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                    Microsystems in Spacecraft Thermal Control                      199


                    densities for future science instruments and engineering equipment on board a
                                                        2
                    spacecraft are expected to exceed 25 W/cm . Some applications, such as higher
                                                                     2
                    power lasers, may involve fluxes in excess of 100 W/cm . Advanced thermal
                    control concepts and technologies are essential to keep future payloads within
                    allowable temperature limits and to provide accurate temperature control.
                       JPL’s MEMS-based pumped liquid cooling is a mechanically pumped cooling
                    system which consists of a working fluid circulated through microchannels by a
                    micropump. Microchannel heat exchangers have been designed and fabricated in
                    silicon. The microchannels are 50 mm deep, with widths ranging from 50 to 100
                    mm. In the development stage, the heat exchangers are subjected to hydraulic and
                    thermal performance testing in simulated microspacecraft heat loads using deion-
                    ized water as the working fluid. The test data will be evaluated and used for
                    numerical thermal model validation. Optimization studies will be conducted using
                    these numerical models on various microchannel configurations, working fluids,
                    and micropump technologies.
                       The MEMS-based pumped liquid cooling is an attractive thermal control device
                    for future missions. It may be particularly beneficial for chip level applications as
                    The working fluid in the cooling loop provides efficient coupling to the hot surface
                    of the electronics, and the cooling loop provides flexibility in locating the heat sink
                    inside the spacecraft. The cooling loop provides a simple mating to semiconductor
                    surfaces through bonding techniques. MEMS cooling system can be easily inte-
                    grated with the overall spacecraft thermal control system.
                       Future spacecraft used for deep space science exploration are expected to
                    reduce in size by orders of magnitude. MEMS-based pumped liquid cooling will
                    be useful in resolving many thermally induced problems.

                    9.4.6 MEMS STIRLING COOLER
                    Stirling cooling, an active thermal control method, is theoretically able to achieve
                    the maximum efficiency in cooling. With a minimum of moving parts, a Stirling
                    cooler consists of a hermetically sealed capsule and a small amount of gas as its
                    working medium. A free piston Stirling cooler has a piston to compress the internal
                    gas and a displacer to move the gas from the cold side to the hot side, where the heat
                    is dissipated.
                       Stirling coolers have been applied in several space missions. The long-life
                    Stirling coolers, either single-stage or two-stage, are available for cooling instrument
                    detectors. The advancement of wavelength infrared and submillimeter imaging
                    instruments for space applications demand further improvement in areas of vibration,
                    electromagnetic interference, and temperature stability. A two-stage linear Stirling
                    cycle cooler has been developed for use by instruments on several Earth Observing
                    System (EOS) spacecraft. Stirling coolers will clearly be of use to many other NASA
                    programs in Earth science, astronomy, microgravity sciences, interplanetary sci-
                    ences, and the Human Exploration Initiative. These conventional coolers are
                    designed to have long mission life, high reliability, and low vibration, as well as
                    being small, light weight, and efficient. A typical cooler has a weight of about 15 kg.





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