A state-of-the-art review of hydroxylammonium nitrate (HAN)-based gelled propellants

This paper presents a systematic review of recent advancements in hydroxylammonium nitrate (HAN)-based gelled propellants, analyzing their advantages and challenges as next-generation green and high-energy propellants. HAN gelled propellants, typically consist of oxidizers (dominated by HAN), fuel components, gelling agents and function additives, exhibiting high energy density, excellent stability and environmental compatibility. Research on HAN-based gelled propellants reveals distinct thermal decomposition characteristics in aqueous solutions, exhibiting multistage behavior governed by complex reaction pathways including proton transfer, nitration and nitrosation. The decomposition kinetics are significantly influenced by factors such as temperature, concentration, pH, and catalytic activity. In terms of formulation design, the selection of gelling agents and the optimization of the gelation process serve as critical determinants, directly governing the mechanical properties of the resultant propellant. Vapor nucleation and hydrodynamic instabilities are effectively suppressed by the gel matrix via hydrogen-bond-mediated water adsorption. Consequently, the multi-stage combustion and combustion instability phenomena typical of liquid HAN solutions are eliminated. HAN gelled propellants exhibit advantageous non-Newtonian rheological behavior, including shear-thinning and thixotropy, which enhance their suitability for propulsion system applications. While HAN-based gelled propellants demonstrate significant promise for applications in microsatellites and reusable launch vehicle propulsion systems, several critical technical bottlenecks remain unresolved. Foremost among these are gel matrix aging during long-term storage and ignition delay under low-temperature initiation conditions. These challenges arise from the complex interplay between the hygroscopicity of HAN, polymer network degradation kinetics, and the thermal activation barriers of ignition chemistry.