The second major upgrade to the Fluid Heat Transport System extension of the ESATAN thermal analyser culminated in significant improvements and extensions to its two-phase capabilities. The third upgrade is nearing conclusion, the emphasis of this work centring around the improvement of the Graphical User Interface and the validation of the numerical algorithms employed.An interactive environment for the creation of ESATAN thermal models has been developed, easing both creation and visualisation of the model hierarchy. To consolidate this facility the current FHTS Graphical User Interface has now been incorporated within the framework of this tool. Many new capabilities have also been added, most notably the ability to configure, manage and control all files required by an ESATAN model. This product is aimed at both new and experienced users and will allow the thermal engineer to concentrate more upon the thermal modelling aspects as opposed to the idiosyncrasies of the software tool.Validation is a key issue throughout the life cycle of any simulation tool, comparing predicted results against physical data. Extensive work has already been carried out in this area, concentrating mainly upon the single-phase application of FHTS. Early in 1994 a two-phase experimental capillary-pumped loop was flown as a “Get Away Special” on the Space Shuttle. From this mission a wealth of data is now available at component and system level for the engineer and scientist alike to extend their knowledge of two-phase flow phenomena in a micro-gravity environment. A validation programme has been defined for FHTS using this data and preliminary results from the work are presented.