Integrated Truss Structure

The Integrated Truss Structure (ITS) of the International Space Station (ISS) consists of a linear 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. It is approximately 110 metres (360 feet) long, weighs slightly under 118 tonnes (260,000 lb) with all its attached equipment, and is made primarily from aluminium and stainless steel.

Truss components

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.

Manufacturing

ISS truss segments were fabricated by Boeing in its facilities at Huntington Beach, California (formerly McDonnell Douglas), Michoud Assembly Facility in New Orleans, Louisiana, Marshall Space Flight Center in Huntsville, Alabama, and in Tulsa, Oklahoma. The trusses were then transported or shipped to Kennedy Space Center's Space Station Processing Facility for final assembly and checkout. The structural framework was made using several manufacturing processes, including the investment casting, steel hot rolling, friction-stir, and TIG welding processes.

Z1 truss

The first truss piece, the Z1 truss, 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. It is made from stainless steel, titanium, and aluminum alloys. While the bulk of the Z1 truss is unpressurized, it features a Common Berthing Mechanism (CBM) port that connects its nadir to the zenith port of Unity and contains a small pressurized dome that allowed astronauts to connect electrical ground straps between Unity and the truss without an EVA. In addition, the dome inside the CBM of Z1 can be used as storage space. The Z1 truss also features a forward-facing Manual Berthing Mechanism (MBM) ring. This MBM is not a port and is not pressurized or electrically powered, but it can be operated with a handheld tool to berth any passive CBM to it. The Z1 truss's MBM was used only once, to temporarily hold PMA-2, while the Destiny lab was being berthed onto the Unity node during STS-98. Since the installation of the nearby S0 truss in April 2002, access to the MBM has been blocked. In October 2007, the P6 truss element was disconnected from Z1 and moved to P5; P6 will now be permanently connected with P5. The Z1 truss is now solely used to house the CMGs, communications equipment, and the plasma contactors; furthermore, Z1 connects now solely to Unity (Node 1) and no longer houses other space station elements. In December 2008, the Ad Astra Rocket Company announced an agreement with NASA to place a flight test version of its VASIMR ion thruster on the station to take over reboost duties. In 2013, the thruster module was intended to be placed on top of the Z1 truss in 2015. NASA and Ad Astra signed a contract for development of the VASIMR engine for up to three years in 2015. However, in 2015 NASA ended plans for flying the VF-200 to the ISS. A NASA spokesperson stated that the ISS "was not an ideal demonstration platform for the desired performance level of the engines". (An example of a spacecraft that used an ion thruster to maintain its orbit was the Gravity Field and Steady-State Ocean Circulation Explorer, whose engine allowed it to maintain a very low orbit.)

S0 truss

The S0 truss, (also called the Center Integrated Truss Assembly Starboard 0 Truss) forms the central 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 to the Destiny Lab module by both the Module-to-Truss Structure Attachment System (MTSAS) and four Module-to-Truss Structure (MTS) extruded aluminum struts. The Module-to-Truss Structure Attachment System is in two halves. The active half Lab Cradle Assembly bolted to the aft and mid-rings of the Destiny Lab and the passive half grapple bar and alignment pins integral to S0.

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 with 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 were 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.

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 Russian parts of the ISS also provided that capability, the reboost capability of the Space Station Freedom design was no longer needed at that location. As such, P2 and S2 were canceled.

P3/P4, S3/S4 truss assemblies

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