Airflow Energy Entering The Engine

With a rocket, all of the fuel and oxidizer are carried onboard the vehicle, so other than atmospheric vehicle drag and the nozzle exit pressure compared to atmospheric pressure, the vehicle's relative speed with respect to the atmosphere does not determine the propulsion system performance. The specific impulse is the thrust per unit propellant mass flow per second. So, if more thrust is required more engine mass flow is required, i.e. a larger engine or increased chamber pressure to increase the mass flow. With an airbreathing propulsion system just the opposite is true. Because for the airbreathing engine air enters the vehicle via an inlet, Figure 4.2, the ability of the inlet to preserve energy, as the flow is slowed down in the inlet (for instance, by passing through a series of shock waves), is absolutely critical. The magnitude of the flow kinetic energy recovered at the end of the inlet determines

Combustion Oxidizer chamber

Combustion Oxidizer chamber

Liquid propellant topping cycle rocket engine

Air-breathing engine air capture inlet

Figure 4.2. Liquid rocket engine carries its fuel and oxidizer onboard. By contrast an air-breathing engine carries only fuel onboard and the oxidizer is atmospheric air captured by the inlet. Ac = geometric capture area; A0 = cowl stream tube area, can be greater or less than Ac; A1 = engine module cowl area; A2 = engine module minimum area.

Air-breathing engine air capture inlet

Figure 4.2. Liquid rocket engine carries its fuel and oxidizer onboard. By contrast an air-breathing engine carries only fuel onboard and the oxidizer is atmospheric air captured by the inlet. Ac = geometric capture area; A0 = cowl stream tube area, can be greater or less than Ac; A1 = engine module cowl area; A2 = engine module minimum area.

how much of the fuel combustion energy is available to be converted into thrust. Because the oxidizer is the oxygen in the air, there is a maximum energy that can be added per unit mass flow of air. The capture area of the inlet and flow speed relative to the vehicle determines how much total energy the burned fuel can add to the air stream. Ultimately, it is the difference between the energy lost in the inlet and the combustion energy that determines the thrust. The energy of the air is a function of two quantities, the energy of the air in the atmosphere (static enthalpy, in kJ/kg) and the kinetic energy of the air stream (kinetic energy, in kJ/kg). In equation form the relationship is:

Total energy = Static enthalpy + Kinetic energy ht = ho + V2 = (kg) Vo = m/s ht = 232.6 +

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