TheIntegrated Truss Structure (ITS) of theInternational Space Station (ISS) consists of a linear arranged sequence of connectedtrusses on which various unpressurized components are mounted such as logistics carriers,radiators,solar arrays, and other equipment. It supplies the ISS with abus architecture. It is approximately 110 meters long and is made fromaluminium andstainless steel.
Alltruss 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 ofDestiny and is neither starboard nor port side.
ISS truss segments werefabricated by Boeing in its facilities atHuntington Beach, California (formerly McDonnell Douglas),Michoud Assembly Facility inNew Orleans, Louisiana,Marshall Space Flight Center inHuntsville, Alabama, and inTulsa, Oklahoma.[citation needed] The trusses were then transported or shipped to Kennedy Space Center'sSpace Station Processing Facility for final assembly and checkout.
The structural framework was made using several manufacturing processes, including theinvestment casting, steelhot rolling, friction-stir, andTIG welding processes.[citation needed]
The first truss piece, the Z1 truss, launched aboardSTS-92 in October 2000. It contains thecontrol moment gyroscope (CMG) assemblies, electrical wiring, communications equipment, and twoplasma 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 aCommon Berthing Mechanism (CBM) port that connects its nadir to the zenith port ofUnity and contains a small pressurized dome that allowed astronauts to connect electrical ground straps betweenUnity and the truss without an EVA.[1][2] In addition, the dome inside the CBM of Z1 can be used as storage space.[3]
The Z1 truss also features a forward-facing Manual Berthing Mechanism (MBM) ring.[4] 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.[5] The Z1 truss's MBM was used only once, to temporarily holdPMA-2, while theDestiny lab was being berthed onto theUnity node duringSTS-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 toUnity (Node 1) and no longer houses other space station elements.
In December 2008, theAd Astra Rocket Company announced an agreement with NASA to place a flight test version of itsVASIMR 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.[6] NASA and Ad Astra signed a contract for development of the VASIMR engine for up to three years in 2015.[7] 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".[8] (An example of a spacecraft that used anion thruster to maintain its orbit was theGravity Field and Steady-State Ocean Circulation Explorer, whose engine allowed it to maintain a very low orbit.)
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The S0 truss, (also called theCenter Integrated Truss Assembly Starboard 0 Truss) forms the central backbone of the Space Station. It was attached on the top of theDestiny Laboratory Module duringSTS-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) stainless steel struts.
The P1 and S1 trusses (also called thePort and Starboard Side Thermal Radiator Trusses) are attached to the S0 truss and contain carts to transport theCanadarm2 and astronauts to worksites along with the space station. They each flow 290 kg (637 lb) ofanhydrousammonia through three heat rejection radiators. The S1 truss was launched onSTS-112 in October 2002 and the P1 truss was launched onSTS-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.
The P2 and S2 trusses were planned as locations for rocket thrusters in the original design forSpace Station Freedom. Since the Russian parts of the ISS also provided that capability, thereboost capability of the Space Station Freedom design was no longer needed at that location. As such, P2 and S2 were canceled.[9]
The P3/P4 truss assembly was installed by theSpace Shuttle AtlantisSTS-115 mission, launched September 9, 2006, and attached to the P1 segment. The P3 and P4 segments together contain a pair ofsolar arrays, a radiator, and arotary 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 bySTS-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 bySpace ShuttleAtlantis during flightSTS-117, mission13A and mounted to the S1 truss segment. It is the heaviest station-bound module ever launched by the Space Shuttle.[10]
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 worksite accommodations for theMobile Transporter. The P3/S3 primary structure is made of a hexagonal-shaped aluminum structure and includes four bulkheads and sixlongerons.[11] The S3 truss also supportsEXPRESS 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 twoSolar Array Wings (SAW), thePhotovoltaic Radiator (PVR), the Alpha Joint Interface Structure (AJIS), and Modified Rocketdyne Truss Attachment System (MRTAS), and Beta Gimbal Assembly (BGA).
Years later, iROSA 3 and 4 was added in front of Old 3A and 4A solar arrays on S4 and P4 truss respectively and iROSA 5 was added in front of Old 1B solar array on S4 truss in December 2022 and June 2023 respectively.
The P5 and S5 trusses are connectors that support the P6 and S6 trusses, respectively. The P3/P4 and S3/S4 truss assemblies' length was limited by the cargo bay capacity of theSpace Shuttle, so these small (3.37 m long) connectors are needed to extend the truss. The P5 truss was installed on December 12, 2006, during the firstEVA of missionSTS-116. The S5 truss was brought into orbit by missionSTS-118 and installed on August 11, 2007.
The P6 truss was the second truss segment to be added because it contains a largeSolar 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 duringSTS-97, but the SAW was folded, one half at a time, to make room for the SAWs on the P4 and S4 trusses, duringSTS-116 andSTS-117 respectively. Shuttle missionSTS-120 (assembly mission10A) 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 sequenceSTS-119) mounted the S6 truss on the S5 truss, which provided a fourth and final set of solar arrays and radiators.
