Advanced Cooling Technologies, Inc.
Lancaster, PA
www.1-act.com/industries/space
Recent initiatives aimed to survive and thrive on the lunar surface will enable space exploration, science, and humanitarian missions for decades to come. While extremely exciting, these missions come with immense engineering challenges. One of the most demanding thermal challenges associated with lunar surface operation is “Surviving Lunar Night.” This phrase has gained popularity as several upcoming exploratory missions seek to explore the South Pole of the Moon, where extreme cold temperatures exist during lunar night and certain areas (craters) are permanently shadowed.
The challenge is amplified by the longevity of the Moon’s rotation, which takes over 27 Earth days to rotate once about its axis. Therefore, when a mission must “Survive Lunar Night,” it must remain safe — such that the critical onboard electronics and instruments maintain functionality — during an approximate 14-day shadowed period where temperatures approach that of liquid nitrogen (-196 °C or -321 °F). The ability to solve this challenge for upcoming missions will provide access to, mapping, and, ultimately, harvesting of the water (ice) known to be at the Southern Pole of the Moon. The availability of water on the lunar surface opens significant opportunities for future missions, making this a critical challenge to solve.
Surviving lunar night poses a colossal thermal challenge. The thermal management system, responsible for minimizing energy loss during lunar night, also dissipates a significant amount of waste heat at the peak of lunar day, where the lunar surface temperatures can approach or exceed those of the hottest environments on Earth.
During lunar day, the thermal management system must comprise traditional advanced space-grade technologies to transfer and reject heat. Using decades of experience from satellite and other NASA missions, passive thermal management is historically used for extended life and reliability.
For very low-power applications, basic conduction and radiation can transfer and reject heat in space system; however, in most applications with highly capable electronics’ systems, heat pipes are used to transfer heat to and along radiator panels. Heat pipes are passive, two-phase (liquid and vapor) heat-transfer devices that transport heat long distances with very low temperature difference across their length. This efficient thermal link from electronics to heat rejection surfaces is critical to keep systems from overheating during operation.
During lunar night, the goal of the thermal control system changes; it becomes critical to keep the electronics warm enough to survive the extreme low temperatures at the lowest survival heater power budget. Using auxiliary heater power for most vehicles is not ideal, as 1W of energy lost continuously requires roughly 5kg of batteries to survive lunar shadow. Therefore, the thermal link transferring heat from the electronics to the radiators, which tends to be in the low hundreds of Watts during operation, must be disabled/reduced and any residual heat must be retained at the electronics.
One methodology, which has been implemented for NASA’s upcoming VIPER (Volatiles Investigating Polar Exploration Rover) mission is to utilize a “warm box” assembly to house the electronics. Developed by the Orbital Space Systems team at Advanced Cooling Technologies, the warm box assembly is thermally coupled to the heat rejection radiators via a Loop Heat Pipe (LHP). This type of heat pipe creates a passive pump at the heat source/evaporator region, known as the pump body, which creates a high-pressure drop enabling fluid vapor to travel long distances through standard, smooth-bored tubing lines.
The tubing path can be geometrically designed to allow tolerance/integration forgiveness and flexibility — one key feature for a thermal management system that links mechanically discrete locations (source electronics and radiator panels) across a vehicle that experiences shock and vibration through launch and during lunar exploration. The enabling thermal control feature is a passive thermal control valve, integrated near the evaporator/pump body before the fluid vapor is transported to the radiator panel.
As the system cools, the thermal control valve redirects flow back to the pump body; this redirection prevents direct energy from making it to, and subsequently being lost from, the radiator panels of the rover. This results in the thermal communication from the LHP to the radiators to be passively controlled based on temperature, manipulating the flow path to reject waste heat during lunar day, and remain in the warm box during lunar night.
These critical, passive thermal control systems have been developed over years of research and product development and will serve as enabling technology for NASA’s VIPER and future missions. Advanced Cooling Technologies, Inc. would like to thank NASA, our team of extraordinary engineers, and our supply chain partners for all the efforts that went into technologies designed to survive lunar night.
This article was contributed by Advanced Cooling Technologies (Lancaster, PA). For more information, visit here .
