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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 need for an on-board
computer
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- [28] The Apollo lunar
landing program presented a tremendous managerial and technical
challenge to NASA. Navigating from the earth to the moon and the
need for a certain amount of spacecraft autonomy dictated the use
of a computer to assist in solving the navigation, guidance, and
flight control problems inherent in such missions. Before
President John F. Kennedy publicly committed the United States to
a "national goal" of landing a man on the moon, it was necessary
to determine the feasibility of guiding a spacecraft to a landing
from a quarter of a million miles away. The availability of a
capable computer was a key factor in making that
determination.
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- The Instrumentation Laboratory of the
Massachusetts Institute of Technology (MIT) had been working on
small computers for aerospace use since the late 1950s. Dr.
Raymond Alonso designed such a device in
1958-19591. Soon after, Eldon Hall designed a computer for an
unmanned mission to photograph Mars and return2. That computer could be interfaced with both
inertial and optical sensors. In addition, MIT was gaining
practical experience as the prime contractor for the guidance
system of the Polaris missile. In early 1961, Robert G. Chilton at
NASA-Langley Space Center and Milton Trageser at MIT set the basic
configuration for the Apollo guidance system3. An on-board digital computer was part of the
design. The existence of these preliminary studies and the
confidence of C. Stark Draper, then director of the
Instrumentation Lab that now bears his name, contributed to NASA's
belief that the lunar landing program was possible from the
guidance standpoint.
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- The presence of a computer in the Apollo
spacecraft was justified for several reasons. Three were given
early in the program: (a) to avoid hostile jamming, (b) to prepare
for later long-duration (planetary) manned missions, and (c) to
prevent saturation of ground stations in the event of multiple
missions in space simultaneously4. Yet none of these became a primary justification.
Rather, it was the reality of physics expressed in the 1.5-second
time delay in a signal path from the earth to the moon and back
that provided the motivation for a computer in the lunar landing
vehicle. With the dangerous landing conditions that were expected,
which would require quick decision making and feedback, NASA
wanted less reliance on ground-based computing5. The choice, later in the program, of the lunar
orbit rendezvous method over direct flight to the moon, further
justified an on-board computer since the lunar orbit insertion
would take place on the far side of the moon, out of contact with
the earth6. These considerations and the consensus among MIT
people that autonomy was desirable ensured the place of a computer
in the Apollo vehicle.
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- Despite the apparent desire for autonomy
expressed early in the [29] program, as the
mission profile was refined and the realities of building the
actual spacecraft and planning for its use became more immediate,
the role of the computer changed. The ground computers became the
prime determiners of the vehicle's position in three-dimensional
space "at all times" (except during maneuvers) in the
missions7. Planners even decided to calculate the lunar orbit
insertion burn on the ground and then transmit the solution to the
spacecraft computer, which somewhat negated one of the reasons for
having it. Ultimately, the actual Apollo spacecraft was only
autonomous in the sense it could return safely to earth without
help from the ground8.
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- Even with its autonomous role reduced, the
Apollo on-board computer system was integrated so fully into the
spacecraft that designers called it "the fourth crew
member"9. Not only did it have navigation functions, but
also system management functions governing the guidance and
navigation components. It served as the primary source of timing
signals for 20 spacecraft systems10. The Apollo computer system did not have as long a
list of responsibilities as later spacecraft computers, but it
still handled a large number of tasks and was the object of
constant attention from the crew.
