Table of contents

Micromachining and micro-electromechanical system (MEMS) technologies can be used to produce complex structures, devices and systems on the scale of micrometers. Initially micromachining techniques were borrowed directly from the integrated circuit (IC) industry, but now many unique MEMS-specific micromachining processes are being developed. In MEMS, a wide variety of transduction mechanisms can be used to convert real-world signals from one form of energy to another, thereby enabling many different microsensors, microactuators and microsystems. Despite only partial standardization and a maturing MEMS CAD technology foundation, complex and sophisticated MEMS are being produced. The integration of ICs with MEMS can improve performance, but at the price of higher development costs, greater complexity and a longer development time. A growing appreciation for the potential impact of MEMS has prompted many efforts to commercialize a wide variety of novel MEMS products. In addition, MEMS are well suited for the needs of space exploration and thus will play an increasingly large role in future missions to the space station, Mars and beyond.

We discuss deep reactive ion etching (DRIE) as a promising technology that can be readily applied in the micromanufacturing of low-thrust propulsion systems to be used on future generations of micro- and nanosatellites. This dry processing technique permits the fabrication of high-aspect-ratio silicon structures and intricate morphologies, both with tight tolerances, in a repetitive and controllable fashion that lightweight space vehicles will exploit with the introduction of smaller thrust components for precise maneuvering and attitude control. The etching approach described herein is counted among the present state of the art techniques utilized in the current trend toward miniaturization of sensors and actuators. This trend is being propelled by the increased technological capability as the enabling factor for size reduction. Scaling laws, especially the cube-square law, can be successfully applied for obtaining macropower from microdevices manufactured with the silicon technology that has developed for microelectronics applications, including DRIE. The application of this plasma etching technique in the fabrication and testing of silicon supersonic micronozzles is also described.

This paper details the design and fabrication of millimeter-scale solid propellant rockets for one-time deployment of wireless sensor platforms, known as Smart Dust. Each microrocket assembly is an integrated system, incorporating a combustion chamber, composite propellant grain, nozzle, igniter, and thermoelectric power converter. Solid propellant is advantageous for a millimeter-scale single-use device because of its simple implementation, unlike liquid propellants, which require a more elaborate system of pumps and valves. Therefore the total system volume and complexity are minimized.

One type of combustion chamber was fabricated in silicon; however, thermal losses to the silicon sidewalls during combustion through a 1.5 mm² cross section of fuel were too high to reliably maintain a burn. Successful combustion was demonstrated in cylindrical alumina ceramic combustion chambers with thermal conductivities five times lower than silicon and cross sections of 1-8 mm². Thrusts of 10-15 mN were measured for ceramic rockets weighing under l g, with specific impulses up to 15 s.

Silicon nozzles integrated with polysilicon microheaters and thermopiles for thermal power conversion were microfabricated in a single process. Fuel ignition by polysilicon microheaters suspended on a low-stress nitride (LSN) membrane was demonstrated. Microheaters require less than 0.5 W of power to ignite a propellant composed primarily of hydroxyl-terminated polybutadiene (HTPB) with ammonium perchlorate (AP) oxidizer. They are suspended for thermal isolation through bulk post-processing by a backside deep reactive ion etch (DRIE). The etched hole beneath the igniter area also serves as a nozzle through which high-velocity combustion gases exit the rocket. Thermopiles, which generate voltages proportional to hot and cold junction temperature differentials, have been fabricated in the same process as igniters, and span backside DRIE thermal isolation cavities. Ten-junction thermopiles produced a maximum power of 20 µW. With potential temperature differences of hundreds of degrees and a total of 120 thermocouple junctions fabricated on the silicon nozzle chip, hundreds of milliwatts of power could feasibly be produced during the microrocket's flight and used to augment the Smart Dust power supply.

