## 2 Research Status on Anhydrous Hydrazine Fuel
### Hydrazine Combinations with Other Species  
[PERSON] et al. [41] presented a chemical mechanism for the early stage of hypergolic ignition in liquid hydrazine/nitrogen tetraoxide mixtures commonly used as propellants in spacecraft. The mechanism determined through quantum chemical calculations showed that the energy barrier to the reaction between N\({}_{2}\)H\({}_{4}\) and N\({}_{2}\)O\({}_{4}\) was much lower than predicted for the same reaction in the gas phase. The study provides insights into the detonation-like ignition of these propellants. However, further experimental data are needed to validate the hypothesis presented in their study. [PERSON] et al. [42] and [PERSON] et al. [43] investigated the gas-phase reaction of N\({}_{2}\)H\({}_{4}\)/NO\({}_{2}\)-N\({}_{2}\)O\({}_{4}\) experimentally by introducing N\({}_{2}\)H\({}_{4}\) and NTO liquid propellant into a vacuum chamber, which created spontaneous combustion at temperatures below 300 K. [PERSON] et al. [44] observed the combustion of N\({}_{2}\)H\({}_{4}\) droplets at room temperature using a high-speed camera. The N\({}_{2}\)O\({}_{4}\) droplets were dropped into the N\({}_{2}\)H\({}_{4}\) liquid pool. Less than 5 \(\upmu\)s after contact and before the onset of ignition, the surface layers of both liquids began to evaporate in an explosion-like reaction. The N\({}_{2}\)O\({}_{4}\) vapor penetrated the N\({}_{2}\)H\({}_{4}\) pool before ignition. For the same mass, the N\({}_{2}\)H\({}_{4}\)/NTO two-component fuel had an explosive power equivalent to 160% of Trinitrotoluene (TNT) [45]. [PERSON] and [PERSON] [46] measured the ignition delay time at 40-90 torr, from 343 to 433 K, in a diffusion flame experiment with N\({}_{2}\)H\({}_{4}\)/NTO. [PERSON] et al. [45; 47] investigated the nature and mechanism of contact explosions of spontaneous liquid propellants with combinations of fuels and oxidizers. The results show that the observed explosion phenomena can be classified into two categories. First, sudden, spontaneous vaporization of the superheated N\({}_{2}\)H\({}_{4}\)/NTO liquid layer formed at the boundary between the two liquids occurs. A burst-like reaction takes place in the resulting reaction mixture. Chemical reactions did not enhance the abrupt vaporization with weak explosions. Second, the shock of local ignition causes the sudden vaporization of MMH/NTO and UDMH/NTO, and a turbulence-combustion reaction occurs in the resulting reaction mixture. [PERSON] et al. [48] developed the gas-phase chemical reaction mechanism [49] for N\({}_{2}\)H\({}_{4}\)/NTO, containing 64 species and 22 chemical reactions. The hydrazine decomposition reaction rate was improved and validated by experimental results from the literature [48]. The model influenced fuel film cooling behavior simulation and improved the combustion chamber wall temperature prediction. The model had good prediction capability for hydrazine and NTO reactions. CFD simulation results show that the nitric oxide reaction affects the ignition delay time and initiates combustion. However, low temperature is insufficient to cause the decomposition reaction of the substances, and the mechanism cannot explain the spontaneous combustion of the system at low temperatures. These studies on two systems highlight the importance of understanding the mass transfer characteristics of hydrazine in mixture systems and other species' effects on hydrazine's combustion and kinetics. [PERSON] et al. [50] evaluated the thermochemical data of chemical species [17] associated with the combustion of N\({}_{2}\)H\({}_{4}\)/NO\({}_{2}\) with quantum chemical calculations at the CBS-QB3 level of theory and constructed a primary chemical kinetic mechanism for the gas-phase combustion of N\({}_{2}\)H\({}_{4}\)/NO\({}_{2}\) mixtures. Kinetic simulations showed that the sequential reactions [51], as shown in Equations (2)-(5), cause low-temperature spontaneous combustion fires.