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

<p dir="ltr">Hypergolic propellants, which ignite spontaneously upon liquid-liquid contact, are</p><p dir="ltr">important for maneuvering applications. Unfortunately, the toxicity of the state-of-the-art</p><p dir="ltr">hypergolic fuel, monomethylhydrazine (MMH), mandates expensive and time-consuming</p><p dir="ltr">handling procedures. Thus, alternatives to MMH that are easier to handle are desirable. Both the</p><p dir="ltr">design of better hypergolic fuels and the kinetic modeling of hypergolic reactions require</p><p dir="ltr">understanding of the reactions that drive hypergolic behavior. Drop-on-drop mass spectrometry</p><p dir="ltr">experiments, where small quantities of hypergolic propellants are allowed to react in the liquid</p><p dir="ltr">phase, reveal spontaneous production of large abundances of ions in the pre-ignition reactions of</p><p dir="ltr">dinitrogen tetroxide (NTO) with low-toxicity fuels. Here, studies on these ionic reactions are</p><p dir="ltr">presented, with the aim of understanding the extent to which these ionic reaction pathways may</p><p dir="ltr">contribute to hypergolic behavior. The reactions of NTO with low-toxicity hydrocarbon fuels,</p><p dir="ltr">amines, and a non-hypergolic compound are studied. Density functional theory (DFT) calculations</p><p dir="ltr">are used to characterize reaction pathways which produce ions detected in drop-on-drop mass</p><p dir="ltr">spectrometry experiments. Reactions of nitrosonium (NO⁺) and/or nitronium (NO₂⁺) formed from</p><p dir="ltr">NTO are found to be likely contributors to the hypergolic behavior of the fuels studied here. Raman</p><p dir="ltr">spectroscopy experiments are conducted to probe potential ionization pathways of NTO induced</p><p dir="ltr">by polar environments which may form NO⁺ and/or NO₂⁺. Ignition delay experiments are</p><p dir="ltr">conducted with solutions of NO⁺ and NO₂⁺ to probe the extent to which ionic reactions cause</p><p dir="ltr">hypergolic behavior. Ionic reactions of hypergolic fuels with NO⁺ and NO₂⁺ are concluded to be</p><p dir="ltr">important contributors to the hypergolic behavior of NTO. Based on comparisons of the studied</p><p dir="ltr">hypergolic and non-hypergolic fuels, important ionic reactions to hypergolic behavior are</p><p dir="ltr">highlighted, and recommendations are made for the design of new hypergolic fuels.</p>