Orbital replacement unit

Orbital replacement unit

Orbital Replacement Units (ORUs) are key elements of the International Space Station that can be readily replaced when the unit either passes its design life or fails. 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 SPDM Arm. All are stored on the three External Stowage Platforms (ESPs) or the four ExPRESS Logistics Carriers (ELCs) mounted on the Integrated Truss Structure (ITS).

Location of ESPs and ELCs on the International Space Station.



ISS Integrated Truss Structure detailing all Orbital Replacement Units in situ

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[1] with External Stowage Platform ESP-1, STS-114[2] with ESP-2, STS-118[3] with ESP-2, STS-129[4] with ExPRESS Logistics Carrier ELC-1 and ELC-2, STS-133[5] with ELC-4 and STS-134[6] 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).[7]

Modes of ORU Transport to the ISS

Types of ORUs

Orbital replacement units are parts of the main systems and subsystems of the external elements of the ISS. Affecting the control of the cooling system, the movement and control of the solar arrays and SARJ as well as the flow of power throughout the station from solar arrays to the heat rejection system as part of the External Active Thermal Control System (EATCS). 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.

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

Orbital replacement unit (ORU) spares

(Weight, description and current location of the spare on the station)

Multiple spares

  • Flex hose rotary coupler (FHRC) weight approx. 900 lbs. × 1 unit each on S1 & P1 Truss. The FHRC provides the transfer of liquid ammonia across the Thermal Radiator Rotary Joint (TRRJ) between the P1 & S1 truss segments and the Heat Rejection System Radiators (HRSRs).

Three spares – ESP-2 FRAM-7 (keel side)[2], ESP-3 FRAM-2 (top side)[8], ELC-4 FRAM-5 (keel side)[7]

  • Pump Module (PM) weight 780 lbs. x 1 unit each on S1 & P1 Truss. The PM is part of the station’s complex External Active Thermal Control System (ETCS), which provides vital cooling to internal and external avionics, crew members, and payloads. The station has two independent cooling loops. The external loops use an ammonia-based coolant and the internal loops use water cooling.

Three spares – ESP-3 FRAM-3 (top side)[9], ELC-1 FRAM-7 (keel side)[4], ELC-2 FRAM-6 (keel side)[4]

  • Ammonia tank assembly (ATA) weight 1,702 lbs. x 1 unit each on S1 & P1 truss. The primary function of the ATA is to store the ammonia used by the external thermal control system (ETCS). The major components in the ATA include two ammonia storage tanks, isolation valves, heaters, and various temperature, pressure, and quantity sensors. There is one ATA per loop located on the zenith side of the Starboard 1 (Loop A) and Port 1 (Loop B) truss segments.

Two spares – ELC-1 FRAM-9 (keel side)[4], ELC-3 FRAM-5 (keel side)[6]

  • Nitrogen tank assembly (NTA) weight 550 lbs. each × 1 unit each on S1 & P1 truss. The NTA provides a high-pressure gaseous nitrogen supply to control the flow of ammonia out of the ATA. The ATA contains two flexible, chambers incorporated into its ammonia tanks that expand as pressurized nitrogen expels liquid ammonia out of them.

Two spares – ELC-1 FRAM-6 (keel side)[4] ELC-2 FRAM-9 (keel side)[4]

  • High-pressure gas tank assembly (HPGTA) weight 1,240 lbs. x 5 units on quest. High-pressure oxygen and nitrogen gas tanks onboard the ISS provide support for EVA and contingency metabolic support for the crew. This high-pressure O2 and N2 is brought to the ISS by the high-pressure gas tanks (HPGT) and is replenished by the Space Shuttle.

One spare – ELC-3 FRAM-6 (keel side)[6], one depleted tank ELC-2 FRAM-4 (top side)[4]

  • Cargo Transport Container (CTC) each can weigh between 1,000 and 1,300 lbs. A container used to transport smaller ORUs such as Remote Power Control Modules in bulk, which may also be used during EVA or by the SPDM. NASA purchased 5 CTCs for such deliveries.

Three units – CTC-3 on ELC-2 FRAM-2 (top side)[4], CTC-2 on ELC-4 FRAM-2 (keel side)[7], CTC-5 on ELC-3 FRAM-1 (top side)[6]

  • Pitch/roll joint (P/R‐J) x 2 units on the SSRMS. A Wrist joint with several degrees of freedom, designed to be replaced on orbit if required.

