Variable Emissivity Surfaces for Micro and Nanosatellites

Abstract

The contemporary satellites usually utilize louvers as variable emissivity surfaces (VES) for the thermal control subsystem. This means is only particularly scalable down and the next generation of small satellites definitely requires new techniques for thermal control. Further, the low mass, volume and cost of the micro and nanosatellite require additional features from the future VES. Besides high reliability, which is an unconditional requirement for a space application, the other criteria are as follows: low mass, deep modulation of emissivity, low heat leak in off-state, fast reaction time, passive action, as well as other lower level criteria. These requirements are complex and sometimes contradictory. The current approaches to find alternatives to the traditional mechanical louvers branch in several directions: electrophoretic, electrochromic, electrostatics actuated VES, MEMS shutters, etc. None of the current solutions is successful in meeting all of the posed criteria. We present a novel VES subsystem, particularly developed for use in micro and nanosatellites. The concept is simple, reliable and very efficient. The bionic structure, a flower-like design, is made from a thin and elastic foil. The artificial flower consists from a peduncle, fixed to the satellite radiator panel and 4–6 petals. The upper surface of the petal is made as the second-surface mirror and the lower surface is the gold plated or the first-surface mirror. The kinematic mechanism which opens and closes the artificial flower is the shape memory actuator located in the petal root. The SMA actuators are trained as the “two way actuators”. The “two way” memory effect has been recognized as difficult to control and suffering from amnesia. However, the new learning process of the shape memory actuators enables more than 350.000 cycles without SMA parameter degradation. The artificial flower works as follow: when the sun irradiates the flower and/or the radiator temperature exceeds the preset value, the SMA actuators bend and open the flower. In such a manner the flower exposes the highly reflective surface to the sun and shadows the satellite radiator until the sun sets again. The flower structure is without any friction-connected kinematic movements, thus the reliability of device should be high. The mass of the flower is less than 450 g/m2, the heat leak trough the open flower is >2% and the efficiency of the closed flower is <80%. The SMA actuator is passive and quite resistant to the radiation, oxygen and EDS.