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Having been in operation for over 15 years, the Hubble Space Telescope (HST) had experienced significant changes in both hardware upgrades and operational modes. The changes were necessary to improve performance of some equipment and to replace failed electronics in others. Hardware replacements were done in several servicing missions. To accommodate the change in physical condition of HST, alterations in the way the telescope is operated were also required. The final opportunity to make any hardware changes on HST is during Servicing Mission 4 (SM-4) which is currently scheduled for December of 2007. It is important to make the most appropriate changes in order to ensure that HST will be in good operating condition until its planned termination. In order to provide manifest input to the HST project for the final servicing mission, the HST thermal team must conduct careful evaluation of every single piece of hardware on HST.

A Small Loop Heat Pipe (SLHP) featuring a wick of only 1.27 cm (0.5 inches) in diameter has been designed for use in spacecraft thermal control. It has several features to accommodate a wide range of environmental conditions in both operating and non-operating states. These include flexible transport lines to facilitate hardware integration, a radiator capable of sustaining over 100 freeze-thaw cycles using ammonia as a working fluid and a structural integrity to sustain acceleration loads up to 30 g. The small LHP has a maximum heat transport capacity of 120 Watts with thermal conductance ranging from 17 to 21 W/°C. The design incorporates heaters on the compensation chamber to modulate the heat transport from full-on to full-stop conditions. A set of start up heaters are attached to the evaporator body using a specially designed fin to assist the LHP in starting up when it is connected to a large thermal mass.