## 1 Introduction  
Hydrazine-based fuels consist of Hydrazine (N\({}_{2}\)H\({}_{4}\)) [1], Monomethylhydrazine (CH\({}_{3}\)NHNH\({}_{2}\)), and Unsymmetrical Dimethylhydrazine ((CH\({}_{3}\))\({}_{2}\)NNH\({}_{2}\)) [1, 2]. They are strong reducing agents that remain in a liquid state at room temperature and are used in rocket propulsion systems due to their desirable characteristics: high energy density; stability; reliable ignition; high thrust-weight ratio; reactivity; efficiency; specific impulse; and storage stability at room temperature. Hydrazine propellants offer two primary combustion modes. In the first mode, a single-component hydrazine propellant decomposes exothermically upon exposure to a catalyst or a hot surface, leading to spontaneous ignition, combustion, or explosion [3, 4, 5]. This mode generates a stable flame, enabling thrust and gas power [6]. The second mode involves mixing hydrazine with a strong oxidizer, such as inhibited red-fuming nitric acid (IRFNA) [7, 8] or dinitrogen tetroxide (N\({}_{2}\)O\({}_{4}\), NTO) [9], to create a two-component propellant. Combustion of these propellants occurs in the gas phase of rocket motors for high orbital transfer and attitude control [6, 10, 11]. However, the combustion of hydrazine fuels and oxidizers, such as NO\({}_{2}\) or N\({}_{2}\)O\({}_{4}\), poses challenges, particularly at or below room temperature [12], due to their high reactivity and combustibility. For instance, mixing highly dilute gas-phase Hydrazine with NTOcan result in spontaneous ignition without ignition devices such as sparks or shock waves. This characteristic makes these fuels highly reliable in low-temperature and low-pressure environments [12]. However, determining the precise ignition mixture concentration and understanding the complete ignition process for engine design remains unclear, posing difficulties in empirically predicting their combustion and decomposition characteristics. Researchers have focused on exploring alternative fuels with reduced toxicity to advance the development of safer and more efficient rocket propulsion systems. One avenue of research involves the utilization of ionic liquids designed with specific cations and anions to control sensitivity and thermal properties [13]. Synthesized hypergolic ionic liquids, utilizing compounds such as dicyanamide anion or 1,2,3-triazole, exhibit favorable characteristics, including high density, increased energy storage capacity, superior performance, and hydrophobic stability [13]. There is also a proposal to use various bio-based fuel alternatives as a more sustainable option [14; 15]. Developing less toxic organic fuels requires a better understanding of hypergolic combustion mechanisms. Visible and near-infrared Raman spectroscopy and drop and mixing tests evaluate the reaction process and kinetics and the effect of enhanced mixing processes reducing ignition delay times. Techniques that allow estimating the production of free radicals can also give information about the chemical mechanisms [16]. A comprehensive analysis of different additives, combustion chamber geometries, ignition systems, combustion instability, flame structure, thermal decomposition products, thermal management requirements, and nozzle designs are necessary to optimize rocket engine fuel performance. Figure 1 highlights the various thermal decomposition mechanisms of hydrazine-based fuels. Figure 1: Thermal Decomposition Mechanisms of hydrazine-based fuels. Deepening our understanding of the reaction kinetics and mechanisms of hydrazine-based fuels is crucial for advancing the development of safer and more efficient rocket propulsion systems. Theoretical approaches, such as high-precision quantum mechanical (QM) calculations and reactive molecular dynamics (MD) simulations, enable the microscopic observation of molecular and intermolecular reactions, prediction of reaction transition states and intermediates, and determination of reaction rates and thermodynamic parameters [17].