Performance characteristics of a rocket-ejector operating with a single or twin thruster configuration

This study focused on the ejector operating mode of a rocket-based combined-cycle (RBCC) engine. In ejector mode, rocket thrusters embedded in a duct fire to generate thrust. The high-momentum rocket exhaust (primary) acts as an ejector and pumps air (secondary) into the duct through the engine inlet. Fuel is injected into the duct to react with the entrained air and generate additional thrust. The engine efficiency resulting from this thrust augmentation can be 50--100% greater than typical rocket engine values. In order to achieve this large efficiency gain, the engine must entrain as much air as possible without increasing the vehicle drag.; The objective of this research was to characterize the primary/secondary mixing and combustion in order to understand the physical processes that affect thrust augmentation. A single-thruster (Single) and two twin-thruster (Twin A and Twin B) configurations were tested. The horizontal RBCC duct entrained quiescent air from the lab through an open, converging inlet.; The main accomplishment was the characterization of a phenomenon that allowed the Twin B configuration to entrain 20% more air and generate 8% more thrust than the Single and Twin A configurations. Complementary computational fluid dynamics (CFD) calculations predicted virtually no difference between the three configurations. High-speed images showed that the experimental flow fields were unsteady, unlike the steady-flow CFD solutions. Frequency measurements helped determine that the unsteadiness was due to transverse acoustic modes in the duct. The acoustic frequencies and power levels were significantly different for each thruster configuration. Unlike the other cases, the Twin B configuration had most of the acoustic energy concentrated in a well-defined fundamental mode. The phenomenon responsible for the Twin B performance improvement is aero-acoustic coupling. It occurs when noise emanating from the primary/secondary shear layer tunes to a fundamental duct mode, causing the ejector performance to change in a non-linear manner.; A numerical model was developed to predict the mode frequencies based on a variable speed of sound in the duct. The predicted frequencies were within +/-10% of the measured values for the Twin B configuration, which had the most coherent mode shapes.