Orbital replacement unit

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.

Payload bay sidewall mounted ORUs, such as BCDUs, were regularly carried and transported to the ISS via EVAs.

The replaceable modular nature of the station allows its life to be extended well beyond its initial design life, theoretically.

The TUS reel assembly (TUS-RA) is basically a large spool much like a garden hose reel that pays out cable when the MT moves away and rolls it back up as the MT returns to the center of the truss. This is the same TUS-RA retrieved during STS-121. It was replaced and this failed unit was returned to earth and refurbished to later fly on ELC-2.

The Heat Rejection Subsystem (HRS) consists of a base, eight panels, torque panel, torque arm, an interconnected fluid system, a scissors-type deployment mechanism and a computer controlled motor/cable deployment system. Part of the station's external active thermal control system (EATCS), the HRS radiator rejects thermal energy via radiation.

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

Plasma Contactor Unit (PCU) is used to disperse the electrical charge that builds up by providing an electrically conductive "ground path" to the plasma environment surrounding the ISS. This prevents the electrical discharges and provides a means of controlling crew shock hazard during EVA. There are two PCUs located on the ISS Zenith 1 Truss, both of which are operated during EVA.