Computers in Spaceflight: The NASA
Experience
- - Chapter One -
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- The Gemini Digital Computer:
First Machine in Orbit
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- [10] Project Mercury
was America's first man-in-space effort. The McDonnell-Douglas
Corporation developed the Mercury spacecraft in the familiar bell
shape. It was barely large enough for its single occupant and had
no independent maneuvering capability save attitude control jets.
Its orbital path was completely dependent on the accuracy of the
guidance of the Atlas booster rocket. Re-entry was calculated by a
realtime computing center on the ground, with retrofire times and
firing attitude transmitted to the spacecraft while in flight.
Therefore, it was unnecessary for the Mercury spacecraft to have a
computer, as all functions required for its limited flight
objectives were handled by other systems.
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- Gemini both continued the objectives of
the Mercury program and served as a test bed for the development
of rendezvous techniques critical to lunar
missions1. At first glance, the Mercury and Gemini spacecraft
are quite similar. They share the bell shape and other
characteristics, partially because Gemini was designed as an
enlarged Mercury and because the prime contractor was the same for
both craft. The obvious difference is the presence of a second
crew member and an orbital maneuvering system attached to the rear
of the main cabin. The presence of a second crewman meant that
more instrumentation could be placed in Gemini and that more
experiments could be performed, as an extra set of eyes and hands
would be available. Gemini's maneuvering capability made it
possible to practice rendezvous techniques. The main rendezvous
target was planned to be the Agena, an upper stage rocket with a
restartable liquid propellant engine that could be launched by an
Atlas booster. After rendezvous with an Agena, the Gemini would
have greatly increased maneuvering capability because it could use
the rocket on the Agena to raise its orbit.
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- Successful rendezvous required accurate
orbital insertion, complex catch-up maneuvering, finely tuned
movements while making the final approach to the target, and
guidance during maneuvers with the Agena. Safety during the
critical powered ascent phase demanded some backup to the ascent
guidance system on the Titan II booster vehicle. The Gemini
designers also wanted to add accuracy to re-entry and to automate
some of the preflight checkout functions. These varied
requirements dictated that the spacecraft carry some sort of
active, on-board computing capability. The resulting device was
the Gemini digital computer.
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- The Gemini computer functioned in six
mission phases: prelaunch, ascent backup, insertion, catch-up,
rendezvous, and re-entry. These requirements demanded a very
reliable, fairly sophisticated digital computer with simple crew
interfaces. IBM built such a machine for the Gemini
spacecraft.
By the early 1960s, engineers were searching
for ways to automate checkout procedures and reduce the number of
discrete test lines connected to launch vehicles and spacecraft.
Gemini's computer....
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[11]
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- Figure 1-1. First orbital
rendezvous: Gemini VI keeps station after using its on- board
computer to maneuver to position near Gemini VII. (NASA photo
S-65-63 l 75)
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....did its own self checks under software
control during the prelaunch phase. It also accepted parameters
needed for the flight during the last 150 minutes before
launch2. During ascent, the computer received information
about the velocity and course of the booster so that it would be
ready to take over from the Titan's computers if they failed.
Switch-over could either be automatic or manual. The computer could
then issue steering and booster cutoff commands to the
Titan3. Even if the updated parameters were not necessary to
boost guidance, they were useful in the calculation of additional
velocity needed after the Titan's second-stage cutoff to achieve the
proper orbit. That velocity difference was displayed to the crew so
that they could use the spacecraft's own propulsion system to reach
insertion velocity4.
- [12] Rendezvous
operations required an on-board computer because the ground
tracking network did not cover all parts of the Gemini orbital
paths. Thus, it would be impossible to provide the sort of
continuous updates needed for rendezvous maneuvers. For example,
Gemini XI was planned as a first-orbit Agena rendezvous, with some
of the critical maneuvers conducted outside of telemetry
range5. That same mission also featured a fully
computer-controlled re-entry, which resulted in a splashdown 4.6
kilometers from the target6. In computer-controlled descents, the roll attitude
and rate are handled by the computer to affect the point of
touchdown and re-entry heating. The Gemini spacecraft had
sufficient lift capability to adjust the landing point up to 500
miles on the line of flight and 40 miles laterally respective to
the line of flight. Five minutes before retrofire, the computer
was placed in re-entry mode and began to collect data. It
displayed velocity changes during and after the retrofire. During
the time the spacecraft traveled from an altitude of 400,000 feet
to when it reached 90,000 feet, the computer controlled actual
attitude7.

