The study of variable emittance materials and coatings for the thermal control of spacecrafts has been an active field of research for the last 20 years. Initially, the preferred solutions had perovskite manganites as the thermochromic material. However, it was soon realized that manganites have very high crystallization temperatures, and pretty low TMIT. The focus was then shifted to continuous-thin-film coatings based on VO2, which requires lower temperatures for crystallization, and provides transition at room temperature if properly doped. One striking characteristic of all the proposed coatings, however, is that they have either extremely complex design, or poor T.O. properties. Most papers and patents focus on Δε, while they hardly mention α, which tends to be too high (above 0.4). High α derives in part from the poor quality of the LESR (which may be difficult to implement with good optical and mechanical properties), in part from the use of a continuous VO2 layer, that has high absorbance across the UV-VIS-NIR spectrum. One strategic advantage of Smart-Flex is that it utilizes a patterned layer of VO2: α decreases with the fill factor of the pattern, and tends by design to be lower than in continuous thin-film solutions. A second advantage derives from a good choice of materials and processes. These advantages translate into experimental results that place Smart-Flex at the state of the art among the smart solutions for thermal control, both in terms of current T.O. performance and prospective evolution, and for the fact that it is the only technology implemented on a flexible substrate.
The Smart-Flex technology can find application on space platforms of any kind and use in replacement of traditional OSRs, as it offers advantages in terms of temperature-variable emissivity and mechanical flexibility. These advantages are appreciable not only at the component level, but also and especially at the system level. In particular, temperature variable emissivity means reduced heat losses in the cold phase, whereas the thermal control subsystem is designed primarily as a cooler during the hot phases. Reduced heat losses translate into reduced electric budget at the heaters level, and/or reduced masses deriving from the integration of louvers. At the proposal preparation stage, Smart-Flex appeared most attractive for small satellites, that cannot take on board massive and power-hungry active emittance control systems. More recently, however, it has been underlined the relevance of the technology also for the new telecom satellites which use electrical orbit raising. Electrical orbit raising to geostationary orbit takes several months and is a cold case because the satellite is not fully operational. The available electrical power is preferably dedicated to electrical propulsion rather than for heating the satellite, and the implementation of temperature variable OSRs would have a great added value.
