International Space Station Structures & Mechanisms

ISS Home Page Components and Devices Index
ISS abbr and Acronyms

Table of Contents


        14.1-1 Introduction
        14.1-2 Structural Principals of ISS
        14.1-3 Structures
                        Pressurized Structures
                        Truss Assemblies
        14.1-4 Mechanisms
                        Active Mechanisms
                        Passive Mechanisms
        14.1-5 External Forces of Space
                        Direct Forces
                        Idirect Forces


        14.2-1 Introduction to ISS Structures
                        Primary Structures
                             Ring Frames
                             Shell Panels
                             Integrated Trunnions
                        Secondary Structures
        14.2-2 The ISS Aluminum Alloy 2219-T87
        14.2-3 Pressurized Modules
                        Example: U.S. LAB
        14.2-4 Truss Assembly Structures
                        Example: Z1 Truss Assembly
                             Command & Tracking Subsystems
                             S-band Communications Systems
                             Ku-band Communications Systems
                             Extravehicular Activity Subsystems
                             Motion Control Subsystems
                             Structural Composition
        14.2-5 Diagrams of Other Truss Assemblies
                        P6 – S6 Truss Assembly
                        P5 – S5 Truss Assembly
                        SO Truss Assembly
                        P1 – S1 Truss Assembly
                        P 3/4 – S 3/4 Truss Assembly


        14.3-1 Mechanisms Introduction
        14.3-2 Hatches & Operations
                        U.S Common Hatch
                             U.S. Common Hatch Components
                             U.S. Pressure Equalization Valves
                                  Positive Pressure Relief Valve (PPRV)
                                  Manual Pressure Relief Valve (MPRV)
                                  Negative Pressure Relief Valve (NPRV)
        14.3-3 Vestibule & Operations
        14.3-4 ISS Racks
                        Rack Types
                             System Rack
                             Payload Rack
                             Stowage Rack
                        Rack Components
                             Upper Attachment Mechanism
                             Knee Braces
                             Lower Attachment Mechaninsm
                             Pivot Mechanism
                             Pivot Pin Fitting
                             Utility Interface Panel Connectors
        14.3-5 Umbilical Connectors & Operations
                        Connection Types
                             NASA Zero-G Lever (NZGL)
                             NASA Zero-G Wing (NZGW)
                             NASA Breech Lock Coupling (NBLC)
                             NASA Threaded Coupling (NATC)
                             Operations of (NZGL, NZGW, and NBLC)
                             Operations of (NATC)
        14.3-6 Common Berthing Mechanism (CBM)
                        CBM Introduction
                        Active Common Berthing Mechanism (ACBM)
                              Active Common Berthing Mechanism Components
                                     ACBM Structural & Sealing Support
                                     ACBM Capture Equipment
                                     ACBM Alignment Aids
                                     ACBM Bolt Assembly
                                     ACBM Motor Controllers
                                     ACBM Thermal Striker Plates
                                     ACBM Covers
                        Passive Common Berthing Mechanism (PCBM)
                              Passive Common Berthing Mechanism Components
                                      PCBM Structural Sealing & Support
                                      PCBM Capture Equipment
                                      PCBM Alignment Aids
                                      PCBM Nut Assembly
                                      PCBM Covers
                        CBM Flight Rules
        14.3-7 Manual Berthing Mechanism (BMM)
                        Manual Berthing Mechanism Components
                              MBM Bolt Assembly
                              MBM Drive Screw Assembly
                              MBM Alignment Aids
                              MBM Latch Assembly
                              MBM Covers
        14.3-8 Segment-to-Segment Attachment System (SSAS)
                        Segment-to-Segment Attachment System Components
                              SSAS Support Structure
                              SSAS Alignment Aids
                              SSAS Capture Equipment
                              SSAS Bolt & Nut Assembly
                              SSAS Motor Controllers


        14.4-1 External Materials of Space
                        Particle Debris Intro
                        Particle Debris Protection for the ISS
                            U.S. MMOD System
                            Russian MMOD System


        14.5-1 Structures Summary
        14.5-2Mechanisms Summary
        14.5-3 External Forces of Space Summary
        14.5-4 Collective Summary of “ISS STRUCTURES AND MECHANANISMS”

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14.1-1 Introduction

Scheduled to be completed in the year 2004, the International Space Station (ISS) will become the largest zero gravity test facility constructed by mankind (ISSUG).  Fully assembled, the ISS will weight approximately 470 tons (lb), and will measure 356 ft wide by 290 ft long (WWW-1).  The ISS assembly began in late 1998 (ISSUG).  Over the course of 45 missions, more than 100 elements will become apart of the ISS (WWW-1).  Each addition requires the ISS be equipped with necessary structures and mechanisms for proper assembly, mating, stowage and control operations (S&MTM).

