Thermal Control System
What is TCS?
Thermal control systems are an essential component of any spacecraft and equipment, ensuring that the temperature remains within the operating range. The instruments, heaters, and solar absorption are the primary heat sources in a spaceship, while radiation is the only heat loss method.
The thermal subsystem maintains the right temperatures in all parts of the spacecraft. That may sound easy, but it turns out that it’s not. The Sun heats up one side of the spacecraft, and black space on the other side pulls the heat out. The hot side is thus hundreds of degrees hotter than the cold side. In addition, parts of the spacecraft that use electrical power will generate heat internally and tend to get very hot.
In spacecraft design, the function of the thermal control system (TCS) is to keep all the spacecraft’s component systems within acceptable temperature ranges during all mission phases. It must cope with the external environment, which can vary in a wide range as the spacecraft is exposed to deep space or to solar or planetary flux, and with ejecting to space the internal heat generated by the operation of the spacecraft itself.
Now, on Earth, when your home gets too hot you can cool things off with fans or air conditioners. Or, when it gets too cold, you turn on the furnace. All these methods work by adding or subtracting heat from air and then moving it around (this is called convection). In space there isn’t any air, so convection doesn’t work. Other physical processes — conduction and radiation — must be used to move heat around the spacecraft and ultimately get rid of the excess.
Conduction is the process by which heat moves through substances and between substances that are in contact, like pool water and your skin, or a cold metal railing and your hand. Heat moves around the spacecraft mainly by conduction (to a lesser extent also by radiation).
Radiation, or more correctly, electromagnetic radiation, includes everything from X-rays to sunlight to radio waves. Our eyes are sensitive to only a very small part of this spectrum, the part we call visible light. The heat you feel on your face when you get close to a fire is the result of infrared radiation, which is a type of electromagnetic radiation just beyond the range your eyes can see. The only way a spacecraft can actually absorb or get rid of heat is by electromagnetic radiation.
Basic Laws Of Thermodynamics
In the basic review of thermal systems, a fast review of two of the basic laws of thermodynamics is important.
The zeroth law of thermodynamics states that two systems in equilibrium with a third are likewise in equilibrium with each other for isolated (closed) systems, implying that heat always flows from hot to cold.
The first law of thermodynamics states that a change in heat in an isolated system is equivalent to a change in energy and temperature. This means that heat-energy is conserved in an isolated system, that heat-energy changes can be measured by temperature, and that different forms of energy and heat are equivalent.
The second law of thermodynamics describes two systems in contact that strive for temperature and energy equilibrium
Temperature Requirements
The thermal control system is designed around the temperature requirements of the instruments and equipment on board. The TCS’s purpose is to maintain all of the instruments operating within their temperature ranges. The working temperature range for all electrical instruments on board the spacecraft, like as cameras, data-collection systems, batteries, and so on, is fixed. Every mission relies on keeping these equipment within their optimal operating temperature range. The following are some instances of temperature ranges:
- Batteries, which have a very narrow operating range, typically between −5 and 20 °C.
- Propulsion components, which have a typical range of 5 to 40 °C for safety reasons, however, a wider range is acceptable.
- Cameras, which have a range of −30 to 40 °C.
- Solar arrays, which have a wide operating range of −150 to 100 °C.
- Infrared spectrometers, which have a range of −40 to 60 °C.
Active and Passive Systems
The thermal control subsystem is made up of both passive and active components, and it functions in two ways:
Thermal insulation from external heat fluxes (such as the Sun or the planetary infrared and albedo flux) or adequate heat removal from internal sources protects the equipment from overheating (such as the heat emitted by the internal electronic equipment).
Thermal insulation from external sinks, improved heat absorption from external sources, or heat release from internal sources protect the equipment against too low temperatures.
Components of a passive thermal control system (PTCS) include:
MLI protects the spacecraft against excessive solar or planetary heating, as well as excessive cooling when exposed to deep space. External surface coatings that affect the thermo-optical characteristics. Thermal fillers are used to improve thermal coupling at specific interfaces (for instance, on the thermal path between an electronic unit and its radiator). Thermal washers are used at specific interfaces to reduce thermal coupling. Thermal doublers distribute heat dissipated by equipment across the radiator surface.
Components of the active thermal control system (ATCS) include:
During the mission’s cold periods, thermostatically controlled resistive electric heaters keep the equipment temperature above its lower limit.
Fluid loops are used to transmit heat from equipment to radiators. They can be single-phase loops with a pump; two-phase loops with heat pipes (HP), loop heat pipes (LHP), or capillary pumped loops; or three-phase loops with heat pipes (HP), loop heat pipes (LHP), or capillary pumped loops (CPL).
Louvers are a type of shutter that is used (which change the heat rejection capability to space as a function of temperature).
Current Technologies
Coating
Coatings are the simplest and least expensive of the TCS techniques. A coating may be paint or a more sophisticated chemical applied to the surfaces of the spacecraft to lower or increase heat transfer. The characteristics of the type of coating depends on their absorptivity, emissivity, transparency, and reflectivity. The main disadvantage of coating is that it degrades quickly due to the operating environment.
