Fundamentals of Propulsion

Interest in propulsion systems for aircraft started with the first powered flight by the Wright brothers. Today, interest in aircraft propulsion systems is driven predominantly by the fact that it is a multi-billion dollar industry, serving both commercial and military applications. In the commercial sector, fuel economy, emissions, noise, safety and ease of maintenance are the primary issues. These issues are important in military applications also, although to a lesser extent. The primary issues are flight speed (usually in the high subsonic, supersonic range), thrust, performance under off-design conditions (important for maneuverability) and acceleration capability (important for combat and short takeoff, landing). In this book, we focus primarily on aircraft engines for subsonic flight speeds. In the last chapter, some propulsion systems used for supersonic flight speeds are discussed briefly, to give an idea of the issues involved in the design of such systems.

Compressible flows are encountered in many applications in Aerospace and Mechanical engineering. Some examples are flows in nozzles, compressors, turbines and diffusers. In aerospace engineering, in addition to these examples, compressible flows are seen in external aerodynamics, aircraft and rocket engines. In almost all of these applications, air (or some other gas or mixture of gases) is the working fluid. However, steam can be the working substance in turbomachinery applications. Thus, the range of applications in which compressible flow occurs is quite large and hence a clear understanding of the dynamics of compressible flow is essential.

Since the compressor, fan and turbine are an integral part of any gas turbine-based engine, a clear understanding of the fundamental fluid- and thermo-dynamic processes occurring in such turbomachines is essential. With this in mind, some basic concepts relating to turbomachines are discussed in this chapter. For a detailed discussion on these and other advanced topics, the reader is referred to the classic treatise on turbomachinery by Shepherd.

In this chapter, some basic concepts in combustion are discussed. The combustion of a hydrocarbon fuel in a combustor is an important aspect of a gas turbine engine. Today, the most important issue relating to the combustor is emission and its control. There are also other issues such as combustion efficiency, stable operation and thermal stresses. Some of the basic ideas on fuels and combustion of the fuels discussed in this chapter will allow the reader to gain a better understanding of these issues. For a more detailed discussion of the topics related to combustion, the reader is referred to the book by Turns.

In this chapter, the operation of a basic turbojet engine is described. This is followed by a discussion of the individual components, namely, the compressor, combustor, turbine and the nozzle. This discussion brings out the details of the operation of these components along with important issues related to their operation. A thermodynamic analysis of the operation of the whole engine as well as the components is deferred till Chap. 7.

In the last chapter, the turbojet engine was discussed in detail. Historically, commercial aviation began toward the end of World War II with propeller-driven planes. It soon became apparent that the flight speeds of the propeller planes were limited to low subsonic values. The use of the gas turbine engine in combination with a propulsion nozzle, i.e., the turbojet engine, allowed higher flight speeds to be realized. Although still used for military applications, turbojets lack the efficiency required for commercial applications. This led to the evolution of the turbofan engine.

In the last two chapters, basic concepts and theory behind the design and operation of turbojet and turbofan engines were discussed. In this chapter, the thermodynamic operation of these engines is discussed. Expressions for the thrust developed as well as the thrust specific fuel consumption are derived. A procedure for calculating these quantities for a given set of operating conditions is also outlined.

The turbojet and turbofan engines discussed in detail in the previous chapters are designed for high subsonic cruise speeds. The addition of an afterburner allows the turbojet engine to operate at transonic speeds for short periods of time. As shown in Fig. 1.​1, for sustained supersonic flight speeds, ramjet or scramjet engines are necessary. These are discussed in this chapter. Keeping the intended audience in mind, only the basic concepts and ideas are presented here. The interested reader should consult the propulsion books given in the list of suggested reading for further details.