Satellites orbiting within few hundred kilometers from the Earth’s surface are multiplying every day due to the rise in demand for communication, navigation and monitoring technologies. Typical applications of these so called Low – Earth Orbit(LEO) satellites include, air , sea and road traffic monitoring, remote sensing, communication services, atmospheric research and weather forecasting. In space, these satellites are exposed to a harsh space environment that varies widely in temperature as the satellite orbits around the Earth. These extreme temperatures pose a major threat to the electronics housed inside the satellite. Generally, the electronic boards are designed to operate optimally within a certain range of temperatures defined by the manufacturer. In addition to the thermal loads from the environment, the electronic components themselves generate heat which has to be managed. Hence, it is critical that the satellites maintain operational temperatures to avoid any subsystem failures.
The responsibility is on the Thermal System Design Engineer to solve these challenges with an efficient and affordable system. This article will present important factors that govern the design of the Low Earth Orbiting satellite from a thermal point of view.
Orbits and their Environments
Before we get into the details of the thermal system design, it is wise to find out about the different types of orbits and the nature of space environment experienced by a satellite in each of these orbits. Most of thermal design choices are heavily dependent on the type of orbit chosen for a particular satellite. Typical LEOs are: Sun – Synchronous Orbits (SSO), Polar Orbits, Inclined Orbits or Elliptical Orbits. Sun – Synchronous Orbits are of two types: Dawn – Dusk SSO and Noon – Midnight SSO. The two orbits are completely different from the thermal perspective as will be explained shortly.
Space thermal environment experienced by a LEO satellite is, for all practical purposes, defined by three parameters: Solar Flux (S), Albedo and Earth’s Infrared radiation. The latter two parameters are a function of altitude of the orbit while the first one is that of distance from sun. A typical orbit of a satellite around earth can be divided into two phases – (1) Sun-lit phase, and (2) Eclipse phase. During the sun – lit phase of the orbit, the satellite heats up from all of these three effects. As a result, the temperature of the satellite goes to a maximum. When in the eclipse phase, the Solar Flux and Albedo effects are not encountered and the satellite is exposed to temperatures as low as the Earth’s Average Infrared temperature of -18 ˚C. Dawn – Dusk SSO doesn’t have an eclipse phase and hence it experiences a high temperature environment for the whole time but Noon – Midnight SSO and all other orbits experiences both hot and cold environments as they have both sun – lit and eclipse phases. The duration of these phases though, depends on the type of orbit. Thus, based on altitude and duration of the two phases, satellite in each type of orbit will experience a different hot and cold environment. However, the typical range of temperatures was found to be from -170 ˚C to 123 ˚C for LEO satellites while -250 ˚C to 300 ˚C could be experienced in other orbits. For better understanding, an example providing different thermal loads and the temperature of the satellite orbiting at an altitude of 1280 km is presented below.
Thermal System Design Considerations
Given the extremely cold and hot ambient temperatures that a satellite is exposed to, it is impossible to design a satellite and sustain its operation without a thermal control system (TCS). Once a specific orbit is selected for the mission, a thorough understanding of the space environment for that orbit coupled with the mission requirements, provide the ideal thermal system to be implemented. Representative operational temperature limits of the typical electronic components used in a satellite is given here.
Thermal Control System (TCS)
Having learnt the “why” and “how” a thermal control system is selected, we can further explore the different thermal system design components available. In general, there are three categories of thermal control systems used in satellites: 1) Passive Thermal Control System, 2) Active Thermal Control System, and 3) Partially – Active Thermal Control System. They differ in the way they function and maintain the temperature of a section or the whole satellite. Passive TCS requires no mechanical moving parts or moving fluids and no power consumption. It is simple to design, implement and test. It has low mass and cost and is highly reliable. However, it has limited temperature control capability. Active TCS requires mechanical moving parts or moving fluids or electrical power. It has complex design and generates constraints on spacecraft design and test configurations. It has a high mass and cost and it is less reliable than Passive TCS. Partially – Active TCS is a hybrid system that uses both Passive TCS and Active TCS components. It uses less power and is of lower cost than the Active TCS. It is comparatively low in mass and offers better reliability than active TCS. It also provides better temperature control as compared to the Passive TCS. The figure on the right highlights all the regularly used passive and active thermal system design components.
Orbits and suggested TCS
Any one or a combination of the thermal system components mentioned earlier can be used to establish the required thermal equilibrium in the satellite. Whether to choose passive or active or both depends on the type of selected orbit. Satellites in Dawn – Dusk Sun – Synchronous orbit will not require heater power to increase the temperature of the spacecraft as it is sun – lit throughout the mission. But, to avoid the temperature from rising above the maximum allowable operating temperature, a cooling down mechanism is required. Heat can be distributed along the structure of the satellite by suitable construction material (Eg. Aluminum) or through heat pipes or fillers. Multi – Layer Insulation (MLI) blankets and paint on the surface with suitable coating material are also used. These techniques come under the Passive thermal control system.
Satellites in Noon – Midnight Sun – Synchronous, Polar, Inclined and Elliptical orbits will require heater power to increase the temperature of the spacecraft during cold eclipse phases. During the rest of the orbit when the satellite is sun – lit, to avoid temperatures from rising above the allowable operating temperature limit, similar thermal control methods as used in the above case can be used. Since, this thermal control system uses both passive components and active electric heater system, the system is Partially-active.
Satellites in any orbit will require a surface coating with a specific surface property (Absorptivity and Emissivity controls the heat load input and output) as required to manage the thermal loads in that orbit. Secondary surface mirrors (SSM) or Optical solar reflectors (OSR) sometimes replace surface coating but add an extra expense to the cost of the satellite. Thermal radiators are used in satellites to manage internal heat generated by electronics. Thermal doublers are usually used in the radiators of large satellites like RADARSAT-2 which generate enormous amount of heat. Louvers are usually not used unless there is a stringent condition to maintain the temperature of the spacecraft as a function of time as in the ROSETTA mission.
Conclusion: Options are too many. A precise thermal control can be achieved using expensive components which in turn affects the satellite cost and mass budget, while reasonable temperature control can be achieved using partially active or passive components that are low cost and more reliable. It is upon the thermal system design engineer to choose the optimal design. He or she will have to explore all the available options to find the most efficient and affordable thermal control system such that the temperature limit constraints are met with good tolerance and the costs are kept within the budget.
Study carried out by Miracle Israel, Intern at Astrome Technologies.