A Critical Review of Limitations and Challenges in Advancement of HNF as a Green Oxidizer for Composite Solid Propellant Formulations

Abstract

Hydrazinium nitroformate (HNF) is a chlorine-free, high-energy oxidizer with promising applications in green propulsion systems. This review examines the thermal decomposition, combustion kinetics, flame structure, and behavior of HNF, while critically evaluating the current body of literature. Thermal studies reveal that HNF undergoes two-stage decomposition, with exothermic peaks at 136°C and 138°C and an activation energy of 150 kJ/mol, determined using Kissinger and Ozawa methods. The decomposition products include nitroform, hydrazine, and nitrogen oxides, with a 72.5% weight loss in the first stage (105°C–142°C) and 24.5% in the second stage (142°C–210°C). Combustion studies identify a three-zone flame structure: fizz zone, dark zone, and luminous flame zone, with temperatures ranging from 1320 to 1540 K near the surface to 2720 K in the outer flame. HNF exhibits a high-pressure exponent (∼0.85), necessitating optimization through burn rate modifiers (BRMs) and advanced formulations. While HNF demonstrates superior specific impulse and reduced exhaust plume radiation compared to ammonium perchlorate (AP), its needle-like crystal morphology poses challenges for handling and propellant integration. This review emphasizes the need for advanced diagnostic techniques, computational modeling, and innovative BRMs, along with polymeric binders that are compatible with HNF and can improve the performance of HNF-based systems, offering valuable insights for green propellant development.