Beyond the orbit of Mars, the average temperature seen by a spacecraft
falls below the normal operating temperature range of the spacecraft's
electronics. Presently, spacecraft operating in the cold environment
of deep space carry onboard a large number of Radioisotope Heating
Units to maintain an operating temperature for the electronics
of approximately 300 K. This is not an ideal solution because
the Radioisotope Heating Units are always producing heat, even
when the spacecraft is already too hot, thus requiring an active
thermal control system for the spacecraft. Those spacecraft with
sufficient power reserves to use electrical heaters experience
problems restarting after a hibernation period during which the
electronics cool significantly below 70 K.
Another approach to problems associated with the cold operating
environment of deep space is to use cryogenic electronics to operate
the spacecraft at the ambient temperature. This approach has several
advantages: semiconductor devices are more efficient at lower
temperatures than at room temperature; high-temperature superconductors
can be integrated with semiconductors to reduce losses; and a
simpler (lighter and cheaper), passive, heat-rejection system
can be employed. It has one significant challenge: cryogenic power
generation and storage.
NASA Lewis Research Center is developing the enabling technologies
for a cryogenic power system in conjunction with the NASA Jet
Propulsion Laboratory (JPL) and universities. In addition, creative
processes that could expand industrial participation in this program
are being explored, such as the Small Business Innovation Research
Program.
Through a National Research Council Fellowship and a NASA Graduate
Student Research Fellowship, demonstrations of two key technologies
have been performed: high-temperature superconductor components
and cryogenic compound semiconductor switch technology. A dc-dc
converter for low-temperature operation was designed, built, and
characterized with commercial, off-the-shelf components and a
custom-built superconducting inductor. A High Electron Mobility
Transistor switch was designed, fabricated, and characterized
at low temperatures. High Electron Mobility Transistor structures
have the potential to handle high current loads at cryogenic temperatures.
Although many silicon power devices that operate from room temperature
to liquid nitrogen temperature (77 K) were used at NASA Lewis
to build dc-dc converters which successfully operated over that
temperature range, none of the commercial silicon or germanium
transistors tested operated successfully significantly below 77
K. Therefore, compound semiconductor devices with the potential
to operate over the temperature range of interest to deep-space
missions (30 to 60 K) are being investigated by NASA Lewis.
Previous articleLast updated April 30, 1997
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