The state-of-the-art, storable bipropulsion system uses hydrazine (typically monomethylhydrazine) as the fuel component. This fuel affords useful performance characteristics and has a fast ignition with the oxidizer. This fast (hypergolic) ignition provides system reliability for on-demand action of the propulsion system. The bipropellant's hypergolic character is very beneficial as it removes the requirement of a separate ignition component. Additional components may be added to bring increased inert mass and reduce system performance. The energy density of the state-of-the-art, storable bipropulsion system is largely limited by the density of the fuel. Storable fuels range in density from 0.88 g/cc (monomethylhydrazine) to 1.00 g/cc (hydrazine). Energetic ionic liquids have established densities that range well above 1.00 g/cc, and thus can confer greater energy density as bipropellant fuels.
There are significant costs and operational constraints associated with handling state-of-the-art fuels (hydrazines) that derive from the fuel's carcinogenic vapor. Fuel transport, loading, and unloading are significantly complicated by vapor toxicity and can require considerable effort and cost in vapor monitoring by trained operations crews employed in expensive personal protection equipment.
One major drawback of recently discovered hypergolic ionic liquids is that the majority are hypergolic only with nitric acid in one of its several formulations. Furthermore, only very few have been shown to be hypergolic with higher performing N2O4. By their very nature, oxidizers are hazardous; however, the toxicity and corrosiveness of the nitric acids make operability quite difficult. While N2O4 is much less corrosive and easier to handle than IRFNA (inhibited, red-fuming nitric acid comprising about 83% HNO3, 14% N2O4, about 2% H2O, and 0.6% HF), it is highly toxic with an even higher vapor pressure than hydrazine (101 kPa at 21° C. for NO2 or N2O4). A true “all-green” bi-propulsion system has to address the toxicity of the oxidizer. Hydrogen peroxide is the only known, high performing, storable oxidizer, which can be considered environmentally benign. Although the OSHA permissible exposure limit for hydrogen peroxide is only 1 ppm in air, the high boiling points of the water solutions, 141° C. (90%) and 148° C. (98%), result in vapor pressures at 25° C. of only 0.5 KPa and 0.3 KPa for 90% and 98% hydrogen peroxide, respectively, which makes handling of the oxidizer considerable less difficult than N2O4.
Accordingly, there is need and market for environmentally enhanced “green” ionic liquid fuels, which overcome the above prior art shortcomings.