Propulsion, Communications and Power

The ISS orbits the Earth at an altitude of 217 to 285 miles (362 to 475 km). At this altitude, the Earth's atmosphere is extremely thin, but still thick enough to drag on the ISS and slow it down. As the ISS slows down, it loses altitude. In addition to atmospheric drag, solar flares also slow the station down and cause it to lose altitude. So the ISS will need to be boosted periodically to maintain its proper altitude. The command and service modules have rocket engines that can be used to boost the ISS in the early stages. However, the Progress supply ships will do most of the reboosting. Each reboosting event requires two rocket engine burns. During the burns, work on the ISS will be suspended. After the burns, station life will return to normal.

The ISS must be able to talk with flight controllers on the ground daily, for the routine operation of the station. In addition, crew members must be able to communicate with each other within the ISS and when conducting spacewalks outside the station.

Talking with the Ground
NASA's Mission Control in Houston will send signals to a 60-foot radio antenna at White Sands Test Facility in New Mexico. White Sands will relay the signals to a pair of Tracking and Data Relay satellites in orbit 22,300 miles above the Earth. The satellites will relay the signals to the U.S. portion of the ISS and/or the space shuttle if it is attached. During the early phase, signals will be sent through the Russian Space Agency's communications system of ground stations and/or satellites.

The ISS has two systems for communicating with the ground:

• S-band - voice, commands, telemetry and data files
• Ku-band (high bandwidth) - video and transfer of two-way data files

Talking Within the ISS and to Spacewalkers
The Internal Audio Subsystem (IAS) will provide intercom, telephone and alarm system communications within the ISS's pressurized modules. The IAS will also connect with the following:

• Ultra-High Frequency (UHF), to talk with spacewalkers
• External connectors, to talk with a docked space shuttle
• Russian segment's audio system

The IAS will carry sound only, and will also feed into the station's video data system (VDS), a series of internal and external video cameras, to provide sound for the videos.

Navigation
The ISS must be able to know precisely where it is in space, where other objects are and how to go from one point in space to another, especially during reboosting. To know where it is and how fast it is moving, the ISS uses both U.S. and Russian global positioning systems (GPS). To know which way it is pointing, its attitude, the ISS has several gyroscopes. The combination of all this information will help the ISS move from one point to another in space. In addition, the Russian navigation system uses sighting on the stars, sun and Earth's horizon for navigation.

Power
We take for granted having electrical power to operate our homes. For example, to use your toaster or coffee maker, you plug it into the wall without a second thought. Like in your home, all of the onboard systems of the ISS will require electrical power. Eight large solar arrays will provide electrical power from the sun. Each array is 109 feet (33 m) long and covers an area of 27,000 ft² (approximately 2508 m²), or about one acre. On each array are two blankets of solar cells. Each blanket is on one side of a telescoping mast that can extend and retract to fold or form the array. The mast turns on a gimbal, so that it can keep the solar cells facing the sunlight. The Russian modules also have 72- to 97-foot (22- to 30-m) solar arrays that provide power.

Like a power grid on Earth, the arrays will generate primary power -- approximately 160 volts of DC electricity. The primary power will be converted by a secondary transformer to provide a regulated 124-volt DC current to be used by the station's equipment. There are also power converters onboard to meet the different currents required by U.S. and Russian equipment. The primary power will also be used to charge the ISS's three nickel-hydrogen battery stations, which will provide power when the ISS passes through the Earth's shadow in each orbit.

Computers
By the time the ISS is completed, there will be more than 100 computers aboard. Computers will be used for the following:

• Operations of the ISS (such as housekeeping functions, payload operations, rendezvous and docking)
• Interface with the crew (IBM Thinkpads with 80386 microprocessors and Windows operating systems)
• Caution and warning systems
• Data acquisition and processing from experiments

The computers will be networked together to coordinate activities and functions.

Re-supply
If we need new supplies, we go to the grocery store or other retailers. In the ISS, they have to call for "home-delivery." Progress supply ships will be used to ferry new supplies (food, water, medicines,oxygen, nitrogen, fuel, equipment, clothing, personal items) to the ISS. Progress ships will also remove solid waste from the ISS. The space shuttle can bring new supplies to the ISS as well, along with equipment for construction. In addition to Progress and the space shuttle, two new supply vehicles are being developed by the European Space Agency (ESA) and National Space Development Agency of Japan. The ESA's vehicle will be like Progress, capable of supplying nine tons of cargo, including food, clothing, fuel, water, oxygen and nitrogen; the vehicle will also be able to reboost the ISS. The Japanese craft, called the Hope Transfer Vehicle, will be capable of delivering pressurized cargo (food, water, clothing), but not fuel, oxygen or nitrogen.

Escape
If a crew member has a serious injury or illness, he or she will need to get back to Earth as soon as possible. The whole crew of the space station might have to evacuate in the case of a serious fire, or some other life-threatening damage to the station. So there has to be a way to escape the station quickly. A Soyuz capsule will always be docked at the ISS, capable of carrying two people in a medical emergency, or three people in other emergencies. A crew will take a fresh Soyuz capsule to the station every six months.

NASA is designing and building a crew-return vehicle (CRV), called the X-38, for emergency use. The X-38 will be capable of transporting seven people to the surface.

Photo courtesy NASA Artist's rendering of the X-38 leaving the ISS.

Photo courtesy NASA X-38 in free flight test.

The X-38 will weigh 20,000 pounds (9,072 kg). Its design is a lifting body style -- that is, the shape of the body, instead of wings, generates lift -- with a de-orbit engine. This engine weighs 95,000 pounds (43,000 kg), and can only slow the craft down for re-entry. The X-38 also has a parafoil parachute and landing skids. The craft will fire its de-orbit engine and throw its engine away once the fuel is gone. When the X-38 re-enters the atmosphere, it will be protected from the heat of re-entry by ceramic tiles, like the space shuttle. Once through the atmosphere, the X-38 will glide toward its landing site, use its parachute to slow down and steer, and touchdown on its skid. While the X-38 is designed to fly automatically, it can also be flown manually.