## 4 Conclusions  
This review summarized recent advances in the kinetics and reaction mechanisms of the thermal decomposition reactions of three hydrazine-based self-ignition propellants. Key experimental data for hydrazine/NTO are missing due to the self-ignition characteristics. Some of the available chemical reaction kinetic mechanisms of hydrazine propellants in the literature (N\({}_{2}\)H\({}_{4}\)/NTO, MMH/O\({}_{2}\), and MMH/NTO) have missing reaction paths and thermodynamic parameters to varying degrees. Some reaction rate constants are only obtained by estimation. The mechanisms are narrowly applicable and can hardly meet the current needs. The following key points could be highlighted:  
(1) The highly reactive reaction of N\({}_{2}\)H\({}_{4}\) and NO\({}_{2}\) is caused by the continuous H-atoms abstraction from N\({}_{2}\)H\({}_{4}\) by NO\({}_{2}\), N\({}_{2}\)H\({}_{4}\) => N\({}_{2}\)H\({}_{3}\) => N\({}_{2}\)H\({}_{2}\) => NNH => N\({}_{2}\), in which a large amount of heat is released during this process, especially by the reaction that produces N\({}_{2}\), and the temperature increase accelerates the reaction N\({}_{2}\)H\({}_{4}\) => N\({}_{2}\)H\({}_{3}\) with a small activation potential;  
(2) The overall decomposition rate of MMH is consistent with first-order kinetics. N-N bond splitting is the most sensitive reaction pathway to simulate the homogeneous decomposition of MMH at high pressure, with pressure dependence at 0.3 to 5 atm. The measured reaction rate constants follow a pressure-dependent trend. In addition, simple C-N bond fission to generate radicals CH\({}_{3}\) + NHNH\({}_{2}\) is also expected to dominate the decomposition kinetics. NO\({}_{2}\) continues to abstract H atoms on the intermediate N of MMH at low temperatures to generate products (CH\({}_{3}\)NNH\({}_{2}\) and HONO), followed by exothermic HONO cleavage to generate OH radicals, which is the main reaction that induces ignition. H-atom abstraction by OH radicals and H are also essential to the MMH reaction kinetics;  
(3) UDMH will slowly oxidize or decompose to produce up to 19 trace oxidation products during long-term storage, of which nitroso dimethylamine (NDMA), unsymhydrazone, dimethylamine (FDMH), water, and \(N\), \(N\)-dimethylformamide (DMF) are the main products. However, the mechanism and pathway of converting UDMH to NDMA are not precise. The dominance of the oxygen addition reaction or the hydrogen capture reaction in the oxidation of UDMH to NDMA is also further discussed.  
A detailed kinetic study of the reaction mechanism of UDMH has not been seen in the available studies. In as much as there is a need for a detailed kinetic study of these fuels, there is also a need to develop more safe and sustainable alternatives for propellant and fuel applications.  
[PERSON] and [PERSON] supervised the project and the provision of financial resources. [PERSON] was responsible for the overall conceptualization of the study, the graphical and tabular representations, and the writing and editing of the final draft. [PERSON] and [PERSON] contributed equally to this work by developing the initial development of manuscript and literature sources for hydrazine fuels. All authors have read and agreed to the published version of the manuscript.  
The authors acknowledge the funding support from the Startup Funds of Aoxiang Oversea Scholar from Northwestern Polytechnical University (0602022 CH02002135).  
Not applicable.  
The authors thank [PERSON] (Chang'an University) and [PERSON] (Xi'an Jiaotong University) for their insightful discussions on environmental sustainability.  
The authors declare no known competing financial interest or personal relationship that could have appeared to influence the work reported in this paper.