Multilayer insulation (MLI)
Multilayer insulation (MLI) is the most common passive thermal control element used on spacecraft. MLI prevents both heat losses to the environment and excessive heating from the environment. Spacecraft components such as propellant tanks, propellant lines, batteries, and solid rocket motors are also covered in MLI blankets to maintain ideal operating temperature. MLI consist of an outer cover layer, interior layer, and an inner cover layer. The outer cover layer needs to be opaque to sunlight, generate a low amount of particulate contaminates, and be able to survive in the environment and temperature to which the spacecraft will be exposed. Some common materials used for the outer layer are fiberglass woven cloth impregnated with PTFE Teflon, PVF reinforced with Nomex bonded with polyester adhesive, and FEP Teflon. The general requirement for the interior layer is that it needs to have a low emittance. The most commonly used material for this layer is Mylar aluminized on one or both sides. The interior layers are usually thin compared to the outer layer to save weight and are perforated to aid in venting trapped air during launch.
The inner cover faces the spacecraft hardware and is used to protect the thin interior layers. Inner covers are often not aluminized in order to prevent electrical shorts. Some materials used for the inner covers are Dacron and Nomex netting. Mylar is not used because of flammability concerns. MLI blankets are an important element of the thermal control system.
Louvers
Louvers are active thermal control elements that are used in many different forms. Most commonly they are placed over external radiators, louvers can also be used to control heat transfer between internal spacecraft surfaces or be placed on openings on the spacecraft walls. A louver in its fully open state can reject six times as much heat as it does in its fully closed state, with no power required to operate it. The most commonly used louver is the bimetallic, spring-actuated, rectangular blade louver also known as venetian-blind louver. Louver radiator assemblies consist of five main elements: baseplate, blades, actuators, sensing elements, and structural elements.
Heaters
Heaters are used in thermal control design to protect components under cold-case environmental conditions or to make up for heat that is not dissipated. Heaters are used with thermostats or solid-state controllers to provide exact temperature control of a particular component. Another common use for heaters is to warm up components to their minimal operating temperatures before the components are turned on.
- The most common type of heater used on spacecraft is the patch heater, which consists of an electrical-resistance element sandwiched between two sheets of flexible electrically insulating material, such as Kapton. The patch heater may contain either a single circuit or multiple circuits, depending on whether or not redundancy is required within it.
- Another type of heater, the cartridge heater, is often used to heat blocks of material or high-temperature components such as propellants. This heater consists of a coiled resistor enclosed in a cylindrical metallic case. Typically a hole is drilled in the component to be heated, and the cartridge is potted into the hole. Cartridge heaters are usually a quarter-inch or less in diameter and up to a few inches long.
- Another type of heater used on spacecraft is the radioisotope heater units also known as RHUs. RHUs are used for travelling to outer planets past Jupiter due to very low solar radiance, which greatly reduces the power generated from solar panels. These heaters do not require any electrical power from the spacecraft and provide direct heat where it is needed. At the center of each RHU is a radioactive material, which decays to provide heat. The most commonly used material is plutonium dioxide. A single RHU weighs just 42 grams and can fit in a cylindrical enclosure 26 mm in diameter and 32 mm long. Each unit also generates 1 W of heat at encapsulation, however the heat generation rate decreases with time. A total of 117 RHUs were used on the Cassini mission.
Panels and radiators (white square panels) on ISS after STS-120
Radiators
Excess waste heat created on the spacecraft is rejected to space by the use of radiators. Radiators come in several different forms, such as spacecraft structural panels, flat-plate radiators mounted to the side of the spacecraft, and panels deployed after the spacecraft is on orbit. Whatever the configuration, all radiators reject heat by infrared (IR) radiation from their surfaces. The radiating power depends on the surface’s emittance and temperature. The radiator must reject both the spacecraft waste heat and any radiant-heat loads from the environment. Most radiators are therefore given surface finishes with high IR emittance to maximize heat rejection and low solar absorptance to limit heat from the Sun. Most spacecraft radiators reject between 100 and 350 W of internally generated electronics waste heat per square meter. Radiators’ weight typically varies from almost nothing, if an existing structural panel is used as a radiator, to around 12 kg/m2 for a heavy deployable radiator and its support structure.
The radiators of the International Space Station are clearly visible as arrays of white square panels attached to the main truss.
Last Words
The thermal control system’s job is to keep component temperatures within acceptable ranges for specific orbits, power demands, and operations. Temperature gradients across the spacecraft and some components, such as lenses, should be reduced as well.
Thermal control is necessary to ensure the mission’s optimal performance and success, because if a component is exposed to temperatures that are too high or too low, it may be damaged or have its performance significantly harmed. Thermal management is also required to keep specialized components (such as optical sensors, atomic clocks, and other similar devices) within a specified temperature stability requirement in order for them to work as effectively as feasible.