astronomy

Orbital replacement unit

Orbital replacement unit is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Orbital replacement unit rather than just read about it. In short: Orbital replacement units (or on-orbit replaceable unit) (ORUs) 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.

Key takeaways

Orbital replacement unit belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Orbital replacement unit to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Orbital replacement unit from memory before moving on to harder problems.

Reference excerpt

Orbital replacement units (or on-orbit replaceable unit) (ORUs) 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 (SPDM) robotic 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).

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 Space Shuttle missions were dedicated to the delivery of ORUs using support carrier structures and pallets. Some of these remained in the cargo bay, others were deployed and later retrieved, and some were designed to be removed from the payload bay by the Remote Manipulator System (RMS) and installed 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 (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), and HTV-4 delivered a Main Bus Switching Unit (MBSU) and a Utility transfer assembly (UTA).

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.

ORUs and robotic arms 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. The H‐fixture is for massive objects and/or to stabilize Dextre, the most common is a Micro‐fixture (also known as a Micro‐square) and the Micro‐Conical Fitting is used in tight spaces. A Modified Truncated Cone (MTC) 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 (FHRC) 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 (TRRJ) between the P1 (FHRC SN1001) & S1 (FHRC SN1002) truss segments and the Heat Rejection System Radiators (HRSRs). Three spares – ESP-2 FRAM-7 (keel side) FHRC SN1003, ESP-3 FRAM-2 (top side) FHRC SN1004, ELC-4 FRAM-5 (keel side) FHRC SN0005 delivered by HTV-2.

Pump Module (PM) weight 780 lb x 1 unit each on S1 (current PM SN0006) & P1 (original PM SN0001 still in situ) Trusses. 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. Four original spares. Two unused Pump Modules remain – ELC-1 FRAM-7 (keel side) PM SN0007, ESP-2 FRAM-1 (top side) PM SN0005. Two utilised – ELC-2 FRAM-6 (keel side) PM SN0004 (Installed on ESP-2 FRAM-1 during STS-121, then removed by the Exp 24 crew to replace the failed original PM SN0002 on the S1 truss. SN0002 was returned to earth by the STS-135 crew. SN0004 replaced by PM SN0006 and moved to MT POA by Exp 38 crew in Dec. 2013. Relocated to ESP-2 FRAM-1 by ISS-41 EVA-27 in Oct. 2014. Swapped positions with SN0005 by SPDM in Mar. 2015.); ESP-3 FRAM-3 (top side) PM SN0006 (Installed on ESP-3 FRAM-3 during STS-127, swapped with failed PM SN0004 from S1 truss by Exp 38 crew Dec. 2013).

Ammonia Tank Assembly (ATA) weight 1,702 lb x 1 unit each on S1 (now ATA SN0004) & P1 (now ATA SN0002) trusses. 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. 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 (keel side), ELC-3 FRAM-5 (keel side) 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 (P1 truss original ATA) and STS-131 SN0002 up/SN0003 down (S1 truss original ATA).

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Orbital replacement unit

Start with the simplest possible case. Write down what Orbital replacement unit claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Orbital replacement unit before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Orbital replacement unit ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Orbital replacement unit

In research: Orbital replacement unit appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Orbital replacement unit in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Orbital replacement unit is common in secondary-school and first-year university syllabi. It links to neighbouring topics Components of the International Space Station, Spacecraft components, so understanding it makes those chapters shorter.
In everyday life: Look for Orbital replacement unit outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Orbital replacement unit in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Orbital replacement unit means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Orbital replacement unit out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Orbital replacement unit in simple terms?

Why does Orbital replacement unit matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Orbital replacement unit?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Orbital replacement unit.