ROD@SU-AI.ARPA (04/11/84)
From: Rod Brooks <ROD@SU-AI.ARPA>
n089 1719 10 Apr 84
AM-SHUTTLE-EXPLAIN
By WILLIAM J. BROAD
c.1984 N.Y. Times News Service
CAPE CANAVERAL, Fla. - After a free-flying astronaut failed to
capture the ailing Solar Max satellite and the shuttle's robot arm
dramatically succeeded, space agency officials have been pointedly
asked why they picked man over machine in the first place. Was it
mere grandstanding?
Space agency officials say it was anything but that. They point to a
surprising chain of events involving unexpected risk, sheer luck and,
in the end, calm determination and self-assurance.
The risk came when Dr. George D. Nelson, flying free in space 200
feet from the shuttle, failed to latch onto the crippled satellite
and made matters worse by putting it into an end-over-end tumble as
he repeatedly bumped it with the docking device.
The luck came when the satellite's equipment performed much better
than anyone expected to reduce its wobble, spin and tumble. And quiet
confidence came when ground simulations showed it would be easy to
capture the stabilized satellite with the robot arm.
According to John Cox, the flight director in Houston, the ''gold
star'' goes to the scientists on the ground who succeeded better than
anyone expected in halting the satellite's gyrations.
The reason for planning a manned rescue in the first place was that
the satellite was spinning too fast for the robot arm, at one
revolution every six minutes. Flight controllers were afraid to slow
down the satellite with its on-board backup control system because
simulated tests on the ground had suggested that such a maneuver
could also increase the rate of tumble.
Cox said at a news briefing that before the rescue attempt,
simulations had shown that slowing the spin that way could increase
the wobble ''to about 40 or 50 degrees.''
That kind of wobble, he said, is what doomed the rescue attempt with
the robot arm Sunday. With the satellite's outspread solar panels
flailing about, Capt. Robert L. Crippen of the Navy, the space
shuttle's commander, had to keep the shuttle popping up and down to
avoiding hitting them while trying to maneuver the robot arm close to
the satellite's grappling fixture. He abandoned that approach after
four attempts.
The situation Sunday was bleak. It looked as though Solar Max was
out of control and might never see the sun again.
Not only was it spinning, as it had been before the first retrieval
attempt, but it was also spinning twice as fast, one rotation every
three minutes. It was also tumbling end over end and wobbling from
side to side.
A second approach by a free-flying astronaut equipped with a
different mechanical jaw to latch onto the satellite was ruled out
because there was not enough fuel left on the shuttle in case the
astronaut accidentally drifted away and had to be rescued.
At this point, late Sunday night, engineers at the Goddard Space
Flight Center in Greenbelt, Md., used the backup control system and a
new computer program to arrest the violent motions of the satellite.
Because the wobble was already so bad there was nothing to lose and
everything to gain, they said.
William Steward, the mission operatons manager, said it was a long
shot nonetheless. ''You don't know whether the spacecraft is going to
end up looking at the sun, which is what you'd like it to do to get
the most power, or looking at some other place in space.''
Unless they faced the sun, the solar panels of the satellite might
not be able to charge its batteries. Meanwhile, power was slipping
away. ''We came within minutes of losing it,'' Stewart said.
One sign of the desperation was the backup plan. If the
stabilization maneuver failed the engineers were prepared, right
before the batteries gave out, to fire explosive bolts on the
satellite and jettison the solar panels in the hope that the blast
would add a bit of stabilty to the satellite's motion and ease an
attempt at retrieval by the robot arm.
But late Sunday night, just as the batteries were about to give out,
Solar Max caught a burst of sunlight, ''a glorious sun,'' as Steward
put it.
An additional bit of luck was that a new computer program sent to
the satellite from the Goddard Space Center allowed the backup
control system to eliminate nearly all the spinning and wobbling of
the satellite. ''The wobble on the satellite is somewhere between 1
and 9 degrees as opposed to the 15, 20, 30, 40 that we expected
preflight if we ever got down to this low rotation rate,'' Cox, the
flight director, said
By late in the day the satellite was rotating once every 12 minutes,
exactly half its original speed.
At this point space agency officials started to exude confidence.
''We're in good shape and expect a good shot at it,'' said Jay
Greene, the chief flight director.
Meanwhile, in Houston, other astronauts were attempting the
grappling maneuver on a mission simulator and finding no problems at
all. Asked how the simulation went, Cox said, ''Oh yes, it went
without a miss.''
''When you get down to those well-behaved dynamics'' on a satellite,
he said, ''it's easy for a well-trained crewman. I can't get in there
and hit a lick with it, but they do it real well.''
nyt-04-10-84 2012est
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