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68 MEMS and Microstructures in Aerospace Applications

expectancies of missions have continued to grow over the years from 6 months on
early TIROS weather project to the current requirements of 30 years for the
International Space Station (ISS). The Telstar 1 launched in 1962 had a lifetime
of 7 months compared to Telstar 7 launched in 1999 with a 15þ year life expect-
ancy. Albeit, the earlier Telstar weighed in at only 78 kg and cost US $6M
compared to the 2770 kg Telstar 7 at a cost of US $200M. The geostationary
operational environmental satellites (GOES) carry life expectancies greater than 5
years while current scientific satellites such as TERRA and AQUA have life
expectancies greater than 6 years. Military-grade satellites such as Defense Satellite
Communication System (DSCS) have design lives greater than 10 years.
To assure long-life performance, numerous factors must be considered relative
to the mission environment when determining requirements to be imposed at
the piece part (MEMS device) level. The high reliability required of all space
equipment is achieved through good design practices, design margins (e.g., de-
rating), and manufacturing process controls, which are imposed at each level of
fabrication and assembly. Design margins ensure that space equipment is capable of
performing its mission in the space environment. Manufacturing process controls
are intended to ensure that a product of known quality is manufactured to meet the
design requirements and that any required changes are made based on a documented
baseline.
MEMS fall under the widely accepted definition of ‘‘part’’ as used by NASA
projects; however, due to their often multifunctional nature, such as electrical and
mechanical functions, they may well be better understood and treated as a com-
ponent. The standard NASA definitions are:

. Part — One piece, or two or more pieces joined, which are not normally
subjected to disassembly without destruction or impairment of designed use.
. Component — A combination of parts, devices, and structures, usually self-
contained, which performs a distinctive function in the operation of the overall
equipment.
. Assembly — A functional group of components and parts such as an antenna
feed or a deployment boom.
. Subsystem — The combination of all components and assemblies that com-
prise a specific spacecraft capability.
. System — The complete vehicle or spacecraft made up of the individual
subsystems.

4.2 MECHANICAL, CHEMICAL, AND ELECTRICAL STRESSES

4.2.1 THERMAL MECHANICAL EFFECTS
Spacecraft may receive radiant thermal energy from two sources: incoming solar
radiation (solar constant, reflected solar energy, albedo) and outgoing long-wave
radiation (OLR) emitted by the Earth and the atmosphere. 1

© 2006 by Taylor & Francis Group, LLC