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3 June 2003

NASDA Report

No.131 2003 MAY.

Part 2 of 3

The following components are typically used to control the satellite
temperature. 

    * Multi-layer insulator or thermal blanket (a gold colored sheet
      wrapped around the satellite body to prevent heat from being
      transferred in and out of the satellite 

    * Heat pipes (pipe-shape devices for transporting heat)

    * Thermal louver (functions like a window blind to emit excessive
      heat and absorb additional heat by opening and closing the
      blades)

    * Heater (heats the equipment to prevent their temperature from
      dropping too low)

    * Thermal insulation material / Adiabatic spacer (protects heat
      transfers from the warm side to the cold side)
 
Through the combined use of these thermal control technologies, space
flight hardware is designed to endure the extreme environment in
space and maintain the satellite at a suitable temperature between 0
and 50 degrees C. 

In order to ensure that the thermal control subsystem will work
properly in space, a thermal vacuum test is conducted in a vacuum
chamber exposing the satellite to temperatures similar to those in
the space environment. In addition, a computer analysis is performed
to estimate the temperature of in-orbit satellites. 

Here, we will present a multi-layer insulator and heat pipe, both of
which are typical thermal control technologies. 


Protecting Satellites from Sunlight! 
- Multi-layer Insulator Works Like a Thermos Bottle

Have you seen a satellite on TV or at an exhibition and wondered what
purpose the gold sheet wrapped around the satellite serves? This
sheet plays an important role in protecting the satellite under the
severe space environment, particularly from sunlight.

The sheet serves as a reflector so that sunlight will not hit
directly on the satellite and also insulates the satellite to prevent
its inside heat from escaping. In short, the sheet protects the
satellite from direct exposure to infrared rays in the sunlight,
thereby keeping the satellite temperature under control.

The sheet is called a Multi-layer Insulator (MLI) because the
insulator is configured of multiple layers of different types of
macromolecule sheets. The MLI is designed to shield more than 90% of
the hot solar energy. Hence, only several percent of the thermal
energy penetrates the MLI. 

By "wearing" the MLI, which works much like a thermos bottle, the
satellite is protected from the very harsh satellite environment. 

The golden sheet in the photo to the right is the MLI. In recent
years most MLI are equipped with an antistatic property and are
colored black for easy use. 


Transporting Heat via Pipeline!
- An Alternative to Wind

We have integrated fans into electronic devices used on the ground,
such as computers, allowing us to reduce the working temperature of
the devices to prevent system failures. Since there is no air in
space, however, we cannot adopt a "convective heat transfer" method
for controlling satellite temperature. 

Onboard equipment that generates a lot of heat is integrated with a
heat pipe that works to transport heat to an external panel (facing
deep space).

The heat pipe is a simple pipe that transports heat. The pipe has no
special structure but contains an internal working fluid. Now, we
will explain how the heat pipe transports heat efficiently. 

The operation principle is shown in the schematic illustration. When
one end of the heat pipe is heated, a vapor flow is generated. As the
evaporated fluid spreads toward the other end of the pipe, the vapor
dissipates heat and then condenses. The condensed working fluid is
flow back into the evaporating section and is reheated. This heat
pipe is a thermal conduction device that takes advantage of the
thermodynamic cycle to conduct heat transport through convection in
the pipe. As space is has no air, we cannot expect convection to
occur. However, heat can be transported efficiently by producing
convection within the heat pipe. 

Moreover, there is no "top and bottom" in a microgravity environment.
Therefore, cooled fluid never drops. For this reason, capillary
effect within the wick is used to return the condensate to the heat
source. Capillary effect achieves the same effect as the aspirating
of liquid into the spaces of a porous material. A good example is
that of water seeping into a sponge well. 

The heat pipe has been developed for use in space flight device.
However, it is commonly used in household appliances, as well. For
example, most of notebook computers employ built-in heat pipes. Take
a look inside your computer sometime!! 


Watching Space Coolly!
- A Method for Cooling Observation Sensor

In addition to controlling the satellite temperature, onboard sensors
must be cooled individually. 

A sensor is the most important element of the onboard equipment and
critical to achieving mission goals. There are various types of
onboard sensors, including sensors to observe the Earth and
atmosphere, X-ray and infrared sensors to investigate the evolution
and structure of space. Among these onboard sensors, high-performance
infrared sensor onboard Earth observation satellites and astronomical
satellites must be cooled down to nearly -273 degrees C (zero Kelvin)
in order to minimize thermal noise. 

Recent equipment and sensors (onboard JEM/SMILES, for example) take
advantage of a superconducting property. Superconductivity requires
us to maintain the sensor at a low temperature. The use of liquid
helium at about -269 degrees C is one way we can reduce the
temperature of a sensor. However, this cooling method cannot be used
after the liquid evaporates, and the cooling medium increases the
satellite's weight and volume. 

NASDA attempts to cool down sensors using a mechanical cryocooler
(similar to the basic function of a household refrigerator) instead
of a cooling medium.(cryogen)

Since the cryocooler needs only a electricity, it is the best
alternative because additional cooling medium cannot be supplied in
space. Unlike household refrigerators, the space cryocooler cannot be
repaired when a malfunction is detected. Accordingly, it is necessary
to develop a cryocooler with a long lifetime and high reliability.

 - Continued -

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