Every part of a spacecraft must remain within a specific temperature range to fulfill mission requirements, with heat moving primarily via conduction and radiation.
On Earth, we cool things by moving air around to dissipate heat; in space however, with no atmosphere available to carry away excess heat, thermal radiation must be used instead to disperse it.
Radiator Materials
Radiators require a surface with high infrared emittivity and low solar absorptivity (radiator properties), in order to dissipate heat efficiently. As these properties depend on wavelength, choosing an appropriate material for this function must be done with great care.
Paints, coatings and surface finishes as well as adhesive tapes are common materials used to customize spacecraft radiator surfaces. For instance, the 6U BioSentinel SmallSat employs second-surface silver FEP tape to control its external thermal radiative properties and overall energy balance (4).
Due to structural constraints, providing sufficient radiator surfaces may not be possible; in such a situation, a deployable radiator could help provide additional radiational surfaces and increase heat rejection capabilities.
Utah State University and JPL developed an ATA radiator that can passively deploy from within a 3U CubeSat without an actuator (5). It features aluminum and copper foil layers as well as ribbons or braids made of high conductivity carbon materials such as pyrolytic graphite sheets for extra heat dissipation needs.
Radiator Shape
Radiators are the primary heat-rejecting elements on any spacecraft. Their size depends on both how much power is dissipated by all components within it as well as thermal conditions of an orbit.
Figure 3 depicts a radiator panel as having a honeycomb structure composed of eight aluminum tubes joined at either end by manifolds; silverized Teflon tapes are applied to its outer surfaces for high infrared emissivity and low solar absorptivity; additionally, these radiator panels are shielded against micrometeoroids and orbital debris by using doublers with thicknesses of 0.5mm aluminum doublers to protect them.
Freon-21 flows through eight pipes of a honeycomb radiator and its performance was tested under different coolant mass flow rates and temperatures to replicate deep space environments. Simulations confirmed these test results as did thermal vacuum tests conducted at the JAXA Tsukuba Space Center.
Radiator Orientation
Modern spacecraft equipment typically has temperature tolerance limits that must be observed to function optimally, and it is the responsibility of TCS to ensure each piece remains within these boundaries throughout all duty cycles.
Passive systems offer low costs, volumes and risks compared to their active counterparts, making them particularly suitable for small spacecraft such as CubeSats. Passive technologies such as MLI (microlithography layer interface), coatings/surface finishes, thermal interface materials (TIM), heat pipes and sunshades (sunshades) are excellent ways of keeping component temperatures consistent without the use of powered equipment.
NASA GSFC developed a passive louver which utilizes bimetallic springs to control a single flap’s position and alter its average infrared emittivity, changing it as part of the CubeSat Dellingr mission in late 2017. As it requires no power source for operation it is considered passive; however, this device should not be relied upon solely as means for spacecraft thermal management as this leaves it vulnerable to micrometeoroids impacting during flight.
Radiator Emissivity
Spacecraft radiator emissivity, measured as its total, hemispherical emissivity or “emissivity e,” is defined as the ratio between equilibrium surface temperature and radiated power emitted from it. It is determined by various factors including physical structure of the radiator, its polarization state and ambient conditions in its vicinity.
Thermal management system must consider the dynamic environment in which spacecraft are operated, including variations in solar intensity and angle as well as thermal loads from deployed avionics and materials processing equipment. Aside from using radiators, additional heaters or heat pipes must also be considered to transfer thermal energy between systems.
Louvred or shuttered radiators like those shown in Figure 5b allow for their opening and closure via actuator, providing several advantages over more common louvres found on European spacecraft such as increased effective emissivity when fully open without multi-reflection effects and improved insulation efficiency when closed.