Research progress on ejector mode of rocket-based combined-cycle engines

The Rocket-based Combined-cycle (RBCC) engine is regarded as one of the most efficient propulsion systems for reusable space transportation vehicles and hypersonic cruise vehicles. It can perform excellently over a very broad range of Mach numbers and flight altitudes through multi-mode operations by effectively integrating the high thrust-to-weight ratio of the rocket engine and the high specific impulse of the airbreathing engine. As one of the most distinctive features and advantages of an RBCC engine, the ejector mode is an important mode for supporting the flight vehicle's rapid acceleration and climb. The engine performance, especially the specific impulse in the ejector mode, plays a vital role in determining the majority of the total fuel consumption, the propulsion efficiency of the entire RBCC engine, and further the whole flight trajectory. Therefore, an excellent performance in the ejector mode sufficiently guarantees the competitiveness and application prospect of an RBCC engine. This paper presents a comprehensive and systematic review of the research progress on the ejector mode of RBCC engines in different countries and different historical periods. In the survey, the summary delivers a progress overview on the ejector mode research, which covers a wealth of landmark experimental and application results on the ejector mode research in different historical periods. On this basis, a number of relevant key technologies regarding the study of the ejector mode are proposed, including efficient ejection, efficient mixing and combustion, widely adjustable rocket, widely applicable variable geometry inlet/nozzle design and control, multi-mode flow passage design, and robust mode transition. Several inherent scientific issues involved in these technologies are refined, such as the dynamic air ejection mechanism, combustion and thermal choke mechanism, and the coupling mechanism between air ejection and thermal choke. Finally, the historical development trend of ejector mode research is clarified, and some recommendations for performance improvement and the key technology breakthrough are provided for future ejector mode research of RBCC engines.