Electrical power system

The EPS consists of three subsystems: power reactant storage and distribution, fuel cell power plants (electrical power generation) and electrical power distribution and control.

The PRSD subsystem stores the reactants (cryogenic hydrogen and oxygen) and supplies them to the three fuel cell power plants, which generate all the electrical power for the vehicle during all mission phases. In addition, cryogenic oxygen is supplied to the environmental control and life support system for crew cabin pressurization. The hydrogen and oxygen are stored in their respective storage tanks at cryogenic temperatures and supercritical pressures. The storage temperature of liquid oxygen is minus 285 F and minus 420 F for liquid hydrogen.

The three fuel cell power plants, through a chemical reaction, generate all of the 28-volt direct-current electrical power for the vehicle from launch through landing rollout. Before launch, electrical power is provided by ground power supplies and the onboard fuel cell power plants until T minus three minutes and 30 seconds. Each fuel cell power plant consists of a power section, where the chemical reaction occurs, and a compact accessory section attached to the power section, which controls and monitors the power section's performance. The three fuel cell power plants are individually coupled to the reactant (hydrogen and oxygen) distribution subsystem, the heat rejection subsystem, the potable water storage subsystem and the EPDC subsystem. The fuel cell power plants generate heat and water as by-products of electrical power generation. The excess heat is directed to fuel cell heat exchangers, where the excess heat is rejected to Freon coolant loops. The water is directed to the potable water storage subsystem.

The EPDC subsystem distributes the 28 volts dc generated by each of the three fuel cell power plants to a three-bus system that distributes dc power to the forward, mid-, and aft sections of the orbiter for equipment in those areas. The three main dc buses-MNA, MNB and MNC-are the prime sources of power for the vehicle's dc loads. Each of the three dc main buses supplies power to three solid-state (static), single-phase inverters, which constitute one three-phase alternating-current bus; thus, the nine inverters convert dc power to 115-volt, 400-hertz ac power for distribution to three ac buses-AC1, AC2 and AC3-for the vehicle's ac loads.

The EPDC subsystem controls and distributes electrical power (ac and dc) to the orbiter subsystems, the solid rocket boosters, the external tank and payloads. Power is controlled and distributed by assemblies. Each assembly is a housing for electrical components, such as remote switching devices, buses, resistors, diodes and fuses. Each assembly usually contains a power bus or buses and remote switching devices for distributing bus power to subsystems located in its area.