AP/KMnO4 dual oxidiser on reduction of HCl gas and combustion properties for composite solid rocket propellant

Composite solid propellant (CSP) is integral to modern rocketry and missile technology due to its high performance, reliability, and ease of use. Their applications span military, space exploration, scientific research, and hobbyist activities, making them one of the most versatile and widely used propellants in the aerospace industry. CSP consists of a mixture of a metal fuel and an oxidiser called Ammonium perchlorate (AP) bound together by a polymer binder, usually Hydroxyl-terminated polybutadiene (HTPB). Fundamentally, AP-based CSP has a very high combustion performance compared to other types of rocket fuel. However, AP-based CSP resulted in some drawbacks, which come from the emission of HCl gases as a combustion by-product. This HCl gas is a toxic gas and can affect a few environmental concerns: (i) ground- based impacts ranging from groundwater contamination to accidents caused by improper propellant processing and handling, (ii) atmospheric impacts resulting from the interaction of exhaust combustion products with the surrounding atmosphere, including the ozone layer, and (iii) biological impacts encompassing propellant toxicity and corrosiveness. The main objective of this research is to synthesise and evaluate KMnO4/AP dual oxidisers that reduce HCl concentration as combustion by-products in AP-based CSP. Potassium permanganate (KMnO4) is a high oxidising agent and was introduced to minimise the usage of AP, which acts as KMnO4/AP dual oxidiser. Incorporating KMnO4 in a specific ratio will reduce the formation of HCl as a combustion by-product of CSP. KMnO4 and AP were synthesised using the agate mortar mixing method and characterised in terms of morphological, elemental composition, crystallinity, molecular interaction, surface area, and thermal properties. Then, KMnO4/AP was incorporated into the CSP system, and four experiments were conducted, which were the HCl concentration test, uniaxial tensile test and burn rate test. The HCl concentration test was evaluated using Mohr’s titration method. Then, the tensile strength was tested using a uniaxial tensile test, and the burn rate was analysed using the Crawford burn rate. Factorial design (FD) and response surface methodology (RSM) were used to design the experiments incorporating three factors, namely ratio of KMnO4 to AP, temperature, and pressure. These were developed to illustrate the relationship between those factors. From the findings, the sample of KMnO4 (40:60) CSP exhibits 39.04% reduction percentage of chlorine content from the conventional AP CSP. In addition, the tensile strength properties of sample KMnO4/AP (40:60) observed a value of 2.51 MPa at a temperature of 20 °C. Besides, the burn rate value resulted in an increase in value with increased pressure. KMnO4/AP (40:60) CSP sample resulted in the highest burn rate value of 7.53 mm/s at 3 atm. This percentage of chlorine content reduction could result in cleaner exhaust emissions, improving air quality and minimising biological health damage caused by HCl acidic by-products. Thus, incorporating KMnO4 as a dual oxidiser with AP proved to be a promising candidate for developing environmentally friendly rocket fuel applications without compromising the combustion and mechanical performances.

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