The objective of this paper is to describe several figures of merit for estimation of loss in work potential based on the second law of thermodynamics and to evaluate their relative merits for propulsion system analysis and design. The loss figures of merit examined are exergy, gas horsepower, stream thrust, and thrust work potential. Definitions and simplified expressions for evaluating each are presented, and the relationships between these four metrics are expressed via contours on a T-S diagram. A general taxonomy classifying the various work potential figures of merit is suggested. The results indicate that that each method is well suited to a particular type of application, with the most appropriate choice of loss figure of merit depending on the particular application. Finally, thrust work potential is shown to be a special case of gas horsepower, which is in turn a special case of exergy.