Synergetic and Performance Characteristics of a High-Speed Precooled Propulsion Concept

Precooled air-breathing cycles are promising candidates to power future high-speed flight as well as single-stage-to-orbit vehicles, due to their increased efficiency over contemporary propulsion systems and launch vehicles. These concepts usually feature complex interactions in the synergy of their thermodynamic cycles, which are not yet well understood, especially at off-design conditions. In this study, a performance model for a precooled, hybrid, air-breathing, rocket-cycle engine is developed for its air-breathing mode of operation. One-dimensional thermodynamic modeling is employed within a component-level approach, to evaluate the performance and operation of the cycle under investigation in the range of $1.35≤M∞≤5$ and conditions of up to 26 km altitude. The model is validated quantitatively and qualitatively for both design and off-design conditions. The specific impulse $Isp$ and specific thrust $Fspec$⁠, as predicted by the model, agree within less than 5% for the design-point conditions at $M∞$ = 5. At off-design conditions, the model captures the trend of $Isp$ and agrees within less than 1% with respect to the data for the maximum value of $Isp$⁠. The maximum gross thrust $Fgross$ point is predicted correctly at $M∞=4$⁠. The fundamental operating principles and synergetic characteristics of the engine at design and off-design conditions are investigated and reported. Parametric analyses quantify the influence of the engine's parameters on the leading performance metrics. A model which does not feature a bypass duct is created and compared for the same inflow conditions and mission profile. It is found that the engine without the bypass duct exhibits reduced specific impulse which can be up to 32% lower at off-design conditions. In addition, the corresponding fuel mass flowrate to achieve the same mission is increased by a factor of 1.5. It is demonstrated that the overall trend of engine efficiency cannot be properly captured without modeling of the bypass duct, especially at the region of $M∞<3.5$⁠, where the ramjet-like operation is critical. This highlights the importance of the bypass, which is typically neglected in the modeling of such high-speed, combined-cycle systems.