knutsen@sri-unix.UUCP (04/22/84)
a013 12-Apr-84 07:34
AM-SHUTTLE-BLACKBOX
By WILLIAM J. BROAD
c.1984 N.Y. Times News Service
CAPE CANAVERAL, Fla. - The balky, 550-pound black box that the
Challenger astronauts replaced on the satellite Solar Max Wednesday
does a crucial job. It adjusts the position of the $235 million
observatory so it can point its telescopes and scientific instruments
with incredible precision as it speeds through space at more than
17,000 miles an hour.
The black box, called the attitude control module, is one of the
most advanced control systems ever built. It failed in 1980 when
three fuses, each less than half an inch long, blew.
The astronauts, Dr. George D. Nelson and Dr. James D. van Hoften,
replaced the entire attitude control unit in a historic repair
mission and fixed another system so that the solar observatory can
resume its job of photographing and analyzing the mysterious storms
and flares that erupt on the surface of the Sun.
''Not only are these attitude controls the most sophisticated of
their kind, but they are the most important system on the
spacecraft,'' said Dr. Stephen P. Maran, a scientist at the Goddard
Space Flight Center in Greenbelt, Md.
''Control is critical for all satellites,'' he added. ''They have to
have it to keep from tumbling. In addition, solar panels have to be
pointed at the Sun or you lose power.'' With Solar Max the job is
even more demanding because the satellite has to be able to track
solar flares that flash across the surface of the Sun, 93 million
miles away.
Many commercial satellites are maneuvered by the firing of jet
thrusters. But the solar observatory needs control that is hundreds
of times more accurate.
According to the scientists at Goddard who designed the Solar Max,
the large black box does the job by using electric motors to spin
precision metal wheels. These look like large gyroscopes. There are
three of them, each about 10 inches in diameter, one for each axis of
desired rotation about the 13-foot-tall solar satellite.
Although they look like gyroscopes, the wheels are different in a
critical way. A gyroscope spins steadily and imparts stability to
whatever it is attached. Ships and planes often have gyroscopes on
board to help keep them steady.
But the wheels on Solar Max spin only when the position of the
satellite needs to be changed. They exert a precise power that is
gently and accurately applied.
''The spacecraft sends out commands to those reaction wheels, which
then spins them up or down,'' explained Dr. Frank J. Cepollina, the
head of satellite servicing at Goddard. ''In the process, they impart
momentum or take it away. And the spacecraft is basically rotated
over and steered and held precisely on the target it's supposed to be
on.''
In 1980, however, an unexpected glitch came up when the fuses of the
system blew. The problem, according to Goddard scientists, lay in a
design flaw.
The fuses are big enough to carry a certain amount of electric
current. But as the Solar Max was being designed, someone increased
the circuitry and thus the electrical load in the attitude control
without increasing the size of the fuses. ''Somebody looked and
thought the fuses were big enough but they weren't,'' said Maran.
Ironically, the designers had originally considered heavier fuses,
not because of expected power flow but because the fuses are so tiny
that designers feared they could easily be damaged during
installation. In the end, the tiny fuses were used anyway.
After the reaction wheels came to a grinding halt in 1980, flight
controllers switched to a backup system that used magnetic torquers,
which are basically short bars that can be magnetized so that they
interact with the Earth's magnetic field to move the satellite very
slowly. These magnetic bars were the heros earlier this week when
they were able to stabilize the satellite after a spacewalking
astronaut accidently put the solar observatory into a violent tumble.
According to Cepollina, ''The torque bars lock on the Earth's
magnetic field and progressively, as we go through the orbit, we
apply current to those torque bars and they torque against the
Earth's magnetic field to allow us to point the spacecraft.''
Also in the black box that was replaced Wednesday were precision
star trackers, tiny telescopes that help the satellite find its
position in space. ''They look out and see the stars,'' said
Cepollina. ''By comparing the stars they should be looking at versus
the stars they are actually seeing, they in effect say 'Ahhaaa, this
spacecraft is looking here and it should be looking there.''
nyt-04-12-84 1025est