Spacecraft equipment must operate within an acceptable temperature range to complete its mission successfully, doing this by either absorbing or rejecting heat from its environment.
Hardware available to accomplish this includes multilayer insulation systems to shield equipment from excessive solar and planetary heating, thermal paints and coatings that change optical properties (such as solar absorptance and infrared emittivity) of surfaces (including solar absorptance and infrared emittivity), fillers, and washers.
Fundamentals
Thermal Control Systems (TCSs) on spacecraft serve to keep all equipment operating optimally by controlling temperatures within their required parameters, whether that means maintaining cryogenic temperatures or typical operating temperatures. They move heat from primary sources such as instruments, heaters and solar absorption onto radiators while limiting radiation loss into deep space and decreasing temperature gradients to provide power-hungry electronics with an acceptable environment.
NASA needs to accommodate for transient thermal environments arising from celestial bodies casting shadows, changes in electronic power dissipation and orientation relative to the Sun, and internal temperatures fluctuating within batteries, sensors and propulsion systems. Passive thermal control techniques are implemented to mitigate transients. They include matt black paints and thermal doublers which maximize radiative and conductive transfer rates respectively.
Cryogenics
Cryogenics is the study of extremely cold temperatures – below 123 Kelvins (K). More specifically, it focuses on turning gases into liquids at such extreme temperatures and their subsequent behaviour in such an extreme environment.
Cryogenically chilled foods are used in humanitarian efforts, such as providing frozen meals to war zones or earthquake-affected regions. Furthermore, certain rare blood groups are preserved through a process known as cryopreservation.
Liquifaction of gases at very cold temperatures can be highly dangerous as it releases large amounts of oxygen that can cause burns. Furthermore, this creates pressure which could result in ruptured tanks or explosions; to ensure safety and make handling the gases easier. Cryogenic storage tanks therefore come equipped with large capacities so that users may safely store and manage gasses.
Radiators
Radiators provide a surface to dissipate excess heat through radiation, with their design optimized to maximize both IR emissivity and solar absorptivity. SmallSats with limited volume often utilize deployable radiators that can be stored away during transit before being deployed when excess heat has dissipated.
Passive thermal control techniques utilize coatings, such as paint or more sophisticated chemicals applied directly to the exterior surface of spacecraft. Their thermal properties depend on factors like their albedo, emissivity, transparency and reflectivity properties; when applied effectively mirrored coatings can reduce thermal coupling at key interfaces and increase efficiency by spreading out dissipated heat more evenly throughout equipment.
Heaters
Based on its mission, spacecraft may need to maintain temperatures that range from cryogenic to typical operating conditions, requiring many components and structures that must be managed to do so effectively, including its own structure.
Thermal designers must predict spacecraft temperatures by solving an energy balance equation and adapting its thermal hardware configurations accordingly to achieve its temperature goals. Their calculations depend on initial configuration choices of this hardware which may have direct implications on other subsystems like mechanical and structural.
Passive thermal control utilizes paints, coatings and tapes designed for thermal control purposes – such as Fluorinated Ethylene Propylene (FEP) tapes with low solar absorptivity but high IR emissivity – that reflect incident solar radiation efficiently while dissipating internal heat efficiently from spacecraft systems. Many manufacturers provide catalogues or guides to help select appropriate products.
Heat Switches
Heat switches allow spacecraft thermal control systems to adjust heat transfer by opening or closing conduction paths to manage its flow. They play an essential role in keeping spacecraft temperatures within their operating and survival limits; temperatures outside these limits could result in electronic failure, sensor pointing errors and fatigue failure in wire bonds – all factors which threaten survival of any small spacecraft mission.
Solar inputs can be managed using electrical resistance heaters and multilayer insulation blankets with various degrees of solar absorptance and infrared emittivity. As solar eclipses occur regularly and sun angles shift quickly, thermal environments may change quickly as a result. Additionally, electronic power dissipation rates vary dramatically in response to solar input changes.
SmallSats often feature biological payloads that require actively-controlled heaters to maintain temperature restrictions, which can be costly and power-hungry solutions.