Dual control over the reignition and combustion performance of hydroxylammonium nitrate-based gel propellants

Hydroxylammonium nitrate (HAN; NH₃OH ⁺ NO₃⁻), a green and non-toxic monopropellant, finds wide application in liquid and controllable solid propulsion. Gel propulsion enjoys advantages such as a high throttling capacity and encouraging operational safety. This study prepared three HAN-based gel propellant samples with gelling agent contents ranging from 2 to 4 wt%. Their decomposition processes were analyzed using thermogravimetry (TG), differential scanning calorimetry (DSC), and mass spectrometry (MS), and a microthruster was designed to investigate their combustion characteristics under varying voltages and flow rates. Results reveal the presence of three exothermic peaks in the HAN-based gel propellants at temperatures of 204, 306 °C and 441 °C. The gel propellants experienced violent decomposition between 100 °C and 400 °C, producing low-molecular-weight organics such as C₃H₈, C₂H₆, H₂O, O₂, and NH₃. For the initial ignition, the flow rate exerts a greater effect of reducing the delay time than the voltage. The reignition exhibited a shorter delay time than the initial ignition, and increasing voltage led to a more significant decrease in the reignition delay time than increasing the flow rate. Under a gelling agent content of approximately 4 wt%, the reignition delay time decreased from 2.35 s to 0.65 s as the voltage increased from 150 V to 250 V, with the flame length and light intensity during the reignition greater than those in the initial ignition. At the end of combustion, the extinguishment delay time changed insignificantly under high voltage. As revealed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analyses, residues with numerous cavities emerged due to incomplete combustion and the severe agglomeration of the gel propellants, exhibiting a maximum chlorine content of up to 77.73%.