With the development of microspacecraft technology micropropulsion concepts are introduced for course correction and orbit insertion as well as attitude control of the microspacecraft. In this context, we have introduced a new concept of MEMS-based technology microthruster responding to the spatial constraints (volume constraints, high level of integration) and MEMS characteristics (miniaturization, low cost, mass production). The originality of these new thrusters is the use of only one solid propellant loaded in a small tank micromachined in a ceramic, glass or silicon substrate and the fabrication of arrays of N independent microthrusters in the same chip. The structure consists of a sandwich of three micromachined silicon substrates: nozzles, igniters and propellant chambers. The thrust force generated can be set from a few hundred µN to a few tens of mN by geometrical and dimensional considerations. In this paper we present the fabrication and assembly of one prototype: it is an array of 36 microthrusters that proved the technological feasibility of this new concept of small-scale thrusters. We also investigate the influence of nozzle geometry on the performances of our thruster.

Microelectromechanical systems (MEMS) techniques offer great potential in satisfying the mission requirements for the next generation of miniaturized spacecraft being designed by NASA and Department of Defense agencies. More commonly referred to as `nanosats', these spacecraft feature masses in the range of 10-100 kg and therefore have unique propulsion requirements. The propulsion systems must be capable of providing extremely low levels of thrust and impulse while also satisfying stringent demands on size, mass, power consumption and cost. We begin with an overview of micropropulsion requirements and some current MEMS-based strategies being developed to meet these needs. The remainder of the paper focuses on the progress being made at NASA Goddard Space Flight Center toward the development of a prototype monopropellant MEMS thruster which uses the catalyzed chemical decomposition of high-concentration hydrogen peroxide as a propulsion mechanism. The products of decomposition are delivered to a microscale converging/diverging supersonic nozzle, which produces the thrust vector; the targeted thrust level is approximately 500 µN with a specific impulse of 140-180 s. Macroscale hydrogen peroxide thrusters have been used for satellite propulsion for decades; however, the implementation of traditional thruster designs on the MEMS scale has uncovered new challenges in fabrication, materials compatibility, and combustion and hydrodynamic modeling. A summary of the achievements of the project to date is given, as is a discussion of remaining challenges and future prospects.

Microelectromechanical system (MEMS) technology promises to improve performance of future spacecraft components while reducing mass, cost, and manufacturing time. Arrays of microcilia actuators offer a lightweight alternative to conventional docking systems for miniature satellites. Instead of mechanical guiding structures, such a system uses a surface tiled with MEMS cilia actuators to guide the satellite to its docking site.

This paper summarizes work on an experimental system for precision docking of a `picosatellite' using MEMS cilia arrays. Microgravity is simulated with an aluminum puck on an airtable. A series of experiments is performed to characterize the cilia, with the goal of understanding the influence of normal force, picosatellite mass, docking velocity, cilia actuation frequency, interface material, and actuation strategy (`gait') on the performance of the MEMS docking system.

We demonstrate a 4 cm² cilia array capable of docking a 41.2 g picosatellite with a 2 cm² contact area with micrometer precision. It is concluded that current MEMS cilia arrays are effective in positioning and aligning miniature satellites for docking to a support satellite.

We have evaluated fluid flow effects induced by off-the-shelf mini-actuators with available microstructure flow, pressure and temperature sensors and demonstrated the feasibility of exciting new sensing approaches. Specifically, we discuss new approaches for experimental and analytical determinations of:

• sub-millisecond flow sensor response time versus flow velocity and microsensor structure,

• compact and affordable composition correction, C_V, for volumetric fluid flow sensors,

• concentration of binary mixtures, based on measurement of C_V, and

• fluid properties based on actuator-induced flow or compression, such as viscosity or γ = c_p/c_v, respectively.

The micromachined thermal flow sensors, i.e. thermal microanemometers, that we used consisted of either:

(a) off-the-shelf, front-etched microbridge sensor chips of ~1.7×1.7 mm, with bridges of ~0.2×0.25 mm, or

(b) developmental, very rugged, Microbrick^TM sensor chips of equal size but without the etched cavities.

For actuators, we used commercially available, 10-12 mm OD, membrane-based, low-cost, earphone speakers, with resonances in the 2 kHz region.

