John L. Champion

MEMS and Microstructures in aerospace applications

OVERVIEW OF MICROELECTROMECHANICAL SYSTEMS AND MICROSTRUCTURES IN AEROSPACE APPLICATIONS Robert Osiander and M. Ann Garrison Darrin VISION FOR MICROTECHNOLOGY SPACE MISSIONS Cornelius J. Dennehy MEMS FABRICATION James J. Allen IMPACT OF SPACE ENVIRONMENTAL FACTORS ON MICROTECHNOLOGIES M. Ann Garrison Darrin SPACE RADIATION EFFECTS AND MICROELECTROMECHANICAL SYSTEMS Stephen P. Buchner MICROTECHNOLOGIES FOR SPACE SYSTEMS Thomas George and Robert Powers MICROTECHNOLOGIES FOR SCIENCE INSTRUMENTATION APPLICATIONS Brian Jamieson and Robert Osiander MICROELECTROMECHANICAL SYSTEMS FOR SPACECRAFT COMMUNICATIONS Bradley Gilbert Boone and Samara Firebaugh MICROSYSTEMS IN SPACECRAFT THERMAL CONTROL Theodore D. Swanson and Philip T. Chen MICROSYSTEMS IN SPACECRAFT GUIDANCE, NAVIGATION, AND CONTROL Cornelius J. Dennehy and Robert Osiander MICROPROPULSION TECHNOLOGIES Jochen Schein MEMS PACKAGING FOR SPACE APPLICATIONS R. David Gerke and Danielle M. Wesolek HANDLING AND CONTAMINATION CONTROL CONSIDERATIONS FOR CRITICAL SPACE APPLICATIONS Philip T. Chen and R. David Gerke MATERIAL SELECTION FOR APPLICATIONS OF MEMS Keith J. Rebello RELIABILITY PRACTICES FOR DESIGN AND APPLICATION OF SPACE-BASED MEMS Robert Osiander and M. Ann Garrison Darrin ASSURANCE PRACTICES FOR MICROELECTROMECHANICAL SYSTEMS AND MICROSTRUCTURES IN AEROSPACE M. Ann Garrison Darrin and Dawnielle Farrar INDEX

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.

MEMS-based resonating xylophone bar magnetometers

A novel magnetometer which utilizes the Lorentz force to measure vector magnetic fields has recently been described. The device, based on a classical resonating xylophone bar, has an extremely wide dynamic range and is ideally suited to miniaturization using a variety of technologies. The overall dimensions of the xylophone bar are limited by the width of the nodal supports which act as current electrodes and ultimately govern the resonance qualities. Minimum xylophone lengths of 10 and 5 mm mare attainable by electrostatic discharge machining and chemical milling of metal foils, respectively. Significantly smaller devices are achievable using polycrystalline silicon-based MEMS processing. However, the sheet resistivity of the silicon restricts the drive current through the xylophone bar and thus limits the sensitivity. This sensitivity can potentially by regained by replacing the silicon xylophone bar with a metal/piezoelectric/metal sandwich structure.

Polysilicon xylophone bar magnetometers

The recently developed JHU/APL magnetometer, which is based on a free-free (xylophone) resonating bar, is simple, small, light weight, has a low power consumption and utilizes the Lorentz force to measure vector magnetic fields. The device is intrinsically linear and has a wide dynamic range such that it can measure magnetic field strengths from nanoteslas to teslas. Furthermore, its sensitivity is independent of size for resonating bars of the same material and aspect ratio. This makes it ideally suited for miniaturization using MEMS techniques. Various polysilicon xylophone bars have been designed, processed, and characterized. The output response has verified the size-independent scaling law and sensitivities of the order of 100 nanoTesla have been achieved with drive currents as low as 20 microamps. This drive current is limited by the sheet resistance of the polysilicon support electrodes and directly affects the sensitivity. The electrodes also have a dramatic effect on the resonant frequency since they act as torsional stiffening members on the resonating bar. For example, for a 500 X 50 micron xylophone the resonant frequency varies from the designed 69 kHz to over 95 kHz for 10 micron wide support electrodes. The electrodes do not affect the mechanical Q-factors observed and values in excess of 20,000 at reduced pressures have been routinely obtained.

A micromachined flat plasma spectrometer (FlaPS)

Through the application of a new approach to energy analysis to microelectromechanical systems (MEMS), the Flat Plasma Spectrometer (FlaPS) presented here provides a solution to the investigation of plasma distributions in space. It is capable of measuring the kinetic energy and angular distributions of ions/electrons in the space environment for energies ranging from a few eV to 50keV. A single pixel of a FlaPS instrument has been designed, built and tested to occupy a volume of approximately one cubic centimeter, and is characterized by a high throughput-to-volume ratio, making it an ideal component for small-scale satellites. The focus of this paper is on the design, fabrication, simulation, and testing of the instrument front end that consists of a collimator, parallel plate energy analyzer, and energy selector mask. Advanced micro-fabrication techniques enable fabrication of the miniature plasma spectrometer with geometric factor 4.9x10-5 cm2-sr per pixel and an entrance aperture area of 0.01cm2. Arrays of narrow collimator channels with 4° angular divergence and high transmission allow energy analysis of ions/electrons without the need for focusing, the key feature that enables large mass reduction. It is also shown that the large plate factors achievable with this approach to energy analysis offers definite advantages in reducing the need for excessively high voltages.

Thermal stressing techniques for flaw characterization with shearography

Controlled heating of a test specimen with a laser source provides several advantages for flaw detection using shearographic detection. This stressing method is non-contacting, can be localized, and allows defect information to be obtained while heating. In addition, the beam profile can be tailored to aid in the detection of different defect types. This paper presents results of simultaneous observations of material response to an applied thermal load using both TRIR and shearographic detection methods. Of particular importance is the demonstration that the depth of a defect can be determined by measuring the time-dependence of the shearographic fringe development during heating.

Localized temperature control of microwave-assisted cure in SCRIMP-manufactured composites

This paper describes current efforts to apply spatially and temporally localized microwave processing techniques to ensure uniformity of material properties in polymer composite materials. In large polymer composite structures, high temperatures caused by exothermic resin cure can degrade the mechanical properties of the composite. In this work, resin cure temperature data was obtained during microwave processing from a series of thermocouples embedded at various lateral locations relative to the microwave source and uniformly through the thickness of the composite structure. Using this temperature information, the potential for localized microwave-accelerated cure to reduce the occurrence of material degradation from resin over-temperature was evaluated.

Analysis of Time-Resolved Shearographic Methods with Controlled Thermal Stressing

Electronic shearographic interferometry is a nondestructive evaluation (NDE) technique in which qualitative detection of subsurface defects is readily achieved. In both industrial and laboratory environments, various full field stressing methods, including vibration, vacuum, thermal and mechanical loading, have been employed to produce characteristic deformations which can be monitored shearographically [1,2]. However, quantitative measurements of parameters such as defect depth are difficult to make with these techniques. This paper presents the results of using controlled thermal stressing with shearography in an effort to expand the quantitative capabilities of the technique. The use of controlled thermal-stressing allows a totally noncontact inspection technique with a large standoff distance to monitor the time-dependent deformations of test specimens. Typically laser power levels of tens of milliWatts are sufficient to generate measurable deformations.