Green Hypergolic Ionic Liquids: Future Rocket Propellants

Hydrazine and its derivatives have been used as standard propellants for spacecraft propulsion systems since the 1960s, despite being highly toxic and carcinogenic. The propellant synthesis community has constantly been looking for green alternatives for the same. Hypergolic ionic liquids (HILs) with several attractive properties, such as high energy content, high bulk density, low vapor pressure, and low toxicity, have been proposed as an alternative to hydrazine and its derivatives. In the present study, the theoretical performance of sixty-eight HILs was studied at a combustion chamber pressure of 3 MPa and a nozzle expansion ratio of 40. The specific impulse and density specific impulse of the HILs were calculated with white fuming nitric acid (WFNA), inhibited red fuming nitric acid (IRFNA), and nitrogen tetroxide (NTO) as oxidizers. The specific impulse of 2,2-dimethyltriazanium nitrate (HIL-1) was found to be 23 s higher than monomethylhydrazine (MMH), whereas its density-specific impulse was found to be 123 g-s/cm³ higher than MMH. The gains in the specific impulse and density specific impulse coupled with other desirable “green” properties for several HILs are expected to establish them as potential replacements for hydrazine and its derivatives.