3D Printed Loop Heat Pipe

The fast-growing space industry results in increasing demand for thermal control systems. Loop heat pipes (LHPs) are a commonly utilized device for spacecraft due to high efficiency and flexibility (ability to integrate with deployable radiator, thermal control valve, etc.). However, they are currently too costly to manufacture to make them viable for most cost-sensitive applications such as CubeSats and SmallSats. Conventional loop heat pipe manufacturing method involves multiple labor-intensive steps including a knife edge seal process to ensure no back leak of vapor that affects the thermal transport capability. Unfortunately, the steps to manufacture the wick, insert it in the LHP evaporator, and seal with a knife edge results in very high manufacturing costs. Advanced Cooling Technologies, Inc. (ACT) has developed a low-cost LHP evaporator using a technique called Direct Metal Laser Sintering (DMLS), otherwise known as 3D printing. With the capability of building a porous wick structure together with a solid wall via additive manufacturing, 3D printed LHP eliminates the knife edge seal as well as many wick manufacturing and testing steps. The 3D printed LHP reduces the fabrication cost by an order of magnitude, which enables the use of LHPs in many emerging space applications, including CubeSats and SmallSats. While the developed 3D printed loop heat pipe shows significant cost benefit compared to the loop heat pipe made by conventional fabrication process, the pore size of 3D printed primary is around 5~7 m, much larger than the 1 m pore size made by sintering process. In addition, the porosity of the 3D printed primary wick is lower than sintering wick (31% vs. 40%). In order to apply this emerging technology to higher power, large satellite applications, which are the current main loop heat pipe market, further reduce pore size and increase porosity are needed. There is an ever-increasing demand for thermal control systems in the rapidly-growing space industry, particularly due to the proliferation of miniaturized satellites for research and commercial applications. Loop Heat Pipes (LHPs) are commonly utilized for thermal management due to their high efficiency and application versatility, given the ease of integration with deployable radiators, thermal control valves, etc. However, conventional LHP manufacturing involves several labor-intensive steps, including wick insertion, vapor groove machining, and the knife edge sealing process, which result in high manufacturing costs and extended lead times. A low-cost LHP evaporator has been developed at Advanced Cooling Technologies, Inc. (ACT) using Direct Metal Laser Sintering (DMLS). With DMLS, the porous wicked region, vapor grooves, and the outer solid wall can all be printed as a single unit leading to an order-of-magnitude savings in manufacturing costs, and thereby making the proposed innovation attractive for emerging space applications such as miniaturized satellites. Further advancement of DMLS wicks Continued improvement of wick fineness down to a 1-2 µm pore radius. Development of an advanced bi-porous primary wick with a high-porosity inner region for increased permeability and a low porosity annular region for high overall capillary limit. DMLS-fabrication of LHP evaporator with combined primary and secondary wicks. Continued improvement of thermal conductance associated with DMLS evaporators Thermal contact improvement between the DMLS evaporator and the saddle through the use of solder joints or other advanced methods. Minimize heat leakage from the evaporator to the compensation chamber through adequate design of the primary wick, in particular by ensuring sufficient thickness of the inner region in the proposed bi-porous wick. Demonstrate the reliability and robustness of DMLS evaporator The reliability of DMLS evaporators will be established through extensive shock and vibration testing and ensuring performance comparability between pre- and post-tested samples. Design, build, and test a mini LHP system with DMLS evaporator A mini LHP system designed around a DMLS evaporator with diameter ∼ 0.5-inch and length ∼ 2–3 inches will be fabricated and tested. The deliverables at the end of the Phase II program will include a detailed final report and a DMLS mini LHP system.

NASA has expressed strong interest in LHPs for various applications, including miniaturized satellites such as SmallSat/CubeSat, traditional large satellites, planetary rovers, and landers. The proposed technology provides an attractive low-cost, DMLS-based LHP evaporator as an alternative to the traditionally-manufactured evaporator, which involves a series of high-skill, labor-intensive processes. An order of magnitude savings in the evaporator cost is obtained by eliminating complex processes such as wick insertion and the knife-edge seal. The DMLS-based LHP evaporators are also applicable to DOD and other government satellites. With the rapid proliferation of cost-sensitive commercial satellites, there is an ever-increasing demand for low-cost thermal management solutions, which can also be addressed through further development of the proposed technology.  Current LHPs cost more than a University CubeSat.

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