power module. Waste heat from the CBC is transferred to a heat rejection/radiator system in a gas-to-liquid heat exchanger. All the SD components are designed and packaged so that two complete SD power modules can be launched in a single Shuttle flight. The receiver/PCU/radiator combination is completely assembled and charged with gas and cooling fluid on earth before launch to orbit. Therefore, there will be no need to make on-orbit gas or fluid connections. The concentrator panels and facets will be pre-aligned and the panels, with facets installed, stowed in the Shuttle orbiter bay before launch. On orbit, the beta gimbal will be installed into the truss and the receiver/PCU/radiator attached to the gimbal. The pre-aligned concentrator panels will then be latched together and the total concentrator attached to the receiver/PCU/radiator. After final electric connections are made and check-out is complete, the SD power module will be ready for operation. Introduction Power for the manned base of Space Station Freedom will be supplied from two Solar Power Elements (SPEs). One SPE will be located on the port side of the transverse boom of the manned base and the other on the starboard side, each joined to the central part of the transverse boom by a single degree-of-freedom rotational gimbal (alpha gimbals). Initially, the SPEs on Freedom will supply a total of 75 kilowatts of electric power using photovoltaic (PV) power sources as shown in Fig. 1. As Freedom evolves and grows, increased power needs will be satisfied by the addition of Solar Dynamic (SD) power modules at the outboard ends of the initial SPEs as shown also in Fig. 1. Each set of SD hardware will add 25 kW of power to the manned base. For the first growth increment, which is expected to be 50 kW, one SD power module will be added on each side of the manned base. The evolution of Freedom is expected to require power capability growth to about 300 kW total. There are two primary reasons for the interest in the solar dynamic system as the source of growth power. A PV/SD hybrid system offers the flexibility of a power system with two types of sources, thus assuring an uninterrupted supply of power in the unlikely event of a major or systematic failure in either type of source. But even more compelling is the potential cost savings that can be realized with SD. SD power generating and storage components have longer lifetimes than photovoltaic arrays and batteries. These SD lifetimes result in substantial cost savings in hardware replacement, launch, and on-orbit installation costs. Because of the significantly higher solar- to-electric power efficiency of a SD system it has about a 60% smaller solar collection area than a PV system for a given power output. Therefore, it will have lower aerodynamic drag and lower reboost requirements. For constant drag operation, SD systems allow the Freedom to operate at lower altitudes. This permits the Shuttle orbiter to rendezvous with the Freedom manned base at lower altitudes, significantly increasing the orbiter's payload capacity and lowering the launch cost per pound to orbit. Studies have shown that the various operations and hardware cost savings resulting from the use of SD power rather than PV power for the growth of Freedom's manned base, amount to a reduction in life cycle costs of 3 to 4 billion dollars over the 30 year life of Freedom. In the present baseline phase of the Space Station Freedom Program, preliminary design of the SD power module will be completed along with developmental testing of critical components and subsystem testing. Also, the design and operational features which must be incorporated into Freedom's software (hooks) and hardware (scars) to allow the later addition of SD power modules will be defined. Inclusion of these hooks
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