Computers in Spaceflight: The NASA
Experience
- - Chapter One -
- - The Gemini Digital Computer:
First Machine in Orbit -
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- Hardware
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- [12] IBM Corporation
received the contract for the Gemini digital computer on April 19,
1962, amounting to $26.6 million. It provided for the construction
of the on-board computer and for integration with other spacecraft
systems 8. The first machine was in its final testing phase
by August 31, 1963, and IBM delivered the last of 20 such machines
by December 19659. Engineers at IBM believe that the main reason why
their company received the contract was the successful development
of a core memory used on the Orbiting Astronautical Observatory
10. One of them, John J. Lenz, said that the contract
for Gemini came just at the right time. The best of the
engineering teams of the IBM Federal Systems Division plant in
Owego, New York were between assignments and were put on the
project, increasing its chance for success.
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- Restrictions on size, power, and weight
influenced the final form of the computer in terms of its
components, speed, and type of memory. The shape and size of the
computer was dictated by the design of the spacecraft. It was
contained in a box measuring 18.9 inches high by 14.5 inches wide
by 12.75 inches deep, weighing 58.98 pounds11. An unpressurized equipment bay to the left of the
Gemini commander s seat held the computer, as well as the inertial
guidance system power supply and the computer auxiliary power
supply. The machine consisted of discrete components, not
integrated circuits 12. However, circuit modules that held the components
were somewhat interchangeable. They were plugged into one of five
interconnection.....
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[13]
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- Figure 1-2. Locations of key
components of the Gemini Guidance System. (From McDonnell Corp.,
Gemini Familiarization Manual)
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- ....boards, and it took 510 of the modules
to build the logic section alone 13. The computer had no redundant circuits, which
meant that failures in the computer canceled whatever activity
needed to be controlled by it. For example, a failure in the power
switch three quarters of the way through the Gemini IV mission
caused cancellation of the planned computer-controlled re-entry.
It was possible to fly the Gemini computer without a backup
because whatever the computer did erroneously could be either
abandoned (such as rendezvous) or handled. albeit more crudely, in
other ways (such as re-entry using Mercury procedures).
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- The machine had an instruction cycle of
140 milliseconds, the time it required for an addition.
Multiplication took three cycles, or [14] 420 milliseconds,
with division requiring double that time14. The arithmetic bit rate was 500 kilocycles, and
the memory cycle rate half that15. The computer was serial in operation, passing bits
one at a time, which explains the relatively slow processing
speeds, slower than some vacuum tube computers such as the
Whirlwind. Also, its fixed decimal point arithmetic unit design
limited the precision of the calculations but greatly reduced
complexity. The Gemini digital computer used ferrite cores for its
primary memory. Core memories store one bit in each ferrite ring
by magnetizing the ring in either a clockwise or counterclockwise
direction. One direction means a one is stored and the opposite
direction is a zero. Each core is mounted at a perpendicular
crossing of two wires. Thousands of such crossings are in each
core plane, consisting of rows of wires running up and down (the x
wires) and others running left and right (the y wires). Therefore,
to change the value of a bit at a specific location, half the
voltage required for the change is sent on each of two wires, one
in the x direction and one in the y direction. This way only the
core at the intersection of the two wires is selected for change.
All the others on the same wires would have received only half the
required voltage. By the use of a third wire it is possible to
"sense" whether a selected core is a one or a zero. In this way,
each individual core can be read.
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- The ferrite core memory in the Gemini
computer had a unique design. It consisted of 39 planes of 64 by
64-bit arrays, resulting in 4,096 addresses, each containing 39
bits. A word was considered to be 39 bits in length, but it was
divided into three syllables of 13 bits. The memory itself divided
into 18 sectors. Therefore, it was necessary to specify sector and
syllable to make a complete address. Instructions used 13 bits of
the word with data representations of 26 bits. Data words were
always stored in syllables 0 and I of a full word, but
instructions could be in any syllable. This means that up to three
instructions could be placed in any full word, but only one data
item could be in a full word16.
