Influence laws on ground zero-velocity ejector performance of two-stage rocket rocket-based combined-cycle engines

The ground zero-velocity takeoff is a critical phase for the ejector mode of a Rocket-Based Combined Cycle （RBCC） engine. At this stage， the mass of the vehicle is at its maximum， and the traditional single-stage rocket operation mode struggles to meet the thrust requirements for takeoff. Therefore， this paper proposes an engine configuration with a two-stage rocket and investigates the influence law of rocket operating parameters on the zero-velocity takeoff phase of the RBCC engine through numerical simulations. The results show that when the chamber pressure of the first-stage rocket is maintained at 5 MPa and the total mass flow of the first-stage rocket is maintained at 0.4 kg/s， the ejector ratio is maximized， with the induced airflow increasing by 30% compared with the condition of the mass flow of the first-stage rocket at 0.2 kg/s. The ejector ratio increases as the chamber pressure of the first-stage rocket increases. When the chamber pressure reaches 20 MPa， the ejector ratio and specific impulse performance are improved by 19% and 26%， respectively. However， increasing the chamber pressure from 10 MPa to 20 MPa only results in an approximately 2% improvement in the ejector ratio， while significantly increasing the design and manufacturing challenges. Reducing the throttling ratio of the second-stage rocket leads to a decrease in the engine-specific impulse， whereas increasing the throttling ratio reduces the induced airflow. Simultaneously， increasing the expansion ratio of the rocket nozzle with a higher throttling ratio significantly enhances the engine’s specific impulse performance. This study identifies the matching rules between rockets and the ramjet duct in the two-stage rocket RBCC engine configuration， providing valuable guidance for the design of the ejector mode in future two-stage rocket RBCC engines.