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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.

Loop Heat Pipes (LHP) are widely used in space applications: deployable radiators, instrument cooling and precise temperature control. These applications are utilizing classical LHP designs with single evaporator and single condenser. Traditionally, the LHP is considered as an ideal solution for heat sources with concentrated power. The rule of thumb for a thermal control system based on the LHP is one heat source – one evaporator. It has been demonstrated several times that LHPs with dual evaporators are feasible, however, the volume of the compensation chamber grows dramatically with the number of evaporators. Therefore, LHPs with more than three evaporators are not practical. These restrictions restrict the area of LHP applications. The LHP evaporator generates vapor, which travels through the vapor line toward the condenser. In principle, this vapor could be used to absorb some additional power from heat sources other than the heat source feeding the evaporator.

Modern spacecraft are using Loop Heat Pipes (LHP) more frequently. However, there are no practical guidelines to help LHP users know how and when to use the active temperature control. Most spacecraft payloads and instruments must operate in a required, sometimes-narrow temperature range. Outside this temperature band the instruments either operate poorly or cease to operate. Radiator (sink) sizing can set the maximum permissible operating temperature, and active temperature control devices (TCD) provide lower margin of operation temperature. The LHP is a component of the spacecraft thermal control system, which, in most cases, functions as a system; therefore, LHP operation requires special consideration when using TCDs. It is shown that two major groups of TCD devices exist. The classification is based on the mechanism TCDs use to control the LHP temperature. A simple classification of different ways to control the temperature of LHP is presented.