Advanced Al Hydrogel Propellants: Preparation, Thermal Stability, and Self-Sustainable Combustion Characteristics

Abstract

A new group of Al-based hydrogel propellants has been prepared and evaluated, aiming to replace currently used Al-ice propellants. The theoretical specific impulse (I_sp) of these formulations is around 235 s. To enhance the ignition and combustion performances of these propellants, a 2% metastable intermixed composite Al@PVDF-CuO and minor ammonium perchlorate (AP) were introduced. The measured maximum ignition temperature of these hydrogel propellants was 2277 °C, higher than that of the Al-ice propellant (1700 °C). This increase in the ignition temperature could be attributed to the enhanced reactivity and energy release provided by the additives. Additionally, the initial mass loss temperature for this kind of hydrogel propellant, ranging from 211 to 235 °C, indicates higher thermal stability compared to the evaporation temperature of moisture water, indicating the strong water-locking capability of the gelling agent triaminoguanidine-glyoxal polymer (TAGP). The two main reaction steps involved in the combustion process are the oxidation of Al by H₂O with H₂ release, which coincides with the decomposition of TAGP at 246 °C, and a subsequent oxidation step with remaining H₂O and TAGP residues peaking at 320 °C. The burn rate of these propellants is stable and controllable, ranging from 2.5 to 5.9 mm·s^–1, which is crucial for maintaining a consistent thrust during propulsion. The combustion condensed products are predominantly α-Al₂O₃, which could influence the overall combustion efficiency and thermodynamic properties of the system. The effect of AP on the Al-water reaction has been further studied using reactive force field (ReaxFF) molecular dynamic simulations. It has been shown that the presence of AP may lead to a slight decrease in H₂ production, which was attributed to the increase in the H₂O concentration as the major decomposition product of AP.