Qualification of Copper Water Heat Pipes for Space Application
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Spacecraft Engineering Department
Introduction: Copper water heat pipes have been extensively used for electronic cooling in laptop computers for several years. Despite its acceptance in terrestrial applications, copper water heat pipes have very limited space flight heritage; publications on space application of copper water heat pipe are rare. The Naval Center for Space Technology (NCST) has implemented this technology in its recently launched low earth orbit (LEO) payload (WindSat) to provide temperature control for its low noise amplifiers (LNAs). The copper water heat pipe is 4 mm in outside diameter and 305 mm in length. Its primary function is to transport heat from the LNAs to the feedbench radiators, where heat rejects into the deep space. As part of the risk mitigation process, a flight qualification test program was established by NCST. This article describes the methodology of the test program, test results, and selected flight data.
Test Program Overview: Figure 1 summarizes the overall sequence of this qualification test program. All of the tests were performed at the component level. These tests were designed to demonstrate that the copper water heat pipes are able to meet the heat transfer performance required by the WindSat program, and that they can withstand the extreme environments experienced during ascent and on-orbit operation. The qualification tests also validated the initial screening technique and procedure, which served as a standard component-level acceptance test for the flight units.
Initial Screening Test: All heat pipes were visually inspected for dents, cracks, and susceptible solder/epoxy joints. Then, the heat pipes were tested in a group of 20 in a thermal vacuum chamber. End-to-end (evaporator-to-condenser) thermal conductance of each heat pipe was evaluated with the applied heat load and the temperature difference measured. A predetermined heat load (0.75W) was applied on one end of the heat pipe, and the other end of the heat pipe was attached to a heat sink controlled at 10°C. T-type thermocouples were placed along the heat pipes to monitor their temperatures. A conductance value between 0.15 W/C and 0.35 W/C is considered acceptable. This is derived from system-level thermal analysis as a conductance requirement to remove waste heat from the LNAs and keep them within their temperature limits (0°C to 40°C). After the conductance measurement, the heat pipes were frozen to -30°C for about 2 h, and then thawed by applying a heat load of 0.75 W on the evaporator end of each heat pipe. Another conductance measurement was then performed to determine whether any degradation in performance resulted from freezing the heat pipe. Statistically, about 50% of the heat pipes passed the initial screening. Most of the failed heat pipes had thermal conductance values fall outside the required range. It is realized that the manufacturing process of these copper water heat pipes is not optimized for flight build quality. The differences in thermal conductance among the heat pipes may suggest inconsistency in the amount of working fluid charged and the presence of noncondensable gas (NCG).
Thermal Performance Test: The purpose of the thermal performance test was to verify heat transport capability and overall thermal conductance of the copper water heat pipe to meet program requirement. In order to provide adequate temperature control for the LNAs, the copper water heat pipe must transport a 0.75 W heat load with an overall thermal conductance between 0.15 W/C and 0.35 W/C. Five heat pipes were selected from the flight lot for this test. They were tested at two sink temperatures (10°C and 30°C), and three heat load levels (0.5 W, 0.75 W, and 1.0 W). The heat pipes were oriented vertically, with the condenser end at the bottom. Test results demonstrated that all five heat pipes operated successfully and met the heat transport and thermal conductance requirements.
It is realized that moderate bending of heat pipe is needed for this flight application. Therefore, the second part of this test was to demonstrate that the copper water heat pipe can tolerate moderate bending. The same five qualifying heat pipes were bent, with two 90-deg bends in the middle. The bend radius is about 0.5 in. Thermal performance test was then repeated. Test data did not indicate any degradation in heat transport capability or overall thermal conductance as a result of the bending.
Pressure Proof Test: After the thermal performance test, the same qualifying heat pipes were tested to verify the pressure containment integrity. For the pressure proof test, internal pressures of the heat pipes were raised to two times the maximum design pressure (MDP) by external heating. The MDP for current applications is 47.4 kPa. Upon completion, the thermal performance test was repeated. Test results did not show any degradation in thermal conductance and heat transport performance. In addition, dimensional measurements were made at controlled locations along the heat pipes both before and after the pressure proof test. No significant deformation was found on the controlled locations.
Pressure Burst Test: The purpose of the pressure burst test was to ensure that the heat pipe would not rupture at four times the MDP. Two new heat pipes in straight configuration were selected for this test. Similar to the pressure proof test, the internal pressure of the heat pipe was raised by external heating. Neither heat pipe burst when they reached four times the MDP.
Three-Axis Random Vibration Test: The objective of this test was to verify that the design and workmanship of the heat pipe was adequate to survive launch vibration loads. Both straight and bend configurations were tested. The heat pipe was assembled on to a test fixture, which allowed one end of the heat pipe to be rigidly fixed while the other end was attached to a cantilevered beam having a natural frequency of approximately 60 Hz. The intent of this test setup was to expose the beam to a conservative simulation of flight-like loading. A three-axis accelerometer was used to measure the response of the cantilevered beam. To document any changes in heat pipe performance, thermal conductance and heat transport capability were measured before and after the random vibration test. All qualifying heat pipe units passed the random vibration test. No physical damage or thermal performance degradation was found on the heat pipes.
Freeze/Thaw Cycling Test: The objective of this test was to determine the survivability of copper water heat pipe under repetitive freeze/thaw cycles. This is an environment that may be seen during flight operation, particularly in the safe hold mode. Five heat pipes were subjected to 100 freeze/thaw cycles, which has been determined to be four times the expected number of freezing that may occur throughout ground operations and flight mission. During the freeze/thaw cycles, the evaporator end of the heat pipe was kept between 3°C to 11°C, which simulates the thermostatically controlled heater operation during flight operation. All heat pipes were tested vertically, with the condenser end at the bottom. Figure 2 shows the overall thermal conductance (G) measured between cycles for the five heat pipes tested. It also provides the state of health of the heat pipes throughout the test. Two of the five heat pipes failed after 60 freeze/thaw cycles. Thermal conductance of the failed heat pipe significantly deteriorated. Figure 2 also shows damage at the condenser end of heat pipe due to repetitive freezing and thawing. Frozen water expanded and weakened the copper shell at each cycle until it ruptured. Although the remaining three heat pipes did not fail, similar deformation was found at the condenser end. It is concluded that the copper water heat pipe can only tolerate freezing to some extent. It is not meant to be used in a situation in which the system is routinely exposed below freezing temperature. To minimize such risk, auxiliary heaters were added to the heat pipe system to prevent freezing.
Conclusions: Copper water heat pipe was flight qualified at NRL for the WindSat program. Thermal performance and structural integrity of the heat pipes were demonstrated to satisfy program requirement. Flight temperature data from the last 10 months since launch indicates that the heat pipe system operates reliably without any anomaly. Figure 3 also shows that all 22 LNAs are well within the 0°C to 40°C temperature requirement. Temperature gradients among the LNAs are able to keep within 5C throughout the seasons.
Acknowledgments: The author expresses his gratitude to the WindSat team who participated in various parts of this test program.
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