Advanced Structurally Embedded Thermal Spreader Oscillating Heat Pipe Micro-Gravity Flight Experiment

Abstract

On 7 September 2017, the U.S. Air Force Research Laboratory launched the second Advanced Structurally Embedded Thermal Spreader (ASETS-II) flight experiment on the fifth flight [Orbital Test Vehicle 5 (OTV-5)] of the Air Force’s X-37B spaceplane. OTV-5 landed on 27 October 2019 after 780 days on orbit. The ASETS-II flight experiment accomplished its mission to demonstrate functionality, characterize behavior, and explore the predicted limits of operation of oscillating heat pipes (OHPs) in a long-term microgravity environment. The three science objectives were to measure initial on-orbit thermal performance, long-duration thermal performance, and any lifetime degradation. The three OHPs on ASETS-II have varying configuration (center heating with large and small heaters and with single- and double-ended cooling) and heat pipe fluids (butane and R-134a) to isolate performance parameters of interest. Data collected during on-orbit operation are presented and favorably compared to predicted OHP limits of operation. Each OHP performed as expected with no notable differences between horizontal and microgravity flight data. All three OHPs started easily, had similar performance on ground and on orbit, showed no hysteresis effects or degradation on orbit, and performed well in several six-week-duration continuous operation tests both on the ground and in orbit.