Currently, to ensure the effective heat transfer of Loop Heat Pipes (LHPs), the Knife Edge Seal (KES) in the LHP evaporator is a key component that seals the primary wick and prevents high-pressure vapor from entering the lower-pressure interior of the evaporator and compensation chamber (CC). However, the KES may be susceptible to failure after long-time exposure to thermal cycles and vibration. Additionally, the KES insertion process is labor intensive and has an intrinsic risk that may increase the lead time and cost. To address the challenge, Advanced Cooling Technologies, Inc. (ACT), in collaboration with FormAlloy, Inc., utilized Direct Energy Deposition (DED), an Additive Manufacturing (AM) technique, to eliminate the KES, improve the LHP reliability and performance, streamline the LHP manufacturing process, and further enable innovative LHP designs. In the Phase I Program, we have successfully demonstrated using DED technology to deposit dense sealing layers on a testing specimen and thus eliminate the KES. The DED seal improves the overall LHP reliability and performance while lowering the cost and the lead time. Further, we have successfully demonstrated the operation of a complete LHP with a DED-sealed evaporator. The Phase II Program will optimize the DED process and further leverage the DED to the LHP fabrication process. Specifically, we will optimize the DED conditions including laser power, scanning paths, etc. In addition, we will conduct reliability and lifetime tests for the DED-sealed parts. ACT will also compare the performance of KES-sealed LHP with that of DED-sealed LHP. Further, we will develop Functional Gradient Material (FGM) that transits from aluminum alloy to nickel to further streamline the LHP manufacturing process. Finally, we will leverage the AM nature of the DED process to enable different CC and the evaporator body shape designs. By the end of Phase II, a complete hybrid manufactured LHP is one major deliverable to NASA.

Ammonia and propylene LHPs are currently used in most NASA and commercial satellites. In comparison with Constant Conductance Heat Pipes (CCHPs), they carry much higher powers (1 kW vs. 100 W) over longer distances (10 m vs. 2-3 m). They also are better suited for ground testability. An LHP can operate with the evaporator 2 meters above the condenser, versus 2.5 mm for a CCHP. With the DED-sealed, hybrid-manufactured LHP, improved performance, shorter lead time, and lower cost are well expected.

For Universities and other Research institutes with a limited budget to perform space research, the reduced cost of the LHP via the hybrid manufacturing process will enable their adoption of the high-performance LHP to conduct high-heat-generation research activities such as space computing.