This webinar explores the evolution of liquid rocket engine cycle designs, from gas-generator cycles to advanced closed-cycle configurations, highlighting their advantages and engineering challenges. It focuses on the RS-25 liquid cryogenic rocket engine, explaining its architecture, operation, and the complex interactions between turbopumps, preburners, cooling circuits, and combustion processes. The presentation demonstrates why system-level simulation is essential for understanding such highly interconnected propulsion systems and discusses the limitations of traditional 3D CFD approaches. Flownex is introduced as a powerful 1D CFD simulation environment capable of modelling fluids, turbomachinery, controls, heat transfer, combustion, and nozzle thrust within a single integrated framework. A live demonstration showcases the creation of a basic rocket combustion model before presenting a full transient RS-25 simulation that illustrates engine performance, control behavior, and vehicle ascent dynamics.