)NH\({}_{2}\) + HNO\({}_{3}\), respectively, are triggered by the rapid attack of ONONO\({}_{2}\) on the hydrazine molecule. NTO is isomerized at an energy barrier higher than that of the reactants by 5.9 kcal/mol and 6.3 kcal/mol. The authors highlight [54] the short half-life of N\({}_{2}\)O\({}_{4}\) in a mixture containing excess N\({}_{2}\)H\({}_{4}\) at atmospheric, reflecting the effective self-ignition initiation of these propellant systems [54]. Using the transition state theory in the temperature range from 298 to 2000 K, the rate constant of the formation of N\({}_{2}\)H\({}_{3}\)NO and HNO\({}_{3}\) was estimated, as shown in Figure 2, as well as the rate constant of the bimolecular reaction of N\({}_{2}\)O\({}_{4}\) with CH\({}_{3}\)NHNH\({}_{2}\), as shown in Figure 2.  
Table 1 highlights the Arrhenius fit parameters calculated in various studies relating to hydrazine. Figure 2: Ignition Reactions of N\({}_{2}\)H\({}_{4}\) with N\({}_{2}\)O\({}_{4}\) and CH\({}_{3}\)NHNH\({}_{2}\).