An Experimental Study on the Base Flow Plume Interaction of Booster Configurations

The integration of the propulsion component plays a key role in the design process of future launchers. At high altitude, conventional rocket nozzles of fixed geometry operate at a non-adapted state. A sub-stantial aerodynamic problem is the interaction between the highly underexpanded plume and the ambient flow field. This interaction may induce boundary layer separation at vehicle base components going along with a significant rise of boat tail drag and may influence the stability and control effectiveness. In the base region “reverse jets” of the hot gas could cause overheating of external nozzle surfaces. To the same extend as the base flow and the shock structures are unstable, oscillations may stimulate “buffeting”. In order to improve the understanding of the interaction of the nozzle flow with the ambient flow, an experimental study on a scaled model representing the base region of a booster configuration has been carried out at the hypersonic wind tunnel H2K. To distinguish between different physical effects, the geometry of the model was kept as simple as possible, i.e. as a single nozzle configuration. The paper covers a description of the experimental set up including the complex internal model design. At Mach 5.3 and realistic external flow conditions wall pressure measurements and high speed Schlieren visualizations were performed to study the flow topology for cold and warm nozzle flow at several pressure ratios. Among others, the study focuses on the identification of shock oscillations as evaluated from Schlieren images. Recorded spectra provide the dominating oscillation frequencies of the external shock as well as the internal shock, linked to the flow condition of nozzle gas. Tests with different exhaust gases as Helium and Argon provide information on the influence of the specific heat coefficient. To support the physical interpretation of flow interactions downstream real nozzles and its potential on buffeting effects the measured frequencies are converted into reduced frequencies