Overview

The purpose of this chapter is to describe in simple physical terms the fundamental characteristics of gas turbines and related flight vehicle propulsion systems the characteristics that control and limit their design and their application. Some of these characteristics are thermodynamic, some fluid-dynamic, some mechanical. It is important to realize that they all play important roles in the engineering choices that enter into the design of propulsion systems, whether the application is on the ground, in an aircraft, or in a launch vehicle.

All aircraft engines and gas turbines are heat engines, in which thermal energy derived from the combustion of fuel with air (or with an oxidizer carried on the vehicle) is converted to useful work in one way or another.

When the useful output of a gas turbine is in the form of shaft power used to drive a wheeled vehicle, a machine, or an electric generator, its efficiency will usually be characterized as thermal efficiency, defined as the fraction of the thermal-energy input converted to mechanical work. This concept should be familiar to those acquainted with thermodynamics.

In aircraft propulsion, of course, the useful work of the engine is work done in propelling the aircraft. It is appropriate then to define a second efficiency, the propulsive efficiency, as the ratio of the propulsive power to the total mechanical power produced by the engine. Analogous efficiencies of utilization can be defined for other applications of gas turbines, but the propulsive efficiency is particularly important to this...