The Significance of Ionic Pre-Ignition Reactions to the Hypergolic Behavior of Low-Toxicity Fuels with Dinitrogen Tetroxide

Hypergolic propellants, which ignite spontaneously upon liquid-liquid contact, are

important for maneuvering applications. Unfortunately, the toxicity of the state-of-the-art

hypergolic fuel, monomethylhydrazine (MMH), mandates expensive and time-consuming

handling procedures. Thus, alternatives to MMH that are easier to handle are desirable. Both the

design of better hypergolic fuels and the kinetic modeling of hypergolic reactions require

understanding of the reactions that drive hypergolic behavior. Drop-on-drop mass spectrometry

experiments, where small quantities of hypergolic propellants are allowed to react in the liquid

phase, reveal spontaneous production of large abundances of ions in the pre-ignition reactions of

dinitrogen tetroxide (NTO) with low-toxicity fuels. Here, studies on these ionic reactions are

presented, with the aim of understanding the extent to which these ionic reaction pathways may

contribute to hypergolic behavior. The reactions of NTO with low-toxicity hydrocarbon fuels,

amines, and a non-hypergolic compound are studied. Density functional theory (DFT) calculations

are used to characterize reaction pathways which produce ions detected in drop-on-drop mass

spectrometry experiments. Reactions of nitrosonium (NO⁺) and/or nitronium (NO₂⁺) formed from

NTO are found to be likely contributors to the hypergolic behavior of the fuels studied here. Raman

spectroscopy experiments are conducted to probe potential ionization pathways of NTO induced

by polar environments which may form NO⁺ and/or NO₂⁺. Ignition delay experiments are

conducted with solutions of NO⁺ and NO₂⁺ to probe the extent to which ionic reactions cause

hypergolic behavior. Ionic reactions of hypergolic fuels with NO⁺ and NO₂⁺ are concluded to be

important contributors to the hypergolic behavior of NTO. Based on comparisons of the studied

hypergolic and non-hypergolic fuels, important ionic reactions to hypergolic behavior are

highlighted, and recommendations are made for the design of new hypergolic fuels.