ABS Infill Structures for Enhancing the Performance of Paraffin Fuel Grains in Hybrid Rocket Engines
An experimental investigation of Hybrid Rocket Engine fuel grains is undertaken to study the impact of ABS gyroid infill structures on the performance of liquefying fuel grains, in particular; chamber pressure, thrust, regression rate, specific impulse, combustion efficiency and thrust coefficients. To facilitate part of this investigation, a new hybrid rocket engine test facility is designed and constructed.
The design of the new facility commences with a system level analysis of the combustion cham- ber operation conditions, followed by the development of the pneumatics system for the oxidiser feed, ignition system to kick-start the combustion process, control systems for the actuation of valves and instrumentation to measure thrust, chamber pressure, oxidiser mass flow rate and optical imaging of the exhaust plume. With the experimental facility in a functional condition, the focus of this study shifts to the experimental campaign to test the performance of a novel fuel grain design utilising 3D printing technology.
With knowledge of the Kelvin-Helmholtz Instability in the liquid layer inducing discrete wavelengths based on the supplied oxidiser mass flow rate, a unique 3D printed ABS-paraffin fuel grain is devel- oped. At particular oxidiser mass flow rates, the liquid layer wave-like structures produced in paraffin fuel grains exhibit a corresponding wavelengths. The hypothesis behind this design is to amplify the wave-like structures to promote further droplet break-up by matching these wavelengths with the gyroid topology on the surface of the fuel grain. Baseline ABS and paraffin wax fuel grains were tested and the results were then compared against literature and past experiments to both verify the performance of the engine as well as establishing a benchmark to better compare the novel gyroid fuel grain. It was confirmed that paraffin wax exhibits regression rates up to 200% greater than ABS, however consequently, the efficiency fell short of the ABS at lower oxidiser mass fluxes.
The performance of the gyroid fuel grain consistently beat the baseline paraffin wax and demon- strated the highest efficiencies of the three fuel formulations evaluated in this study, achieving 80% combustion efficiency and 240 s of specific impulse. The predicted thrust coefficients also imply that the efficacy of the gyroid fuel grain might theoretically be nearly 1.5 times greater than the values obtained in this study. A closer examination of the gyroid fuel grains after hot fire reveals structures of the paraffin wax at the surface that are similar to wave-like structures in Kelvin-Helmholtz Instabilities (KHIs). The wavelengths are observed to follow the topology of the gyroid structure, with tiny break-up waves at each peak. This was an intriguing result, and the ramifications could imply that the better performance found in the data could be linked to the gyroid structure’s improved entrainment and break-up of droplets from the liquid layer. A final observation from solidified paraffin samples after firing showed uniform deposits of carbon, implying passive carbon impregnation, which might potentially replace the need for carbon infusion into the paraffin mixture.
