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The Two Phase Flow (TPF) Thermal Control Flight Experiment is an integrated two-phase thermal control system designed to address capillary pumped loop (CPL) component and system performance issues in the microgravity environment of space. The TPF experiment was flown aboard the Space Shuttle Discovery (STS-85) in August 1997 as part of the Technology Applications and Science-01 (TAS-01) mission. Ground and flight tests were performed to demonstrate reliable system start-up from the flooded state and sustained operation under various heat loads and conditions. Use of any of the large evaporator elements as a sacrificial starter pump in conjunction with the capillary vapor flow valve (CVFV) resulted in successful system start-up subsequent to which normal operation was demonstrated for an extended period of time. As expected, start-up from the flooded state with smaller evaporator elements was consistently unsuccessful.

The Capillary Pumped Loop (CAPL) experiment, consisting of four tubular evaporator elements configured in parallel, a dedicated starter pump, four condenser elements, and a heatpipe radiator, was tested in microgravity onboard the Space Transportation System (STS). All twenty attempts at starting the system from the flooded state using a ground-proven procedure were unsuccessful. These unexpected events are attributed to the failure of the startup procedure to clear the liquid flooding the vapor spaces of the evaporator elements before heat was applied to the evaporators. Emptying of these vapor spaces prior to the application of a heat load had been found necessary to prevent a postulated vapor blow-by mechanism that can potentially lead to arterial depriming by injection of bubbles into the liquid core of the capillary pumps.