To maintain structural integrity, ISS structures are engineered and manufactured to withstand its enormous size and load forces, while providing crewmember protection.  Mechanisms on Station are designed to mate/demate, supply stowage and enable higher-complex task to be accomplished by much restricted Extravehicular Activity (EVA)/Intravehicular Activity (IVA) operations.

14.1-2 Structural Principals of the ISS

The structural principals of the ISS are to use lightweight materials, maintain low resource cost, while assuring quality, reliability and durability (Wertz & Larson).  Aluminum alloys are used to manufacture the majority of the ISS elements due to their low molecular mass—(lightweight), resistance to corrosion, and electrostatic conductivity (S&MTM).  Table 14.1-2-I briefly describes the ideal requirements of spacecraft structures.

Ideal Characteristic Level
Corrosion/Oxidation Resistance
Electrical Conductivity
Thermal Conductivity
Normally High
Thermal Expansion
Radiation Tolerance


Table 14.1-2-I Required Characteristics of Spacecraft (WWW-2, Erickson 11-15-00)

Using the same principals, as in Table 14.1-2-I, engineers design ISS elements with low material and resource cost vs. high strength and radiation tolerances (Wertz & Larson).  The opportunity-cost for each element’s structural support, manufacturing cost, and performance are weighed by engineers against each material’s usefulness and abilities; this system enables the ISS to be equipped and efficiently constructed with respect to conserving both time and money (Larson & Wertz).

The ISS elements also use aluminum alloys due to their low material cost and easy machineability.  However, for different missions, loads and durations of external forces, different materials are chosen to replace the relatively low strength of aluminum alloys.  Table 14.1-2-II (Commonly Used Materials), is a listing of advantages and disadvantages of different types of structural materials.

High strength vs. weight

Easy to machine

Relatively low strength 

Low hardness 
High coefficient of thermal expansion

High strength 

Wide range of strength, hardness, and ductility obtained by treatment

Not efficient for stability (high density) 

Most are hard to machine 

Heat resistant
High strength vs. volume 

Strength retained at high temperatures 

Not efficient for stability (high density) 

Not as hard as some steels

Low density- very efficient for stability
Susceptible to corrosion 

Low strength vs. volume

High strength vs. weight 

Low coefficient of thermal expansion

Hard to machine 

Poor fracture toughness if solution treated and aged

High stiffness vs. density
Low ductility & fracture toughness 

Low short transverse properties 

Can be tailored for high stiffness, high strength, and extremely low coefficient of thermal expansion
Costly; requires developmental program 

Strength depends on workmanship; requires individual proof testing

Table 14.1-2-II Commonly Used Materials (WWW-2, Erickson after Larson & Wertz, 1992, p436)

14.1-3 Structures

ISS structures protect crewmembers and provide structural support.  Structures of the ISS are required to withstand multiply/simultaneously acting loads (S&MTM).  Loads are internal and thermal forces applied on structures (S&MTM).  Loads are created by: pressure differences, mechanical interfacing, accelerations, and vibrating energy waves (S&MTM).  Structures are designed to safely absorb, efficiently control, and effectively manage load forces (S&MTM).  The ISS is equipped with two main types of structures: pressurized structures and assembly trusses (S&MTM).

Pressurized Structures

Pressurized structures are designed to protect the crew from the environment of space.  Pressurized structures provide a workable atmosphere for crewmembers and house experiments, payloads and tools for protection (S&MTM).  The majority of the ISS pressurized structures are pressurized modules.  Pressurized modules are generally round in shape and supply crewmembers with protected mobility throughout the ISS (S&MTM).

Pressurized structures are classified into two categories: primary and secondary structures (S&MTM).  Primary structures provide structural integrity to the ISS (S&MTM).  Secondary structures are generally transitional aids for crewmembers, both inside and outside of the ISS (S&MTM).  However, certain structures switch between primary and secondary structure classification according to their operation, usage, and current structural position—(further discussed in section 14.4 INTERNATION SPACE STATION MECHANINSMS).

