Spacecraft thermal control


In spacecraft design, the function of the thermal control system 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 the extreme coldness found in the shadows of deep space or to the intense heat found in the unfiltered direct sunlight of outer space. A TCS must also moderate the internal heat generated by the operation of the spacecraft it serves.
A TCS can eject heat passively through the simple and natural infrared radiation of the spacecraft itself, or actively through an externally mounted infrared radiation coil.
Thermal control is essential to guarantee the optimal performance and success of the mission because if a component is subjected to temperatures which are too high or too low, it could be damaged or its performance could be severely affected. Thermal control is also necessary to keep specific components within a specified temperature stability requirement, to ensure that they perform as efficiently as possible.

Active or passive systems

The thermal control subsystem can be composed of both passive and active items and works in two ways:
  • Protecting the equipment from overheating, either by thermal insulation from external heat fluxes, or by proper heat removal from internal sources.
  • Protecting the equipment from temperatures that are too low, by thermal insulation from external sinks, by enhanced heat absorption from external sources, or by heat release from internal sources.
Passive thermal control system components include:
  • Multi-layer insulation, which protects the spacecraft from excessive solar or planetary heating, as well as from excessive cooling when exposed to deep space.
  • Coatings that change the thermo-optical properties of external surfaces.
  • Thermal fillers to improve the thermal coupling at selected interfaces.
  • Thermal washers to reduce the thermal coupling at selected interfaces.
  • Thermal doublers to spread on the radiator surface the heat dissipated by equipment.
  • Mirrors to improve the heat rejection capability of the external radiators and at the same time to reduce the absorption of external solar fluxes.
  • Radioisotope heater units, used by some planetary and exploratory missions to produce heat for TCS purposes.
  • Barbecue roll - a slow roll along the spacecraft's long axis to prevent the Sun over heating one side, like a rotisserie chicken rotating as it is cooked.
Active thermal control system components include:
  • Thermostatically controlled resistive electric heaters to keep the equipment temperature above its lower limit during the mission's cold phases.
  • Fluid loops to transfer the heat emitted by equipment to the radiators. They can be:
  • * single-phase loops, controlled by a pump;
  • * two-phase loops, composed of heat pipes, loop heat pipes or capillary pumped loops.
  • Louvers.
  • Thermoelectric coolers.

    Thermal control systems

A thermal control system can have various purposes, the most notable of which are:
  • Environment interaction
  • * Includes the interaction of the external surfaces of the spacecraft with the environment. Either the surfaces need to be protected from the environment, or there has to be improved interaction. Two main goals of environment interaction are the reduction or increase of absorbed environmental fluxes and reduction or increase of heat losses to the environment.
  • Heat collection
  • * Includes the removal of dissipated heat from the equipment in which it is created to avoid unwanted increases in the spacecraft's temperature.
  • Heat transport
  • * Is taking the heat from where it is created to a radiating device.
  • Heat rejection
  • * The heat collected and transported has to be rejected at an appropriate temperature to a heat sink, which is usually the surrounding space environment. The rejection temperature depends on the amount of heat involved, the temperature to be controlled and the temperature of the environment into which the device radiates the heat.
  • Heat provision and storage
  • * Is to maintain a desired temperature level where heat has to be provided and suitable heat storage capability has to be foreseen.

    Environment

For a spacecraft, the main environmental interactions are the energy coming from the Sun and the heat radiated to deep space. Other parameters also influence the thermal control system design, such as the spacecraft's altitude, orbit, attitude stabilization, and spacecraft shape. Different types of orbit, such as low earth orbit and geostationary orbit, also affect the design of the thermal control system.
  • Low Earth orbit
  • * This orbit is frequently used by spacecraft that monitor or measure the characteristics of the Earth and its surrounding environment and by uncrewed and crewed space laboratories, such as EURECA and the International Space Station. The orbit's proximity to the Earth has a great influence on the thermal control system needs, with the Earth's infrared emission and albedo playing a very important role, as well as the relatively short orbital period, less than 2 hours, and long eclipse duration. Small instruments or spacecraft appendages such as solar panels that have low thermal inertia can be seriously affected by this continuously changing environment and may require very specific thermal design solutions.
  • Geostationary orbit
  • * In this 24-hour orbit, the Earth's influence is almost negligible, except for the shadowing during eclipses, which can vary in duration from zero at solstice to a maximum of 1.2 hours at equinox. Long eclipses influence the design of both the spacecraft's insulation and heating systems. The seasonal variations in the direction and intensity of the solar input have a great impact on the design, complicating the heat transport by the need to convey most of the dissipated heat to the radiator in shadow, and the heat-rejection systems via the increased radiator area needed. Almost all telecommunications and many meteorological satellites are in this type of orbit.
  • Highly eccentric orbits
  • * These orbits can have a wide range of apogee and perigee altitudes, depending on the particular mission. Generally, they are used for astronomy observatories, and the TCS design requirements depend on the spacecraft's orbital period, the number and duration of the eclipses, the relative attitude of Earth, Sun and spacecraft, the type of instruments onboard and their individual temperature requirements. Molniya orbits, with perigee altitudes lying in the LEO range of about 550 km and apogee altitudes lying in the vicinity of the GEO altitude, are an example of this type of orbit.
  • Deep space and planetary exploration
  • *An interplanetary trajectory exposes spacecraft to a wide range of thermal environments more severe than those encountered around Earth's orbits. Interplanetary mission includes many different sub-scenarios depending on the particular celestial body. In general, the common features are a long mission duration and the need to cope with extreme thermal conditions, such as cruises either close to or far away from the Sun, low orbiting of very cold or very hot celestial bodies, descents through hostile atmospheres, and survival in the extreme environments on the surfaces of the bodies visited. The challenge for the TCS is to provide enough heat-rejection capability during the hot operating phases and yet still survive the cold inactive ones. The major problem is often the provision of the power required for that survival phase.

    Temperature requirements

The temperature requirements of the instruments and equipment on board are the main factors in the design of the thermal control system. The goal of the TCS is to keep all the instruments working within their allowable temperature range. All of the electronic instruments on board the spacecraft, such as cameras, data-collection devices, batteries, etc., have a fixed operating temperature range. Keeping these instruments in their optimal operational temperature range is crucial for every mission. Some examples of temperature ranges include
  • 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.

    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. Coatings can also be applied in the form of adhesive tape or stickers to reduce degradation.

Multilayer insulation (MLI)

Multilayer insulation 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 contaminants, 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.