Title of Presentation: Miniature Loop Heat Pipe with Multiple Evaporators for Small Spacecraft Thermal Control

Primary (Corresponding) Author: Jentung Ku

Organization of Primary Author: Laura Ottenstein, Thai Pham, Donya Douglas

Co-Authors: NASA Goddard Space Flight Center

Abstract: Loop Heat Pipes (LHPs) are very versatile heat transfer devices that have been used for thermal control of many commercial communications satellites and NASA’s spacecraft, including ICESAT, AURA, SWIFT, and GOES. All LHPs currently servicing orbiting spacecraft have a single evaporator with an outer diameter of 25mm. For small spacecraft applications, miniaturization of the LHP is necessary in order to meet the stringent requirements of low mass, low power and compactness. Also important in the thermal subsystem development are the need for design flexibility which allows for optimum placement of components. Under NASA’s New Millennium Program Space Technology 8 (ST 8) Project, the Thermal Loop experiment will validate in space an advanced heat transport system consisting of a miniature loop heat pipe (MLHP) with multiple evaporators and multiple condensers for thermal control of small spacecraft. An MLHP Breadboard having two evaporators and two condensers has been successfully built and tested. The MLHP evaporators have capillary wicks with an outer diameter of 6.35mm. Thermoelectric converters (TECs) are used to control the MLHP operating temperature.

The MLHP Breadboard has demonstrated excellent performance in laboratory and thermal vacuum environments. The loop started successfully every time with heat loads of 2W to 50W to either one or both evaporators. The loop operated steadily with heat loads of 2W to 130W. The TECs could maintain the loop operating temperature within 0.5K of the desired set point temperature at all power levels and all sink temperatures, and could save more than 50% of the control heater power when compared to traditional electrical heaters. The loop could adapt to rapid changes in the evaporator heat loads or condenser sink temperatures. The un-powered evaporator would automatically share heat from the other powered evaporator, thus eliminating or reducing the supplemental heater power needed to maintain the temperature of the un-powered instrument. The Thermal Loop technology has achieved a technology readiness level of 5.

The MLHP combines the functions of variable conductance heat pipes, thermal switches, thermal diodes, and the state-of-the-art LHPs into a single integrated thermal system. It retains all the performance characteristics of state-of-the-art LHPs and offers additional advantages to enhance the functionality, performance, versatility, and reliability of the system. In addition, steady state and transient analytical models have been developed, and scaling criteria have also been established.