## 2 Research Status on Anhydrous Hydrazine Fuel
### Research Status on UDMH Fuel  
Unsymmetrical Dimethylhydrazine (UDMH) is widely used as the primary fuel for rocket engines in strategic missile and space fields. However, during the actual propellant storage, re-injection, and end-of-life disposal, a large amount of exhaust gas of UDMH enters the atmosphere. Based on the data for improvement kinetic reaction constants, UDMH is mainly converted to nitroso dimethylamine (NDMA) by ozone oxidation [85]. For the mechanism of UDMH oxidation to NDMH, [PERSON] et al. [86] and [PERSON] et al. [87] suggest that the pathway of NDMH generation is to generate (CH3)2 NN(H) radicals after the hydrogen atoms are removed from UDMH. Then the radicals undergo further oxygenated dehydrogenation. However, [PERSON] et al. [88] and [PERSON] et al. [89] believe that the initiation process of UDMH is adding an oxygen atom to the NH\({}_{2}\) functional group. The resulting (CH\({}_{3}\))\({}_{2}\)NN(O)H\({}_{2}\) is further oxygenated, hydrogen transferred, and the dehydration reaction is converted into NDMA. However, the atmospheric environment is a complex system with various oxidants. The mechanism and path of UDMH conversion to NDMA under the simultaneous action of various oxidants are not precise. The oxygenation or hydrogen capture reaction in the process of UDMH oxidation to generate NDMA is dominant. Therefore, there is a need to explore multiple reaction pathways under the action of different oxidants to determine the main pathways for NDMA formation from UDMH in the atmosphere. In addition, UDMH is the primary fuel for conventional liquid missiles. It will slowly oxidize or decompose during long-term storage to produce trace oxidation products, such as dimethylamine, meta-hydrazone, and other impurities. Currently, the interaction of UDMH with constant oxygen has been studied more. [PERSON] et al. [90] analyzed the initial oxidation products of UDMH using the gas chromatography-mass spectrometry method. The results showed seven oxidation products in the initial stage of UDMH oxidation in the air not larger than 100 h. The concentrations of dimethylamine, meta-hydrazone, and nitroso dimethylamine increased slowly. Then, as the oxidation of UDMH continued, the concentration of dimethylamine decreased. The concentration of nitroso dimethylamine increased rapidly, and new oxidation products appeared. The mass spectra of nitrogen-containing organic compounds all contained fragment ion peaks with m/z = 42 to 43, indicating that the N-N bonds of UDMH and nitrogen-containing organic oxide molecules were easily broken by heat. [PERSON] et al. [91] further analyzed the primary oxidation products and oxidation mechanism of UDMH in the gas-liquid phase by gas chromatography-mass spectrometry. The results showed as many as 19 oxidation products of UDMH, among which nitroso dimethylamine (NDMA), meta-hydrazone, dimethylamine, water, N, and N-dimethylformamide were the main products. The oxidation of oxygen with gas-phase UDMH significantly increased the production of the potent carcinogen NDMA, in which 30% of the degraded UDMH was converted to NDMA. The mass fraction of NDMA in the gas phase was as high as 1.3%. In comparison, the production rate of NDMA was about 5% when UDMH interacted with air. Therefore, the failure of UDMH could be avoided by reducing the oxygen content. [PERSON] et al. [92] proposed the mechanism of preliminary reaction to form peroxide in low-temperature autooxidation research, which, in turn, led to a chain reaction. However, the effect of these impurities on the combustion performance and the detailed combustion reaction mechanism of the gas phase is unclear. [PERSON] et al. [93] used DFT to investigate the chemical kinetics of UDMH degradation in wastewater by OH radicals, proposing potential by-products and generation pathways. The study aimed to provide a theoretical foundation for the plasma degradation of UDMH wastewater. It explored the potential of plasma as an advanced oxidation process. The study highlights the importance of considering solvent effects.