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5

A maneuver planned for Saturday, June 10, was postponed a day

so the Marshall engineering team could verify by ground computer

simulation that a problem which occurred about noon on Friday

wouldn't

repeat.

On

Friday

morning

the

vehicle

had

been

commanded

into

the

solar

inertial

position.

The

maneuvers

to

chieve

a

stable

_

space station had consumed a considerable amount of the nitrogen

stored

aboard.

Flight

controllers

wanted

to

enable

the

redundant

computer onboard which could take over control if the prime computer

should

fail.

In

the

process

of

commanding

the

vehicle

to

turn

on

the redundant computer a decoder onboard the space station issued

an extraneous command which shut off the control servo unit on one

of the momentum gyros, making it to be non-responsive to computer

commands.

This

occurred

as

the

space

station was

passing

away

from

the

ground

tracking

station.

The

problem

was

corrected

during

the

next

station pass but resulted in using about one fifth of the nitrogen

gas remaining.

The computer aboard Skylab continued to compute attitude

control based on its programmed knowledge of two working momentum

gyros.

With

only

one

momentum

gyro

the

computer

was

getting

into

a situation where it sensed the vehicle wasn't maintaining its

position.

When

the

computer

sensed

the

vehicle

had

failed

to

hold

its position, it automatically fired the nitrogen system to regain

attitude

control.

This

happened

three

times

before

the

space

station

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