Thermal Performance of a Miniature Variable Conductance Loop Heat Pipe 2000-01-2490

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 prototype of a miniature variable conductance LHP with a three-way valve was developed and built in 1999 for evaluation as the thermal control unit for the Mars ‘03/’05 rover. This LHP has two condensers, one integrated with the internal equipment which needs to be heated and the other serving as an external radiator. A three-way valve is used to activate the external radiator whenever the evaporator temperature exceeds a pre-set value. If the vapor temperature drops below the valve's set-point, the valve opens a by-pass line and the heat is no longer rejected at the external condenser. Warm vapor bypasses the condenser and enters the evaporator from the liquid side and keeps the evaporator warm. If the system's temperature has increased and exceeded the set-point, the valve closes the bypass line and opens the line to the external condenser. The miniature VCLHP was subjected to a test program that verified the reliability of temperature control provided by this type of LHP; it also highlighted the pluses and minuses of this technology. The design of the LHP, the test setup, test plan and test results are discussed in this paper. An interesting behavior was demonstrated during the course of the evaluation program. It was demonstrated that the three-way valve approach to control LHP temperatures is very attractive for the thermal control systems.