Variable emittance (e) is a property vital for the increasing needs in thermal control of future microspacecraft. This article describes fabrication, function, and performance of thin-film, flexible, variable-emittance (V-E) electrochromic skins that use a conducting polymer/-Au/-microporous membrane (CP/Au/mP) base, and a new, unique ionic liquid electrolyte (IonEl). Poly(aniline-codiphenyl amine) with a long-chain polymeric dopant is used as the CP. A unique, patented device design yields no barrier between the active, electrochromic CP surface and the external environment, except for a thin, infrared-transparent semiconductor/polymer film that lowers solar absorptance [a(s)] and protects from atomic-O/far-UV. Use of the IonEl requires special activation methods. Data presented show tailorable e variations from 0.19 to 0.90, De values of >0.50 (which is the highest reported thus far for any functional V-E material, to our knowledge), a(s) < 0.35, and nearly indefinite cyclability. Extended space durability testing, including calorimetric thermal vacuum and continuous light/dark cycling over >7 months under space conditions (<10 25 Pa vacuum, far-UV), show excellent durability. Other data show resistance to solar wind, atomic-O, electrostatic discharge, and micrometeoroids. These lightweight, inexpensive, advanced polymeric materials represent the only technology that can work with micro-(<20 kg) and nano-(<2 kg) spacecraft, thus eventually allowing for much greater flexibility in their design and potentially "democratizing" the entire space industry, for example, allowing small firms to launch their own, dedicated satellites. V C 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40850.

Thermal control is an important aspect of spacecraft design, particularly in the case of crewed vehicles, which must maintain a precise internal temperature at all times in spite of significant variations in the external thermal environment and internal heat loads. Future missions beyond low Earth orbit will require radiator systems with high turndown ratios, defined as the ratio between the maximum and minimum heat rejection rates achievable by the radiator system. Current radiators are only able to achieve turndown ratios of 3:1, far less than the 12:1 turndown ratio requirement expected for future missions. An innovative morphing radiator concept uses the temperature-induced phase transformation of shape memory alloy (SMA) materials to achieve turndown ratios that are predicted to exceed 12:1 via substantial geometric reconfiguration. Developing mathematical and computational models of these morphing radiators is challenging due to the strong two-way thermomechanical coupling not present in traditional fixed-geometry radiators and not widely considered in the literature. Although existing simulation tools are capable of analyzing the behavior of some thermomechanically coupled structures, general problems involving radiation and deformation cannot be modeled using publicly available codes due to the complexity of modeling spatially evolving boundary fields. This paper provides important insight into the operational response of SMA-based morphing radiators by employing computational tools developed to overcome previous shortcomings. Several example problems are used to demonstrate the novel radiator concept. Additionally, a prototype morphing radiator was designed, fabricated, and tested in a thermal environment compatible with mission operations. An associated finite element model of the prototype was developed and executed. Model predictions of radiator performance generally agree with the experimental data, giving confidence that the tools developed are able to accurately represent the thermomechanical coupling present in morphing radiators and that such tools will be useful in future designs.

In order to control system and component temperatures, many spacecraft thermal control systems use a radiator coupled with a pumped fluid loop to reject waste heat from the vehicle. Since heat loads and radiation environments can vary considerably according to mission phase, the thermal control system must be able to vary the heat rejection. The ability to "turn down" the heat rejected from the thermal control system is critically important when designing the system. Electrochromic technology as a radiator coating is being investigated to vary the amount of heat rejected by a radiator. Coupon level tests were performed to test the feasibility of this technology. Furthermore, thermal math models were developed to better understand the turndown ratios required by full scale radiator architectures to handle the various operation scenarios encountered during a mission profile for the Altair Lunar Lander. This paper summarizes results from coupon level tests as well as the thermal math models developed to investigate how electrochromics can be used to increase turn down ratios for a radiator. Data from the various design concepts of radiators and their architectures are outlined. Recommendations are made on which electrochromic radiator concept should be carried further for future thermal vacuum testing.

Spacecraft designed for missions beyond low earth orbit (LEO) face a difficult thermal control challenge, particularly in the case of crewed vehicles where the thermal control system (TCS) must maintain a relatively constant internal environment temperature despite a vastly varying external thermal environment and despite heat rejection needs that are contrary to the potential of the environment. A thermal control system may be required to reject a higher heat load to warm environments and a lower heat load to cold environments, necessitating a relatively high turndown ratio. A modern thermal control system is capable of a turndown ratio of on the order of 12:1, but crew safety and environment compatibility have constrained these solutions to massive multi-loop fluid systems. This paper discusses the analysis of a unique radiator design that employs the behavior of shape memory alloys (SMAs) to vary the turndown of, and thus enable, a single-loop vehicle thermal control system for space exploration vehicles. This design, a morphing radiator, varies its shape in response to facesheet temperature to control view of space and primary surface emissivity. Because temperature dependence is inherent to SMA behavior, the design requires no accommodation for control, instrumentation, or power supply in order to operate. Thermal and radiation modeling of the morphing radiator predict a turndown ranging from 11.9:1 to 35:1 independent of TCS configuration. Coupled thermal-stress analyses predict that the desired morphing behavior of the concept is attainable. A system level mass analysis shows that by enabling a single loop architecture this design could reduce the TCS mass by between 139 kg and 225 kg. The concept has been demonstrated in proof-of-concept benchtop tests.

No abstract