Start Limits and Positive Control of RBCC Inlet

Abstract

A rocket-based combined-cycle (RBCC) inlet is always required to start at a relatively low Mach number in consideration of the fuel consumption in the ejector mode and the comprehensive engine performance throughout the flight trajectory. However, the practical start of an RBCC inlet is a dynamic process easily and significantly affected by the embedded rocket operating at a high chamber pressure in the ejector mode. Thus, the real start limits, in terms of the Kantrowitz limit and isentropic limit, and the relevant operational characteristics in the practical applications of a central strut–based RBCC inlet were studied through theoretical analysis and numerical simulations. A two-dimensional analysis model of the RBCC inlet fully concerning the expansion and aerodynamic compression of the rocket jet was established, and the theoretical calculation methods for the inlet start limits were carefully validated and corrected through numerical simulations. It was found that the underexpanded rocket jet expanded rapidly outside the nozzle and compressed the aerodynamic configuration of the flow passage, thereby increasing the actual internal compression ratio (ICR) of the RBCC inlet and increasing the difficulty of the start. The influence rules of the operational pressure and the nozzle expansion angle of the embedded rocket on the RBCC inlet start limits were also theoretically studied. As these key parameters kept increasing, the start limits were increased gradually. Moreover, the isentropic limit was more sensitive to the operational chamber pressure, whereas the increasing rate of the Kantrowitz limit was much more sensitive to the enlarging nozzle expansion angle. Finally, a positive control method based on the rocket operation during the inlet start process was promoted, which could not only guarantee the self-start requirement of the RBCC inlet as the designed scheme in the practical operation, but could also improve the compression efficiency and back-pressure resistance effectively in the subsequent ramjet operation.