We found the useful operating frequency range of both sensors and actuators, with due consideration to resonance effects, to be in the 40-100 Hz range and the one most free of disturbances for the actuators used. The flow sensors themselves showed the capability of operating beyond 500 Hz, especially the rugged version, which showed response times down to ~0.2 ms. This Microbrick^TM sensor is burst-proof and designed for operation in harsh environments featuring gas or liquid mass fluxes up to 500 g cm^-2 s^-1, with condensible vapors and suspended sand or dust.

With the above devices we demonstrated a new approach for on-line fluid flow sensor composition correction, which is needed to correct errors caused by fluid composition changes. Previously developed, time-consuming and costly composition correction for thermal flow sensors relied on either individual calibration or via measurement of thermal conductivity, specific heat and Prandtl number. Those methods can now be replaced by this one-step, on-line, low-cost, actuation-based normalization, which can be adapted as well to other flow sensing technologies, such as orifice flow sensors.

Using the same mini-actuators to induce flows in laminar flow restrictors, we also report on the demonstration of a very compact and affordable approach to the measurement of viscosity, which is a coveted gaseous fuel property for feed-forward combustion control. The demonstration included the design and fabrication of associated circuitry to prove satisfactory operation after temperature cycles, shock and vibration, and to provide an accurate, temperature-compensated output, despite changes in supply voltage, gas pressure or temperature.

Rockwell Science Center (RSC) has designed and implemented a microelectromechanical-system- (MEMS-) based radio frequency switch experiment in a miniature satellite format (picosat) as an initial demonstration of MEMS for space applications. This effort is supported by DARPA-MTO, and the mission was conducted with Aerospace Corporation and Stanford University as partners. MEMS surface-micromachined metal contacting switches were manufactured and used in a simple, yet informative, experiment aboard the miniature satellites to study the device behavior in space, and its feasibility for space applications in general. Communication links between multiple miniature satellites, as well as between the satellites and ground, were also achieved using communications circuits constructed and provided by RSC. Details of both the MEMS and radio communications and networking efforts will be discussed in this paper.

A light and fast two-axial fine-pointing mirror has a number of space applications, especially in intersatellite optical links. The fine pointing of laser beams in optical links is currently realized with electromagnetic or piezoelectric actuators, which are relatively large and heavy. Micro-electro-mechanical system technology bears a high potential in space applications, offering a reduction in device size, mass and power consumption. Microtechnology facilitates batch mode fabrication, yielding a low cost per unit. VTT Automation has designed and partially tested a silicon micromachined electrostatically actuated two-axial mirror, which can be controlled with microradian resolution and large bandwidth over the angular range of ±3 mrad.

In this paper, design concepts of reconfigurable and electronically steered antennas based on a new fractal antenna and RF-MEMS devices are presented. As modern telecommunications extend towards higher frequencies, the advantages of employing RF-MEMS switches, phase shifters, and miniaturized fractal antennas become more significant. The input characteristics of the Hilbert curve fractal antenna can be made frequency agile by incorporating RF switches along its length. In addition, due to the large number of connected segments in this antenna geometry, reconfigurable radiation characteristics can be obtained by adding just a few additional line segments to interconnect these through semiconductor or RF-MEMS switches. The beam peak direction can be shifted by 63° and the beam width can be changed by up to 25° by this approach. An electronically steered antenna with micromachined phase shifters using tunable ferroelectric barium strontium titanate thin film is also discussed. These MEMS-based antenna systems find applications in communications satellites and electronically scanned arrays for space-based radars.

A distributed microelectromechanical system (MEMS) phase shifter has been fabricated on a high-resistivity silicon substrate using a microstereolithography technique. This technique enables us to decrease the early development cost by removing the need for photomasks used for defining the MEMS bridges. The distributed MEMS phase shifter fabricated consists of a high-impedance coplanar waveguide transmission line and ten MEMS bridges of 4.5 µm height, which gives a phase shift of 6° at 18 GHz with a bias voltage of 30 V. This MEMS bridge design can be improved further using the microstereolithography technique to give a phase shift of nearly 90° at 18 GHz with a bias voltage of 20 V. The fabrication procedure described here shows significant promise for the use of microstereolithography techniques for RF MEMS devices for ground and space telecommunication applications.

