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The Integrated Truss Structure forms the backbone of the International Space Station, with mountings for unpressurized logistics carriers, radiators, solar arrays, and other equipment. In the initial Space Station Freedom plans, a variety of designs for the truss were used, all of them intended to be shipped up as girders where they would be assembled and their equipment installed by astronauts on spacewalks once it had been launched. After the 1991 redesign, NASA switched to shorter, prefabricated pieces that would require minimal installation. Z1 Truss The first truss piece, the Z1 truss, launched on STS-92 in October 2000 is being used as a temporary mounting position for the P6 truss and solar array, attached on the subsequent shuttle flight, STS-97. The Z1 truss was the first permanent lattice-work structure for the ISS, very much like a girder, setting the stage for the future addition of the station's major trusses or backbones. It contains the control moment gyro (CMG) assemblies, electrical wiring, communications equipment, and two plasma contractors designed to neutralize the static electrical charge of the space station. S0 Truss The S0 Truss, (also called the Center Integrated Truss Assembly Starbord 0 Truss) forms the center backbone of the Space Station. It was attached to the Destiny Laboratory Module during STS-110 in April 2002. S0 is used to route power to the pressurized station modules and conduct heat away from the modules to the S1 and P1 Trusses. P1, S1 Trusses The P1 and S1 trusses (also called the Port and Starboard Side Thermal Radiator Trusses) are attached to the S0 truss, and contain carts to transport the Canadarm2 and astronauts to worksites along the space station. They each flow 290 kg (637 lb) of anhydrous ammonia through three heat rejection radiators. The S1 truss was launched on STS-112 in October 2002 and the P1 truss was launched on STS-113 in November 2002. P2, S2 Trusses The P2 and S2 trusses were planned as locations for rocket thrusters in the original design for Space Station Freedom. Since the Russians are part of the ISS, the reboost capability was no longer needed at that location. So P2 and S2 were canceled. * P3/P4, S3/S4 Truss Assemblies The P3/P4 truss assembly was installed by the Space Shuttle ''Atlantis'' STS-115 mission, launched September 9, 2006, and attached to the P1 segment. The P3 and P4 segments together contain a pair of solar arrays, a radiator and a rotary joint that will aim the solar arrays, and connects P3 to P4. Currently no power is flowing across the rotary joint, so the electricity generated by the P4 solar array wings is only being used on the P4 segment, and not the rest of the station. A major electrical rewiring of the station, scheduled for STS-116 in late 2006, will fix this. Major P3 subsystems include the Segment-to-Segment Attach System (SSAS), Solar Alpha Rotary Joint (SARJ), and Unpressurized Cargo Carrier Attach System (UCCAS). The primary functions of the P3 truss segment are to provide mechanical, power and data interfaces to payloads attached to the two UCCAS platforms; axial indexing for solar tracking, or rotating of the arrays to follow the sun, via the SARJ; movement and work site accommodations for the Mobile Transporter. The P3 primary structure is made of a hexagonal shaped aluminum structure and includes four bulkheads and six longerons.• Major subsystems of the P4 Photovoltaic Module (PVM) include the two Solar Array Wings (SAW), the Photovoltaic Radiator (PVR), the Alpha Joint Interface Structure (AJIS), and Modified Rocketdyne Truss Attachment System (MRTAS), and Beta Gimbal Assembly (BGA). The S3/S4 truss assembly, identical to P3/P4, will attach to the truss on the starboard side, and is scheduled to arrive with mission 13A on STS-117. P5, S5 Trusses The P5 and S5 trusses are connectors which will support the P6 and S6 trusses, respectively. The P3/P4 and S3/S4 truss assemblies' length was limited by the cargo bay capacity of the Space Shuttle, so these small connectors are needed to extend the truss. P6, S6 Trusses The P6 truss was the second truss segment to be added, because it contains a large Solar Array Wing (SAW) that generates essential electricity for the station. It is currently mounted to the Z1 truss but it will be moved far along the port side main truss and mounted on the P5 truss during assembly mission 10A, STS-120. A later assembly mission will mount the S6 trusses on the starboard side and provide a fourth set of solar arrays and radiators. Truss subsystems Solar arrays The International Space Station's main source of energy is from two of the four large U.S.-made photovoltaic arrays currently on the station, sometimes referred to as the Solar Array Wings (SAW). The first pair of arrays are attacted to the P6 truss segment, which was launched and installed late 2000 during STS-97. The second pair was launched and installed in September 2006 during STS-115, but they won't provide electricity until STS-116 in late 2006 when the station gets an electrical rewiring. Two more nearly identical pairs of solar panels are planned to arrive on future missions. More solar power would be available via the Russian-built Science Power Platform, but it was cancelled. Each of the Solar Array Wings are 34 m (112 ft) long by 12 m (39 ft) wide, and are capable of generating nearly 32.8 kW of DC power. They are split into two photovoltaic blankets, with the deployment mast in between. Each blanket has 16,400 silicon photovoltaic cells, grouped into 82 active panels, each consisting of 200 cells, and each cell is 8 cm² with 4,100 diodes. Each pair of blankets is folded like an accordion for compact delivery to space. Once in orbit, the deployment mast between each pair of blankets unfolds the array to its full length. Gimbals, known as the Beta Gimbal Assembly (BGA) are used to rotate the arrays so that they face the Sun to provide maximum power to the Space Station. Solar Alpha Rotary Joint The Solar Alpha Rotary Joint (SARJ), is a joint located between the P3 and P4 truss segments that continuously rotates to keep the solar array wings on P4 oriented towards the sun as the station orbits the earth. The SARJ is a 10 foot diameter and weighs approximately 2,500 pounds. The SARJ can spin 360 degrees using bearing assemblies and a servo control system to turn. All of the power will flow through the Utility Transfer Assembly (UTA) in the SARJ. Roll ring assemblies allow transmission of data and power across the rotating interface so it never has to unwind. The SARJ was designed, built and tested by Lockheed Martin and its subcontractors. Truss and Solar Array Assembly Sequence See also | |||||||
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