Loop Heat Pipes for ASTRO-H/SXS

It is becoming increasingly difficult to meet the challenging thermal control requirements of modern spacecraft missions with only existing thermal control devices such as conventional heat pipes. A loop heat pipe (LHP) is an effective method to overcome some of these thermal control constraints. The LHP is a passive two phase heat transfer device that utilizes the evaporation and condensation of a working fluid to transfer heat and capillary force to circulate the fluid. The LHP can transport much heat for a long distance against gravity among many other excellent characteristics, such us the possibility of complex routing or the diode operation. Due to these excellent characteristics, LHPs are used in many space applications. In JAXA’s X-ray astronomy satellite ASTRO-H, mechanical cryocoolers were used in the thermal control system for the Soft X-ray Spectrometer (SXS) which was one of the most important missions in ASTRO-H. The cryocoolers were installed on the dewar and cooled down the dewar to prevent the evaporation of the liquid helium, which keeps the temperature of the SXS sensor at low temperature. 60W of heat dissipated by the cryocloolers as maximum should be transported to the heat sink which is located about 700mm under the dewar. Thus, the heat dissipated by the cryocoolers should be managed by a heat transfer device in top heat mode on ground at the launch pad, as the cryocoolers operated on the launch vehicle until right before launch. It was difficult to manage all the design and testing scenarios with conventional heat pipes whose performances are seriously affected by gravity, so LHPs were chosen for the thermal control of cryocoolers. The overview of the development of LHP (specifications of LHP, development schedule, overview of testing on ground before launch) and the on-orbit performance are presented in this paper.