Ballistics, Performances and Stability Analysis of a Flight Hybrid Rocket Engine

Hybrid rocket engines have been known for over 50 years, but only in the last years there is a growing interest on them. This is primarily due to the fact that they have some advantages in certain applications compared to solid or liquid systems, but they are characterized by poor regression rate performance and low combustion efficiency due to the diffusive nature of the combustion process. \\Recently a new class of fuels that burn faster than conventional polymeric hybrid fuels has been identified. A research at Stanford University led to the discover that materials forming a thin, hydrodynamically unstable liquid layer on the burning surface show an entrainment of droplets from the liquid-gas interface which increases the regression rate. These materials are called liquefying fuels and are characterized by low viscosity and surface tension of the melt layer since this makes them more susceptible to shear layer instabilities that cause the droplet formation. The entrainment mass transfer is not affected by the blocking phenomenon induced by blowing from the surface and this leads to an increase in the regression rate of this class of fuels. Among them there are cryogenic fuels (such as solid methane, solid pentane and solid oxygen), paraffin-based fuels and polyethylene waxes. The intrinsic safety, simplicity and low cost advantages make hybrid systems appreciable for research at university level with students. At the University of Stuttgart a paraffin/nitrous oxide hybrid rocket engine has been developed by the HyEnD (Hybrid Engine Development) student project for a small sounding rocket. The aim of this work is to provide a ballistics characterization of paraffin-based hybrid rocket fuels with different additives in combination with nitrous oxide. Moreover a performances evaluation of the HyEnD hybrid rocket engine is carried out and a comparison with theoretical results is done. Effects of different diaphragm and injector geometries on engine performances are also investigated. Finally a stability analysis using different injector geometries is performed. The test campaign was carried out at the experimental set-up in the test bench M11.5 at DLR (Deutsches Zentrum fur Luft-und Raumfahrt) Lampoldshausen, where the HyEnD hybrid engine is available for the combustion tests. Cylindrical paraffin-based fuel samples with a single central port perforation were tested. Chamber pressure was about 30-35 bar and the area ratio of the engine was 5. The space-time average regression rate and all the averaged performances were computed using measurements taken from different pressure transducers, a flow-meter and a load cell. Theoretical performances were computed with the software CEA, setting equilibrium or frozen conditions. Combustion stability was evaluated by using Fast-Fourier Transformations and spectrograms of the pressure signals. It was observed that paraffin waxes with lower viscosity show a higher regression rate due to the higher fuel mass involved in the entrainment process. Moreover the test results show that the combustion efficiency increases with increasing the length of the post-chamber and adding a diaphragm (especially cross-like one) at the end of the fuel grain. This is due to the increase of the turbulence level in the chamber and, thus, in the better mixing of the oxidizer with the fuel vapours. By using an impingement injector there is also an increase in the combustion efficiency with respect to the shower one, since a better atomization of the oxidizer is reached. This also leads to an increase in the regression rate. \\For what concerns the combustion instability, it was observed that oscillations of the chamber pressure increase if a diaphragm is put at the end of the fuel grain, while they decrease if the impingement injector is used. However no relevant instabilities were observed during the test campaign. In almost every test a low frequency oscillation was present often accompanied by a lower frequency which is thought to be a vortex-shedding oscillation in the pre-chamber. An intermediate frequency was also seen in some cases. This can be the Helmholtz frequency or the vortex-shedding in the post-chamber.