Ionic Liquids as Hypergolic Fuels

After seminal work presented a decade ago, ionic liquids (IL) have now received a lot of attention as energetic materials for propellant applications.1–3 In bipropellant rocket engines, it is desirable to achieve ignition by means of a hypergolic reaction and therefore to minimize system complexity. Hypergolic bipropellants are defined as fuel and oxidizer combinations that, upon contact, chemically react and release enough heat to spontaneously ignite, eliminating the need for an additional ignition source. This also makes them highly reliable for spacecraft and satellites, which need to fire their rocket engines hundreds or even thousands of times during their lifetime.

Unfortunately, no reliable a priori method for prediction of hypergolicity for fuel-oxidizer pairs is available today. Here, we report the first ILs to manifest hypergolic ignition.

The initial “hunting for the hypergol”, as John Clark entitled one of the chapters in his book, Ignition!, took place mainly during World War II.4 At that time, such toxic systems as “CStoff” (a mixture of N2H4 ·H2O, methanol, and water) and others consisting of triethyl amine, aniline, toluidine, xylidine, and N-methyl aniline were developed. Today, environmental and health concerns are becoming more and more pressing in the propellant world. Nevertheless, hydrazine and its methylated derivatives are still the state-of-the-art fuels for bipropellant applications. Most of the problems handling hydrazine and its derivatives are related to their volatility, because they are carcinogenic vapor toxins. For these reasons, it is exceedingly attractive to replace hydrazine with ILs, which are regarded as paragons of environmental friendliness, green chemistry, and low vapor toxicity.

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