This article presents a novel and comprehensive approach to conceptual design of expander cycle liquid engines. One advantage of the computational code developed is the possibility of considering the working regime without cavitation of pumps by maximizing specific impulse of the engine system. Based on the proposed algorithm, the steady state regime of the mathematical modeling engine becomes static. This is important to increase reliability and to reduce costs during conceptual design phase, as well as optimization. One of the factors affecting the reliability of a system is its simplicity, which eliminates complexity. Here, the generator has been eliminated. In addition to the dimensional and functional estimations of the engine system, the proposed algorithm examines the performance of the propulsion system (engine specific impulse, turbine efficiency, and pumps) at various work stations. The computational algorithm of the static mathematical model was developed in Microsoft Visual Studio language.