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

Electrical Power Distribution and Control

Space Shuttle electrical power is provided by three fuel cells, each capable of generating an average power of 2 to 7 kilowatts at a nominal voltage of 28 volts dc with a peak power output of 12 kilowatts for short periods. This power is controlled, monitored, and distributed to loads throughout the Space Shuttle vehicle by the electrical power distribution and control (EPDC) subsystem. Figure 4-45 contains an overall block diagram of the system showing the major components and their relative locations in the vehicle. (Note: To reduce congestion, the power distribution system is not represented on the overall avionics system block diagram.) Within the EPDC, solid-state inverters convert 28-volt dc power to 117/200-volt, 400-hertz, three-phase ac power, which is also distributed by way of a separate redundant bus system to loads requiring alternating current. The EPDC is fail operational/fail safe and therefore capable of providing sufficient power for safe operation after sustaining two failures. The dc and ac distribution systems are described in this section. The events control and pyrotechnic sequencing functions which are included as part of the EPDC were covered in the Sequencing section.

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Figure 4-45. - Electrical power system (single string).

Direct Current Distribution

Five classes of buses are used to control and distribute dc power. These include main dc, bus-tie, essential, control, and preflight test buses. The three redundant main dc buses are each connected separately to a fuel cell by motor-driven contactors in the main distribution assemblies (MDA's), which are located near the fuel cells in the midbody area. The main dc buses deliver power, protected by fuses, from the MDA's to distribution centers as shown in figure 4-45. The bus-tie buses shown on this figure indicate the motor-driven contactors which allow manual interconnection of the buses for failure management. The ground support equipment (GSE) connections shown use the time zero (T-0) umbilical to deliver ground power when the vehicle is not operating on internal power or when the internal and external systems are sharing the load; i.e., during the prelaunch countdown. When a main bus is energized, either from the ground or from a fuel cell, all associated distribution assemblies also are energized. The system uses multipoint grounding and structural return; however, all loads are required to maintain isolation between primary power returns and chassis, and these returns are led to controlled points on the fore and aft payload bay bulkheads.

The essential buses, also established in the MDA's, are low-power buses used primarily to control critical vehicle functions and to provide power to loads considered critical in an emergency. Each receives triply redundant power directly through a switch from one fuel cell and indirectly from the other two main buses through remote power controllers (RPC's) in the respective bus power control assemblies (PCA's) (fig. 4-46). They provide control power for selected switching devices, such as those that service the onboard computers, and operating power to small, critical loads such as the C&W unit, the fuel cell electrical control units and reactant valves, smoke detectors, and selected cabin lighting. Normally, all essential buses are energized whenever any fuel cell/main bus circuit is operating.

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Figure 4-46. - Essential bus distribution (one of three).

Three control (CNTL) buses originate in each of the three forward PCA's; each is powered through RPC's from two main buses and through a circuit breaker from the third (fig. 4-47). Therefore, loss of two main buses will not interrupt control power to any function serviced. Typically, the control buses provide control power to RPC's servicing multiply redundant loads such as the guidance, navigation, and control system and those in the auxiliary power unit (APU) controllers, valves, and heaters; main propulsion system valves; RCS and OMS valves and heaters; air data probe heaters and actuators; hydraulic controls; and landing gear. During checkout and turnaround, each control bus may be selectively deenergized; this capability provides a means of verifying redundancies in a given circuit.

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Figure 4-47. - Control buses.

Two preflight test buses, powered from GSE power supplies, originate in aft PCA's 4 and 5. These buses can be energized only by ground power and are used to activate an inert vehicle for ground checkout or prelaunch testing.

Three types of distribution boxes provide the required services to the various Space Shuttle loads. These include the MDA, the PCA, and the load control assembly (LCA). As indicated previously, three MDA's are located in the midbody area beneath the payload bay liner on shelves near their respective fuel cells. The primary functions of these devices are to control power from the associated fuel cell, to control the bus-tie bus, to protect the main bus wires fore and aft and the payload umbilical, to establish the associated essential bus, and to control power to the respective midbody PCA.

Twelve PCA's are installed, three in the forward avionics bays, three in the midbody area, and six in the aft avionics bays. Each PCA receives power from its respective MDA or from another PCA and distributes it to loads requiring as much as 135 amperes. They provide overload protection for loads, wires, and power sources, and the means to switch loads remotely through RPC's or relays. The forward PCA's provide power to the static inverters, which generate ac. Both fore and aft PCA's distribute power to associated LCA's. The aft PCA's — 4, 5, and 6 — also contain motor switches which control ground power.

The LCA's, located in the fore and aft avionics bays, contain hybrid load drivers (HLD's) for control of current loads as great as 5 amperes. The HLD's provide capability for computer control of selected functions.

Alternating Current Generation and Distribution

Each forward avionics bay contains three power static inverter modules, which are connected in a phase-locked array to produce 117/200-volt, 400-hertz, three-phase, y-connected, four-wire ac power. Direct current input power to drive the inverter arrays is furnished by the respective forward PCA's, which contain circuitry to limit in-rush current to an acceptable level when the highly capacitive inverters are activated. Output current is limited to 20 amperes by circuitry within the inverters. The inverters are synchronized by an internal oscillator.

The output of each three-phase inverter array is monitored and controlled by an inverter distribution and control assembly (IDCA). Relays within the IDCA's connect the inverter arrays to the respective three-phase buses. These relays can be controlled manually by the crew, remotely from the ground during checkout, or automatically by internal circuitry in the event of an overload or an overvoltage. This automatic disconnect feature may be inhibited by the crew during critical mission phases to avoid disconnects caused by spurious signals or transients. Normal C&W monitoring and alarms will continue to operate when in the "inhibit" mode.

Each of the three redundant three-phase ac buses is isolated, is capable of supplying nominal power of 2.25 kilovolt-amperes, and is grounded to structure in a single point. All current is confined to the bus wiring except for some navigation equipment which uses an internal chassis ground. No provisions have been made for cross-tying the ac buses to accommodate inverter failures. Power reliability for critical ac loads is obtained either by providing a switch which allows access to more than one bus or by providing duplicate hardware operating off separate buses.

Ten motor control assemblies (MCA's), three each in the fore and aft avionics bays, and four in the midbody area, provide power and control to motors and other three-phase and single-phase loads in the Orbiter. Approximately 250 three-phase motors are required to drive deployment/retract mechanisms, latches, actuators, motorized valves, positioning devices, etc. Remote switching capability is provided by three-phase hybrid relays, which can be controlled by MDM commands.

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