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- The arithmetic and logic circuit boards
and the core memory made up the main part of the Gemini computer.
These components interfaced to a plethora of spacecraft systems,
most of which were concerned with guidance and navigation
functions. This system was the Gemini digital computer through the
Gemini VII mission. Beginning with Gemini VIII, the computer
included a secondary storage system, which had impact on the
spacecraft computer systems built by IBM and flown on the Skylab
and Shuttle.
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- During the 1950s and well into the 1960s,
the most ubiquitous method of providing large secondary storage
for computers was the use of high-speed, high-density magnetic
tape. By 1980, tape was used mainly to store large blocks of data
unneeded on a regular basis or to mail programs and data between
sites. Disk Systems have considerably faster access times and have
rapidly increased in storage....
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[15]
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- Figure 1-3. Cores like these were
used in Gemini's memory. (IBM photo)
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- ....capacity, rivaling or even exceeding
tape, and thus supplanting it in common use. In 1962, disk systems
were large, expensive, and far from fully reliable. When the
software for the Gemini computer threatened to exceed the storage
capacity of the core memory, IBM proposed an Auxiliary Tape Memory
to store software modules that did not need to be in the computer
at lift-off. For example, programs that provided backup booster
guidance and insertion assistance would be in the core memory for
the early part of the flight. The re-entry program could be loaded
into the core shortly before it was needed, thus writing over the
programs already there. This concept, fairly common in earth-bound
computer usage, was a first for aerospace computing.
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- IBM chose the Raymond Engineering
Laboratory of Middletown, Connecticut to build the
device17. The unit weighed 26 pounds and filled about 700
cubic inches of space in the adapter module of the Gemini
spacecraft18. The tape memory increased the available storage of
the Gemini computer by seven and one-half times with its capacity
of l, l 70,000 bits. Programs loaded from the tape would fill
syllables 0 ....
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[16]
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- Figure 1-4. Layout of the Gemini
Digital Computer core memory. (From McDonnell Corp., Gemini
Familiarization Manual)
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- [17] ...and l of the
core memory locations19 . It took 6 minutes to load a program from the tape
drive into core20.
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- NASA's natural insistence on high
reliability in manned spaceflight operations challenged the
computer industry of the early l 960s. Tape error rates were l bit
in 100,000 and IBM wanted to raise this rate to l bit in
1,000,000,00021. The method used was to triple record each program
on the tape, pass each set of three corresponding bits through a
voter circuit, and send the result of the vote to the core
memory22. This scheme was later used on the Shuttle.
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- Gemini VIII was the first mission with the
Auxiliary Tape Memory on board. Shortly after a successful
rendezvous with an Agena, the combined spacecraft began to spin
out of control. Mission Control decided to disengage the Agena and
bring the Gemini down, as large amounts of attitude control
thruster fuel had been wasted trying to regain control of the
spacecraft. Thus, the first attempt to load a program from the
tape was made while the spacecraft was spinning. Even though the
Auxiliary Tape Memory design parameters specified low vibration
levels,23 the re-entry program was successfully loaded and
used in the subsequent descent.
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- IBM obtained this sort of reliability
beyond the original specifications as a result of an extensive
testing program. For example, the Auxiliary Tape Memory had failed
prequalification vibration tests, so IBM added a brass flywheel
and weights on the tape reels to increase
stabilization24. This ensured a
successful program load under adverse conditions. There were also
problems with transistors shorting out due to loose particles too
small to be seen on x-rays but which shook loose during
acceleration25. Increased cleanliness in manufacturing was one
solution to this problem.
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- The only in-flight failure of a computer
component was on the 48th revolution of the Gemini IV mission,
when astronaut James McDivitt tried to update the computer in
preparation for re-entry. The machine would not turn off, and it
could not be used for the planned "lifting bank"
re-entry26. IBM could not duplicate the failure on the ground,
but the manufacturers did install a manual switch that bypassed
the failure for Gemini V27.

