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Temperature SystemHow is DS1's heat balance maintained? |
Many different materials are used for thermal regulation on DS1. The purpose of the thermal control materials is to maintain the electronics and mechanical moving parts of a spacecraft within the design limits of the temperature range that will allow them to function normally. The following is a detailed description of three kinds of thermal regulating materials. All three of the examples are used in passive thermal control systems. However, the radiator material is so effective that an active heater is used to heat the radiator so as to stop cooling the spacecraft when necessary. It is important to realize that a spacecraft absorbs the Sun's radiation, which is predominantly in the optical wavelengths (400 to >700 nm), and radiates heat at infrared wavelengths (the heat spectrum: >800 nm). The absorbance and emittance performed by the thermal control materials are somewhat wavelength-sensitive and the component materials are selected to be optimum for their particular application.
1. Silver Teflon radiator coating material. This material is referred to as a second surface mirror. The material consists of a 0.010 inch layer of transparent Teflon coated with a few Angstrom thick layer of silver and a protective layer of inconel. The material is applied to the space side of a metal (usually aluminum) plate. Spacecraft internal heat is conducted to or radiated to the inside of the plate to heat the radiator material. The Teflon has a high emissivity and radiates the heat to space. In addition, the sunlight may strike the first surface, which is Teflon, pass through to the silver layer, the second surface, and be reflected back to space. Radiators such as this frequently have an active heater on the inside of the metal plate, controlled by a thermostat, to prevent spacecraft heat from reaching the plate when it is not desirable to further cool the spacecraft.
2. "Gold" thermal blanket. Thermal blankets are officially known as Multilayer Insulation (MLI). The outer layer of the MLI faces space, and is on the Sun side of DS1. The outer layer of the blanket is a type of second surface mirror material with high reflectivity and high emissivity. It is a 0.001 inch Kapton film backed with a few Angstroms thick layer of silver, which results in the gold color. This layer is structurally supported by a coarse fiberglass netting, called scrim, to prevent tearing. The multiple layers in the blanket are 0.00025 inch Kapton with a few Angstrom thick layer of silver on each side. These layers are separated by a dacron netting which prevents contact of the adjacent silver layers. The inside blanket layer makes contact with the spacecraft body. The blanket layers conduct heat only laterally in their respective silver layers as long as there is no contact between layers, but they do exchange heat between adjacent layers by radiation. They will radiate some spacecraft heat to space but their insulating properties increase with the number of layers. All the layers except the outer layer are perforated. This allows the entrapped air to escape during launch; otherwise the blankets would literally blow up! The blankets are made using paper patterns and then cut and sewn on a sewing machine. The edges are finished with a tape prior to sewing. There are a number of individual blankets on the spacecraft. These blankets are held together and to the spacecraft using dacron Velcro.
3. Black thermal blanket The black thermal blanket is used on the shade side of the DS1 spacecraft. It is identical to the gold blanket except for the outer layer. The outer layer is 0.001 inch Kapton filled with carbon powder. This material has a higher absorptivity and lower emissivity than the gold Kapton outer layer, to cut down on radiation of spacecraft heat to space. This layer is also electrically conductive because of the carbon fill. By grounding the outer layer to the spacecraft frame the static charge buildup which occurs on the anti-Sunside is dissipated, whereas the Sun reduces static charge buildup because of photoemission of electrons from the Kapton
Why do we have to control heat on a spacecraft if there are no people on board?
