The third part of our problem had to do with finding the best ways to
mount and protect the cells on the Telstar satellite itself. Since a
satellite’s solar power plant usually has several thousand cells, we
find it best to mount the cells in groups, or modules. These can be
pretested as a unit after individual interconnections have been made.
For Telstar I, we decided to mount the 3600 solar cells in 12-cell
modules like those shown in the _figure below_.
[Illustration: _The satellite uses 300 modules of twelve solar
cells, in groups of six or three modules per facet._]
[Illustration: _Lengthwise diagram of a solar cell module, showing
how individual cells were fixed in place._]
Each of the cells has a top contact along one edge and a bottom contact
all over its base, so we were able to assemble the 12-cell groups like
shingles, with the bottom edge of one cell covering the top edge of the
next, leaving only the active area of each cell exposed. But this meant
that each module would be over four inches long and only 14 thousandths
of an inch thick—far too weak to withstand stress and vibration. To
support the cells, we decided to mount them on a metallized ceramic
base. But this presented a problem: If the cells were soldered directly
to the base, the different thermal expansion rates of the silicon and
the ceramic would cause the structure to break during the cycles of
extreme changes in temperature that Telstar would pass through. We
remedied this by connecting each cell to the ceramic support by a thin
U-shaped strip of silver (_see above_). Since silver has a much higher
thermal expansion coefficient than silicon, we added tiny sandwiches of
Nilvar or Invar (36% nickel, 64% iron) where the cells were attached.
With this mounting method, the cell modules withstood thermal and
mechanical shocks much more severe than those they would undergo in
actual use. In one test, for instance, an entire cell module with its
cover plates was first dipped in hot water, then plunged into liquid
nitrogen at a temperature of -195° Centigrade. In orbit, the temperature
range for the satellite was not expected to be more than from +80° to
-100°C, with a rate of change of no more than three degrees a minute.
Finally, we needed to find the right kind of transparent protective
cover for the Telstar solar cells, both to keep micrometeorites from
damaging the sensitive and very thin diffused layer and to slow down the
incoming electrons to nondestructive energy levels. For micrometeorite
protection, only a thin layer of hard transparent substance was needed;
for electron protection, the cover plates should have a mass of no less
than 0.3 gram per square centimeter (as we explained above). And there
were two other important considerations: The material we used should not
be darkened or discolored by prolonged exposure to ultraviolet
radiation, and it should have good thermal conductance, so that some of
the heat absorbed by the solar cells could be conducted out to the cover
plates and re-radiated. All these requirements led us to the choice of
clear, man-made sapphire. Although sapphire is more expensive and
difficult to make than the equivalent quartz or glass, it only has to be
30 mils (three hundreds of an inch) thick. Twice this thickness would be
required if quartz or glass were used.
We have had space to describe only a few of the things involved in
designing a solar cell power plant that would work unattended out in
space. We have not mentioned a good many of the tough problems that had
to be worked on. But we are glad to report that we could find answers to
almost all our questions. And the most significant answer is shown in
_the figure below_, where you can see how the Telstar I solar power
plant slowly diminished in power almost exactly as we predicted it
would.
[Illustration: _Very gradual decay due to radiation effects of the
Telstar I solar cell plant in the first months after the satellite
went into orbit; it was extremely close to the predicted rate (solid
line)._]
Kenneth D. Smith _was born in Galesburg, Illinois, and received a B.A.
from Pomona College in 1928 and an M.A. from Dartmouth College in
1930. He joined Bell Telephone Laboratories in 1930, and has worked on
the development of proximity fuzes, radar bombing systems, broadband
microwave radio systems, and various semiconductor devices, including
radiation-resistant solar cells for the Telstar satellite._
CASE HISTORY NO. 5
Would Time Delay Be a Problem in Using a Synchronous Satellite?
Peter D. Bricker
_Psychologist—Member of Staff, Behavioral Research Laboratory_