Investigation of the Combustion Products of Ionic Liquid-Based Green Propellants Using Infrared Spectroscopy

Abstract

In rocketry today, conventional hypergolic propellant combinations typically use hydrazine-derived fuels and oxidizers based on nitrogen tetroxide. Due to their high toxicity and consequently expensive handling, safer alternatives, so-called green hypergolics, are currently being developed. The ionic liquid-based fuels [EMIM][SCN], HIP_11 and HIM_30, paired with highly concentrated hydrogen peroxide as an oxidizer, are three candidates for such green hypergolics, which are currently under research at the German Aerospace Center (DLR). These combinations have been shown to exhibit reliable hypergolic ignition. For a better understanding of the reaction process and to assess the risks in working with these propellants, it is desirable to determine their combustion products. A test setup was designed to extract the gaseous combustion products from hypergolic drop tests. The gas samples were analyzed using Fourier-transform infrared spectroscopy and the gaseous combustion products were determined from the infrared spectra. Additional tests with varied oxidizer concentration or alternative fuels were conducted to further investigate detailed aspects of the findings. It was concluded that [EMIM][SCN], HIP_11 and HIM_30 produce very similar sets of combustion products with hydrogen peroxide, including water vapor, carbon dioxide, carbon monoxide, hydrogen cyanide and sulfur dioxide. Finally, the combustion products were compared to the substances produced when thermally decomposing the fuels. This confirmed that the previously detected substances were caused by a reaction between hydrogen peroxide and the fuels, rather than by their thermal decomposition due to heating.