The worldwide growing attention to reduce the environmental impact of civil aviation in the next future is clearly visible. At the same time, the goal of achieving high flight speeds is becoming ever more important. During the first parts of the design process, when a quick response to each design modification is required, it is important to have simple and reliable tools to estimate the performances of such vehicles. The aim of the thesis is the development of methodologies and tools for the characterization of propulsive performance of high-speed aircraft in conceptual design. The results of this work allow the evaluation of basic performances for different hypersonic air-breathing propulsive systems architecture and the upgrade of the mathematical models implemented in ASTRID-H tool, a conceptual design tool currently under development by Politecnico di Torino.