Truss Assemblies

Truss assemblies are structures that endure the majority of the ISS’s structural loads generated or translated from pressurized elements/modules (S&MTM).  Truss assemblies are also responsible for providing “attachment points for external payloads” (S&MTM, 1.1-6).

“The Integrated Truss Structure (ITS) consists of 10 individual segments” (S&MTM, 1.1-6).  Truss assemblies are labeled according to their structural direction.  Three examples of truss assembly labeling are S6, P6, and Z1.  S labeled trusses represents starboard positioned trusses.  P labeled trusses represents port positioned trusses.  Z labeled trusses represents their alignment in the “Z” axis of the ISS’s Cartesian coordinate system.

14.1-4 Mechanisms

The International Space Station’s Structures and Mechanisms Training Manual describes the purpose of mechanisms as: structures that allow “the Orbiter, Progress and Soyuz to dock to Station” (S&MTM, 1.1-7).  They also execute “temporary attachment for external payloads” (S&MTM, 1.1-7).

Mechanisms are designed to aid crewmember in assembly of the ISS (S&MTM).  The majority of structures, elements, payloads and modules cannot be attached to the ISS by way of crewmember hand tools during EVAs; mechanisms are used in such cases to aid in large-tasked crewmember obligations—such as module mating/demating (S&MTM).  ISS mechanisms generally work as an assembly of multi-structured and multi-operational sub-systems, composed of generally two essential elements.  The two major mechanism elements are classified as either active mechanisms or passive mechanisms (S&MTM).

Active Mechanisms

Active mechanisms are equipment controlled by internal crewmember panels (further discussed in section 14.3 INTERNATIONAL SPACE STATION MECHANISMS) (S&MTM).  Generally, active mechanisms are equipped with multiple berthing, docking, mating/demating, or capturing latches.  During mating/demating, active mechanisms serve as control platforms for orbital docking operations.  Their primary involvements are docking control and locking operations (S&MTM).  However, they are used for multiple purposes throughout Station.

For most operations, active mechanisms house the majority of necessary motorized, pneumatic, pressurized and electrical components used during docking procedures (S&MTM).  An example of an active mechanism is the Active Common Berthing Mechanism (ACBM) half of the U.S. Common Berthing Mechanism (CBM)—later discussed in section 14.3 INTERNATIONAL SPACE STATION MECANISMS.

Passive Mechanisms

Passive mechanisms are generally non-motorized, non-mechanical and have no electrical structural components (S&MTM).  Passive mechanisms are used by active mechanisms during mating/demating and mobile operations (S&MTM).  Passive mechanisms are constructed with multiple capturing bars, nut assemblies, and alignment aids (S&MTM).  Passive mechanisms due to their lack of moving parts and components are generally less expensive (Larson & Wertz).

14.1-5 External Forces of Space

While on orbit, the ISS’s interfaces are in constant contact with the space environment.  The lifetime of a spacecraft is can generally be determined according to its capability to withstand the external forces in spaceflight (Larson & Wertz).  Space presents multiple hazards to both the ISS’s structural integrity and its crewmember’s heath (Larson & Wertz).  The environment of space presents an enormous list of external forces: continuous temperature variations, fluxing atmospheric drag densities, gravity gradient torques, wide ranges and forms of energy radiation, and highly corrosive atomic oxygen (Larson & Wertz).  Independent of their form (in most cases), the majority of external forces can be classified as: direct forces, indirect forces, or combined multiple forces (Larson & Wertz).

Direct Forces

Direct forces are loads generated by direct interface between Station and the surrounding space environment (Wertz & Larson).  Focused concentrations of direct forces, place considerably higher loads on pressurized modules, truss assemblies and payloads (S&MTM).  In return, pressurized modules, truss assemblies and payloads transfer the direct loads into internal indirect loads, to be distributed throughout the ISS.  The effects accumulate and over time, they determine a spacecraft’s End of Life (EOL) (Larson & Wertz).

Direct forces include aerodynamic drag, solar flare winds, wide ranges and forms of radiation, gravitational fields, particle debris, corrosive interactions with Station’s structures, and galactic cosmic rays (Larson & Wertz).  Protective layers and coatings are placed on the ISS to reduce deconstruction (S&MTM).  However, multiple subsystems are required on each ISS’s element to assure crewmember safety (S&MTM).

