Preliminary design and comparative study of thermal control in a nanosatellite through smart variable emissivity surfaces

Abstract

The thermal radiation that is rejected or absorbed into deep space is highly variable. Ultralight smart surfaces with arrays of unit cells can be designed to change their effective emissivity and absorptivity without energy consumption, actuators, and controllers, and can be used for the temperature control of satellites. The smart surfaces work in a similar manner to thermal louvers but they are hingeless, lighter, and their activation depends on their anisotropic mechanical properties and multilayer structure. The generated thermal stresses between layers that have a high mismatch in the coefficient of thermal expansion cause large deformations and rotations within small temperature changes. The arrays of the surface open or close, and transform their geometry as a function of temperature; therefore, coatings of different thermo-optical properties are revealed or concealed, thus creating variable emissivity surfaces. The emissivity and absorptivity curves of the smart surfaces can be entirely designed as a function of temperature. Theoretically, an emissivity change equal to Δε = 0.8 can be achieved. The small thermal capacitance renders nanosatellites very susceptible to temperature fluctuations. In this study, different emissivity curves were generated to re-calculate the worst cold and hot cases, and to redesign the thermal control system of a certain nanosatellite. We studied a plethora of design cases based on the energy balance equation in steady state while considering the nanosatellite as one-node geometry. In two ideal designs, the temperature deviation of the nanosatellite in the worst cold and hot cases is limited to Δ Τ = 37 ℃ or 43 ℃ without the use of heaters. Moreover, with a power equal to 0.7 W the temperature deviation is limited to Δ Τ = 20 ℃. Consequently, the thermal fatigue is minimized and the energy consumption during the eclipse phase is reduced.