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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.

A loop heat pipe (LHP) was flight tested as a Hitchhiker experiment on board the space shuttle Columbia. The STS-87 mission lasted from November 19 to December 5, 1997. A total of 213 operating hours of the LHP were accumulated during the 16 day mission. Testing consisted of numerous start-ups, step power changes ranging from 15 to 400 Watts, 18 hours steady state operation at low power (20 W), 49 hours steady state operation at high power (200 W), and temperature control tests using a built-in thermostat. The operating temperatures of the loop ranged from -27°C to +66°C; the lowest sink temperature was -34°C. All objectives of the test plan were met or exceeded. The LHP functioned flawlessly throughout the entire test program.

Loop Heat Pipes have been shown to be reliable, self-starting, high capacity heat transport devices which offer significant performance improvements over conventional heat pipes. They are also capable of both constant and variable conductance operation, providing both temperature control and heat transport functions. Two different Loop Heat Pipes were tested to characterize their temperature control capabilities. Operation in both a passive, autoregulating mode and an active control mode were investigated. The test results demonstrate that temperature regulation in each mode is attainable. The effects of the operational environment on temperature control characteristics were found to be important and are discussed.