Indirect Forces

Indirect forces are loads transferred from direct forces through a medium (S&MTM).  Indirect forces are primarily internal oscillations and coriolis forces—(internal forces caused by material movement within a spacecrafts frame (SMAAD)) throughout Station.  However, they also include: internal thermal temperature variations, differential pressurizations, internal noise, constructive and deconstructive resonance frequencies, and microwave interferences (Larson & Wertz).  Indirect forces are deconstructive mostly to sub-systems (S&MTM).

Combined Multiple Forces

Combined forces are collections of internal forces, direct forces or any combination of the two (Mess & Bertrand).  Scientific research, testing and simulated recreations of combined forces are crucial to the structural integrity of the ISS (Mess & Bertrand).  Damage to the ISS will occur if individual load capabilities of structures and elements are surpassed by load forces (S&MTM).

Combined forces are the primary danger to the International Space Station’s structural integrity (Mess. & Bertrand).  The principal of stability on Station is to not allow collective loads to be reinforced, by damping their amplified loads with multiple subsystems (Mess & Bertrand).  Combined forces are generally larger than most isolated forces, due to constructive properties of jointing energy waves (Mess. & Bertrand).  However, combined forces can also be smaller than isolated forces due to deconstructive properties of jointing energy waves (Mess. & Bertrand)

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    International Space Station Main Structures

14.2-1 Introduction to ISS Structures

International Space Station structures are designed to protect crewmembers against the environment of space (S&MTM).  International Space Station structures provide structural support while managing multiple and simultaneous loads (S&MTM).  Each load must be accurately and safely absorbed, controlled, and transformed (S&MTM).  ISS structures are both pressurized and non-pressurized (S&MTM).  ISS structures differ in both their sizes and usages (Wertz & Larson).  Most of the ISS’ structures are manufactured from aluminum alloys (S&MTM).  There are two main types of structures that support the International Space Station: pressurized structures and truss assemblies (S&MTM).

Pressurized Structures

Pressurized structures, such as modules, are responsible for providing ISS crewmembers with protection, stabilized thermal control, and a comfortable living environment (S&MTM).  Pressurizing elements/modules increases their structural loads (Wertz & Larson).  Therefore, the design and manufacturing of pressurized elements/modules requires increased engineering and manufacturing precision, thus time and cost are also increased (Larson & Wertz).

Pressurized structures are classified into two categories: primary structures and secondary structures (S&MTM).  Primary structures are responsible for the structural integrity of the ISS (S&MTM).  Secondary structures are supplemental structures that are not responsible for the structural integrity of Station (S&MTM).  An example of a secondary structure, handrails, are designed to provide crewmembers with translational assistance during EVAs (S&MTM).

Due to the continuous assembly process(until 2006), the ISS’s number of structures, at any give time, is constantly increasing.  According to their operations, specific structures change their classification from primary structures to secondary structures—and vise versa (S&MTM).  The classification of structures as primary or secondary structures is determined according to its current usage during an exact time or operation (S&MTM).  The International Space Station Structures and Mechanisms Training Manual gives this example:

“A hatch is considered a primary structure when it is the
interface between a pressurized module and the vacuum of
 space.  That same hatch becomes a secondary structure
when it is surrounded by pressurized air on both sides.”

Primary Structures

Primary structures are responsible for maintaining structural integrity of pressurized elements/modules (S&MTM).  Primary structures are evaluated by their protection to crewmembers and ISS structural integrity against the vacuum of space (S&MTM).  There are four foundational components of primary structures: ring frames, longerons, shell panels, and integrated trunnions.

Unlike the principle four components, there is the rare addition of windows to some specialized modules.  Figure 14.1-3.1 (Primary Structures Components), is an illustration of a general-form pressurized module, showing the various locations of primary structural components (except windows).

Figure 14.1-3.1 Primary Structural Components (S&MTM, 1.1-2 Figure 1.1-1)

Ring Frames

Ring frames are circular structures that supply the majority of structural support for pressurized modules (S&MTM).  There are usually two ring frames to each pressurized modules.  Ring frames are located at the ends of pressurized modules; and serve as the module’s structural end components.

Ring frames also provide “attachment points for the longerons and the shell panels” (S&MTM, 1.1-2).  Ring frames are equipped with hatches, mating/demating and storage components (S&MTM).  Ring frames are also the connection point for berthing mechanisms, such as the Common Berthing Mechanism (CBM) (S&MTM).  Ring frame construction is vital to module structural integrity; the ISS’s proper pressurization control system relays on each segment/module to provide its own structural integrity (S&MTM).


