In this paper, a simple model is proposed to predict the performance of an ideal H2-Air air-breathing rotating detonation engine (RDE) and compare it with an ideal conventional ramjet engine. Furthermore, trajectory optimization is done for a rocket-based combined cycle (RBCC) engine (based on the proposed model) installed in an aircraft. The engine model is created by extending an axial flow model that shows the upper limit of the ideal rocket type RDE performance. The Mach number at the combustor entrance is an important factor for proposed model feasibility. Supplied air Mach number is constrained by below two conditions, auto-ignition temperature, and air supply for sustained detonation wave propagation By formulating the above conditions, the supply Mach number can be obtained as a solution to the nonlinear programming problem (NLP). Modeling in the range of the flight Mach number 0.6 to 5 shows that the operative upper limit of flight Mach number is about 3.5, and the operative lower limit depends on the combustor diameter of the RDE. For the minimum proposed size of RDE, the lower limit of flight Mach number is about 2.5; although when the diameter is twice that value, the lower limit of flight Mach number is 0.6. The air-breathing RDE has a specific impulse advantage over the conventional ramjet engine in the operative range. The flight trajectory optimization is formulated assuming a current business jet scale aircraft with a rocket to air-breathing engine switchable RBCC engine. Its objective function is maximizing payload through 2500 km flight. As a result of trajectory optimization, the conventional ramjet does not generate payloads, but air-breathing RDE produces payloads. These results suggest that air-breathing RDE have the potential to improve the performance of RBCC engine installed flight system.
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