The demanding of a holistic view for the engines in the fuel direct precooled family was clarified. A unified cycle model was proposed so as to provide a consistent thread of insight for the engine family. The model indicates that the engines in the fuel direct precooled family are not collections of different cycles, but essentially the same cycle operating under different conditions. Theoretical analysis of the unified cycle was carried out to make clear the dependencies of the engine performance parameters. Numerical model was developed also to take into account the actual conditions including the real fluid properties, the mass addition effects, etc. Performance evaluation and optimization based on the numerical model were completed for three typical cycles under the flight condition of Mach 5.0 with the dynamic pressure q(0) = 45 kPa. The results show that the PC-RT cycle possesses superiority on both the geometry and performance sides among the cycles evaluated. The selection of precooling temperature is a tradeoff between the specific thrust and the specific impulse. Compared to the conventional ramjet engine, the precooled cycles have the potential to provide both high specific thrust and high specific impulse. (C) 2017 Elsevier Ltd. All rights reserved.