Overview

Cycle analysis is the study of the thermodynamic behavior of air as it flows through an engine, without regard for the mechanical means used to effect its motion. Rather than deal with the inlet, the compressor and the turbine, themselves, we characterize them by the results they produce. Thus, for example, the compressor will be specified by a stagnation pressure ratio and efficiency. The behavior of a real engine is in fact determined by its geometry, in that for a given geometry the compressor will produce some well-defined pressure ratio depending, of course, on operating parameters such as speed and airflow, the latter being controlled by some characteristic of the flow path, such as a downstream orifice. Thus, when we plot the results of cycle analysis in this chapter as curves of thrust and specific impulse versus, say, Mach number, we are not portraying the behavior of a real engine as a function of those variables. Rather, each point on such a curve represents a potential performance for some engine, which can be realized if an engine can be assembled from components that develop the airflows, pressure ratios, and efficiencies assumed in the calculation. There is no guarantee that this is possible. We sometimes say that the cycle analysis represents a "rubber engine" to emphasize the point that the computed performance characteristics are not those associated with an actual engine. The main purpose of cycle analysis is to determine which characteristics to choose for the various components of an engine...