Shuttle Set for International Mission

SPACE shuttle Discovery is poised for launch tomorrow on an unprecedented international mission. Space agencies in Canada, Europe, Japan, and the United States are joining in a set of experiments that take advantage of the near-weightlessness aboard the first International Microgravity Laboratory.

The first in a series of such missions, it involves more than 200 scientists in 16 countries.

One of the most intriguing experiments will ride in a small canister outside the astronaut-tended laboratory: a refrigerator that uses a different physical process than does the refrigerator in your kitchen. It uses sound waves for cooling and doesn't need the ozone-destroying chlorofluorocarbons (CFCs) used in most refrigerators and air conditioners. This Space Thermoacoustic Refrigerator (STAR) points toward a new possibility for getting needed cooling when CFCs are phased out internationally eight yea rs from now.

The National Aeronautics and Space Administration (NASA) has planned this mission in response to continued demand from Congress, industry, and scientists for research - without waiting for the space station to become operational - on the behavior in space of various non-living and living materials, including humans, under microgravity conditions.

Mission scientists talk about microgravity rather than weightlessness because small disturbances from spacecraft thrusters and other sources prevent a true weightless condition. To minimize these disturbances, Discovery will assume a tail-down orientation while it travels a circular orbit 302 kilometers (188 miles) high and inclined 57 degrees to the Equator.

Mission Commander Ron Grabe and his six-astronaut crew will be studying one another - gathering data on adaptation to weightlessness, needed to prepare for future extended spaceflights.

Meanwhile, Steven Garrett and Thomas Hofler at the Naval Postgraduate School in Monterey, Calif., will be awaiting the results of their STAR refrigerator experiment. It is only a decade since the late John Wheatley at the Los Alamos National Laboratory in New Mexico discovered that sound waves can be used to drive heat pumps. Now Drs. Garrett and Hofler are building on that discovery to try to develop a refrigerator to use under weightless conditions in space where the normal refrigeration vapor expans ion process doesn't work well.

In this system, a modified loudspeaker, vibrating at frequencies between 350 and 450 Hertz, drives a nickel bellows to set up a standing sound wave in a helium/xenon gas mixture. In such a wave, regions of gas compression and expansion remain in the same place. An expansion region is a region of cooling which can be used for refrigeration. This STAR system has cooled to as much as 80 degrees C below room temperature in ground tests. Now the experimenters want to see what it will do in space.

Hofler says it's too early to know whether or not this process could be used for commercial refrigerators. The question, he says, is "will it be efficient enough" to be economically attractive.

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