Multi-Evaporator Miniature Loop Heat Pipe for Small Spacecraft Thermal Control, Part II: Validation Results

Jentung Ku, Laura Ottenstein, Donya Douglas, Triem Hoang
2010 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition   unpublished
Under NASA's New Millennium Program Space Technology 8 (ST 8) Project, Goddard Space Fight Center has conducted a Thermal Loop experiment to advance the maturity of the Thermal Loop technology from "proof of concept" to "prototype demonstration in a relevant environment", i.e. from a technology readiness level (TRL) of 3 to a level of 6. The thermal Loop is an advanced thermal control system consisting of a miniature loop heat pipe (MLHP) with multiple evaporators and multiple condensers
more » ... d for future small system applications requiring low mass, low power, and compactness. The MLHP retains all features of state-of-the-art loop heat pipes (LHPs) and offers additional advantages to enhance the functionality, performance, versatility, and reliability of the system. An MLHP breadboard was built and tested in the laboratory and thermal vacuum environments for the TRL 4 and TRL 5 validations, respectively, and an MLHP proto-flight unit was built and tested in a thermal vacuum chamber for the TRL 6 validation. In addition, an analytical model was developed to simulate the steady state and transient behaviors of the MHLP during various validation tests. The MLHP demonstrated excellent performance during experimental tests and the analytical model predictions agreed very well with experimental data. All success criteria at various TRLs were met. Hence, the Thermal Loop technology has reached a TRL of 6. This paper presents the validation results, both experimental and analytical, of such a technology development effort. 2 I. Introduction loop heat pipe (LHP) is a very versatile heat transfer device which can transport a large heat load over a long distance with a small temperature difference [1, 2] . The LHPs currently servicing NASA and commercial orbiting spacecraft have a single evaporator with a 25-mm outer diameter primary wick [3] [4] [5] . For small spacecraft applications, miniaturization of the LHP is necessary in order to meet the stringent requirements of low mass, low power and compactness. When the heat source has a large thermal footprint, or several heat sources need to be maintained at similar temperatures, an LHP with multiple evaporators is highly desirable. Multiple evaporators also provide an inherent heat load sharing function among several heat source components [6] . Under NASA's New Millennium Program Space Technology 8 (ST 8) Project, Goddard Space Fight Center has conducted a Thermal Loop experiment to advance the maturity of the Thermal Loop technology from "proof of concept" to "prototype demonstration in a relevant environment", i.e. from a technology readiness level (TRL) of 3 to a level of 6. The Thermal Loop is an advanced thermal control system consisting of a miniature loop heat pipe (MLHP) with multiple evaporators and multiple condensers. Each evaporator has a primary wick with 6.45 mm outer diameter. In addition, thermoelectric converters (TECs) are used to control the MLHP operating temperature. With multiple evaporators and multiple condensers, the MLHP also offers design flexibility, allowing the thermal subsystem components to be placed at optimal locations. TECs provide active heating and cooling to the MLHP compensation chambers (CCs) and allow the MLHP operating temperature to be controlled over a wider range. The MLHP retains all features of state-of-the-art loop heat pipes (LHPs) and offers additional advantages to enhance the functionality, performance, versatility, and reliability of the system. A Thermal Loop experiment Technology Review Board (TRB), consisting of a group of independent outside LHP experts, has been assembled by the New Millennium Program Office to perform the following functions: 1) establish the criteria by which the Thermal Loop experiment will be judged to have achieved TRL 4, TRL 5, and TRL 6; 2) assess the achievement of TRL 4, TRL 5, and TRL 6 by the Thermal Loop experiment; and 3) evaluate the efficacy of the Thermal Loop experiment Technology Validation Plan. An MLHP Breadboard was built and tested in the laboratory and thermal vacuum environments for the TRL 4 and 5 validations, respectively, and an MLHP proto-flight unit was built and tested in a thermal vacuum chamber for the TRL 6 validation. In addition, an analytical model was developed to simulate the steady state and transient operation of LHPs. The Thermal Loop concept, technical advances and benefits, objectives, Level 1 requirements, performance characteristics, analytical model, and the validation approach to verify the attainment of TRL 6 by the Thermal Loop experiment were described in a separate paper. This paper presents the validation results, both experimental and analytical, of the Thermal Loop technology development. The MLHP Breadboard used for TRL 4 and TRL 5 validations will be described first. Results of TRL 4 and TRL 5 validations are presented next. This will be followed by the description of the MLHP proto-flight unit used for TRL 6 validation, and the validation results.
doi:10.2514/6.2010-1494 fatcat:zdhbgwj5drcdzlt33oz6qlsqua