Effect of Iron (III) Oxide Catalyst on Ageing Behaviour of Composite Solid Propellants

This case study investigates the ageing behaviour of hydroxyl-terminated polybutadiene (HTPB)-based solid propellants, containing 0.5% iron oxide and a bimodal ammonium perchlorate (AP) distribution (300 µm coarse AP and 40 µm fine AP). To achieve higher burning rates in large solid rocket motors, burning-rate catalysts were preferred over ultra-fine oxidiser compositions due to processing advantages. Characterisation of the iron oxide burning rate catalyst, specific to its surface area, was attempted. The role of surface characteristics in burning rate augmentation and ageing reactions was studied. Particle size and surface area estimates were obtained, and propellant burning rates, pressure exponents, and propellant ageing behaviour were evaluated to support the study. Accelerated thermal ageing of composite solid propellant samples at three different temperatures is carried out. The Arrhenius equation was used to model the dependence of the initial modulus on ageing time and temperature. An activation energy of 68.18 kJ/mole was obtained, which is approximately 4–8% lower than previously reported values for conventional propellants. The study concludes that iron oxide with a specific surface area of 10 m2/g and proportions up to 0.5% by weight can be safely used in propellant formulations without a significant reduction in the propellant’s shelf life.