Hybrid Thermal Control System for Lunar Habitation Systems

Project Objective

This project is developing a hybrid thermal control system for lunar habitats that combines an internal active system with passively variable external heat rejection, enabling survival in extreme lunar environments.

Project Description

Sustainable long-term presence on the lunar surface is reliant on habitation and exploration systems that can survive in the extremely challenging thermal environments on the Moon. Traditional thermal control systems for crewed vehicles rely on two actively pumped fluid loops – the first to collect waste heat within the crewed volume and the second to reject the waste heat to space. The internal loop is easily serviceable by the crew and uses a non-toxic working fluid. The external loop, however, is not easily serviceable – making it very vulnerable to degradation. The hybrid thermal control system takes a “best of both worlds approach” and combines the internal pumped fluid loop that crews are familiar with and adds a completely passive loop heat pipe for heat rejection through the radiators. This passive loop heat pipe is more robust, more redundant, and less complicated than an external pumped fluid loop. Additionally, it can passively shut off the heat rejection to the environment, allowing for habitats to survive in extremely cold periods when heat rejection should be minimized. This is critical for maintaining environmental control and life support systems during initial dormancy periods and future operational periods during Lunar night.

Project Results and Conclusions

The current development year resulted in two significant accomplishments, both building upon lessons learned from the first development cycle of this technology. The first accomplishment was entering into an SBIR-III contract with Advanced Cooling Technologies for a new loop heat pipe. This loop heat pipe retains the passive shutdown capabilities of the thermal control valve from the first effort, but improves the heat transfer interface between the internal pumped fluid loop and the loop heat pipe. A major finding from the first effort was a very high temperature difference between the pumped fluid loop and loop heat pipe, which indicates a difficulty in transferring heat across the interface. The new loop heat pipe utilizes channels for the pumped fluid loop working fluid built directly into the evaporator, significantly reducing the overall thermal resistance through the interface. This new loop heat pipe will be delivered in the summer of 2025.

The other major accomplishment for this year was the design and assembly of a new pumped fluid loop. The original pumped fluid loop was using an off-the-shelf pump from an industrial supply house. During first stage testing it was discovered that the pump was introducing a significant amount of waste heat into the system, resulting in the test being unable to reach its minimum power setpoint. The new pumped fluid loop, designed and built by three summer interns, was designed with more consideration to selection of the pump and other components. A detailed analysis of the flow conditions was performed to verify selections. Final assembly of the pumped fluid loop was completed, but the internship period ended before testing was able to take place.

This project will enable sustainable, long-duration exploration of the lunar surface. The loop heat pipe technology being developed could also be applicable anywhere a variable link is needed between an actively pumped fluid loop and a radiator.

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