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Microsystems in Spacecraft Thermal Control 191

control can be achieved passively (i.e., a given temperature can be achieved) by the
a/« of the surfaces. This analysis must be repeated for all conditions, as a space-
craft’s thermal environment (and internal load) will typically change as it moves
through its orbit.

9.4 MEMS THERMAL CONTROL APPLICATIONS

The use of nano- and picosatellites in present and future space missions require a
new approach to thermal control. Small spacecraft have low thermal capacitance,
making them vulnerable to rapid temperature fluctuations. At the same time, many
traditional thermal control technologies, such as heat pipes, do not scale well to
meet the constrained power and mass budgets of smaller satellites. MEMS are well
suited for applications in small spacecraft; they are lightweight, rugged, reliable,
and relatively inexpensive to fabricate. 5
The first MEMS experiments have flown on Space Shuttle Mission STS-93 in
1999 to evaluate the effect of exposure to the space environment on the MEMS
materials. During the STS-93 flight, MEMS experiments were carried in the shuttle
middeck locker. These experiments examine the performance of MEMS devices
under launch, microgravity, and reentry conditions. These devices included
accelerometers, gyros, and environmental and chemical sensors. These MEMS
experiments provide in-flight information on navigation, sensors, and thermal
control necessary for future small scale spacecraft. Spacecraft MEMS thermal
control applications are emerging with the Department of Defense (DoD), NASA,
academia, and aerospace industry as major contributors in research and develop-
ment. Most MEMS thermal control applications are developmental with technology
readiness levels (TRL) up to TRL 6. Several potential applications for MEMS
devices in thermal control are described below.

9.4.1 THERMAL SENSORS
At the present, many conventional MEMS thermal devices have been designed and
used as thermal sensors. 6,7 MEMS thermal sensors are transducers that convert
thermal energy into electrical energy. They are devices that measure a primary
thermal quantity: either temperature or heat flow or thermal conductivity. One
technique is to take advantage of the difference in the coefficients of thermal
expansion between two joined materials. This causes a temperature-dependent
deflection, creating stress on a piezoelectric material and generating an electrical
signal or actuating a switch. A good example for such a MEMS thermostat or
thermal switch is the Honeywell Mechanically Actuated Field Effect Transistor
(MAFET) 1 technology. 8,9 The MAFET is a microthermal switch that is low cost,
2
of small size (< 3.0 mm ), and has a long operational life (1,000,000 cycles).
Unlike typical thermal switches, this device uses electronic switching, thus elimin-
ating the arcing and microwelding that occur while making or breaking metal-to-
metal contact. The MAFET thermal switch uses fundamental MEMS processing
technology. The heart of the thermal switch is a temperature-sensitive deflecting

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