Saturday, October 13, 2018

Miscellaneous notes on AGC design from my research



All the connectors on the Apollo Guidance Computer were constructed of the Malco mini-WASP components. These were individual pins, male and female, set in a small rectangular nylon insulator with a circular base. Individual holes were drilled in a metal chassis for each pin position, the nylon with its pin was pressed into the hole, and the holes were arranged in lines with .125" spacing.

The male pins looked like a small standard screwdriver blade. The female pins were a tuning fork shape, at right angles to the male blade. The male slide into the fork with the two tines pressing onto the male to make contact.

Female 'tuning fork' pin

Male 'screwdriver blade' pin

The nylon insulators had a circular bottom section to press into a hole in metal, with a squared off top section and a center hole into which the pin was pressed.
Female insulator body

Male insulator body
It was quite common to build multiple lines of connectors, side by side, to form arrays such as the 24 by 4 connector for core rope modules to attach to the rest of the AGC. The largest connector was the main connection between the AGC and the rest of the Apollo spacecraft. It had 360 of these pins installed in an array, a pair of 180 pin halves.

Connectors but from the predecessor block I version of the AGC
Interaction with the rest of the spacecraft

The AGC was connected to many other parts of the spacecraft. The most visible connection to people was the DSKY, the display and keyboard module that the astronauts interacted through to control the software on the AGC. However, the system was quite central in the operation of the spacecraft and interacted with many subsystems.

The inertial guidance unit or IMU provided a stable platform that would maintain its orientation in space in spite of movement or rotations of the spacecraft. Driven by gyroscopes, this platform would would hold its position as the IMU rotated around it in three dimensions. The pivots, called gimbals, had measuring devices to indicate how the spacecraft pivoted around the stable member, plus a motor that could force the platform to pivot on the gimbal.

The motors forced the stable member to pivot on the three gimbals until the member was oriented in a desired direction. This was done on the ground by uplinks from Launch Control and during flight from mission control, but the most important method of forcing the member to a known position was by use of a star sighting mechanism in the lower bay of the Command Module.

An astronaut would rotate the spacecraft to point at a known star, and through an offset scope look at a second known star. The computer would swing the spacecraft to the position it though would place the stars correctly, but the astronaut would then move the craft around until the stars were precisely in the crosshairs of the scope. Pushing a button at this point told the computer what orientation to set the IMU and what location to record for the spacecraft. 

Since gyroscopes are not perfect and drift a bit over time, the stable platform would be realigned periodically using the optics system described above. This set the IMU to a stable reference alignment and synced the computer view with the physical reality.

Three accelerometers were installed in the IMU and recorded the acceleration of the spacecraft on three axes. The computer recorded the accelerations and used them to calculate the change in position of the spacecraft. The combination of the pivot angles from the stable reference orientation and the accelerations in three dimensions allowed the computer to keep track of the spacecraft position and speed.

The computer had a pair of hand controllers that the astronauts used to command changes in the spacecraft position. One would request pitch, yaw or roll around the current position. The other would request translation along one of the axes - move left, move up, speed up, etc.

These hand controller inputs were read by the computer and caused requests to be sent to the Reaction Control System, small rockets that thrust to cause the spacecraft to translate or rotate in space. Thus, the Apollo spacecraft was a fly-by-wire system, with the AGC or a backup SCS system connecting the pilots inputs on the controllers with the RCS jets.

Telemetry was assembled and disassembled by the AGC over the links with the ground. Uplinks could even insert new data into the AGC and was used often during missions. For example, the various burns for navigation, such as Trans Earth Insertion, were calculated on the ground using the Real Time Computer Complex (a pair of IBM 360/75 mainframes) and uploaded to the AGC for execution by the astronauts.

The AGC even had a mode where it could force a takeover of the Saturn IVB computer during launch, if the astronauts or ground decided that the computer navigation inside the Saturn V was malfunctioning. The hand controller would then swing the engine of the SIVB motor and control the duration of a burn, so that an astronaut could hand fly the system if the booster navigation and control failed.

All this with only 360 signal connections from the AGC to the rest of the spacecraft. 

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