Longerons are aligned parallel with pressurized module’s longitudinal or long axis (S&MTM).  Longerons are the primary structural support of pressurized module’s walls (S&MTM).  Longerons are composed of higher density aluminum alloys which provide greater structural support (S&MTM).  Each longeron spans from structural ring to structural ring, acting as a supporting frame for multiple shell panels (S&MTM).

Shell Panels

Shell panels attach to parallel-running longerons.  Shell panels are thin aluminum alloy sheets that are manufactured to form the outer walls of pressurized modules.  Shell panels are also responsible for serving as a passive defense system against space debris and deferential temperatures due to continuously changing thermal exposure conditions (S&MTM).  Shell panels are coated with protection layers which provide provide thermal control and protection—(Micro-Meteoroid Orbital Debris (MM/OD)) against ballistic particle debris—(discussed further in section 14.3 INTERNATIONAL SPACE STATION MECHANISMS) (S&MTM).

Integrated Trunnions

Integrated trunnions are linear mounted brackets used for pressurized module attachment within the cargo bays of United States Space Shuttles (S&MTM).  The integrated trunnions are visible from the outside of pressurized modules.  They are mounted directly above longerons on top of shell panels.  Trunnion bolts are passed through shell panels by way of secure pressurized mounting assemblies, and become an additional stiffness support for pressurized modules (S&MTM).


Windows are found in only a few locations on the ISS, due to their increased risk to crewmembers (ISSUG).  Windows are considered psychologically important to the crewmembers and important to research module testing (Larson & Wertz).

The single window aboard the U.S Laboratory Module (USLAB), for example, is 20 inches in diameter and constructed of the “highest-quality optical glass ever used in a crewed spacecraft” (WWW-1).  Figure 14.1-3.2, illustrates the layering system and internal components of an ISS window.

Figure 14.1-3.2 Cross-Sectional Area of Window (ISSUG, 22 Figure 5.3.4-1)

ISS’s orbital inclination of 51.6° allows crewmembers to view Earth’s surface from 51.6° South to 51.6° North—(inclination and ground track is displayed in Figure 14.1-3.3 Orbital Inclination & Ground Track) (WWW-1).  ISS’s orbit covers “85% of the globe and 95% of the Earth’s population” (ISSUG).  Due to the ISS’s orbital velocity relative to the Earth’s surface, crewmembers are able to view nearly the entire world “every 92 minutes, 24 seconds” (WWW-3).  ISS’s inclination and velocity, combined with an altitude, ranging from 129.3 km (208 mi.) to 177.1 km (285 mi.) enables crewmembers a view of nearly the entire surface of the Earth (WWW-3).

Figure 14.1-3.3 Orbital Inclination & Ground Track (ISSUG, 13 Figure4.1-1)

However, a small circular region located on the Earth’s surface farthest away and perpendicular to the ISS’s current path of travel (when directly over equator=0°) cannot be view by crewmembers.  This region’s hypotenuse inclination is approximately ± 6°, tangent to the farthest point on the Earth’s surface (Mess. & Bertrand).

Secondary Structures

Secondary structures are designed to “transfer their loads to primary structures” (S&MTM, 1.1-4).  Secondary structures surround both the inside and outside of the ISS.  Secondary structures are defined by their lacking necessity in structural integrity.  That is to say, crewmembers lives are not in danger due to failure of secondary structures.  Such secondary structures are grappling fixtures, handrails and hatches at certain times and operations (S&MTM).

Hatches are an example of one structure that can be considered a primary or secondary structure.  The determination of a hatch’s classification is based on the whether or not the hatch is providing safety to crewmembers and structural integrity to the ISS (further discussed in section 14.3 INTERNATIONAL SPACE STATION MECHANISMS) (S&MTM).

14.2-2 The ISS Aluminum Alloy 2219-T87

The International Space Station’s structures are manufactured primarily of aluminum alloys (S&MTM).  The NASA Space Station Fracture Assessment Technology Development research headed by: Roy W. Hampton; Dr. James C. Newman Jr.; and Robert Dodds; have posted extensive fracture analysis and research on the aluminum alloy 2219-T87 (SSFATD).  The analysis and research is vital, since aluminum alloy 2219-T87 is a primary manufacturing contribution in ISS structures (SSFATD).  The chemical contents of Aluminum Alloy 2219-T87 are listed in Table 14.2-2-I.  Table 14.2-2-II, is a collection of the alloy’s most noticeable characteristics which include tensile tension, fatigue endurance, and modules elasticity.  Each characteristic contributes to the alloy’s overall selective character.

