page_title: Orbital replacement unit
section_title: Introduction
hierarchical_section_title: Orbital replacement unit / Introduction
caption_attribution_description: English: Truss Breakdown and ORUs in situ
context_page_description: Orbital replacement units are key elements of the International Space Station that can be readily replaced when the unit either passes its design life or fails. ORUs are parts of the main systems and subsystems of the external elements of the ISS, none are intended to be installed inside the pressurised modules. Examples of ORUs are: pumps, storage tanks, controller boxes, antennas, and battery units. Such units are replaced either by astronauts during EVA or by the Dextre robotic arm. All are stored on the three external stowage platforms or the four ExPRESS Logistics Carriers mounted on the Integrated Truss Structure.
context_section_description: While spare parts/ORUs were routinely brought up and down during the ISS life-time via Space Shuttle resupply missions, there was a heavy emphasis once the Station was considered complete.
  Several Shuttle missions were dedicated to the delivery of ORUs using support carrier structures/pallets of which some remained in the cargo bay, some that were deployed and retrieved and other pallets that were designed to be removed from the payload bay by RMS and placed onto the station.
  Deployable pallet flights included STS-102 with External Stowage Platform ESP-1, STS-114 with ESP-2, STS-118 with ESP-2, STS-129 with ExPRESS Logistics Carrier ELC-1 and ELC-2, STS-133 with ELC-4 and STS-134 with ELC-3.
  Other modes of ORU delivery included:
  Payload bay sidewall mounted ORUs, such as BCDUs, were regularly carried and transported to the ISS via EVAs.
  Also, three flights of the  Integrated Cargo Carrier (ICC) which remained in the cargo bay on flights STS-102, STS-105 and STS-121; one use of the ICC-Lite on STS-122 (a shortened version of the ICC); two uses of the ICC-Vertical Light Deployable on STS-127 as ICC-VLD and STS-132 as ICC-VLD2, which were deployed and retrieved during the mission; and five uses of the Lightweight MPESS Carrier (LMC) on STS-114, STS-126, STS-128, STS-131 and STS-135, the LMC was not designed to be deployed and remained in the shuttle payload bay throughout the flight.
  To date other than the Space Shuttle missions, only one other mode of transportation of ORUs was utilised by the station, the Japanese cargo vessel HTV-2 delivered an FHRC and CTC-4 via its Exposed Pallet (EP), and HTV-4 delivered a Main Bus Switching Unit (MBSU) and a Utility transfer assembly (UTA).
caption: ISS Integrated Truss Structure detailing all Orbital Replacement Units in situ
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