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New green hypergolic propellant combinations are developed by DLRs Institute of Space Propulsion. The development focuses on a suitable replacement of conventional hypergolic propellants like hydrazine, monomethylhydrazine (MMH) or unsymmetrical dimethylhydrazine (UDMH) as fuels with dinitrogen tetroxide (NTO) as oxidizer. Hydrazine and its derivatives are classified as toxic, carcinogenic and environmentally harmful which makes the usage of these substances hazardous and costly. In 2011, Hydrazine was listed as a substance of very high concern in the European REACH (registration, evaluation, authorization of chemicals) regulation, so a prohibition of this substance could be forthcoming. [1] For this development, hydrogen peroxide was chosen as oxidizer, as it is considered greenest compared to other candidates such as NTO, mixed oxides of nitrogen (MON) and white or red fuming nitric acid (WFNA/RFNA). Moreover, it is characterized by a very low toxicity and vapor pressure. Hydrogen peroxide is a metastable compound, which decomposes to oxygen and water in an exothermic disproportionation under certain conditions. [2] Ionic liquids were identified as promising hypergolic fuels. Ionic liquids are usually defined as salts that melt below 100 C and commonly consist of an organic or inorganic anion and a sterically demanding, asymmetric organic cation. The beneficial characteristics of ionic liquids such as high thermal stability, low vapor pressure and manifold design possibilities make them promising candidates for new propellants. [3] Recently, thiocyanate based ionic liquids were identified as excellent fuels for hypergolic propellants with hydrogen peroxide as oxidizer in our working group. Research on these auspicious substances is ongoing. [4-5] In this work, guanidinium thiocyanate a nitrogen-rich salt was tested as energetic additive for hypergolic ionic liquids in different amounts. Viscosities and densities of the new fuels were measured and the heat of formation as well as the specific impulse was calculated. Furthermore, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) as well as IR-spectroscopy were utilized for a detailed characterization of these fuels. Ignition behavior of the highly energetic propellants was evaluated via drop-on-pool tests. The ignition delay time time between first contact of oxidizer and fuel and first formation of a flame of the different hypergolic propellant combinations was compared and the influence of guanidinium thiocyanate on the ignition capability was investigated. References [1] J. Malm, Inclusion of Substances of Very High Concern in the Candidate List, European Chemicals Agency (EACH), Helsinki (2011). [2] S. Ricker, M. Kurilov, D. Freudenmann, C. Kirchberger, T. Hertel, H. Ciezki, and S. Schlechtriem, Novel gelled fuels containing nanoparticles as hypergolic bipropellants with HTP, Proceedings of 8th European Conference for Aeronautics and Space Sciences (EUCASS 2019), Madrid 1-4 July 2019. [3] Q. Zhang and J. Shreeve, Energetic Ionic Liquids as Explosives and Propellant Fuels: A New Journey of Ionic Liquid Chemistry, Chem. Rev. 114, 10527-10574 (2014). [4] F. Lauck, J. Balkenhohl, M. Negri, D. Freudenmann and S. Schlechtriem, Green bipropellant development A study on the hypergolicity of imidazole thiocyanate ionic liquids with hydrogen peroxide in an automated drop test setup, Combust. Flame 226, 87-97 (2021). [5] S. C. Ricker, D. Freudenmann, S. Schlechtriem, The Impact of Cation Structures on Hypergolicity of Thiocyanate Ionic Liquids with Hydrogen Peroxide, Energy Fuels 35 (19), 16128-16133 (2021). |
