COMBINED CYCLE AIRBREATHING ENGINES :

The favoured solution to the hypersonic air breathing propulsion problem of maintaining acceptable thrust and fuel consumption over the entire flight spectrum is to unite several different propulsion concepts within the same internal fiowpath. Airbreathing engines of this type are known as combined cycle or composite engines. As a general rule, the concepts incorporated in combined cycle engines are based upon familiar turbojet, ramjet, scramjet, and rocket propulsion.

The Turbo Ramjet

The most widely used method of generating thrust at very low flightspeeds rests upon the mechanical compression of turbojet engines,which both induces a flow of air and leads to a positive thermal cycleefficiency for any ensuing energy release. The increased temperaturesand pressures resulting from the mechanical compression are alsoconducive to the efficient and stable combustion of fuel. Figure 8.13contains a diagram of the turbo ramjet, frequently called the air turboramjet, a combined cycle engine configuration that takes advantageof these principles.The main components of the turbo ramjet, moving in the streamwisedirection from left to right, are:

1. A modest pressure ratio, high throughflow axial flow compressor,more commonly known as a fan because it resembles onein form and function. At high freestream Mach numbers, say> 5.0 or so, the compression ratio of the fan is no longer neededfor thermal efficiency, and its survivability is threatened by thehigh stagnation temperature of the captured air. Consequently,if the engine is intended for use at even higher freestream Machnumbers, provision must be made to remove or retract the fanfrom the main fiowpath.

2. A turbine that is driven by high pressure, high temperaturegases generated in a separate combustion chamber. This turbineprovides the power required by the fan, and enjoys thespecial advantage that its inlet conditions are isolated from andtherefore independent of the flight conditions. The turbine flowwill be referred to here as the primary stream. The presenceof the fan and turbine require that at least the forward portionof this engine be axisymmetric, rather than two-dimensional,which can complicate integration with the vehicle.

3. A mixer that blends the airflow with the primary flow. Thisincreases the total temperature of the airflow, and distributesany unreacted fuel throughout the air, where it can burn. Theprimary flow can be deliberately designed to be overly rich infuel, and thus to provide hot, gaseous fuel for easy combustionwith the air.4. Fuel injectors to provide additional unreacted fuel to the flow,as desired.

5. A burner, including fiameholders, with sufficient residence timeto bring the chemical reactions to completion.

6. A nozzle that properly expands the exhaust flow to the ambientstatic pressure.

One might expect from its superficial appearance that the turboramjet behaves like a turbojet at very low flight speeds and like aramjet or rocket engine at higher flight speeds. This conjecture willbe tested by the forthcoming analysis.

The Ejector Ramjet

The basic property of ordinary ejectors is that they multiply the originalor primary mass flow by drawing in a supplemental or secondarymass flow from the surrounding atmosphere, as indicated in Fig. 8.9.In the same process, the total pressure of the secondary flow is raisedto a value between that of the ambient and that of the primary flow.Ejectors are mechanically simple, requiring only an enclosing passageor shroud around the primary flow long enough to enable completemixing with the secondary flow. The ejector is therefore crudely analogousto the mixed exhaust flow bypass turbofan engine, althoughthe energy transfer efficiency is low because it is accomplished byviscous shear forces rather than rotating turbomachinery. 8aSince ramjets produce little or no thrust during takeoff, a rocketengine or its equivalent must be part of the vehicle. This devicecould either operate independently or act as the primary of an ejectorfor which the existing ramjet or scramjet passage could serve as ashroud. In the latter case, the pressurized flow leaving the ejectorcan be decelerated, mixed with fuel, and burned in a combustor, andthen accelerated through a nozzle to produce thrust. The net effectof the ejector is to supply the burner with a flow of pressurized airthat would be roughly equivalent to the ram conditions of a muchhigher forward speed. Thus, the ejector-burner-nozzle combinationis aptly referred to as the ejector ramjet.Ejector ramjets are attractive low speed propulsion candidatesbecause of their mechanical simplicity and because they can easilybe integrated into the existing flowpath. You will soon see that theycan increase the thrust substantially beyond that of the primaryflow acting alone. Thus, they meet all the stated criteria of desirablethrust augmentation devices.