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Impact of Space Environmental Factors on Microtechnologies 69

High temperature causes adverse effects such as cracking, separation, wear-out,
corrosion, and performance degradation on spacecraft system parts and components.
These temperature-related defects may affect the electronic parts, the mechanical
parts, and the materials in a spacecraft.
Although spacecraft environments rarely expose devices to temperatures below
558C, a few spacecraft applications can involve extremely low temperatures.
These cryogenic applications may be subjected to temperatures as low as
1908C. Cryogenic environments may be experienced by the electronics associated
with solar panels or with liquid nitrogen baths used with ultrasensitive infrared
detectors. The reliability of many MEMS improves at low temperatures but their
parametric characteristics could be adversely affected. At such low temperatures
many materials strengthen but may also become brittle. MEMS at cryogenic
temperatures must be carefully selected. Evaluation testing is required for parts
where cryogenic test data are not available.
It is important to evaluate the predicted payload environments to protect the
system from degradation caused by thermal effects during ground transportation,
hoisting operations, launch ascent, mission, and landing. The thermal effects on the
spacecraft must be considered for each payload environment.
Spacecraft must employ certain thermal control hardware to maintain systems
within allowable temperature limits. Spacecraft thermal control hardware including
MEMS devices are usually designed to the thermal environment encountered on
orbit which may be dramatically different from the environments of other phases of
the mission. Therefore, temperatures during transportation, prelaunch, launch, and
ascent must be predicted to ensure temperature limits will not be exceeded during
these initial phases of the mission. 2
The temperature of the spacecraft prelaunch environment is controlled by the
supply of conditioned air furnished to the spacecraft through its fairing. Fairing air
is generally specified as filtered air of Class 10,000 in a temperature range of 9 to
3
378C and 30 to 50% relative humidity (RH). The launch vehicle also controls the
prelaunch thermal environment.
The design temperature range will have an acceptable margin that spacecraft
typically require to function properly on orbit. In addition to the temperature range
requirement, temperature stability and uniformity requirements can play an import-
ant role for conventional spacecraft hardware. The thermal design of MEMS
devices will be subject to similar temperature constraints.
For the first few minutes, the environment surrounding the spacecraft is driven
by the payload-fairing temperature. Prior to the fairing jettison, the payload-fairing
temperature rises rapidly to 90 to 2008C as a result of aerodynamic heating. The
effect of payload-fairing temperature rise may be significant on relatively low-mass
MEMS devices if they are exposed. Fairing equipped with interior acoustic blankets
can provide an additional thermal insulating protection. 2
The highest ascent temperatures measured on the inside of the payload fairing
have ranges from 278C for Orbiter to 2048C for Delta and Atlas vehicles. For space
flight missions, the thermal design for electronics is very critical since mission

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