Ashwin-Ushas (United States)

Initially, the primary drivers of development for applications of the microwave-region and conductivity properties of CPs were the following: Electromagnetic Impulse (EMI) shielding; conductive coatings and composites for electrostatic charge dissipation (ESD) or antistatic applications; and passive absorbers for microwave-region radiation for radar cross section (RCS) reduction. To these have recently been added: Dynamic (switchable) microwave absorption and RCS control; microwave-smoke camouflage; and several other applications.

Carbon nanotubes (CNTs) were first widely popularized following the report of Iijima, working at Japan’s NEC (Nippon Electric Company) Corporation’s research laboratories, in 1991 [1–2]. Iijima first prepared multi-walled CNTs in the arc discharge of graphite rods at high temperature, apparently while attempting to produce fullerenes (C60, see below) [1–2], although this same author had published electron micrographs of “carbonaceous materials” which were clearly multi-walled CNTs earlier, in 1980 [3]. Their first discovery and recognition as a different form of carbon appears, however, to have occurred considerably earlier. Radushkevich and Lukyanovich [4] published an account in 1952 that clearly appears to describe CNTs. And Schutzenberger and Schutzenberger published an account in 1890 describing the production of “carbon microfilaments” obtained by passing cyanogen over porcelain at very high temperature [5].

Variable emittance (ε) is a property useful for external thermal control in future micro- (< 20 kg) spacecraft; current methods, used in large spacecraft, do not work. This contribution describes fabrication, function, performance of thin-film variable emittance electrochromic devices fabricated using an ionic liquid as electrolyte and conducting polymer (CP) as active electrochromic. The spacecraft skins are thin (< 0.5 mm), flexible, lightweight, inexpensive and variable-area. A thin surface film further functions to lower the solar absorptance (α(s), to protect from overheating) and protect from space atomic-O/far-UV. The ionic liquid imparts unique durability to space conditions. Data presented show tailorable ε variations from 0.19 to 0.90, α(s) < 0.3, and nearly indefinite cyclability. Extended thermal vacuum studies (continuous cycling over months under space conditions --- 10-6 Torr vacuum, far-UV) and other studies show excellent durability and resistance to solar wind and atomic-O. Electrochemical properties of the devices are described in detail.

The advent of very small satellites, such as nano and microsatellites, logically leads to a requirement for smaller thermal control subsystems. In addition, the thermal control needs of the smaller spacecraft/instrument may well be different from more traditional situations. For example, power for traditional heaters may be very limited or unavailable, mass allocations may be severely limited, and fleets of nano/microsatellites will require a generic thermal design as the cost of unique designs will be prohibitive. Some applications may require significantly increased power levels while others may require extremely low heat loss for extended periods. Small spacecraft will have low thermal capacitance thus subjecting them to large temperature swings when either the heat generation rate changes or the thermal sink temperature changes. This situation, combined with the need for tighter temperature control, will present a challenging situation during transient operation. The use of off-the-shelf commercial spacecraft buses for science instruments will also present challenges. Older thermal technology, such as heaters, thermostats, and heat pipes, will almost certainly not be sufficient to meet the requirements of these new spacecraft/instruments. They are generally too heavy, not scalable to very small sizes, and may consume inordinate amounts of power. Hence there is a strong driver to develop new technology to meet these emerging needs. Variable emittance coatings offer an exciting alternative to traditional control methodologies and are one of the technologies that will be flown on Space Technology 5, a mission of three microsatellites designed to validate enabling technologies. Several studies have identified variable emittance coatings as applicable to a wide range of spacecraft, and to potentially offer substantial savings in mass and/or power over traditional approaches. This paper discusses the development of the variable emittance thermal suite for ST-5. More specifically, it provides a description of and the infusion and validation plans for the variable emittance coatings.