Spacecraft are subjected to an array of thermal environments, which necessitate unique design considerations for them. These may include long interplanetary journeys near or far from the Sun, descent through hostile atmospheres or extended eclipse durations.
Satellite thermal control systems use various techniques to maintain acceptable operational temperatures throughout each mission phase for spacecraft components. Surface finishes, insulation materials, radiators and louvers are often employed as methods of temperature regulation.
Coatings
Coatings can help satellites maintain consistent internal temperatures by reflecting and absorbing solar radiation, helping prevent overheating of key components and maintaining consistent internal temperatures over time.
Thermal control coatings can be applied to various surfaces, including satellite structures and electronics. The ideal choice depends on factors like geometry, material requirements and thermal requirements – aerogels provide lightweight insulation while keeping sensitive electronic components cool.
Metallic coatings, on the other hand, are designed to reflect sunlight and dissipate heat efficiently while withstanding rapid temperature changes found in space. Manufacturers conduct tests on thermal shock resistance, thermal conductivity and emissivity (the latter of which involves subjecting coated samples to solar irradiance in an evacuated chamber at cold temperatures) in order to assess performance of such coatings.
Sunshields
Space thermal environments vary across planetary orbit or interplanetary missions, with most spacecraft components having temperature ranges they must stay within throughout their flight phases. The amount of solar and infrared (IR) radiation a spacecraft absorbs and radiates back out is dictated by Eq. 1.
Solar absorptivity and total hemispherical IR emissivity aS/eH are among the key properties that play into heat exchange between spacecraft and their respective planets. These two properties determine what proportion of an object’s infrared radiation is emitted into space compared to what would happen with an ideal blackbody emitter at that temperature.
To maintain spacecraft component temperatures, active devices are needed to expel heat away from its components into space. These must be lightweight and consume no power when operating; louvers are commonly used as these active devices – though other devices such as variable emissivity materials and thermochromic materials could also work.
Louvers
Given the increasing sophistication of instruments on small spacecraft with long missions, thermal control is becoming an indispensable technology. Since smaller volumes absorb and release heat more rapidly than larger bodies do, game-changing solutions must be found to maintain stable thermal environments aboard spacecraft.
Louvers are an efficient passive thermal control component designed to maintain internal spacecraft temperature stability by creating a difference of several watts between closed and open louvers, dissipation difference created between closed and open louvers, and creating power dissipation differences of several watts between their closed and open states. Their design is inspired by full-sized radiators modified for CubeSat form factor; bimetallic clock springs whose thermal expansion rates differ between their fused metals create bimetallic clock springs which open upon heating; once the springs expand upon rising temperatures when temperatures increase springs expand opening the louvers while changing average surface IR emittivity of radiator surface by altering its average surface average surface average IR emission level by several percent or so.
Other passive thermal control technologies include thermal straps made of Mylar foil or Boyd’s k-core pyrolytic graphite sheets. Utah State University has utilized these straps to increase thermal rejection and zonal temperature regulation on satellite busses and payloads.
MLI
Spacecraft components must maintain specific temperatures in order to meet survival and functional requirements across all mission stages, which requires balancing heat absorbed, stored, produced, dissipated or exchanged via solar absorptivity, infrared (IR) emission from surface optical properties or thermal capacitance.
Passive thermal control enables spacecraft with limited cost, volume, weight, and power requirements such as SmallSats and CubeSats to maintain component temperatures without using powered equipment, making this approach particularly appealing. Passive technologies include MLI (Mass Loaded Instrumentation), coatings/surface finishes thermal straps interface conductance as well as sunshades.
MLI blankets are typically constructed of multiple layers of thin material with low infrared (IR) emissivity that have been embossed or alternated with thinner netting to limit conduction, as well as perforations to vent trapped air once in orbit. MLI offers high TRL ratings and fits many different SmallSat form factors, though its performance drops considerably when compressed into compact sizes, potentially creating hazards when deployed via pusher spring deployers found on some SmallSats.