Ionic Liquids with Thiocyanate Anions as Space Propellants: How Cation Structure Affects Ignition Delay with Hydrogen Peroxide

Abstract

Hypergolic propellants, which ignite spontaneously upon contact between fuel and oxidizer, play a crucial role in space propulsion due to their reliable ignition and storability. However, conventional hypergolic fuels, such as hydrazine and its derivatives, pose significant environmental and health hazards due to their high toxicity and volatility. As a result, research has increasingly focused on developing "green" hypergolic fuels that maintain high performance while reducing environmental impact. Ionic liquids (ILs) have emerged as promising candidates for next-generation hypergolic propellants due to their low volatility, high density and high thermal stability. Since the discovery of the first hypergolic ionic liquid by Schneider et al. 1 a lot of progress has been made in this field 2-4. In this study, we explore the influence of the cationic structure of ILs with thiocyanate anions on their hypergolic ignition behaviour with hydrogen peroxide, a green oxidizer. A series of ILs with different cationic backbones, including imidazolium, pyridinium, and ammonium derivatives with varying alkyl chain lengths and functional groups, were synthesized and characterized. Through ignition delay time measurements, we established correlations between cation structure and hypergolic reactivity. Our results demonstrate that physical properties, steric hindrance and hydrogen bonding interactions significantly influence the ignition process. These findings contribute to the rational design of hypergolic fuels with optimized performance, paving the way for safer and more sustainable space propulsion systems.