Years later, iROSA 1 and 2 was added in front of Old 4B and 2B solar arrays on P6 truss and iROSA 6 was added in front of Old 1B solar array on S6 truss in June 2021 and June 2023 respectively.
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TheInternational Space Station's main source ofenergy is from the four large U.S.-madephotovoltaic arrays currently on the station, sometimes referred to as theSolar Array Wings (SAW). The first pair of arrays are attached to the P6 truss segment, which was launched and installed on top of Z1 in late 2000 duringSTS-97. The P6 segment was relocated to its final position, bolted to the P5 truss segment, in November 2007 duringSTS-120. The second pair of arrays was launched and installed in September 2006 duringSTS-115, but they didn't provide electricity untilSTS-116 in December 2006 when the station got an electrical rewiring. The third pair of arrays was installed duringSTS-117 in June 2007. A final pair arrived in March 2009 onSTS-119. More solar power was to have been available via theRussian-builtScience Power Platform, but it was canceled.[11]
Each of the Solar Array Wings are 34 m (112 ft) long by 12 m (39 ft) wide, have roughly 1,100 kg (2,400 lb) of mass, and are capable of generating nearly 30kW ofDC power.[12] They are split into two photovoltaic blankets, with the deployment mast in between. Each blanket has 16,400siliconphotovoltaic cells, each cell measuring 8 cm x 8 cm, grouped into 82 active panels, each consisting of 200 cells, with 4,100diodes.[11]
Each pair of blankets was folded like anaccordion 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 theBeta Gimbal Assembly (BGA) are used torotate the arrays so that they face the Sun to provide maximum power to the International Space Station.[citation needed]
Over time, the photovoltaic cells on the wings have degraded gradually, having been designed for a 15-year service life. This is especially noticeable with the first arrays to launch, with the P6 and P4 Trusses in 2000 and 2006. To augment the P6 truss' wings, in June 2021 and November 2022, NASA launched four of a scaled-up version of theRoll Out Solar Array, in two pairs, aboard theSpaceX Dragon 2 missionsSpaceX CRS-22,-26 and-28. These arrays are more lightweight and generate more energy than the existing arrays. They are intended to be deployed along the central part of the wings up to two thirds of their length. Work to install support brackets for the new arrays on the P6 truss mast cans was initiated by the members ofExpedition 64.[13] Work to install and deploy the first two arrays themselves on the P6 brackets was successfully conducted over three spacewalks byShane Kimbrough andThomas Pesquet ofExpedition 65.[14][15][16] In November and December 2022, astronautsFrancisco Rubio andJosh A. Cassada ofExpedition 68 installed the second set of brackets and arrays, one each on the P4 and S4 Trusses.[17][18][19][20] In June 2023, astronautsStephen Bowen andWarren Hoburg ofExpedition 69 installed the third set of brackets and arrays, one each on the S6 and S4 Trusses.[21] A final set of arrays will be installed on the P4 and S6 trusses in 2025.[22]
TheAlpha joint is the main rotary joint allowing the solar arrays to track the sun; in nominal operation the alpha joint rotates by 360° each orbit (however, see alsoNight Glider mode). One Solar Alpha Rotary Joint (SARJ) is located between the P3 and P4 truss segments and the other is located between the S3 and S4 truss segments. When in operation, these joints continuously rotate to keep the solar array wings on the outboard truss segments oriented towards the Sun. Each SARJ is 10 feet in diameter, weighs approximately 2,500 pounds and can be rotated continuously using bearing assemblies and a servo control system. On both the port and starboard sides, all of the power flows 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 byLockheed Martin and its subcontractors.[11]
The Solar Alpha Rotary Joints contain Drive Lock Assemblies which allow the outer segments of the ITS to rotate and track theSun. A component of the DLA is apinion which engages with the race ring that serves as abull gear. There are two race rings and two DLAs in each SARJ providing on-orbit redundancy, however a series ofspace walks would be required to reposition the DLAs and theTrundle Bearing Assemblies (TBAs) to utilize the alternate race ring. A spare DLA was brought to the ISS onSTS-122.[23]
In 2007, a problem was detected in the starboard SARJ and in one of the two beta gimbal assemblies (BGA).[24] Damage had occurred due to excessive and premature wear of a track in the joint mechanism. The SARJ was frozen during problem diagnosis, and in 2008 lubrication was applied to the track to address the issue.[25]
The sequential shunt unit (SSU) is designed to coarsely regulate the solar power collected during periods of insolation—when the arrays collect power during sun-pointing periods. A sequence of 82 separate strings, or power lines, leads from the solar array to the SSU. Shunting, or controlling, the output of each string regulates the amount of power transferred. The regulated voltage setpoint is controlled by a computer located on the IEA and is normally set to around 140 volts. The SSU has an overvoltage protection feature to maintain the output voltage below 200 V DC maximum for all operating conditions. This power is then passed through the BMRRM to the DCSU located in the IEA. The SSU measures 32 by 20 by 12 inches (81 by 51 by 30 cm) and weighs 185 pounds (84 kg).[citation needed]
Each battery assembly, situated on the S4, P4, S6, and P6 Trusses, consists of 24 lightweightlithium-ion battery cells and associated electrical and mechanical equipment.[26][27] Each battery assembly has a nameplate capacity of 110 Ah (396,000 C) (originally 81 Ah) and 4 kWh (14 MJ).[28][29][30] This power is fed to the ISS via the BCDU and DCSU respectively.