Inflatable and other membrane structures are expected to become increasingly important in space exploration due to their light weight and low cost. Unlike rigid structures, these structures are typically fabricated of flexible polymers and require internal pressurization to achieve structural integrity. Due to this, inflatable structures are vulnerable to the harsh space environment and catastrophic failure from structural vibration. A MEMS-based health monitoring and control system (HMCS) for space inflatables has been developed at the University of Arkansas. Fabricated mostly from polymeric materials, the HMCS is lightweight, flexible and can be attached directly to the external surface of an inflatable to provide health monitoring. Structure-wise, the HMCS is a three-dimensional multichip module with a sensor layer at the top, a common polyimide substrate in the middle and an actuator layer at the bottom. The sensor layer consists of an interconnected network of MEMS sensors for monitoring the environmental conditions around the inflatable and also the structural vibration of the inflatable. The actuator layer, fabricated from electroactive polymers, provides a two-dimensional shape control capability to the HMCS. When operated with strain and vibration sensors in the sensor layer, the polymer actuator can deform the surface contour of the inflatable to remove `wrinkles' and dampen structural vibration.

The PDF file provided contains web links to all articles in this volume.

The past decade has seen two key developments that will have a lasting impact on the character of space missions.

(i) The 'faster-better-cheaper' initiative adopted by the National Aeronautics and Space Administration (NASA), with its goal to deliver programs with higher value at lower expense, shorter turn-around times, and increased safety.

(ii) The rapid advances in microelectromechanical systems (MEMS), which promise the integration of a multitude of functions in sensing, actuation, computation, and communication into microscopic components.

This special issue on Space Applications for MEMS shows the synergy between these two areas and demonstrates, with contributions provided from an extensive list of researchers, the potential of MEMS components and devices in a new generation of smaller, lighter, and more intelligent satellites. But let us first clarify some naming conventions from each respective field, which might otherwise cause some confusion. MEMS are micro-electromechanical systems, i.e., devices at a typical scale from hundreds down to fractions of micrometers. Thus, micro stands for one millionth of a meter. On the other hand, when referring to micro-satellites, the aeronautics and astronautics community usually denotes systems less than 100 kg mass. Furthermore, satellites below 10 kg are often dubbed nano-satellites, and so-called pico-satellites comprise less than 1 kg mass. In summary, the terms micro-satellite, nano-satellite, or pico-satellite are somewhat informal denominations.

As recent research efforts have shown, MEMS technology can be the basis for spacecraft components with improved performance in vastly reduced volume, by compressing increased functionality into single microchips. Applications span from propulsion and navigation to scientific instrumentation and communications. Arguably, in no other field is reduction of size and mass as important as in space, where lower mass or power consumption can translate into huge cost savings. For example, the launch cost is a significant factor in every space mission. To haul one kilogram out of Earth's gravity well, approximately 63 MJ of energy are needed. While this number in itself is not `astronomical', the cost for this task is high because of the specialized and sophisticated launch vehicle required. To put 1 kg into low earth orbit (LEO) currently costs approximately $20~000. To reach higher orbits or even interplanetary space presents significantly higher costs. These numbers give an indication that MEMS technology, with its potential to shrink complex instruments into a single microsystem, is extremely attractive.

This special journal issue starts with a thorough and up-to-date overview of the state of the art in MEMS, written by Jack Judy. It is followed by a discussion of the recently developed MEMS-specific processing technique of deep reactive ion enhanced (DRIE) etching and its application to the fabrication of microthrusters. The four subsequent papers also address micro propulsion, during free flight as well as during docking maneuvers. Sensors and actuators for in-flight monitoring are discussed in the following contribution, followed by four papers on MEMS-based communications systems in space. The final paper proposes microsystems to support a novel inflatable satellite architecture.

I would like to thank all the authors for their hard work, their excellent contributions, and their patience with the refereeing process. I would also like to express my gratitude to the anonymous reviewers who gave many valuable comments and suggestions, ensuring and improving the quality of this special issue. Finally, my special thanks go to Mary Ann Romig who handled the administrative parts of the editorial process.