Orbital replacement unit

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.

All are stored on the three external stowage platforms (ESPs) or the four ExPRESS Logistics Carriers (ELCs) mounted on the Integrated Truss Structure (ITS).

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[clarification needed] once the Station was considered complete.

Other modes of ORU delivery included: Payload bay sidewall mounted ORUs, such as BCDUs, were regularly carried and transported to the ISS via EVAs.

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),[8] and HTV-4 delivered a Main Bus Switching Unit (MBSU) and a Utility transfer assembly (UTA).

[9] Orbital replacement units are parts of the main systems and subsystems of the external elements of the ISS.

As well as storage tanks for oxygen as part of the station Environmental Control and Life Support System (ECLSS).

ORUs can be hardware such as radiators, or simply batteries or communication antennas, essentially any element that can readily be removed and replaced when required.

ORUs to be handled by Dextre have attachments designed to be gripped with the ORU/Tool Changeout Mechanisms (OTCM) on the end of each arm.

A Modified Truncated Cone (MTC) Target is used to visually line up Dexter's arm to grab a fixture.

The LDU provides drive and stopping forces for the mobile transporter along the integrated truss structure rail.

This prevents the electrical discharges and provides a means of controlling crew shock hazard during EVA.