The performance of an airbreathing engine is governed principally by the state properties of air and from vehicle characteristics that include: the captured inlet air mass flow, the entry air kinetic energy, the energy released to the cycle by combustion of the fuel, and the internal drag and energy losses through the engine flowpath [Yugov et al., 1990]. The energy losses in the air stream, the internal wave drag and friction drag of the engine module can be a dominant factor. Evaluating these factors permits the establishment of performance boundaries based on first principles. The result is an altitude-speed representation of performance potential and constraints for Brayton cycle airbreathing engines defined by two parameters, altitude and velocity. Performance is constrained by an altitude boundary (based on the entropy state of exhaust gas) and a velocity boundary (based on the air kinetic energy to combustion energy ratio). In order to define these boundaries we need to first establish the magnitude of the engine internal flow losses.
Energy input into the combustion chamber must overcome all the losses that are a result of the external drag of the vehicle, energy losses associated with the internal engine flow, and irreversible losses in the thermodynamic cycle plus supply the excess thrust minus drag required for acceleration to orbital speed. As shown in Figure 4.3, as the flight speed in increased, the kinetic energy becomes increasingly greater than the energy added by the fuel. As the flight speed is increased, the internal drag of the engine increases more rapidly than the airframe drag, so there is a point where the total drag is just equal to the thrust potential of the airbreathing propulsion system (which is decreasing with increasing speed because the fuel added energy is becoming a smaller fraction of the kinetic energy). That is the maximum speed of the air-breathing engine. The losses are represented as a fraction of the flight kinetic energy. The drag losses are given as drag areas referenced to an area related to the propulsion system (see Figure 4.2). Drag area is a universal way to represent drag energy losses. Multiplying the drag area by the local dynamic pressure, q, yields the total drag
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