Investigation of the Combustion Products of Ionic Liquid-Based Green Propellants Using Infrared Spectroscopy
Abstract
:1. Introduction
1.1. Green Propellants
1.2. Related Developments and Studies
2. Test Setup and Campaign
2.1. Analysis of Combustion Products
- As a benchmark, for each fuel—[EMIM][SCN], HIP_11 and HIM_30—six tests were conducted using 98 wt% HTP. These spectra were of high quality, consistent and rich in information. Therefore, it was decided to reduce the number of drop tests for each condition from six to three for subsequent campaigns, in order to shorten the time needed for the experiments.
- Three tests each were carried out with the ionic substances trimethylsulfonium thiocyanate ([S111][SCN]) and triethylsulfonium thiocyanate ([S222][SCN]) [17]. The goal of these was to further investigate a proposed sulfur dioxide peak detected in the benchmark spectra by using fuels containing an increased amount of sulfur.
- HIP_11 and HIM_30 were tested with 80.5 wt% HTP for three drop tests each. This was conducted to investigate the effect of a longer ignition delay time, in order to compare the results to the spectra from the [EMIM][SCN] benchmark. [EMIM][SCN] had a longer ignition delay time than the other fuels under the same conditions, so diluted oxidizer was used to artificially slow the ignition of HIP_11 and HIM_30.
- The additives used in HIP_11 and HIM_30—CuSCN and [HIM][SCN]—were tested as pure substances for three tests each to investigate whether they would produce separate combustion products from [EMIM][SCN], which might have been obscured in the more complex benchmark spectra.
2.2. Analysis of Decomposition Products
2.3. Infrared Spectra and Data Processing
2.4. Propellant Sourcing
3. Results
3.1. Benchmark Spectra of [EMIM][SCN], HIP_11 and HIM_30
3.1.1. Water Vapor (Peaks 1 and 9)
3.1.2. Carbon Dioxide (Peaks 5, 12, 17, 18 and 19)
3.1.3. Carbon Monoxide (Peak 8)
3.1.4. Hydrogen Cyanide (Peaks 2, 4, 13, 14, 15 and 16)
3.1.5. Sulfur Dioxide (Peak 10)
3.1.6. Alkenes (Peaks 3 and 11)
3.1.7. Thiocyanates, Isocyanates and Nitriles (Peaks 6 and 7)
3.2. Decreased Hydrogen Peroxide Concentration
3.3. Spectra of Pure CuSCN and [HIM][SCN]
3.4. Spectra of Thermal Decomposition Products
4. Conclusions and Outlook
- The gaseous combustion products of all three fuels contain water vapor, carbon dioxide, carbon monoxide, hydrogen cyanide, sulfur dioxide and, likely, ethene. They also possibly contain isocyanates, thiocyanates or nitriles.
- Artificially slowing the ignition of HIP_11 and HIM_30 using a lower concentration of HTP results in more intensive IR peaks of hydrogen cyanide, sulfur dioxide and the uncertain substances for which isocyanates, thiocyanates or nitriles have been proposed.
- The additives present in HIP_11 and HIM_30—CuSCN and [HIM][SCN]—do not produce any detectable gaseous combustion products that are not also produced by [EMIM][SCN], the base of all three fuels.
- Apart from hydrogen cyanide, the thermal decomposition of the fuels results in an entirely different set of gaseous products. This confirms that the substances identified from the drop tests are formed from the direct chemical reaction between the fuels and HTP, rather than potential thermal decomposition caused by heating of the fuel pool prior to ignition.
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
Abbreviations
CuSCN | Copper(I) thiocyanate |
DLR | Deutsches Zentrum für Luft- und Raumfahrt e.V. German Aerospace Center e.V. |
[EMIM][SCN] | 1-Ethyl-3-methyl-imidazolium thiocyanate |
FTIR | Fourier-transform infrared (spectroscopy) |
[HIM][SCN] | Imidazolium thiocyanate |
HTP | High-test peroxide |
IDT | Ignition delay time |
IL | Ionic liquid |
IR | Infrared |
MMH | Monomethylhydrazine |
MON | Mixed oxides of nitrogen |
NTO | Nitrogen tetroxide |
[S111][SCN] | Trimethylsulfonium thiocyanate |
[S222][SCN] | Triethylsulfonium thiocyanate |
TGA | Thermogravimetric analyzer |
UDMH | Unsymmetrical dimethylhydrazine |
Appendix A
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Test Type | Fuel | Number of Tests | Other |
---|---|---|---|
Benchmark spectra | [EMIM][SCN] | 6 | |
HIP_11 | 6 | ||
HIM_30 | 6 | ||
Sulfonium-based ionic liquids | [S111][SCN] | 3 | |
[S222][SCN] | 3 | ||
Decreased HTP concentration | HIP_11 | 3 | 80.5 wt% HTP concentration |
HIM_30 | 3 | ||
TGA decomposition products | [EMIM][SCN] | 4 | Thermogravimetric analyzer tests, two per fuel in pure oxygen atmosphere and two in pure nitrogen atmosphere |
HIP_11 | 4 | ||
HIM_30 | 4 |
HIP_11 | HIM_30 | |
---|---|---|
Base component (concentration) | [EMIM][SCN] | [EMIM][SCN] |
(97.5 wt%) | (70 wt%) | |
Base component purity cation/anion | 99.9%/99.9% | 99.9%/99.9% |
Additive (concentration) | CuSCN | [HIM][SCN] |
(2.5 wt%) | (30 wt%) | |
Additive purity cation/anion | 99.4%/99.4% | 97.3%/99.4% |
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Oberndorfer, J.; Teuffel, P.; Stölzle, S.C.; Freudenmann, D.; Kirchberger, C.U. Investigation of the Combustion Products of Ionic Liquid-Based Green Propellants Using Infrared Spectroscopy. Aerospace 2025, 12, 507. https://doi.org/10.3390/aerospace12060507
Oberndorfer J, Teuffel P, Stölzle SC, Freudenmann D, Kirchberger CU. Investigation of the Combustion Products of Ionic Liquid-Based Green Propellants Using Infrared Spectroscopy. Aerospace. 2025; 12(6):507. https://doi.org/10.3390/aerospace12060507
Chicago/Turabian StyleOberndorfer, Jacob, Philipp Teuffel, Sophie C. Stölzle, Dominic Freudenmann, and Christoph U. Kirchberger. 2025. "Investigation of the Combustion Products of Ionic Liquid-Based Green Propellants Using Infrared Spectroscopy" Aerospace 12, no. 6: 507. https://doi.org/10.3390/aerospace12060507
APA StyleOberndorfer, J., Teuffel, P., Stölzle, S. C., Freudenmann, D., & Kirchberger, C. U. (2025). Investigation of the Combustion Products of Ionic Liquid-Based Green Propellants Using Infrared Spectroscopy. Aerospace, 12(6), 507. https://doi.org/10.3390/aerospace12060507