# Heat Sink Equivalent Thermal Test Method and Its Application in Low-Orbit Satellites
## 1 Introduction  
Commercial satellite constellations not only need to achieve reliability, but also require low-cost and time-saving approaches. Among the costs of traditional satellite development, environmental tests account for about 30% to 40% of the total investment in spacecraft development [1, 2]. Satellite thermal testing is the most complicated, costly, and time-consuming project in the spacecraft development process. However, it is an effective and necessary way to improve the reliability of the satellites [3, 4]. On the basis of traditional thermal tests, exploring an efficient and reliable thermal test method for commercial satellites is of great significance to achieve low cost and reduce time consumption. Satellite thermal tests include two parts: a thermal balance test and thermal vacuum cycling. The purpose of the thermal balance test is to verify the thermal design, and thermal vacuum cycling tests the reliability of the satellite. Test cutting is a good approach to saving costs. In the 1980s, the United States manufactured eight identical FLTSATCOM communication satellites [5]. After summarizing the thermal test data and the on-orbit temperature data of the satellites, it was concluded that the thermal balance test, which is an important part of the thermal test, could be eliminated to meet the purpose of reducing expenses. Subsequent mass-produced satellites, such as SUPERBIRD [6], have not conducted thermal balance tests for flight models. Moreover, some tiny satellites, such as FASTSAT-HSV01, have not undergone initial thermal balance tests [7]; rather, only thermal analysis is performed on the satellite instead of the initial thermal balance test and is then directly launched into orbit. However, test simplification is not a good choice for reliability [8]. The space heating flow is the key simulation element of the thermal test. In low-Earth orbit, space heating flow is composed of solar heating, solar heating reflected by Earth, and IR heating from Earth. Solar simulators are commonly used to simulate the space heating flow in tests, which are expensive to build and maintain [9]. In recent years, there have also been a number of cases that use infrared waves to simulate space heating flow. For example, the GPM satellite developed by NASA and JAXA uses heating panels to simulate the space heating flow in different regions of the satellite, achieving the purpose of the experiment [10]. In thermal tests, ultra-low temperatures are simulated using a heat sink cooled by liquid nitrogen, while the background of space is simulated using a heat sink with a high-emissivity surface. The temperature of the heat sink with liquid nitrogen refrigeration cannot be adjusted during the test, and can only be fixed below 100 K, making the start (cooling) and end (heating) processes of the tests more time-consuming. In addition, infrared heating cages have poor versatility and only fit one type of satellite. If the shape of the satellite is complicated, the error of the heating cage's heat flow simulation will be considerably increased, and the design of the heating cage would become complicated, which will reduce the safety of the test [11; 12]. Some satellite development groups are inclined to propose new temperature stabilization criteria or apply unstable test methods to shorten the duration of thermal tests [13; 14]. The algorithm of the unstable thermal balance test has poor portability and cannot adapt to various types of satellites well. Moreover, the unstable thermal balance test method can only shorten the duration of the thermal balance test and does not reduce the length of thermal vacuum cycling or the time it takes to change the heat sink temperature, so the entire thermal test still long. With the diversification of satellites, it is no longer feasible to simplify the tests according to different types of satellites [15], and a universal, simplified test method must be proposed.