A comparative study of high-speed engine performance assessment techniques based on exergy (available work) and thrust potential (thrust availability) is summarized. Simple one-dimensional flowfields utilizing Rayleigh heat addition and friction are used to demonstrate the inability of conventional exergy techniques to predict engine component performance, aid in component design, or accurately assess flow losses. The thrust-based method yields useful information in all of these categories for these flows. The conventional definition of exergy includes work that is inherently unavailable to an aerospace Brayton engine. An engine-based exergy is developed that accurately accounts for this inherently unavailable work; performance parameters based on this quantity yield design and loss information identical to the thrust-based method.