AFRL researchers proposing ionic liquid/hydrogen peroxide combination as more environmentally-friendly high-performance fuel for rockets
|Ignition drop test. The ILs presented in Schneider at al. are hypergolic with H2O2 and are hydrogen-rich, thus helping to boost performance. Credit: Schneider et al. Click to enlarge.|
Researchers at the Air Force Research Laboratory (AFRL) are proposing a more environmentally friendly high-performance fuel for rockets: a hydrogen-rich ionic liquid that self ignites (hypergolic) in the presence of hydrogen peroxide. Stefan Schneider and his colleagues report on their work in a paper in the journal Angewandte Chemie.
Many rockets, satellites, and spacecraft are driven by hydrazine, sometimes with an oxidizing agent like nitric acid or dinitrogen tetroxide. When filling tanks with these highly toxic substances, technicians must wear full protective clothing; a failed launch can lead to significant environmental damage. Researchers are looking for alternatives that are more environmentally friendly and less toxic, but just as powerful—requirements that are hard to meet in a single material.
Despite the potential danger, hydrazine is used as a rocket fuel because it delivers high performance, can be stored for a relatively long time, and spontaneously ignites upon contact with an oxidizing agent or a suitable catalyst. The oxidizing agents used as rocket fuels are also dangerous. Dinitrogen tetroxide is less corrosive than nitric acid, but it is toxic and highly volatile. Hydrogen peroxide is a promising alternative because it is less corrosive and leads to much less toxic gas at room temperature. Its decomposition produces only water and oxygen.
All rocket oxidizers are hazardous by their very nature, and so reduction of those hazards, even though the resulting materials might not be completely harmless, is at the heart of green initiatives in propulsion. The corrosivity of nitric acid is well known, and, although N2O4 is much less corrosive, it combines high toxicity with high vapor pressure. A significant step to a lower-toxicity bipropulsion system would be the demonstration of hypergolicity (spontaneous ignition) between an ionic liquid (IL), which is a paragon of low vapor toxicity, and a safer oxidizer. Apart from cryogens, hydrogen peroxide seems to be especially promising because of its high performance, less-toxic vapor and corrosivity, and its environmentally benign decomposition products, which make handling this oxidizer considerably less difficult than N2O4 or nitric acid.—Schneider et al.
Ionic liquids are compounds that consist of ions, like a salt. However, they are not crystalline; they remain “molten” as a liquid at room temperature. Ionic liquids essentially do not vaporize, which prevents the formation of toxic vapors. It has previously not been possible to produce an ionic liquid that is flammable when partnered with hydrogen peroxide.
Schneider and his team have now overcome this barrier. The positively charged ion of their ionic liquid is a phosphorus atom bound to four hydrocarbon chains. At the core, however, lies the negatively charged ion made from one aluminum, four boron, and sixteen hydrogen atoms. The hydrogen-rich composition raises the power of the fuel component.
This aluminum borohydride ion can be viewed as a densified form of hydrogen stabilized by metal atoms. In fact, for a given tank size, liquids with this ion contain even more hydrogen than pure liquid hydrogen, without the difficult cooling requirements.—Stefan Schneider
In order to test the ignitibility, the researchers applied drops of the novel ionic liquid onto various oxidizing agents. Upon contact with hydrogen peroxide, ignition was nearly instant; with fuming nitric acid it exploded. Says Schneider, “It is thus interesting as a potential component for greener high-performance fuels.”
Schneider, S., Hawkins, T., Ahmed, Y., Rosander, M., Hudgens, L. and Mills, J. (2011), Green Bipropellants: Hydrogen-Rich Ionic Liquids that Are Hypergolic with Hydrogen Peroxide. Angewandte Chemie International Edition, 50: n/a. doi: 10.1002/anie.201101752