Donya Douglas

Development of the variable emittance thermal suite for the space technology 5 microsatellite

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.

Variable emissivity through MEMS technology

All spacecraft rely on radiative surfaces to dissipate waste heat. These radiators have special coatings that are intended to optimize performance under the expected heat load and thermal sink environment. Typically, such radiators will have a low absorptivity and a high infrared-red emissivity. Given the dynamics of the heat loads and thermal environment it is often a challenge to properly size the radiator. In addition, for the same reasons, it is often necessary to have some means of regulating the heat rejection rate of the radiators in order to achieve proper thermal balance. The concept of using a specialized thermal control coating which can passively or actively adjust its emissivity in response to such load/environmental sink variations is a very attractive solution to these design concerns. Such a system would allow intelligent control of the rate of heat loss from a radiator. Variable emissivity coatings offer an exciting alternative that is uniquely suitable for micro and nano spacecraft applications. This permits adaptive or “smart” thermal control of spacecraft by varying effective emissivity of surfaces in response to either a passive actuator (e.g., a bi-metallic device) or through active control from a small bias voltage signal. In essence the variable emittance surface would be an “electronic louver.” It appears possible to develop such “electronic louvers” through at least three different types of technologies: Micro Electro-Mechanical Systems (MEMS) technology, Electrochromic technology and Electrophoretic technology. This paper will concentrate on the first approach using both MEMS and Micromachining technology to demonstrate variable emissivity.

Controlling Variable Emittance (MEMS) Coatings for space applications

Small spacecraft, including micro and nanosats, as they are envisioned for future missions, will require an alternative means to achieve thermal control due to their small power and mass budgets. One of the proposed alternatives is Variable Emittance (Vari-E) Coatings for spacecraft radiators. Space Technology-5 (ST-5) is a technology demonstration mission through NASA Goddard Space Flight Center (GSFC) that will utilize Vari-E Coatings. This mission involves a constellation of three (3) satellites in a highly elliptical orbit with a perigee altitude of /spl sim/200 km and an apogee of /spl sim/38,000 km. Such an environment will expose the spacecraft to a wide swing in the thermal and radiation environment of the earths atmosphere. There are three (3) different technologies associated with this mission. The three technologies are electrophoretic, electrochromic, and Micro ElectroMechanical Systems (MEMS). The ultimate goal is to make use of Vari-E coatings, in order to achieve various levels of thermal control. The focus of this paper is to highlight the Vari-E Coating MEMS instrument, with an emphasis on the Electronic Control Unit responsible for operating the MEMS device. The Test & Evaluation approach, along with the results, is specific for application on ST-5, yet the information provides a guideline for future experiments and/or thermal applications on the exterior structure of a spacecraft.

Electrostatic radiator for satellite temperature control

An objective for advanced satellites and spacecraft is to continually reduce both their size and mass. This reduction can place severe constraints on the thermal control systems. In addition, mission requirements also may dictate the need to alter the spacecraft energy profile during the course of the mission. To facilitate these advances, significant research has been conducted to develop variable emittance coatings and devices to provide active spacecraft thermal control. Several of these technologies have matured to a level where space based testing is feasible and will be performed as part of the ST5 new millennium spacecraft mission. One of these technologies utilizes electrostatic hold-down of a high emissivity composite film to actively control spacecraft skin temperature. This electrostatic radiator (ESR) device functions as a thermal switch and changes the mode of heat transfer between the spacecraft skin and the radiator film from conduction to radiation. This device has demonstrated large changes in effective emissivity in laboratory cold thermal vacuum testing. In this paper, the theory of operation of the ESR and the construction of the device as configured for demonstration on the ST5 mission is presented. The role of the ST5 mission in the maturation process for VEC technologies is discussed. This paper also describes test results for the ESR through flight qualification testing for the ST5 mission. Finally, anticipated operational characteristics and mission reliability estimates are provided.

MEMS Shutters for Thermal Control — Flight Validation and Lessons Learned

Mechanical thermal louvers are active thermal control devices that have been used to regulate the area of a radiator in response to its temperature. Shutters and louvers were suggested as a means of thermal control using MEMS for nano and pico satellites. JHU/APL, together with NASA/GSFC and Sandia National Laboratory, developed a MEMS shutter design which was flown on NASA/GSFC’s Space Technology 5 (ST‐5) technology demonstration mission as a variable emittance coating. Fabricated with Sandia’s SUMMIT 5 process, six electrostatic comb drives, using Sandia’s high performance design, will move an array of shutters, each 150 μm long and 6 μm wide, to expose either a gold surface (emissivity < 0.1) or the silicon substrate (emissivity < 0.6). To qualify the MEMS louver, several environmental tests were conducted on the final flight articles. The device needed to pass various performance tests (i.e. vibration, thermal vacuum, EMI/EMC, and magnetics) to verify its survival. In addition, component‐level life cy...

