Orbital replacement unit

Orbital Replacement Units

Introduction

ISS Truss srructure detailing all Orbital Replacement Units in situ

In support of the International Space Station Orbital Replacement Units (ORUs) are key elements of the ISS 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.

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 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:

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).

Five Modes of ORU Transport to the ISS

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  External Stowage Platform ESP-2 platform mounted to the Quest module
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  ICC-VLD on the RMS during STS-127
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  FHRC and CTC4 on the HTV-2 on the Exposed pallet preflight

Orbital Replacement Unit (ORU) Spares on the ISS

Orbital Replacement Units (ORUs) weights and locations on the station

Multiple Spares

• Flex Hose Rotary Coupler (FHRC) weight approx. 900 lbs. x 1 unit each on S1 & P1 Truss. The FHRC provides the transfer of liquid ammonia across the rotary joint between truss segments.

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

• ETCS/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), ELC-1 FRAM-7 (keel side), ELC-2 FRAM-6 (keel side)

• 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), ELC-3 FRAM-5 (keel side)

• Nitrogen Tank Assembly (NTA) weight 550 lbs. each x 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) ELC-2 FRAM-9 (keel side)

• High Pressure Gas Tank (HPGT) weight 1,240 lbs. x 5 units on Quest. High Pressure oxygen Gas Tanks onboard the ISS provide support for EVA and contingency metabolic support for the crew. This high pressure O2 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), one depleted tank ELC-2 FRAM-4 (top side)

• Main Bus Switching Unit (MBSU) weight 220 lbs. x 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), ESP-2 FRAM-6 (keel side)

• 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, ESP-2 FRAM-2 (top side), ESP-2 FRAM-3 (top side)

• Battery Charge/Discharge Unit (BCDU) weight 235 lbs. x 6 each on each of the 4 IEAs. The Battery Charge Discharge Unit (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), ELC-1 FRAM-4 (top side)

• 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), ESP-2 FRAM-5 (keel side)

• Control Moment Gyroscope (CMG) weight 600 lbs.x 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), ELC-2 FRAM-5 (top side)

• S band Antenna Sub-System Assembly (SASA) weight 256 lbs each x 3 units on ISS. The S-band Antenna Support Assembly (SASA) is an assembly that 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), ELC-3 FRAM-7 (keel side)

• 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), CTC-2 on ELC-4 FRAM-2 (keel side), CTC-5 on ELC-3 FRAM-1 (top side)

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  Flex Hose Rotary Coupler (FHRC)
• 
  ETCS/Pump Module (ETCS/PM)
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  Nitrogen Tank Assembly (NTA)
• 
  Ammonia Tank Assembly (ATA)
• 
  High Pressure Gas Tank Assemblies (HPGTA)
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  Main Bus Switchin Unit (MBSU)
• 
  Direct Current Switching Units (DCSU) on ESP-2
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  Battery Charge/Discharge Unit (BCDU)
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  SSRMS Pitch Roll Joint on the ESP-3 high-lighted by the sun on the left
• 
  Control Moment Gyroscope (CMG) lower right hand corner of the pallet
• 
  S-Band Antenna Support Assembly (SASA)
• 
  Cargo Transportation Container (CTC-2)

Single Spares

• Pump Flow Control System (PFCS) weight 235 lbs. at ESP-1 FRAM-1 x 12 units in total on IEAs & P6/S6 Spacers
• Utility Transfer Assembly (UTA) at ESP-2 FRAM-8 (keel side)
• Space-to-Ground Antenna (SGANT) weight 194 lbs. at ESP-3 FRAM-5 (keel side) x 2 units on Z1 Truss
• Latching End Effector (LEE) weight 415 lbs. at ELC-1 FRAM-1 (top side) 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.

• Linear Drive Unit (LDU) weight 255 lbs. at ESP-3 FRAM-4 (top side) x 1 on the MT
• Mobile Transporter Trailing Umbilical System-Reel Assembly (MT TUS-RA) weight 354 lbs. at ELC-2 FRAM-8 (keel side) 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.

• Special Purpose Dextrous Manipulator (SPDM) Arm at ELC-3 FRAM-2 (top side) x 2 arms on SPDM
• Plasma Contactor Unit (PCU) weight 350 lbs. at ELC-1 FRAM-2 (top side)

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.

• Heat Rejection System Radiator (HRSR) weight 2,475 lbs. at ELC-4 (top side) 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.

• 
  Pump Flow Control System (PFCS)
• 
  Space-to-Ground Antenna (SGANT) upper left in photo
• 
  Latching End Effector (LEE)
• 
  Linear Drive Unit (LDU) right hand corner of this photo
• 
  Mobile Transporter Trailing Umbilical System-Reel Assembly (MT TUS-RA) at the back of the pallet
• 
  SPDM Arm
• 
  Heat Rejection System Radiator (HRSR)

See also

• ExPRESS Logistics Carrier
• External Stowage Platform
• Integrated Cargo Carrier
• Scientific research on the ISS