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Computers in Spaceflight: The NASA
Experience
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- - Chapter Two -
- - Computers On Board The Apollo
Spacecraft -
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- The Apollo computer
systems
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- [30] The mission
profile used in sending a man to the moon went through several
iterations in the early 1960s. For a number of reasons, planners
rejected the direct flight method of launching from the earth,
flying straight to the moon, and landing directly on the surface.
Besides the need for an extremely large booster, it would require
flawless guidance to land in the selected spot on a moving target
a quarter of a million miles away. A spacecraft with a separate
lander would segment the guidance problem into manageable
portions. First, the entire translunar spacecraft would be placed
in earth orbit for a revolution or two to properly prepare to
enter an intercept orbit with the moon. Upon arriving near the
moon, the spacecraft would enter a lunar orbit. It was easier to
target a lunar orbit window than a point on the surface. The
lander would then detach and descend to the surface, needing only
to guide itself for a relatively short time. Alter completion of
the lunar exploration, a part of the lander would return to the
spacecraft still in orbit and transfer crew and surface samples,
after which the command module (CM) would leave for earth.
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- With a lunar orbit rendezvous mission,
more than one computer would be required, since both the CM and
the lunar excursion module (LEM) needed on-board computers for the
guidance and navigation function. The CM's computer would handle
the translunar and transearth navigation and the LEM's would
provide for autonomous landing, ascent, and rendezvous
guidance.
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- NASA referred to this system with its two
computers, identical in design but with different software, as the
Primary Guidance, [31] Navigation,
and Control System (PGNCS pronounced "pings"). The LEM had an
additional computer as part of the Abort Guidance System (AGS),
according to the NASA requirement that a first failure should not
jeopardize the crew. Ground systems backed up the CM computer and
its associated guidance system so that if the CM system failed,
the spacecraft could be guided manually based on data transmitted
from the ground. If contact with the ground were lost, the CM
system had autonomous return capability. Since the lunar landing
did not allow the ground to act as an effective backup, the LEM
had the AGS to provide backup ascent and rendezvous guidance. If
the PGNCS failed during descent, the AGS would abort to lunar
orbit and assist in rendezvous with the CM. It would not be
capable of providing landing assistance except to monitor the
performance of the PGNCS. Therefore the computer systems on the
Apollo spacecraft consisted of three processors, two as part of
the PGNCS and one as part of the AGS.
