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Computers in Spaceflight: The NASA
Experience
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- - Chapter One -
- - The Gemini Digital Computer:
First Machine in Orbit -
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- Crew interfaces to the Gemini
digital computer
- [21] Gemini's digital
computer had three sets of interfaces: the computer's controls,
the Manual Data Insertion Unit (MDIU), and the Incremental
Velocity Indicator (IVI). The controls consisted of a mode switch,
a start button, a malfunction light, a computation light, and a
reset switch. The mode switch had seven positions for selection of
one of the measurement or computation programs. The start button
caused the computer to run the selected program loaded in its
memory. The reset switch caused the computer to execute its
start-up diagnostics and prepare itself for action. The MDIU
consisted of two parts: a 1 0-digit keyboard and a 7-digit
register. The first two digits of the register, a simple
odometerlike rotary display, were used to indicate a memory
address. Up to 99 such logical addresses could be accessed. The
remaining five digits displayed data. Errors caused all zeroes to
appear. Negative numbers were inserted by making the first digit a
nine; the other digits contained the value. The IVI displayed
velocity increments required for, or as a result of, a powered
maneuver. It had three-digit feet-per-second displays for each of
forward-and-back, up-and-down, and left-to-right
axes39.
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[22]
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- Figure 1-6. Manual Data Insertion
Unit. (From McDonnell Corp., Gemini Familiarization Manual)
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[23]
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- Figure 1-7. Incremental Velocity
Indicator. (From McDonnell Corp., Gemini Familiarization
Manual)
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- [24] On a typical
mission the computer would be in operation during ascent as the
backup to the booster. On orbit, if no powered maneuvers were
imminent, it could be shut down to save electrical power. Due to
the nature of core memory, programs and data stored magnetically
in the cores would not disappear when the power was off, as in
present day semiconductor memories. This made it possible to load
the next set of modules, if necessary, from the Auxiliary Tape
Memory, enter any needed parameters, and then shut down the
machine until shortly before its next use. It took 20 seconds for
the machine to run its start-up diagnostics upon restoration of
power. After the diagnostics ran successfully, the current program
load was ready for use, all parameters intact.
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- GT-IV was following such a procedure in
preparing for re-entry on June 7, 1965. The computer was placed in
the RNTY mode, and the crew received and entered updated
parameters given to them when they were in contact with the ground
stations. But when they tried to turn the machine off, the manual
on/off switch did not function. The power had to be cut off by
another means, and the re-entry handled manually
40.
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- Using the computer for catch-up and
rendezvous was a relatively simple task. The difference between
catch-up and rendezvous is that catch-up maneuvers are executed to
put the spacecraft into position to make an orbit-change maneuver.
After the orbit change the spacecraft is prepared to rendezvous
with the target 41. Crews began the catch-up by entering the
ground-calculated rendezvous angle desired into address 83. The
rendezvous angle indicated how much farther along in a 360-degree
orbit the rendezvous was to take place. For example, if the crew
desired rendezvous one-third orbit ahead, 12000 was entered into
address 83 using the MDIU. The interval at which the pilot wanted
to see updates was then entered in address 93. For example, if
04000 was entered, the computer would display on the IVI any
required velocity changes at 120 degrees from the rendezvous point
(the start), 80 degrees to go, and 40 degrees to go. If the IVI
indicated that the computer had calculated that such a rendezvous
was possible within the designated fuel limits, the astronauts
pressed the START button and the IVI displayed the first set of
velocity differentials. The pilot then fired the thrusters until
the displays were all at zero (Astronaut John Young reported that
there was a tendency to "overshoot" in trying to burn to zero
42.). After that,
nothing was done unless the next update indicated a need for more
velocity adjustments 43. The astronauts also did paper-and-pencil
calculations of the velocity changes as a backup by using special
nomographs based on time and angles to the target
44. These backup calculations were compared with the
ground-calculated solution as well as the computer solution.
However, the figures computed on-board were considered the primary
solution for the terminal-phase intercept [25] maneuver
45. In the rendezvous mode, the radar would feed
information to the computer, which used it to calculate the
velocity adjustments needed for final approach
46.
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- These examples of the use of the computer
on a typical flight demonstrate that it was a relatively
straightforward assistant in guidance and navigation. It permitted
the Gemini astronauts to be independent of the ground in
accomplishing rendezvous from the terminal-phase intercept
maneuver to station keeping, a valuable rehearsal for the lunar
orbit rendezvous required for the Apollo program. The astronauts
participated in both the hardware and software design of the
computer and its interfaces, and they were able to go to Owego and
be put in the man-in-the-loop simulations. By flight time, like
everything else in the cockpit, use of the computer was second
nature.
