New Device Provides Active Thermal Control of Spacecraft Surfaces

  • Published
  • By Mindy Cooper
  • AFRL Materials and Manufacturing
Wright-Patterson Air Force Base, Ohio --  The Air Force Research Laboratory's Materials and Manufacturing Directorate (AFRL/RX) successfully integrated technologies to develop a thermal emission management system suitable for use in space. The management system requires very little power (<2 milliwatt), is lightweight and compact, and has a very fast response time.

The space environment is a harsh one that includes radiation, atomic oxygen, and rapid changes in temperature that can be harmful to satellites and spacecraft. To ensure that a satellite functions properly, its temperature must be controlled.

"Methods for controlling temperature are either passive or active," explained Dr. Ming-Yung Chen, the program manager and RX materials research engineer, "Various surfaces and coatings have been developed as passive thermal management devices that have a constant emissivity. They radiate heat at a constant rate allowing no control. Active thermal management devices generally use heaters and mechanical refrigerators to control spacecraft temperature. They have the ability to change the spacecraft's temperature in real-time in order to protect it from extreme environments. However, these active systems are complex, heavy and require power supplies. This integrated new technology has the following advantages: it can be switched between passive and active thermal control upon demand, it weighs considerably less than state-of-the-art active control systems, and it requires very little power to operate."

According to Dr. Chen, RX scientists have been supporting research to develop these devices. Recently, the scientists recognized that pairing two technologies developed under separate Small Business Innovation Research contracts would together comprise an efficient system. The system combines Sensortex Inc.'s (Kennett Square, PA) electrostatic radiator device with the heat flux based emissivity measuring method (HFB) used by Advanced Thermal and Environmental Concepts, Inc (ATEC, College Park, MD).

"Sensortex's electrostatic radiator (ESR) device is a thin film membrane, which consists of a thin layer of polyimide metalized with aluminum which is sprayed with a layer of high emissivity conductive paint," Chen said. "The membrane can be charged by the application of a voltage. Once charged, it attaches to spacecraft surfaces by static electricity, establishes thermal contact, and provides a high emissivity rate."

Dr. Chen also explained that ATEC's HFB method provides a direct measurement of the heat flow through the emitting surface. The heat flux passing through the surface, the surface temperature and the ambient temperature are the only information needed to calculate the surface emissivity. There is no need for heaters and their control systems. The method allows for emissivity values of multiple surfaces to be measured at the same time.

"AFRL brought the two technologies together. The ESR was made compatible with the HFB by eliminating many adhesive layers and much of the thermal mass. A flexible polyimide membrane is brought into contact with the surface by application a voltage. The effective emissivity of the surface is tuned according to the voltage applied," said Dr. Chen.

The newly integrated device was tested in a simulated space environment where the device was mounted on the inside of a large vacuum chamber. Once a steady-state temperature was obtained, a voltage was supplied to one side of the ESR structure, and the membrane was drawn down to contact the rigid surface of the ESR structure. The heat flux sensor had a very fast response time, so the scientists were able to monitor the emissivity of the hybrid device as its temperature was changing. The results from the ESR structures demonstrated a large difference in emissivity values, which is required to allow for a wide range of active thermal control.

As a result of these successful tests, the device was installed in the MISSE-6 (Materials on the International Space Station Experiment-6) package at NASA Langley and deployed in 2008. The results from this mission will inform scientists if this system would be viable for official missions.