After the satellite enters space, the environment is completely different from that on the ground. It faces three main challenges. One is extreme temperatures and vacuum. When the sun shines directly, the surface temperature of the satellite is extremely high. But when the satellite enters the shadow of the earth, the temperature will drop instantly. There is no air convection in space, and heat can only be transferred through radiation and conduction. Second, the internal and external heat sources are complex. There are periodically changing solar radiation, earth infrared radiation, etc. outside the satellite. There is heat generated when electronic equipment is working inside, and its power and heating time are also changing dynamically. The third is the stringent weight and reliability requirements. Every additional gram of weight on the satellite is very costly, and the satellite is almost impossible to repair after it is launched. Therefore, reliable thermal control is very important in determining the success or failure of a space mission. A poorly overlapped heat conduction surface, a blocked heat dissipation surface, or a pressed heat pipe may cause on-orbit failure.<\/p>\n
Generally speaking, a typical satellite flight process goes through the ground segment, ascent segment, orbital segment, etc. The internal and external thermal loads endured by the satellite at each stage are different and changing. The actual temperature of the satellite is determined by these factors. In the ground segment, the satellite is in the preheating (or precooling) state of some instruments; in the ascent segment, the star body is subject to aerodynamic heating; in the orbital segment, the satellite is in the space environment (such as extra-space heat flow, vacuum, microgravity, low temperature and space particle irradiation, etc.). Satellite thermal control is to take various thermal control measures to organize the heat exchange process inside and outside the satellite based on the requirements of the satellite mission and the internal and external heat loads that the satellite will endure during operation to ensure that the temperature levels of all instruments, equipment and structural parts of the satellite are maintained within the specified range during the entire operation period.<\/p>\n
For modern satellite thermal control systems, they usually adopt a “layered progressive, redundant and fault-tolerant” technical architecture, with passive thermal control as the main body and active thermal control as the precise control method. Generally speaking, passive thermal control methods (such as coatings, multi-layer insulation materials, etc.) are relatively simple in technology, reliable in operation (no moving parts), and have long service life. However, because passive thermal control methods do not have the ability to automatically adjust, they cannot overcome the impact on the temperature of instruments and equipment caused by changes in heat flow inside and outside the satellite. Therefore, it cannot adapt to large changes in internal and external heat flow, especially changes in internal heat sources. At this time, active thermal control methods must be considered. Active thermal control technology has a certain temperature adjustment capability, which can greatly reduce the temperature fluctuations of instruments and equipment caused by changes in heat sources.<\/p>\n
Passive thermal control methods mainly include thermal control coatings, multi-layer insulation materials, heat pipes, phase change thermal control materials, contact thermal resistance and thermal conductive fillers, foam insulation materials and inorganic insulation materials. Paint-type coating is the most widely used type of thermal control coating on satellites so far. When used, the pre-prepared paint is applied to the surface of the satellite’s skin, components and instruments to obtain certain thermal radiation properties. According to different binders, coatings can be divided into organic paints and inorganic paints. Organic paint is the most widely used type of thermal control coating, and can be divided into organic white paint, organic black paint, and organic gray paint. Organic white paint has low solar absorptivity and emissivity, organic black paint has high solar absorptivity and emissivity, and organic gray paint is formulated with organic binders and white, black and other pigments. According to different ratios, various gray paints with absorption-emission ratios between white paint and black paint can be obtained. The heat pipe is a device that utilizes the phase change and cyclic flow of the working fluid in the tube to work. Its application on spacecraft is mainly to achieve temperature control of the instruments and equipment within the satellite and local or overall isothermalization within the satellite through the transfer of heat. Since 1976, almost all satellites in our country have been equipped with heat pipes. The basic form of thermal control of phase change materials is to place them between the controlled equipment and the external environment. When the interface temperature between the phase change material and the heating element rises to the melting point of the phase change material, it melts and absorbs heat, keeping the interface temperature near the melting point. When the interface temperature drops due to internal or external reasons, the phase change material releases latent heat and solidifies. As long as there are two phases, the interface temperature will remain near the melting point. A typical application of phase change materials is the three phase change material thermal control systems on the “Apollo 15” lunar rover.<\/p>\n
Active thermal control methods are divided into three types: radiation, conduction and convection. The practical applications of the radiant type on satellites are mainly louvers and rotating disks. The louvers use low-emissivity movable blades to block high-emissivity heat dissipation surfaces to varying degrees to control the temperature. The blades are driven by bimetal springs that sense changes in the temperature of the heat source. The temperature control mechanism of the rotating disk is basically the same as that of the louvers. The advantages are small mass, small space occupation, and simple assembly. It can be used not only on flat surfaces, but also on spherical curved surfaces. The conductive type achieves temperature control by controlling the thermal resistance on the heat conduction path. When the heat source (instrument, equipment) generates more heat, the thermal resistance of the conduction path is reduced, allowing more heat to be dissipated through the satellite’s skin or a special heat radiator. When the heat source decreases, the thermal resistance on the conduction path increases, reducing the heat transferred and preventing the instrument from being overcooled. The convection type generally has a gas circulation system (such as the former Soviet Union’s “Sputnik 1” and “Lunokhod 1”) and a liquid circulation system (so far, all manned spacecraft have adopted liquid circulation active thermal control systems, such as the “Gemini” manned spacecraft, space shuttle, “Salyut” and “Shenzhou”)<\/p>\n
With the continuous development of technology and industry, my country has built a complete thermal control technology ecosystem. 703 Institute leads the research and development of thermal control coatings, MLI, and composite materials. 510 Institute, 508 Institute, etc. carry out thermal control integration of heat pipes, heaters, and loads. 511 Institute provides full-spectrum environmental simulation capabilities. At the same time, under the wave of commercial aerospace, a number of commercial aerospace companies are gradually participating, and thermal control technology is evolving towards standardization, shelf-like, and low-cost.<\/p>\n","protected":false},"excerpt":{"rendered":"
After the satellite enters space, the environment is completely different from that on the ground. It faces three main challenges. One is extreme temperatures and vacuum. When the sun shines directly, the surface temperature of the satellite is extremely high. But when the satellite enters the shadow of the earth, the temperature will drop instantly. […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"inline_featured_image":false,"footnotes":"","_links_to":"","_links_to_target":""},"categories":[2],"tags":[],"class_list":["post-25305","post","type-post","status-publish","format-standard","hentry","category-news"],"acf":[],"_links":{"self":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts\/25305"}],"collection":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/comments?post=25305"}],"version-history":[{"count":0,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts\/25305\/revisions"}],"wp:attachment":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/media?parent=25305"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/categories?post=25305"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/tags?post=25305"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}