Advanced Al\nHydrogel Propellants: Preparation, Thermal\nStability, and Self-Sustainable Combustion Characteristics

A new group of Al-based hydrogel propellants has been\nprepared\nand evaluated, aiming to replace currently used Al-ice propellants.\nThe theoretical specific impulse (<i>I</i><sub>sp</sub>)\nof these formulations is around 235 s. To enhance the ignition and\ncombustion performances of these propellants, a 2% metastable intermixed\ncomposite Al@PVDF-CuO and minor ammonium perchlorate (AP) were introduced.\nThe measured maximum ignition temperature of these hydrogel propellants\nwas 2277 °C, higher than that of the Al-ice propellant (1700\n°C). This increase in the ignition temperature could be attributed\nto the enhanced reactivity and energy release provided by the additives.\nAdditionally, the initial mass loss temperature for this kind of hydrogel\npropellant, ranging from 211 to 235 °C, indicates higher thermal\nstability compared to the evaporation temperature of moisture water,\nindicating the strong water-locking capability of the gelling agent\ntriaminoguanidine-glyoxal polymer (TAGP). The two main reaction steps\ninvolved in the combustion process are the oxidation of Al by H<sub>2</sub>O with H<sub>2</sub> release, which coincides with the decomposition\nof TAGP at 246 °C, and a subsequent oxidation step with remaining\nH<sub>2</sub>O and TAGP residues peaking at 320 °C. The burn\nrate of these propellants is stable and controllable, ranging from\n2.5 to 5.9 mm·s<sup>–1</sup>, which is crucial for maintaining\na consistent thrust during propulsion. The combustion condensed products\nare predominantly α-Al<sub>2</sub>O<sub>3</sub>, which could\ninfluence the overall combustion efficiency and thermodynamic properties\nof the system. The effect of AP on the Al-water reaction has been\nfurther studied using reactive force field (ReaxFF) molecular dynamic\nsimulations. It has been shown that the presence of AP may lead to\na slight decrease in H<sub>2</sub> production, which was attributed\nto the increase in the H<sub>2</sub>O concentration as the major decomposition\nproduct of AP.