High-power heat transport systems for large space platforms such as Space Station require the use of complex fluid loops to effectively and efficiently move waste heat energy from source to sink. In particular, use of two-phase heat acquisition and transport systems offers significant advantages such as reduction of pump power, automation of control systems, constant sink temperatures at the load, and flexible load placement. Analytical tools are needed for design analysis and performance prediction of these systems. Moreover, environmental considerations and insulation systems need to be taken into account, especially when subcooling and superheating become important parameters in the overall design. This paper will discuss the development and use of FLOSIN, a system-level, two-phase fluid loop analyzer. It will explain the modeling approach for systems utilizing a Rotary Fluid Management Device (RFMD), Back Pressure Regulating Valve (BPRV), and cavitating Venturis. It will descibe the unique components and special subroutines for processing input and output, for handling flow splits, and for generating Systems improved Numerical Differencing Analyzer (SINDA) thermal networks. It will also provide some comparisons with data taken during Independent Research and Development (IR&D) testing of the Boeing Aerospace prototype ammonia thermal bus Ground Test Article (GTA).