thermal engineering branch:groups:technology development group:technology development activities:variable emittance coatings

Vary Emittance Coatings

All spacecraft rely on radiative surfaces to dissipate waste heat. These radiators have special coatings that are intended to optimize performance under the expected heat load and thermal sink environment. Typically, such radiators will have a low absorptivity and a high infrared-red emissivity. Given the dynamics of the heat loads and thermal environment it is often a challenge to properly size the radiator. In addition, for the same reasons, it is often necessary to have some means of regulating the heat rejection rate of the radiators in order to achieve proper thermal balance. One of the Thermal Engineering Branch's current efforts is development of specialized thermal control coatings which can adjust their emissivity based upon an on-board passive controller or from ground control. These coatings change the effective infrared-red emissivity of a thermal control surface to allow the radiative heat transfer rate to be modulated upon command. This permits adaptive or "smart" thermal control of spacecraft by varying effective emissivity of surfaces in response to either a passive actuator (e.g., a bi-metallic device) or through active control from a small bias voltage signal. In essence the variable emittance surface would be an "electronic louver." The objectives of the program are to develop highly innovative thermal control coatings that can repeatedly change their properties in response to an external signal. The program includes three alternative concepts (Electrochromic, MEMS, and Electrophoretic) with both ground testing and flight experiment verification.

MEMs Mini-Louvers
Micromachined devices which are similar in function and design to conventional mechanical louvers where a mechanical vane or window is opened and closed to vary the radiative view to space
Electrochromics Devices
Emittance modulation is achieved by applying a bias voltage (~1v) to a conducting polymer material whose reflectance can be tuned over a broad infrared wavelength (2 to 40 microns);color changes with the redox state
Electrophoretics Devices
Movement of suspended particles (i.e., very small flakes) through a highly absorptive fluid under the application of a small electrical field overlap and form a highly reflective surface

The goal is to mature all three technologies as far as possible; perceive different benefits/applications for each:

  • >6:1 Variation in radiator emissivity (e = 0.2 to 0.8 or better)
  • System analysis shows a significant savings over conventional methods
    • 50 — 90% heater power savings
    • Similar mass/cost savings over vchp/louver applications
  • Major reduction in component temperature variations

The Variable Emiitance Suite flew as one of the technologies for validation on ST5.

For more information contact:
Principal Investigator: Ted Swanson, 301.286.8168
Theodore dot D dot Swanson at nasa dot gov
Co-Investigator: Donya Douglas, 301.286.6952
Donya dot M dot Douglas at nasa dot gov

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