Two spares – ESP-3 FRAM-1 (top side)[3], ESP-2 FRAM-5 (keel side)[2]

  • Control moment gyroscope (CMG) weight 600 lbs. × 4 units on Z1 Truss. A CMG consists of a single-piece 25-inch diameter, 220-pound stainless steel flywheel that rotates at a constant speed of 6,600 rpm and develops an angular momentum of 3,600 ft-lb-sec (4,880 N-m-s) about its spin axis. The CMGs can also be used to perform attitude maneuvers. The CMGs rely on electrical power provided by the solar powered electrical subsystem.

Two spares – ELC-1 FRAM-5 (top side)[4], ELC-2 FRAM-5 (top side)[4]

  • S-band antenna support assembly (SASA) weight 256 lbs each × 2 active units and one other spare on ISS. The SASA consists of the assembly contingency radio frequency group (RFG, or ACRFG), SASA boom and avionics wire harness.

Two spares – ELC-3 FRAM-4 (top side)[6], ELC-3 FRAM-7 (keel side)[6]

  • Direct-current switching unit (DCSU) weight 218 lbs. x 2 units each on the 4 IEAs. The DCSU routes battery power to the MBSU distribution bus to satisfy power demands. In addition to primary power distribution, the DCSU has the additional responsibilities of routing secondary power to components on the PV modules.

Three spares – ESP-1 FRAM-2[1], , ESP-2 FRAM-2 (top side)[2], ESP-2 FRAM-3 (top side)[2]

  • Battery charge/discharge unit (BCDU) weight 235 lbs. × 6 each on each of the 4 IEAs. The BCDU is a bidirectional power converter that serves a dual function of charging the batteries during solar collection periods (isolation) and providing conditioned battery power to the primary power buses during eclipse periods.

Two spares – ESP-3 FRAM-6 (keel side)[3], ELC-1 FRAM-4 (top side)[4]

  • Main bus switching unit (MBSU) weight 220 lbs. × 4 units on S0 Truss. The MBSUs act as the distribution hub for the EPS system. The four MBSUs onboard the ISS are all located on the starboard zero (S0) truss. Each of MBSU receives primary power from two power channels and distributes it downstream to the DDCUs.

Two spares – ESP-2 FRAM-4 (top side)[2], ESP-2 FRAM-6 (keel side)[2]

Single Spares

  • Mobile transporter trailing umbilical system-reel assembly (MT TUS-RA) weight 354 lbs. at ELC-2 FRAM-8 (keel side)[4] x 1 unit on MT

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.

  • Latching end effector (LEE) weight 415 lbs. at ELC-1 FRAM-1 (top side)[4] x 3 units on ISS

The SSRMS has two identical grapple end points called LEE that enable it to reattach either end to the station as its new base.

  • Special-purpose dextrous manipulator (SPDM) arm at ELC-3 FRAM-2 (top side)[6] x 2 arms on SPDM
  • Heat Rejection System Radiator (HRSR) weight 2,475 lbs. at ELC-4 (top side)[5] x 3 units each on S1 & P1 Truss

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.

  • Linear drive unit (LDU) weight 255 lbs. at ESP-3 FRAM-4 (top side)[3] x 1 on the MT

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

  • Space-to-ground antenna (SGANT) weight 194 lbs. at ESP-3 FRAM-5 (keel side)[9] x 2 units on Z1 Truss
  • Plasma contactor unit (PCU) weight 350 lbs. at ELC-1 FRAM-2 (top side)[4] x 2 units on the Truss

The 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.

  • Pump flow control subassembly (PFCS) weight 235 lbs. at ESP-1 FRAM-1[1] x 12 units in total, 2 on each IEA x4 & 2 each on P6/S6 Spacers

Each external loop contains a pump and flow control system (PFCS) which contains most of the controls and mechanical systems that drive the EEATCS. There are 2 pumps per PFCS which circulate ammonia throughout the external coolant loops

  • Utility transfer assembly (UTA) at ESP-2 FRAM-8 (keel side)[2] x 2 units on the truss

The UTA is a processor that allows power, signals and data to flow across the SARJ by roll rings incorporated within.

See also


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