NASA Office of Logic Design

NASA Office of Logic Design

A scientific study of the problems of digital engineering for space flight systems,
with a view to their practical solution.

NASA SP-504: Space Shuttle Avionics System

Section 4  System Mechanization/Operation


The Space Shuttle avionics system features a central computer complex, which provides software services to all vehicle subsystems that require them; a serial digital data bus network, which distributes the computer input/output (I/O) function throughout the vehicle; and dedicated hardware unique to each subsystem. Depending on the mission, the system includes more than 274 line-replaceable units (LRU's), the term used to describe a case or chassis, containing electronic parts, with connectors, and having mounting and cooling provisions, which is replaceable as an entity in case of failure. These are distributed primarily among six equipment bays located in the Orbiter as shown in figure 4-1. Some sensors and other LRU's are located outside the bays because of unique location constraints and some in the solid rocket boosters (SRB's) and the external tank (ET). Dually redundant main engine controllers are mounted on each of the main engines. Three inertial measurement units (IMU's) and two star trackers are mounted on alignment pads on the navigation base, a rigid structure located just forward of Orbiter equipment bays 1 and 2. To the extent possible, redundant LRU's are physically separated to prevent damage to more than one string if a problem should occur. The equipment is arranged to facilitate checkout and for easy access and replacement. Cooling by both forced air and coldplate is available in the forward bays. All equipment in the unpressurized aft bays is mounted on coldplates.

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Figure 4-1. - Avionics equipment locations

Additional detail on the distribution of LRU's is contained in the system block diagram, the foldout located inside the back cover, which should be extended at this time. Because of its size and complexity, the Space Shuttle avionics system is very difficult to describe without becoming engulfed in details. Therefore, reference to the block diagram will be made frequently throughout this section in an attempt to maintain overall system perspective. The reader is urged to spend a few minutes to become familiar with its features. Note that the block diagram is organized to reflect the physical location of the avionics equipment in the vehicle. To facilitate reference to various features of the system, letters (across the top and bottom) and numbers (along the sides) define zones in the drawing. To the left and outside the drawing is a list of acronyms and abbreviations used. Below this list is a legend indicating the color codes used to identify data buses.

The crew interface devices are grouped in the flight deck/ cockpit area of the diagram, zones [A,1] through [D,7]. Below the Right deck is the nose area and along the bottom of the drawing is the payload bay, each depicted with the LRU's located in that area. The three forward avionics bays occupy the center of the drawing; the three aft bays, the upper right portions of the drawing. Below the aft bays are the left and right SRB'S, and the isolation amplifiers and umbilicals used to interface with the launch processing system (LPS) while on the ground and with the SRB's before separation.

The five central general-purpose computers (GPC's) are distributed among the forward avionics bays, zones [E,4] through [L,4]. The serial digital data buses, which connect the computers into the rest of the system, are located above and below the computers and generally run right and left across the drawing. They are grouped and color coded into categories as listed in the legend. These buses are connected to various bus terminal units (BTUs), which serve as the interface between the computers and the subsystem LRU'S.

The data processing system (DPS) provides services to all of the avionics subsystems and functions as shown in figure 4-2. Each of the subsystems has unique, dedicated hardware, but all associated applications software modules are resident in the central computers and all functions use features of the DPS. In the following subsections, first, the major functions of the avionics system are described as they relate to the Space Shuttle mission; then, each subsystem or function is discussed together with its unique hardware, its integration with the central data processing complex, the redundancy features employed, and the associated crew interface requirements.

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Figure 4-2. - Avionics functional categories

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