Development of a green hypergolic propellant based on hydrogen peroxide and ionic liquids

Kurzfassung

This thesis aims to develop a green hypergolic propellant combination with similar performance compared to conventional hypergolic propellants but at a reduced hazard potential. Hypergolic propellants are widely used in space propulsion because they are well-suited for in-space propulsion, as they offer virtually unlimited restarts, good storability and excellent performance. However, such propellants have significant drawbacks: the oxidizers, based on nitrogen tetroxide and its derivatives, are highly toxic and volatile, and the fuels, based on hydrazine and its derivatives, are toxic and suspected to be carcinogenic, which may lead to restricted use in Europe. Operations associated with conventional storable propellants are costly and timeconsuming as they require extensive protective equipment and special procedures. To overcome these limitations, alternative "green" propellants are currently being developed at many research centres, but there is still no widely accepted replacement for conventional hypergols. This thesis aims to fill this gap. The development process began by defining the novel propellant’s requirements and boundary conditions. The oxidizer selected was hydrogen peroxide, a green alternative to the conventional oxidizers. The fuel candidates were selected among ionic liquids since they have a neglectable vapour phase at ambient conditions, which facilitates handling, and they can be tuned by selecting a combination of anions and cations. In the next development phase, suitable fuel candidates were screened, focusing on commercially available substances. Based on the screening, a selection of promising ionic liquid was made. Next, these ionic liquids were tested in a lab-scale drop test. This procedure allows the initial evaluation of the hypergolicity of two substances. Ionic liquids with the thiocyanate anion were found to be hypergolic with highly concentrated hydrogen peroxide. In the drop test, the ignition delay of 1-ethyl-3-methylimidazolium thiocyanate was 26-32 ms. Adding an additive, such as copper thiocyanate, reduced the ignition delay to about 13 ms. A study on the influence of various parameters in the drop test showed that the most promising fuel is based on the ionic liquid 1-ethyl-3-methylimidazolium thiocyanate with 5 wt% copper thiocyanate (E5C). The ignition behaviour of this fuel was studied under more relevant conditions in specifically designed impinging injectors. Here, ignition delays in the order of several milliseconds were observed, which is expected to be sufficient to provide smooth ignition in thrusters. The fuel E5C was characterized in terms of density, viscosity and thermal behaviour. A maximum theoretical specific impulse of 323 s (frozen supersonic expansion, combustion chamber pressure 10 bar, expansion ratio 330) was calculated. Thus, the specific impulse is 5 % lower compared to the conventional hypergolic propellant combination of monomethyl hydrazine and dinitrogen tetroxide. However, the density-specific impulse is 10 % higher. Finally, the application of the ionic liquid fuel selected in a dual-mode propulsion system was investigated, with an electrical mode using the ionic liquid as a propellant. For a geostationary mission, it was found that a dual-mode system offers significant mass savings compared to a full chemical solution. To conclude, the group of thiocyanate ionic liquids is a promising class of hypergolic substances with hydrogen peroxide. Further, the fuel E5C is the best candidate in terms of performance and hypergolicity to substitute conventional hypergolic propellants.