The batteries ensure that the station is never without power to sustain life-support systems and experiments. During the sunlight part of the orbit, the batteries are recharged. The nickel-hydrogen batteries had a design life of 6.5 years which means that they were replaced multiple times during the expected 30-year life of the station.[31][29] The batteries and the battery charge/discharge units are manufactured bySpace Systems/Loral (SS/L),[32] under contract toBoeing.[33] Ni-H2 batteries on the P6 truss were replaced in 2009 and 2010 with more Ni-H2 batteries brought by Space Shuttle missions.[30] The nickel-hydrogen batteries had a design life of 6.5 years and could exceed 38,000 charge/discharge cycles at 35% depth of discharge. Each battery measured 40 by 36 by 18 inches (102 by 91 by 46 cm) and weighed 375 pounds (170 kg).[34][29]
From 2017 to 2021, the nickel-hydrogen batteries were replaced bylithium-ion batteries.[30] On January 6, 2017,Expedition 50 membersShane Kimbrough andPeggy Whitson began the process of converting some of the oldest batteries on the ISS to the new lithium-ion batteries.[30]Expedition 64 membersVictor J. Glover andMichael S. Hopkins concluded the campaign on February 1, 2021.[35][36][37][38] There is a number of differences between the two battery technologies. One difference is that the lithium-ion batteries can handle twice the charge, so only half as many lithium-ion batteries were needed during replacement.[30][29] Also, the lithium-ion batteries are smaller than the older nickel-hydrogen batteries.[30] Although Li-ion batteries typically have shorter lifetimes than Ni-H2 batteries as they cannot sustain as many charge/discharge cycles before suffering notable degradation, the ISS Li-ion batteries have been designed for 60,000 cycles and ten years of lifetime, much longer than the original Ni-H2 batteries' design life span of 6.5 years.[30]
The Mobile Base System (MBS) is a platform (mounted on theMobile Transporter) for the robotic armsCanadarm2 andDextre carrying them 108 metres down rails between the S3 and P3 truss.[39] Beyond the rails Canadarm2 can step over the alpha rotary joint and relocate tograpple fixtures on the S6 and P6 truss. During STS-120 Astronaut Scott Parazynski rode theOrbiter Boom Sensor to repair a tear in the 4B solar array.
The first truss segment to be launched was Z1, which was mounted to theUnity module's zenith (facing away from Earth)Common Berthing Mechanism. It was followed by P6, which was mounted atop (zenith side) the Z1 truss. Next, the S0 truss was mounted atop theDestiny module. The other truss elements were attached sequentially to either side of S0. As the truss neared completion, the P6 truss was relocated from Z1 to the end of P5.
| Element[40] | Flight | Launch date | Length (m) | Diameter (m) | Mass (kg) |
|---|---|---|---|---|---|
| Z1 | 3A—STS-92 | October 11, 2000 | 4.6 | 4.2 | 8,755 |
| P6 | 4A—STS-97 | November 30, 2000 | 18.3 | 10.7 | 15,824 |
| S0 | 8A—STS-110 | April 8, 2002 | 13.4 | 4.6 | 13,971 |
| S1 | 9A—STS-112 | October 7, 2002 | 13.7 | 4.6 | 14,124 |
| P1 | 11A—STS-113 | November 23, 2002 | 13.7 | 4.6 | 14,003 |
| P3/P4 | 12A—STS-115 | September 9, 2006 | 13.7 | 4.8 | 15,824 |
| P5 | 12A.1—STS-116 | December 9, 2006 | 3.37 | 4.55 | 1,864 |
| S3/S4 | 13A—STS-117 | June 8, 2007 | 13.7 | 10.7 | 15,824 |
| S5 | 13A.1—STS-118 | August 8, 2007 | 3.37 | 4.55 | 1,818 |
| P6 (relocation) | 10A—STS-120 | October 23, 2007 | 18.3 | 10.7 | 15,824 |
| S6 | 15A—STS-119 | March 15, 2009 | 13.7 | 10.7 | 15,824 |
This article incorporates text from this source, which is in thepublic domain. This article incorporates text from this source, which is in thepublic domain. This article incorporates text from this source, which is in thepublic domain. This article incorporates text from this source, which is in thepublic domain. This article incorporates text from this source, which is in thepublic domain.Components of theInternational Space Station | ||||||||||
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