Published on Apr 02, 2024
In a conventional ramjet, the incoming supersonic airflow is slowed to subsonic speeds by multiple shock waves, created by back-pressuring the engine. Fuel is added to the subsonic airflow, the mixture combusts, and exhaust gases accelerate through a narrow throat, or mechanical choke, to supersonic speeds.
By contrast, the airflow in a pure scramjet remains supersonic throughout the combustion process and does not require a choking mechanism, which provides optimal performance over a wider operating range of Mach numbers. Modern scramjet engines can function as both a ramjet and scramjet and seamlessly make the transition between the two .
The scramjet provides the most integrated engine-vehicle design for aircraft and missiles. The engine occupies the entire lower surface of the vehicle body. The propulsion system consists of five major engine and two vehicle components: the internal inlet, isolator, combustor, internal nozzle, and fuel supply subsystem, and the craft's forebody, essential for air induction, and aftbody, which is a critical part of the nozzle component.
The high-speed air-induction system consists of the vehicle forebody and internal inlet, which capture and compress air for processing by the engine's other components. Unlike jet engines, vehicles flying at high supersonic or hypersonic speeds can achieve adequate compression without a mechanical compressor. The forebody provides the initial compression, and the internal inlet provides the final compression. The air undergoes a reduction in Mach number and an increase in pressure and temperature as it passes through shock waves at the forebody and internal inlet.
The isolator in a scramjet is a critical component. It allows a supersonic flow to adjust to a static back-pressure higher than the inlet static pressure. When the combustion process begins to separate the boundary layer, a precombustion shock forms in the isolator. The isolator also enables the combustor to achieve the required heat release and handle the induced rise in combustor pressure without creating a condition called inlet unstart, in which shock waves prevent airflow from entering the isolator.
The combustor accepts the airflow and provides efficient fuel-air mixing at several points along its length, which optimizes engine thrust.
The expansion system, consisting of the internal nozzle and vehicle aftbody, controls the expansion of the highpressure, high-temperature gas mixture to produce net thrust. The expansion process converts the potential energy generated by the combustor to kinetic energy.
The important physical phenomena in the scramjet nozzle include flow chemistry, boundarylayer effects, nonuniform flow conditions, shear-layer interaction, and three- dimensional effects. The design of the nozzle has a major effect on the efficiency of the engine and the vehicle, because it influences the craft's pitch and lift.
In a conventional ramjet, the incoming supersonic airflow is slowed to subsonic speeds by multiple shock waves, created by back-pressuring the engine. Fuel is added to the subsonic airflow, the mixture combusts, and exhaust gases accelerate through a narrow throat, or mechanical choke, to supersonic speeds. By contrast, the airflow in a pure scramjet remains supersonic throughout the combustion process and does not require a choking mechanism, which provides optimal performance over a wider operating range of Mach numbers. Modern scramjet engines can function as both a ramjet and scramjet and seamlessly make the transition between the two.
The scramjet provides the most integrated engine-vehicle design for aircraft and missiles. The engine occupies the entire lower surface of the vehicle body. The propulsion system consists of five major engine and two vehicle components: the internal inlet, isolator, combustor, internal nozzle, and fuel supply subsystem, and the craft's forebody, essential for air induction, and aftbody, which is a critical part of the nozzle component.
The high-speed air-induction system consists of the vehicle forebody and internal inlet, which capture and compress air for processing by the engine's other components. Unlike jet engines, vehicles flying at high supersonic or hypersonic speeds can achieve adequate compression without a mechanical compressor. The forebody provides the initial compression, and the internal inlet provides the final compression. The air undergoes a reduction in Mach number and an increase in pressure and temperature as it passes through shock waves at the forebody and internal inlet.
The isolator in a scramjet is a critical component. It allows a supersonic flow to adjust to a static back-pressure higher than the inlet static pressure. When the combustion process begins to separate the boundary layer, a precombustion shock forms in the isolator. The isolator also enables the combustor to achieve the required heat release and handle the induced rise in combustor pressure without creating a condition called inlet unstart, in which shock waves prevent airflow from entering the isolator.
The combustor accepts the airflow and provides efficient fuel-air mixing at several points along its length, which optimizes engine thrust.
The expansion system, consisting of the internal nozzle and vehicle aftbody, controls the expansion of the highpressure, high-temperature gas mixture to produce net thrust. The expansion process converts the potential energy generated by the combustor to kinetic energy.
The important physical phenomena in the scramjet nozzle include flow chemistry, boundarylayer effects, nonuniform flow conditions, shear-layer interaction, and three-dimensional effects. The design of the nozzle has a major effect on the efficiency of the engine and the vehicle, because it influences the craft's pitch and lift.
Changing from subsonic to supersonic combustion, the kinetic energy of the freestream air entering the scramjet engine is large compared to the energy released by the reaction of the oxygen content of the air with a fuel (say hydrogen). Thus the heat released from combustion at Mach 25 is around 10% of the total enthalpy of the working fluid. Depending on the fuel, the kinetic energy of the air and the potential combustion heat release will be equal at around Mach 8. Thus the design of a scramjet engine is as much about minimising drag as maximising thrust.
An air-breathing hypersonic vehicle requires several types of engine operations to reach scramjet speeds. The vehicle may utilize one of several propulsion systems to accelerate from takeoff to Mach 3. Two examples are a bank of gas-turbine engines in the vehicle, or the use of rockets, either internal or external to the engine. At Mach 3-4, a scramjet transitions from low-speed propulsion to a situation in which the shock system has sufficient strength to create a region(s) of subsonic flow at the entrance to the combustor.
In a conventional ramjet, the inlet and diffuser decelerate the air to low subsonic speeds by increasing the diffuser area, which ensures complete combustion at subsonic speeds. A converging- diverging nozzle behind the combustor creates a physical throat and generates the desired engine thrust. The required choking in a scramjet, however, is provided within the combustor by means of a thermal throat, which needs no physical narrowing of the nozzle. This choke is created by the right combination of area distribution, fuel-air mixing, and heat release.
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