Thermal System

Satellites are exposed to extreme thermal boundary conditions. The electronic devices in the satellite however have small operation ranges. To guarantee perfect operation of the electronic devices it is essential to have a thermal control system that regulates the temperatures in the satellite. As Dartsat has limited power and a very low mass, a passive thermal control system is needed.

Environment

Two opposed factors, the sun and the deep space mainly determine the satellite's heat exchange. Direct solar radiation can warm up the spacecraft. Here also reflected solar radiation, the albedo should be mentioned. In the shadow of the earth very low temperatures are reached. Planetary radiation that is produced by the earth's black body radiation is the only heat source in this zone.

Electronic Devices

The operation range of the electronic devices is between -40°C and 85°C. For space application this is a very small range. A second problem is that two of the devices produce heat at a high level. The temperature of these hot spots has to be controlled during operation.

Analysis

For analysis the satellite is divided up into its skin and the interior with the heat sources. Each system that is analyzed has to be simulated separately. To do this, heat balances are set up and are calculated numerically. This delivers the time dependent temperature distribution. Extensive investigations resulted in the following design.

Skin

The skin will consist of a sandwich panel, which has a honeycomb core and surface plates. Both will probably consist of a fiber material. Sandwich panels combine low weight with high stiffness. For the solar cells that are mounted on the skin, the stiffness is very important, because the satellite will experience strong vibrations during the launch. On the inner surface the sandwich panel will be coated with gold. As gold has a very low absorptance, most of the radiation inside the satellite will be reflected. Therefore the skin is insulated from the interior of the satellite and local temperature differences on the skin will not affect the temperature of the electronic devices.

Heat Sources

The heat sources produce the high heating power only in the transmission mode, which lasts for 15 minutes. This makes it possible to use phase change capacitors. Phase change capacitors are little boxes that contain a frozen substance. During operation the heat sources melt the substance. Because of the high crystallization enthalpy the temperature can be held constant with a low mass of the capacitor. Using water a mass of 4 grams can store the energy produced by a 1.4 watts heat source during 15 minutes. After transmission the substance will freeze again.

For further information please contact Ernesto Kriesten.