FLUIDNET-G is a “workstation” based graphic's orientated software package, developed by AEROSPATIALE under a C.N.E.S. contract on joint funding. The package performs single phase fluid loops thermal and hydraulic preliminary design. The main characteristics of this tool are: the user friendliness of the graphic's and the workstation concept. the “upstream compatibility” towards others analysis tools (FLUINT, ESATAN, …) This paper illustrates an application for which FLUIDNET-G has increased engineer performance to perform trade-offs and sensitivity studies in the frame of the preliminary thermal and hydraulic design of HERMES spaceplane.