 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 and generating oxygen. The Station will also recycle water from the sinks, shower, urine, and condensation for oxygen generation, an in-orbit first.
Modules and nodes
The ISS will eventually grow to include over a dozen modules for living quarters, labs, and storage. They will be connected by airlocks and nodes, all tied to the main truss and its solar arrays.
The first module in the jigsaw-puzzle construction was the 42,600-lb Zarya, designed in the U.S. but built in Russia. It is 41-ft long and 13.5 ft at its widest, and provides power and propulsion while the ISS is being built. On the outside, it is studded with 36 steering jets for adjusting attitude, and it carries two larger main engines for reboosting and changing orbit. There are also 16 fuel tanks with a total capacity of 6 tons and two solar arrays putting out 3 kW of electrical power. Most of Zayra's functions will be carried out by the Zveda module once it is in place, leaving the Zayra as storage space and a fuel depot. Zayra's two side docking ports will accommodate Soyuz and Progress spacecraft.
There should always be enough Soyuz reentry vehicles on the station ready to act as lifeboats for the entire crew. The piloted, three-person Soyuz must always be flown by a Russian cosmonaut, according to the Russian space agency. It could eventually be replaced by spaceplanes based on the X-38, a NASA project which recently had its funding deferred.
Progress cargo rockets will deliver up to 6 tons of reboost propellant and dry cargo to resupply the station every two months. They will also serve as self-incinerating trash cans. ISS personnel will fill the Progress rockets with refuse, launch them toward Earth, and they will burn up on reentry.
The 43-ft long Russian Zveda module connects to one end of Zayra. During construction, Zveda houses the crew and main computers, distributes power, and maintains life support. All these functions will be redistributed once the ISS is completed. But Zveda will remain the heart of the Russian segment of the ISS, which w