Electrochromic Variable Emittance Devices on Silicon Wafer for Spacecraft Thermal Control

Small light‐weight satellites and space vehicles under development for future NASA missions have reduced thermal mass and are strongly affected by changes in orbital conditions, resulting in large temperature variations. Restrictions on payload weight and volume limit the usefulness of many thermal control technologies. One thermal control approach, being considered by NASA in both nano‐ and micro‐ spacecraft applications, involves the use of electrochromic (EC) variable emittance devices (VEDs). VEDs operating in the harsh space environment (UV radiation, atomic oxygen) must be properly protected if they are to reach their design operational life. In this paper, we discuss the design of an all‐solid‐state EC VED built on a silicon wafer. The silicon wafer serves as a window for IR radiation and protects EC layers from the space environment. This paper also discusses the expected limits of emittance modulation of the EC VED on the silicon substrate as well as possible impact of an antireflective coating o...

Technology Overview of a Multi-Evaporator Miniature Loop Heat Pipe for Spacecraft Applications

Aminiature loop heat pipe with multiple evaporators andmultiple condensers was developed for thermal control of small spacecraft and instruments requiring low mass, low power, and compactness. Multiple evaporators afford flexible placement of instruments inside the spacecraft and facilitate heat-load sharing among instruments. Multiple condensers allow the radiators to be placed at various locations on the spacecraft surface and exposed to different thermal environments. Thermoelectric converters are used to provide heating and cooling to the reservoir for loop operating temperature control. A breadboard and a protoflight unit of the miniature loop heat pipe with two evaporators and two condensers were built and tested in a thermal vacuum chamber to demonstrate the thermal performance. In addition, an analytical model was developed to simulate the steady-state and transient behaviors of the miniature loop heat pipe during thermal performance tests.

Multi-Evaporator Miniature Loop Heat Pipe for Small Spacecraft Thermal Control

Under NASA s New Millennium Program Space Technology 8 (ST 8) Project, four experiments Thermal Loop, Dependable Microprocessor, SAILMAST, and UltraFlex - were conducted to advance the maturity of individual technologies from proof of concept to prototype demonstration in a relevant environment , i.e. from a technology readiness level (TRL) of 3 to a level of 6. This paper presents the new technologies and validation approach of the Thermal Loop experiment. The Thermal Loop is an advanced thermal control system consisting of a miniature loop heat pipe (MLHP) with multiple evaporators and multiple condensers designed for future small system applications requiring low mass, low power, and compactness. The MLHP retains all features of state-of-the-art loop heat pipes (LHPs) and offers additional advantages to enhance the functionality, performance, versatility, and reliability of the system. Details of the thermal loop concept, technical advances, benefits, objectives, level 1 requirements, and performance characteristics are described. Also included in the paper are descriptions of the test articles and mathematical modeling used for the technology validation. An MLHP breadboard was built and tested in the laboratory and thermal vacuum environments for TRL 4 and TRL 5 validations, and an MLHP proto-flight unit was built and tested in a thermal vacuum chamber for the TRL 6 validation. In addition, an analytical model was developed to simulate the steady state and transient behaviors of the MLHP during various validation tests. Capabilities and limitations of the analytical model are also addressed.

Effect of Variable Emittance Coatings on the Operation of a Miniature Loop Heat Pipe

As the size of spacecraft shrink to accommodate small and more efficient instruments, smaller launch vehicles, and constellation missions, all subsystems must also be made smaller. Under NASA NRA 03‐OSS‐02, Space Technology‐8 (ST 8), NASA Goddard Space Flight Center and Jet Propulsion Laboratory jointly conducted a Concept Definition study to develop a miniature loop heat pipe (loop heat pipe) thermal management system design suitable for future small spacecraft. The proposed loop heat pipe thermal management system consists of a miniature loop heat pipe (LHP) and deployable radiators that are coated with variable emittance coatings (VECs). As part of the Phase A study and proof of the design concept, variable emittance coatings were integrated with a breadboard miniature loop heat pipe. The entire system was tested under vacuum at various temperature extremes and power loads. This paper summarizes the results of this testing and shows the effect of the VEC on the operation of a miniature loop heat pipe.

Variable Emittance Materials Based on Conducting Polymers for Spacecraft Thermal Control

Ashwin‐Ushas has developed a unique, patented Variable Emittance technology based on the infrared (IR) electrochromism of unique Conducting Polymers. This has features of: very thin ( 104 cycles; low power consumption ( 104 cycles; low power consumption (< 40 μW/ cm2); and most importantly, space environment durability (space vacuum and −40 °C to + 75 °C, Solar Wind, gamma radiation to 7.6 Mrad). A demonstrator spaceflight is tentatively planned on NASA‐Goddard’s ST5 mission. This paper describes the features and current status of the technology, including results from the most recent tests. It is shown that the technology is the most promising among proposed new Variable Emittance technologies, and possibly one of the only technologies applicable to microspacecraft, besides also being applicable to large spacecraft, space bas...