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Loop Heat Pipes (LHP) of different designs are currently used in aerospace applications worldwide. Historically, LHPs were considered primarily as candidates for high power, high adverse elevation and high heat flux applications such as deployable radiators for large satellites, thermal bus devices, high heat flux payloads, etc. A new look at the LHP technology was presented in 1998 (Ref. 1), and the miniature LHP concept was introduced to the industry. Because miniature LHPs are frequently serving as “thermal shunts” for payloads and instruments, controllability issues played an important role from the very beginning of their development. For instance, the electrical power that is available for thermal control of Mars rovers on the Martian surface is limited. Because of this limitation, the thermal control systems for the new generation of Mars rovers were required to be absolutely passive.

A miniature Loop Heat Pipe (LHP) has been designed, built and tested at Dynatherm Corp. Inc. (DCI). This small, 229 mm by 127mm, ammonia charged LHP is intended to provide thermal management and temperature control for electronic components, especially for spacecraft applications. The mini-LHP has been successfully tested at DCI in various orientations, and has demonstrated the ability to maintain the LHP evaporator temperature at the simulated electronic heat source within ±0.25°C for heat loads between 5 and 10 watts, and evaporator minus condenser temperature differences as high as 82°C. This paper discusses the steady state and transient behavior of the mini LHP during different operational modes. The nature of the temperature fluctuations observed during mini LHP testing is dependent on the specific temperature control concept. In order to minimize these instabilities, several LHP design and control strategy options are discussed.

It is already well known that Loop Heat Pipes (LHPs) are able to transport heat over long distances, through flexible, small diameter tubes and against high gravitational heads. The existing experience of the conventional Heat Pipe history says that during the time some amount of non condensable gases (NCG) can be generated inside the closed system with the working fluid inside. It was shown in many previous publications how sensitive conventional heat pipes with composite wicks are on the presence of NCG. In spite LHPs are already widely applied in spacecraft, the NCG issue was not studied yet. The paper describes the work aimed at LHP operation study when NCG are injected. The experimental study of the LHP operation was performed. The LHP response on NCG injection with some increments was monitored in order to register any changes of the LHP performance. The End of Life (EOL) NCG amount in the ammonia LHP was calculated. The way of theoretical analysis is discussed in the paper.