The Constrained Vapor Bubble (CVB) is a wickless, grooved heat pipe and is the first, full-scale fluids experiment flown on the U.S. module of the International Space Station. The CVB promises to provide new insight into the operation of a heat pipe in space. It is a relatively simple device constructed from a spectrophotometer cuvette and uses pentane as the working fluid. The pentane flows within the corners of the cuvette due to a curvature gradient in the liquid menisci associated with the cuvette corners. The curvature of the liquid interface can be determined by viewing the meniscus through the transparent quartz walls. Extremely accurate temperature and pressure measurements were obtained in addition to the images. In the article, the results from the first two CVB modules-a dry calibration module and a wet heat pipe module-are presented. We show that the axial temperature profiles are significantly different in space. The heat pipes were seen to operate at a higher pressure and higher temperature in space primarily because radiation was the only heat loss mechanism. A fin model was developed to model the data, and Churchill's correlations for natural convection were used to determine the external heat transfer coefficient. Inside evaporation and condensation heat transfer coefficients were regressed from the temperature data. We show that the heat transfer coefficient in microgravity was higher.