Molecular Element
Composition Percentages (%)
Silicon (Si) 
Iron (Fe) 
Copper (Cu) 
5.8 - 6.8
Manganese (Mn) 
Magnesium (Mg) 
Chromium (Cr) 
Nickel (Ni) 
Zinc (Zn) 
Titanium (Ti) 
0.02 - 00.1
Gallium (Ga) 
Vanadium (Va) 
0.05 - 0.15
Other Specified 
00.1 - 0.25 Zr
Other Each 
Other Total 
Aluminum (Al) 
Remainder—(bulk of composition)

Table 14.2-2-I Chemical Content of Aluminum Alloy 2219-T87
(After the Asia Aluminum Extrusion Council (AAEC), 11-17-00)


Ultimate Tensile Tension
Typical Ultimate
MPa, (N/mm2)
Yield Tensile Tension
Typical Yield
MPa, (N/mm2)
Typical Elongation
% in 2 inches specimen
Fatigue Endurance
MPa, (N/mm2)
Modules of Elasticity
ksi x 103

Table 14.2-2-II Physical Properties of Aluminum Alloy 2219-T87
(After the Asia Aluminum Extrusion Council (AAEC), 11-17-00)

14.2-3 Pressurized Structures

Fully completed, the International Space Station will contain 46,000 cubic feet of pressurized volume (WWW-1).  Pressurized structures are generally round in shape.  The round shape provides a much safer enviroment to crewmembers  due to equal pressurized hull walls.  Pressurized sturcture’s primary obligation is to provide crewmembers with protection ( S&MTM).  The United States Laboratory Module is one example of a pressurized structure.

Example:  United States Laboratory Module

The United States Laboratory (U.S. LAB) is scheduled to be launched in January 01’ (WWW-1).  The research laboratory is considered the centerpiece of the International Space Station (WWW-1).  The ISS will become the largest zero gravity*1 test facility and  considered “a world class, state-of-the-art research facility” (WWW-1).  The U.S. Laboratory is designed to provide a “shirtsleeve atmosphere” (WWW-1) for crewmembers.  The U.S. Laboratory’s major testing facilities will include the Human Research Facility, Fluids and Combustion Facility, Biotechnology Facility, Materials Science Facility, and the Optical Window Rack Facility (WWW-1).  Table 14.2-2-I is an assorted listing of five testing facilities and their research purposes.

The U.S. Laboratory measures 28 ft in length by 14 ft in diameter and weights 32,000 (lbs) (WWW-1).  The research module is equipped with 24 experimental racks: 13-scientific racks and 11-systems racks, only three of these will be attached on launch; six racks are located on each of the four sides (WWW-1).  The U.S. Laboratory’s center cylinder is equipped with a single 20-inch diameter window (WWW-1).

Figure 14.2-2-1 U.S. Laboratory Module (WWW-1)

* Photo of the U.S. Laboratory module for the International Space Station, fall of 1997 at the Marshall Space Flight Center station manufacturing facility in Huntsville, Al.

The exterior of the U.S. Laboratory is the largest passive system of the module (WWW-1).  The paneling of the laboratory has a “waffle” pattern that strengthens the outer-hull (WWW-1).  The lab’s paneling is manufactured of an aluminum alloy, similar to most Station structures (WWW-1).  To provide thermal protection to the module, the paneling is covered with blanket insulation similar to the Space Shuttle’s thermal blanketing (WWW-1).  An “intermediate debris shield” (WWW-1) is affixed on top of the blanket insulation.  An aluminum alloy debris shield is manufactured over the intermediate debris shield (WWW-1).

All objects have emissivity and absorptivity properties.  “Emissivity deals with the ability of an object to emit radiant energy (to radiate), while absorptivity describes the ability of an object to absorb radiant energy falling upon it.”  The emissivity and absorptivity of aluminums, aid the module’s thermal control systems in providing a comfortable environment (S&MTM).

