[Illustration: _Cutaway view of the inside of the Telstar I
satellite, showing the electronics canister covered with its
protective blanket of many layers of Mylar. To control temperature,
shutters automatically open all the way if the canister gets hotter
than 80°F, close completely if it goes down to 50°F._]
shutter (closed)
electrical heat transfer
insulation blanket
heat transfer by radiation
shutter (open)
solar cells
electronic chassis
heat transfer by circulation
thermal control mechanism
nylon lacing
microwave antennas
frame
In designing the Telstar satellite, both its internal and external
temperatures had to be controlled. The electronics canister inside the
satellite operates best if it stays at approximately room temperature of
65 to 75°F. This much heat is supplied in the canister by dissipation of
electrical energy from the solar cells. The container is well insulated
to keep its temperature relatively stable, and it has shutters that open
automatically if it begins to get overheated (_see above_). The
operating characteristics of the solar cells on Telstar’s surface also
had to be considered; they work better at rather cool temperatures. So
we decided to keep the satellite’s skin at an average temperature of
about 0°F, although temperatures actually will range quite a bit above
and below the average as the satellite moves from sun to shadow.
Now, using this average temperature of 0°F (converted to 460°R) as _T_
in our formula, we can solve for α/ε. We find that this gives us a ratio
of approximately 0.7 for the satellite’s surface. However, this presents
a problem. Almost 40 per cent of Telstar’s surface is taken up by its
power plant of 3600 sapphire-covered solar cells. These cells,
unfortunately, have a relatively high α/ε ratio—their α is 0.8 and their
ε is 0.54, for an α/ε of 1.5. This means that the portion of the surface
not used by either solar cells or antenna openings must, in order to
give us an over-all average of about 0.7, have a very low α/ε ratio—less
than 0.3.
To get this sort of ratio, we had to select carefully the material for
the outer surface of the Telstar satellite. There were many kinds of
surfaces that might have been used; they could have been metal or
non-metal, rough or smooth, shiny or dull. And they could have been any
color from black to white. However, to get a 0.3 ratio we needed
something with a relatively high emissivity for the low-frequency
electromagnetic radiation that the satellite emits and a rather low
absorptivity for the high-frequency radiation coming from the sun. High
emissivity meant that we should use a nonmetal surface rather than
polished metal, since the emissivity of nonmetals is quite high at the
temperatures in which we were interested, while that of polished metals
is relatively low. And, to get low absorptivity, we decided that the
color of these surface areas should be very close to a pure white.
[Illustration: _Partially molten aluminum oxide particles being
sprayed onto aluminum outer surface panels._]
There were several substances that met our requirements. After testing a
number of them, we decided to use aluminum panels coated with a thin
layer of aluminum oxide (Al₂O₃). This coating is very pure, hard, and
stable, and we left it rough to minimize changes due to meteoroid
abrasion. Its α/ε ratio is 0.24. The aluminum oxide coating can be
applied by means of the plasma jet process—particles of aluminum oxide
are heated to a partially molten state, mixed with gases, and then
sprayed onto the cleaned, pre-coated aluminum panels (_see illustration
above_).
Using this carefully selected outer surface has helped solve the
temperature-control problem. Since Telstar has been in orbit its
internal and skin temperatures have kept well within the ranges we
wanted them to. Thus you can see how some basic formulas from classical
physics helped us choose the right material for the satellite’s
surface—and even what color it should be. The blue-and-white checkered
appearance that Telstar I finally took on was no accident—it was the
result of carefully combining various colors and materials in just the
right amounts to obtain the temperature balance we needed.
Peter Hrycak _was born in Przemysl, Western Ukraine, and received a
B.S. in 1954, an M.S. in 1955, and a Ph.D. in 1960 from the University
of Minnesota. He joined Bell Telephone Laboratories in 1960, and has
worked on low temperature refrigeration problems and thermal design
and thermal testing of the Telstar satellite._
CASE HISTORY NO. 3
How Can We Make Optical Measurements on a Satellite?
Jeofry S. Courtney-Pratt
_Physicist—Head, Mechanics Research Department_