Several efforts are presently underway to reduce the size and weight of space systems by transitioning from single-phase to two-phase thermal management. Because of large density differences between liquid and vapor, buoyancy can play an important role in defining the motion of vapor relative to liquid and, therefore, influence heat transfer effectiveness. This is particularly the case for future manned missions to Mars. The emphasis in two-phase fluid physics is evident from several recent NASA workshops that culminated in critical recommendations concerning Rankine cycle power conversion, thermal control systems and advanced life support systems. Among others, key fluid physics mechanisms that have been identified are flow boiling critical heat flux (CHF) and condensation. A key strategy is to develop mechanistic models for minimum flow conditions that would ensure insensitivity to gravity level for these important heat transfer mechanisms. This would allow performing most of the required confirmatory experiments on Earth. These topics and the minimum flow requirements constitute key objectives for the proposed study. Unfortunately, most of the two-phase heat transfer know-how amassed over nearly a century of research comes from experiments that were conducted in Earths gravity. The proposed study aims to develop an experimentally validated, mechanistic model for the minimum flow criteria to ensure gravity independent flow boiling critical heat flux (CHF) and condensation. This will require performing extensive parabolic flight experiments in preparation of long-term International Space Station (ISS) experiments. This project will be a joint effort between the Purdue University Boiling and Two-Phase Flow Laboratory (BTPFL) and the NASA Glenn Research Center. Personnel from the two organizations combine extensive experience in research and development of both flow boiling and condensation systems, and in conducting microgravity experiments. A novel flow boiling and condensation apparatus has been developed jointly by Purdue University and NASA Glenn research Center to perform experimentation to better understand and evaluate the mechanistic models governing such phenomena in terrestrial and reduced gravity. The flow boiling system consists of a transparent flow boiling module attached to a closed FC-72 flow loop. The flow boiling module facilitates simultaneous heat transfer measurements and high-speed video imaging of flow boiling behavior. The rig is also equipped with two separate condensation modules and a separate water cooling loop. One module is instrumented mainly with thermocouples and pressure transducers and is occluded from visual observation. The second condensation module is instrumented with window ports, high-speed cameras, optical diagnostics and illumination systems. The purpose of the second condensation module is to observe and characterize the interfacial wave motion of the FC-72 condensate film during the important annular flow regime.