ISS Integrated Truss Structure

The Integrated Truss Structure (ITS) of the International Space Station (ISS) consists of a linearly arranged sequence of connected trusses on which various unpressurized components are mounted, such as logistics carriers, radiators, solar arrays, and other equipment. It supplies the ISS with a bus architecture. All truss components were named after their planned end-positions: Z for zenith, S for starboard and P for port, with the number indicating the sequential position. The S0 truss might be considered a misnomer, as it is mounted centrally on the zenith position of Destiny and is neither starboard nor port side.

The Z1 truss, first truss piece, launched aboard STS-92 in October 2000. It contains the control moment gyroscope (CMG) assemblies, electrical wiring, communications equipment, and two plasma contactors designed to neutralize the static electrical charge of the space station. Another objective of the Z1 truss was to serve as a temporary mounting position for the “P6 truss and solar array” until its relocation to the end of the P5 truss during STS-120. Though not a part of the main truss, 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. In October 2007, the P6 was moved to its permanent position next to P5 – the Z1 truss is now solely used to house the CMGs, communications equipment and the plasma contactors; Z1 no longer connects other space station elements to Unity (Node 1).

The S0 truss (also called the Center Integrated Truss Assembly Starboard 0 Truss) forms the center backbone of the Space Station. It was attached on the top of 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. The S0 truss is not docked to the ISS, but is connected with four Module to Truss Structure (MTS) struts.

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. Detailed design, test and construction of the S1 and P1 structures was conducted by McDonnell Douglas (now Boeing) in Huntington Beach, CA. First parts were cut for the structure in 1996, and delivery of the first truss occurred in 1999.

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. Upon its installation, no power was flowing across the rotary joint, so the electricity generated by the P4 solar array wings was only being used on the P4 segment, and not the rest of the station. Then in December 2006 a major electrical rewiring of the station by STS-116 routed this power to the entire grid. The S3/S4 truss assembly—a mirror-image of P3/P4—was installed on June 11, 2007 also by Space Shuttle Atlantis during flight STS-117, mission 13A and mounted to the S1 truss segment.

Major P3 and S3 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/S3 primary structure is made of a hexagonal shaped aluminum structure and includes four bulkheads and six longerons. The S3 truss also supports EXPRESS Logistics Carrier locations, first to be launched and installed in the 2009 time frame. Major subsystems of the P4 and S4 Photovoltaic Modules (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 P5 and S5 trusses are connectors which 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. The P5 truss was installed on December 12, 2006 during the first EVA of mission STS-116. The S5 truss was brought into orbit by mission STS-118 and installed on August 11, 2007.

The P6 truss was the second truss segment to be added, because it contains a large Solar Array Wing (SAW) that generated essential electricity for the station, prior to activation of the SAW on the P4 truss. It was originally mounted to the Z1 truss and had its SAW extended during STS-97, but the SAW was folded, one half at a time, to make room for the SAWs on the P4 and S4 trusses, during STS-116 and STS-117 respectively. Shuttle mission STS-120 (assembly mission 10A) detached the P6 truss from Z1, remounted it on the P5 truss, redeployed its radiator panels and attempted to redeploy its SAWs. One SAW (2B) was deployed successfully but the second SAW (4B) developed a significant tear that temporarily stopped deployment at around 80%. This was subsequently fixed and the array is now fully deployed. A later assembly mission (the out of sequence STS-119) mounted the S6 truss on the S5 truss, which provided a fourth and final set of solar arrays and radiators.