ications. Lithium batteries limit the Sprint to 7-hr missions, which is also the length of most space walks. A 0.6-in.-thick covering of Nomex should cushion it against inadvertent bumps and collisions, while a yellow LED and surface markings make it easy for operators to see and position.
Securing the basics
Power for the ISS currently comes from the Zarya, a Russian module with solar arrays. But plans call for attaching four solar wings (or 16 arrays) to the truss that will span the station. The 16 arrays, covering 27,000 sq ft, will be the largest deployable space structures ever built, according to NASA. Rotating joints on the truss will let station operators keep the arrays optimally pointed to receive sunlight. When new, each array generates 31 kW of dc voltage, which slowly degrades to 24 kW over their operational life.
Although the rotary joints let data and power pass through, they can't handle fluids. Therefore, each pair of arrays is equipped with a thermal-control system that draws conditioned electricity from the arrays and keeps temperatures within operating limits. (Temperatures range from –184 to 149°C at the ends of the truss.) The thermal systems use pumps, coldplates, ammonia as a liquid coolant, and radiators jutting into space at right angles from the arrays to shed heat. A small heater keeps fluid lines from freezing while the ISS traverses the Earth's shadow.
Total power generation at peak conditions will be about 240 kW, a third of which powers the thermal controls for the arrays. Another third or so recharges the nickel-hydrogen batteries (six batteries per array). These batteries provide power while the ISS is out of the sunlight, or 30 min out of every 90-min revolution around the Earth. By design, the batteries shouldn't fall below 35% of a full charge. This lets them fully charge in the 60 min the ISS is in sunlight, and lets them power the station at a reduced level for one entire orbit following an orbital eclipse.
The last third of the power is distributed throughout the station at 160 V to prevent transmission losses, and then converted to secondary power, a tightly regulated 124 Vdc, near where it will be used. The two-level design of the power-distribution system compensates for line losses, hardware degradation, and aging solar arrays on the primary system while the secondary voltage remains constant for equipment.
This is a change from past practices in which most spacecraft, Russian and U.S., relied on 28 Vdc. The higher voltage lets NASA engineers meet the ISS's power needs and use smaller, lighter power lines. Appliances, computers, and other electrical devices will have to be designed to operate on 124 Vdc or use an adapter. Russian modules will still use 28 Vdc and converters will ensure power is at the proper voltage depending on whether crews are in the American or Russian sectors of the station.
The power system will use single-point ground architecture, making the ISS metal structure the electrical common point inside the station. But the potential difference between the ISS and the plasma environment in space could be as much as 140 Vdc during eclipse, enough to cause microarcing and damage to the arrays and thermal coatings. Two plasma contactors mounted on the truss, one operational and one backup, will generate a stream of electrons from xenon gas and emit it into space, effectively grounding the ISS to space.
The station generates oxygen primarily by electrolyzing water into oxygen and hydrogen. The hydrogen will be vented overboard. If necessary, oxygen can also be generated using chemical cartridges in an exothermic reaction.
Oxygen is also transferred from the Shuttle to the Station whenever the Shuttle repressurizes itself and the station prior to undocking and heading to Earth. The Shuttle will also leave its excess water at the ISS. Water is a byproduct of the Shuttle's fuel cells, which convert hydrogen and oxygen into electricity. NASA figures it should only take three Shuttle flights per year to keep the ISS supplied with water, which will be used for drinking a