3The Auxiliary Propulsion System Analysis Tool (APSAT) is a compilation of preexisting, stand-alone propulsion system component sizing modules that are merged into a single tool for use in integrated propulsion system design. The focus of APSAT is toward on-orbit main stage and reaction control liquid chemical propulsion systems, which can include orbital vehicles, propellant depots, and planetary landers. APSAT uses a one-dimensional equilibrium analysis capability to provide rocket engine performance characterization that is then used in the parametric or point design functions for system-level sizing. Consequently, APSAT can be used to rapidly estimate mass and volume requirements during conceptual propulsion system sizing studies and detailed propellant trades. Details of the model capabilities, sizing routines, assumptions used, and future plans for the continued development of APSAT is presented.