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. .
Introduction
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 which remained in the cargo bay on flights STS-102, STS-105 and STS-121; one use of the ICC-Lite on STS-122 ; 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 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, and HTV-4 delivered a Main Bus Switching Unit and a Utility transfer assembly.
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. As well as storage tanks for oxygen as part of the station Environmental Control and Life Support System. 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.
ORUs and robotic arms
ORUs to be handled by Dextre have attachments designed to be gripped with the ORU/Tool Changeout Mechanisms on the end of each arm. The H‐fixture is for massive objects and/or to stabilize Dextre, the most common is a Micro‐fixture and the Micro‐Conical Fitting is used in tight spaces. A Modified Truncated Cone Target is used to visually line up Dexter's arm to grab a fixture. Any ORU with a grapple fixture can be moved by the Canadarm2.
Orbital replacement unit (ORU) spares
(Weight, description and current location of the spare on the station)
Multiple spares
Flex Hose Rotary Coupler weight approx. 900 lb × 1 unit each on S1 & P1 Truss. The FHRC provides the transfer of liquid ammonia across the Thermal Radiator Rotary Joint between the P1 & S1 truss segments and the Heat Rejection System Radiators.
Three spares – ESP-2 FRAM-7 FHRC SN1003, ESP-3 FRAM-2 FHRC SN1004, ELC-4 FRAM-5 FHRC SN0005 delivered by HTV-2.
Pump Module weight 780 lb x 1 unit each on S1 & P1 Trusses. The PM is part of the station's complex External Active Thermal Control System, 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.
Four original spares. Two unused Pump Modules remain – ELC-1 FRAM-7 PM SN0007, ESP-2 FRAM-1 PM SN0005. Two utilised – ELC-2 FRAM-6 PM SN0004 ; ESP-3 FRAM-3 PM SN0006.
Ammonia Tank Assembly weight 1,702 lb x 1 unit each on S1 & P1 trusses. The primary function of the ATA is to store the ammonia used by the external thermal control system. 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 and Port 1 truss segments. 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-9, ELC-3 FRAM-5 Also note – other than these two spares, two other Shuttle missions brought up new ATAs and then returned the failed ATAs: STS-128 ATA SN0004 up/SN0002 down and STS-131 SN0002 up/SN0003 down.
Nitrogen Tank Assembly weight 550 lb 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.
Two spares – ELC-1 FRAM-6 NTA SN0002 ELC-2 FRAM-9 NTA SN0003 Also note – other than these two spares, two other Shuttle missions replaced NTAs. STS-122 delivered new NTA SN0004 and then returned the depleted P1 Truss NTA SN0003. STS-124 swapped the new NTA SN0005 from ESP-3 FRAM 2 with the depleted NTA SN0002 from the S1 Truss. The STS-126 crew returned this depleted NTA.
High-Pressure Gas Tank Assembly weight 1,240 lb x 5 units on quest. High-pressure oxygen and nitrogen gas tanks on board 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 and is replenished by the Space Shuttle.
One spare – ELC-3 FRAM-6, one depleted tank ELC-2 FRAM-4 Note the depleted tank was swapped with the original HPGTA launched on ELC-2 at FRAM-4.
Cargo Transport Container each can weigh between 1,000 and 1,300 lb 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 formerly on ELC-2 FRAM-2, was later moved to ESP-2 FRAM-3 via SPDM. CTC-2 on ELC-4 FRAM-2, CTC-5 on ELC-3 FRAM-1
Control Moment Gyroscope weight 600 lb × 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 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 CMG SN104, ELC-2 FRAM-5 CMG SN102 Note: STS-118 crew delivered a CMG on ESP-3, swapping it for a failed unit on the ITS-Z1 truss. That failed unit was placed on ESP-2 FRAM-5 until it was returned by STS-122.
S-band Antenna Support Assembly weight 256 lb each × 2 active units and one other spare on ISS. The SASA consists of the assembly contingency radio frequency group, SASA boom and avionics wire harness.
Two spares – ELC-3 FRAM-4, ELC-3 FRAM-7
Direct Current Switching Unit weight 218 lb 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, ELC-2 FRAM-2
Battery Charge/Discharge Unit weight 235 lb × 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 and providing conditioned battery power to the primary power buses during eclipse periods.
Two spares – ESP-3 FRAM-6, ELC-1 FRAM-4
Main Bus Switching Unit weight 220 lb × 4 units on S0 Truss. The MBSUs act as the distribution hub for the EPS system. The four MBSUs on board the ISS are all located on the starboard zero truss. Each of MBSU receives primary power from two power channels and distributes it downstream to the DDCUs.
Two spares – ESP-2 FRAM-4, ELC-2 FRAM-7. The MBSU on ESP-2 FRAM-6 was added by STS-120 crew, then swapped with a failed unit from the S0 truss by the Exp 32 crew in late 2012.
Utility Transfer Assembly a processor that allows power, signals and data to flow across the SARJ by roll rings incorporated within.
Two spares – ESP-2 FRAM-8 ELC-4 FRAM-4 Utility Transfer Assembly
Pump Flow Control Subassembly weight 235 lb Each external loop contains a pump and flow control system which contains most of the controls and mechanical systems that drive the EATCS. There are 2 pumps per PFCS which circulate ammonia throughout the external coolant loops. There are 2 on each IEA for a total of 8 active units.
Three original spares, now two available spares – ESP-1 FRAM-1 plus 2 on ITS-P6 that were initially used by the Early External Active Thermal Control System. One EEATCS spare on ITS-P6 swapped out for a leaky unit on the 2B power channel during an Exp 35 EVA May 11, 2013. Other EEATCS spare developed electrical fault and was replaced by an additional spare launched on SpaceX CRS-14.
Single spares
Mobile transporter trailing umbilical system-reel assembly weight 354 lbs. at ELC-2 FRAM-8 x 1 unit on MT
The TUS reel assembly 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.
Heat Rejection System Radiator weight 2,475 lbs. at ELC-4 x 3 units each on S1 & P1 Truss
The Heat Rejection Subsystem 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, the HRS radiator rejects thermal energy via radiation.
Linear Drive Unit weight 255 lbs. at ESP-3 FRAM-4 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 weight 194 lbs. at ESP-3 FRAM-5 x 2 units on Z1 Truss
Plasma Contactor Unit weight 350 lbs. at ELC-1 FRAM-2 x 2 units on the Truss
Plasma Contactor Unit 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.