Testing Facility
Facility Usage
Human Research Facility


To access crew health and survey how the human body respond and adapts to microgravity—particularly the heart and lungs, muscles and bones, sense of balance and body regulatory systems such as temperature control and wake-sleep cycles.
Fluids and Combustion Facility


To study the uses of microgravity for improving processes employed in producing semiconductor crystals, glass fiber, energy and other products, as well as weather prediction and environmental monitoring.
Biotechnology Facility


For the application of engineering and technology research in: improved protein crystal growth in microgravity for more effective medicines, and human cell tissue culturing for studying diseases and their cultures.
Materials Science Facility


For the study of the atomic and molecular structures of materials in microgravity, as well as their magnetic, thermal and chemical properties, and processes by which they come into being.
Optical Window Rack Facility


For testing and using cameras, sensors and other devices in search of identifying pollution sources, and monitoring forest conditions.

Table 14.2-2-I (U.S. Laboratory Testing Facilities) (After WWW-4)

14.2-4 Truss Assembly Structures

The truss assemblies provide attachment points for payloads, elements and modules, and solar array panels, while serving as the presidential backbone of Station (S&MTM).  Truss assemblies are integrated constructions of rails, rods, antennas, communication (CPUs), batteries, umbilical jumpers and hardware and support frames (S&MTM).  Truss assemblies hold the bulk of the ISS together.  They provide attachment points for ISS elements/modules for directional change (S&MTM).  Truss assemblies are labeled according to the directional position that they extend the ISS elements/modules (S&MTM).  One of the most important truss assemblilies is the Z1 Integrated Truss Assembly.

Example:  Z1 Integrated Truss Assembly

“The Z1 is the base structure for the U.S. solar array” (STS92).  The Z1 truss assembly is labeled according to its directional positioning relative to the ISS’s Cartesian coordinate system.  The Z1 truss assembly serves several purposes.  The Z1 truss assembly supports two plasma contactors, two DC-to-DC converters units (DDCUs), one Ku-band communications system, primary and secondary power distribution, thermal control system hardware, mechanical interfaces, EVA/extravehicular robotics (EVR) hardware, components of single string S-band communications system, and four control moment gyro (CMG) assemblies (STS92).  Figure 14.2-4.1, is an illustration of the truss assembly revealing its Ku-band boom and antenna assembly.

Figure 14.2-4.1 Z1Truss Assembly (S&MTM, 1.1-6 after Figure 1.1-5)

“The Z1 truss structure is designed to maximize component packaging and support load paths during launch and on orbit” (STS92).  The truss is equipped with mechanical interfaces are located on four sides of the Z1 truss assembly (STS92).  Each side is responsible for providing stowage/mating for other truss segments, modules and payloads (S&MTM).  The Z1 zenith face provides the interface connection for the P6 truss assembly (STS92).  The nadir face is equipped for providing interface contact with Node 1 (STS92).  The forward face is equipped for providing connection to PMA-2 and allowing a hinged cable tray connection to the U.S. Laboratory (STS92).

Command & Tracking Subsystems

The Z1 command and tracking subsystem is composed of two main types of communication bands: S-band and Ku-band (S&MTM).

S-band Communications System

“The S-band communications system consist of two redundant strings, each of which comprises three on-orbit replaceable units (ORUs) and two antennas, the baseband signal processor (BSP), the Tracking and Data Relay Satellite System (TDRSS) transponder, and the antenna RF group, which includes a low-gain antenna and a high-gain directional antenna” (STS92, 11-18-00).

Contribution to S-band Communication System 
Radio Frequency Group (RFG)
Amplifies and filters radio signals; controls antenna switching; and antenna pointing.  Composed of two antennas, High-gain antenna (HGA)—(supports TDRSS, and GN&C pointing), and Low-gain antenna (LGA)—(fixed position, requires no pointing).
Baseband Signal Processor (BSP)
Provides data and voice processing (downlink & uplink).  Downlink capabilities of constant-rate data stream of either 192 kbps or 12 kbps.  Is considered the “heart” (STS92) of the S-band communications system.

Table 14.2-4-V S-band Communications Systems Subsystems
(After STS92: Z1 Integrated Truss Segment, 11-18-00)

Ku-band Communications System

Due to the higher frequency, the Ku-band communication system is the primary return link for the International Space Station video and digital information (STS92).  For video downlink the Ku-band provides a 50-Mbps fixed–rate downlink with the ability to handle up to four video signals (up to 43 Mbps high-rate with 7 Mbps overhead) (STS92).  In short, the ISS requires 7 Mbps leaving 43 Mbps for user usage.  Higher order transmissions are routed through the 1553 bus and I/F data outlet by way of a the Video Baseband Signal Processor (VBSP) (STS92).

