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In this paper, experimental work performed on a breadboard Loop Heat Pipe (LHP) is presented. The test article was built by DCI for the Geoscience Laser Altimeter System (GLAS) instrument on the ICESat spacecraft. The thermal system requirements of GLAS have shown that ammonia cannot be used as the working fluid in this LHP because GLAS radiators could cool to well below the freezing point of ammonia. As a result, propylene was proposed as an alternative LHP working fluid since it has a lower freezing point than ammonia. Both working fluids were tested in the same LHP following a similar test plan in ambient conditions. The thermal performance characteristics of ammonia and propylene LHP's were then compared. In general, the propylene LHP required slightly less startup superheat and less control heater power than the ammonia LHP. The thermal conductance values for the propylene LHP were also lower than the ammonia LHP. Later, the propylene LHP was tested in a thermal vacuum chamber.

Loop Heat Pipes (LHPs) are being considered for cooling of military combat vehicles and spinning spacecraft. In these applications, it is important to understand the effect of an accelerating force on the performance of LHPs. In order to investigate such an effect, a miniature LHP was installed on a spin table and subjected to variable accelerating forces by spinning the table at different angular speeds. Several patterns of accelerating forces were applied, i.e. continuous spin at different speeds and periodic spin at different speeds and frequencies. The resulting centrifugal accelerations ranged from 1.2 g's to 4.8 g's. This paper presents the first part of the experimental study, i.e. the effects of an accelerating force on the LHP start-up. Tests were conducted by varying the heat load to the evaporator, condenser sink temperature, and LHP orientation relative to the direction of the accelerating force.