Thiocyanate-based ionic liquids: Fuels for hypergolic propellants with hydrogen peroxide

Abstract

Hydrazine as well as its derivatives MMH (monomethylhydrazine) and UDMH (unsymmetrical dimethylhydrazine) are still state-of-the-art for storable space propellants. Despite their high performances those substances show crucial disadvantages. Due to their toxic and carcinogenic character interest in alternative propellants increased significantly in the last years. One of the most promising approaches for green space propellants are ionic liquids (ILs). ILs show several advantageous properties such as low vapor pressures, high densities and flexible cation and anion design opportunities. ILs with thiocyanate anions are able to react hypergolically (or self-igniting) with highly concentrated hydrogen peroxide as oxidizer. Hypergolic propellant combinations offer multiple benefits, most important are the omission of an external ignition device as well as reliable re-ignition capability. Within this work we will give an overview of different classes of ionic liquids which are under investigation as sustainable fuel candidates. First, the synthesis and characterization methods of these ionic liquids are presented. Subsequently, calculations of the theoretical performance properties such as specific impulse and combustion temperatures are outlined based on the experimental determination of heats of combustion. Particular focus of this research is the comparison of hypergolic ignition properties of ILs with different cationic structures. For this purpose, drop tests with highly concentrated hydrogen peroxide were carried out. Correlations were drawn between the influence of the organic frameworks of the cations and the ignition delay times. Furthermore, differences in the hypergolic ignition behavior were also identified by variation of the alkyl moieties which affect the physical and chemical properties of the ionic liquids.