ACT at Spacecraft Thermal Control Workshop 2021

The Spacecraft Thermal Control Workshop is held annually by the Aerospace Corporation. It brings together government, industry, and academic experts to discuss all aspects of spacecraft thermal control.

“In addition to technical papers by ACT and others, there was an excellent series of thermal roadmaps by NASA Goddard, NASA JPL, the Air Force Research Laboratory, and the European Space Agency.  Blue Canyon Technologies discussed Thermal Management Challenges for CubeSats and MicroSats, while Dave Poston gave a review of previous work on Space Fission Power and Propulsion.” ACT’s Cheif Engineer Bill Anderson

As part of the Workshop, ACT prepared four presentations:

• Development of 3D Printed Loop Heat Pipes (Rohit, Chen, Bill Anderson)
• A Variable View Factor Two-Phase Radiator manufactured via Ultrasonic Welding (Jeff Diebold, Calin Tarau, et al.)
• Hot Reservoir Variable Conductance Heat Pipe with Advanced Fluid Management (Kuan-Lin, Calin Tarau, et al.)
• Thermal Management System for Lunar Ice Miners (Kuan-Lin Lee and Calin Tarau)

Development of 3D Printed Loop Heat Pipes

Loop heat pipes (LHPs) are commonly used as thermal control devices in spacecraft due to their high efficiency and flexibility (ability to integrate with deployable radiators, thermal control valves, etc.). In general, however, they are too expensive and therefore less viable for most cost-sensitive applications such as CubeSats and SmallSats. Conventional loop heat pipe manufacturing methods involve multiple labor-intensive steps including a knife-edge seal process to ensure reverse vapor flow that can adversely affect the thermal transport capability.  Unfortunately, the conventional steps needed to manufacture the wick, insert it in the LHP evaporator, and seal with a knife-edge, results in very high manufacturing costs.

To address this issue, 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, a 3D printed LHP eliminates the knife-edge seal as well as many wick manufacturing and testing steps typically seen in 3D printing. By 3D printing the LHP, the fabrication costs can be reduced by an order of magnitude, which enables its use in many emerging spacecraft applications including, CubeSats and SmallSats.

A Variable View Factor Two-Phase Radiator Manufactured via Ultrasonic Welding

To provide adequate cooling in the most challenging thermal environments, radiators for manned spacecraft, satellites, planetary rovers, and unmanned spacecraft are typically oversized for moderate thermal environments and prone to freezing at low sink temperatures. To address the need for light-weight and efficient radiators capable of a significant heat rejection turndown ratio, ACT has developed a novel vapor-pressure-driven variable-view-factor and deployable radiator that passively operates with variable geometry (i.e., view factor). The device utilizes two-phase heat transfer and novel geometric features that passively (and reversibly) adjust the view factor in response to internal pressure in the radiator.

The advantages of the variable-view-factor radiator over a conventional flat panel radiator include:

• Passive temperature control: Variable thermal resistance minimizes temperature swings despite changes in operational or environmental conditions. This feature will keep power electronics above the minimum operating temperature, even during times of low-heat loads and low-heat sink temperatures.
• Survival: In the fully closed position, heat rejection from the radiator is minimized resulting in a reduction in the required survival heater power.
• Deployable: During launch, the radiator is in a compact configuration allowing for simplified storage and overall size during transportation.

Hot Reservoir Variable Conductance Heat Pipe with Advanced Fluid Management

The highly variable temperature environment of the lunar surface presents a significant challenge to small, low power (500W or less) scientific payloads, landers, and rovers envisioned for many future scientific missions. The environmental sink temperature can be as low as 100K during the lunar night. The slow rotation rate of the lunar surface, with a 28-earth day cycle, can subject the scientific payload or rover to this extreme temperature drop for extended periods. Innovative thermal management concepts are required to allow future missions to operate throughout the entire lunar cycle.

Lunar Landers and Rovers are often solar-powered, doing most of their research during the day.  At night, or when traveling in a shadowed crater, only batteries are available to provide survival heater power. Variable thermal links are required to maintain the Lander/Rover electronics temperature, rejecting large amounts of heat during the day, while shutting down to minimize losses during the Lunar night. A conventional Variable Conductance Heat Pipe (VCHP) with a cold reservoir at the end of the condenser can be used, however, it requires several Watts of heater power to maintain the reservoir at the setpoint temperature.

Since 1W of power requires 5 kg of additional solar cells and batteries during the 14-day-long Lunar night, ACT has developed a Warm Reservoir VCHP that does not require any electrical power.  Since the hot reservoir cannot be wicked, it becomes a challenge to properly manage the presence of working fluid within the reservoir. The Warm Reservoir VCHP discussed in the presentation solves this problem.

Thermal Management System for Lunar Ice Miners

The Thermal Management System (TMS) for Lunar Ice Miners uses the waste heat of nuclear power sources for ice extraction and uses the lunar cold environment as the heat sink for ice collection. As shown in the figure below, the proposed TMS consists of the following components:

• A mechanically pumped fluid loop (MPFL) is a “thermal bus” to deliver and distribute the waste heat from an MMRTG to components that required thermal energy. The majority of the waste heat will be used for ice extraction.
• Multiple coring drills (i.e., thermal corers) with embedded miniature flow channels. Heat delivered by the pumped loop will travel through the drills via mini-channels and directly warm up and sublimate the ice within the regolith
• A cold trap tank with integrated variable conductance heat pipes (VCHPs) that can switch between two modes of operation (ice collection and ice removal modes) without using any electricity and moving parts. The benefits of this feature are:
  1. Capability to “clean off” the collection surface (i.e., de-icing) to maintain a high harvesting rate
  2. Better control of ice layer within the tank to improve packing, which would allow a smaller tank design
  3. Efficient discharge of ice to the processing site
• Rotary unions that couple the pumped fluid loop, thermal corer, and the cold trap

Overview

The presentations at this year’s conference sparked a lot of discussion at ACT about future trends in advanced cooling concepts and materials. There is an amazing amount of exciting work being performed by our fellow researchers focused on the space industry, and we appreciate the platform provided by STCW to learn from each other. We look forward to both presenting at and attending next year’s event in person!

For more information about ACT’s innovative technologies for spacecraft thermal control, visit: https://www.1-act.com/markets/spacecraft-thermal-control/