Spacecraft thermal control

Sunshade of MESSENGER, orbiter of planet Mercury

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.

Thermal control is essential to guarantee the optimum 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 (such as optical sensors, atomic clocks, etc.) within a specified temperature stability requirement, to ensure that they perform as efficiently as possible.

The thermal control subsystem can be composed both of passive and of active items and works in two ways:

Passive Thermal Control System (PTCS) components include:

Active Thermal Control System (ATCS) components include:

Thermal Control Systems

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.

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

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 spacecrafts. MLI prevent 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 that is 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 minimum operating temperatures before the components are turned on.

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.[1]

Heat pipes

Heat pipes use a closed two-phase liquid-flow cycle with an evaporator and a condenser to transport relatively large quantities of heat from one location to another without electrical power.

Future of Thermal Control Systems

Events

A major event in the field of space thermal control is the International Conference on Environmental Systems, organized every year by AIAA.

Sun shield

Sunshield full-size test for the James Webb Space Telescope

In spacecraft design, a sun shield restricts or reduces heat caused by sunlight hitting a spacecraft.[2] An example of use of a thermal shield is on the Infrared Space Observatory.[3] The ISO sunshield helped protect the cryostat from sunlight, and it was also covered with solar panels.[4]

Not to be confused with concept of a global scale sun shield in geoengineering, often called a Space sunshade or "sun shield", in that case the spacecraft itself is used to block sunlight on a planet, not as part the spacecraft's thermal design.[5]

An example of a sunshield in spacecraft design is the Sunshield (JWST) on the planned James Webb Space Telescope.[6]

See also

Bibliography

References

  1. "Radiators". International Space Station. NASA. Retrieved September 26, 2015.
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