A test rig is assembled and operational at the Heat Pipe Testing Laboratory at Glenn Research Center’s (GRC) Electric Propulsion Lab to evaluate the thermal interface between a pumped single-phase fluid (water) and a heat pipe evaporator. Several different titanium-water heat pipes with various external fin geometries were manufactured at GRC. The heat pipes can be installed in the Thermal Interface Evaluation (TIE) test rig with the evaporators heated by the pumped water, representative of a pumped cooling loop for a Fission Surface Power (FSP) power conversion unit. The condenser section of the heat pipes is installed in a gas-gap calorimeter to measure heat transfer. The test plan specifies obtaining thermal performance characteristics of titanium-water heat pipes of approximately the same evaporator diameter and length as those envisioned for FSP radiators, operating in the temperature range of 400 K.