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200 MEMS and Microstructures in Aerospace Applications

As the spacecraft size reduced, the scaling in heat transfer is prominent and
solving high power heat transfer problems within small spacecraft becomes rather
difficult. The rapidly expanding capabilities of semiconductor processing in gen-
eral, and microsystems packaging in particular, present a new opportunity to extend
cooling to the MEMS domain. Several MEMS-based active cooling systems to
support future small spacecraft missions have been suggested.
Nakajima and his team have demonstrated a micro Stirling cycle engine having
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a high thermal efficiency. The tiny gas filled engine can be operated with any heat
source and can be driven in reverse to make an active cooling system. Thermo-
mechanical actuators generally require the removal of heat energy to reestablish the
previous condition. Because heat dissipation is directly related to the volume to be
cooled, thermal cycling occurs much faster in microdevices than in macrodevices.
In parallel, NASA Glenn Research Center (GRC) has developed a MEMS
device for active cooling and temperature control. 21 This active cooling device is
aimed for future nano- or microsatellite missions with predicted efficiencies that are
an order of magnitude better than current and future thermoelectric coolers. 22
GRC’s MEMS-based device uses a Stirling thermodynamic cycle to provide cool-
ing or heating directly to a thermally loaded surface. The device can be used strictly
in the cooling mode or can be switched between cooling and heating modes in
milliseconds for precise temperature control. Fabrication and assembly employ
techniques routinely used in the semiconductor processing industry. Benefits of
the MEMS cooler include scalability to fractions of a millimeter, modularity for
increased capacity and staging to low temperatures, simple interfaces, limited
failure modes, and minimal induced vibration.
A working model of a MEMS cooler device has been assembled and tested at
the JHU/APL for MEMS regenerator performance. This 1-by-1-cm regenerator was
fabricated for NASA by Polar Technologies Commercial. Piezoelectric actuators
(non-MEMS) are used to drive the compression and expansion diaphragms, which
are the only moving parts of the device. The diaphragms are deflected toward and
away from the regenerator region in phase-shifted sinusoidal fashion to produce the
Stirling cycle.
NASA GSFC is developing a small, innovative instrument, LEISA, that will
incorporate a miniature cooler. To perfect the cooler technology, GSFC works with
commercial cooler vendors on long life, low vibration miniature coolers. The
reduction of vibration is a significant objective for cooler technology because
commercial coolers presently have unacceptably large vibration which can ser-
iously disrupt sensor readings. GSFC has specified the changes required to allow
a commercial cooler to be used with the existing GSFC vibration control system.
The goal at GSFC is to develop a lightweight, low-cost cooler which will meet the
requirement of small satellites.

9.4.7 ISSUES WITH A MEMS THERMAL CONTROL
While controlling the temperatures of other spacecraft components, MEMS-based
TCS also need to be maintained at proper temperature range. Given their small size,

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