Extravehicular Activity Subsystems

“The Z1 truss segment is equipped with several spacewalk aids: two EVA tool stowage devices (ETSDs), 22 worksite interface (WIF) sockets, one flight-releasable grapple fixture (FRGF), 11 trusses, two tray launch restraints, numerous standard handholds and handrails, and several custom handles” (STS92).

Electrical Power Subsystems

The Z1 truss assembly is equipped with two initialization diode assemblies (IDAs), two secondary power distribution assemblies (SPDAs), two patch panels, two plasma contactor units (PCUs), and two DC-to-DC converters (DDCUs) (STS92).  Table 14.2-2-III is a brief description of the primary electrical subsystems.

Motion Control Subsystems

The motion control subsystems hardware is affixed to Z1 before launch (STS92).  The motion control subsystems consist of primarily two components: the Control Moment Gyros (CMGs) and the CMG assemblies (STS92).  Table 14.2-2-IV is a listing of the two main components and their contributions to the International Space Station.

Number of Units
Contribution to Station
Initialization Diode Assembly (IDA)
Initialization diode assemblies protect power from the shuttle assembly power conversion unit (APCU).
Secondary Power Distribution Assembly (SPDA),
Secondary power distribution assemblies (SPDA) control, protect, and isolate secondary distribution by way of remote power controller modules (RPCMs), power and data connections and cold plates.
Patch panel
Patch panels allow three interchangeable input connections for allowing Z1 input power source control.
Plasma Contactor Unit (PCU)
Plasma connector units emit electros trough self-generation, and control the voltage of space plasma onto ISS structures.
DC-to-DC Converter (DDCU)
DDCUs provide power feed to the RPCMs (via 1553 data bus) through the central utility railing.

Table 14.2-2-III Electrical Power Subsystems
(After STS92: Z1 Integrated Truss Segment, 11-18-00)


Number of elements
Contribution to Station
Control Moment Gyro (CMG)
Equipped with four (4); ISS flight rules require two (2) for operational attitude control
One large flat wheel that rotates at a constant speed (6,600 rpm) and develops an angular momentum of 3,500 ft-lb/sec—(CMGs are responsible for attitude control of the ISS).
Control Moment Gyro Assembly

(Contains all four CMGs)

Total momentum storage capacity of completed assembly is 14,000 ft-lb/sec.  Manages all four (4) CMGs, used for attitude orbital maneuvers of the ISS.

Table 14.2-2-IV Motion Control Subsystems (MCSs)
(After STS92: Z1 Integrated Truss Segment, 11-18-00)

CMGs are equipped with a thermostatically controlled survival heater (STS92).  These heaters consume 120 watts and operate at a temperature range of -42° to 35°F (STS92).  Temperature control is crucial to distribution of expanded/contracted mass on CMGs due to metallic properties from interacting changing temperatures (Wertz & Larson).  In case of oversaturation, the Russian segment thrusters can be used to desaturate the CMGs” (STS92, 11-18-00).

Structural Composition

The Z1 truss segment’s frame and shell are manufactured out of aluminum alloy 2219-T851 (STS92).  Used for connection with the Space Shuttles, the trunnion of Z1 is constructed of INCO 718 (STS92).  The keel pin beam is also constructed of the same material (STS92).  However, due to their (relatively weaker) structural framework, the Ku-band antenna and boom beam are manufactured from steel (STS92).

14.2-5 Diagrams of Other Truss Assemblies

P6 – S6 Truss Assembly


P5 – S5 Truss Assembly


SO Truss Assembly


P1 – S1 Truss Assembly


P 3/4 – S 3/4 Truss Assembly

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International Space Station Mechanisms

14.3-1 Mechanisms Introduction

ISS mechanisms have three main purposes (S&MTM).  Connecting structures to one another is the first and primary function of mechanisms on board the International Space Station.  The Orbiter, Progress and Soyuz require mechanisms to successfully dock with ISS.  The third functional usage of mechanisms on board ISS is to provided attachment for external payloads, such as experiments.  The components and limitations of each mechanism are regulated individually for each mission, module, structure or experiment they are responsible for connecting to ISS.

The number of missions, modules, structures and experiments is quite numerous and therefore, the number and function of each mechanism varies from connection-to-connection.  In light of their complex engineering and increased cost, not all mechanisms are reusable.  The Segment-to-Segment Attach System (SSAS) is an example of a mechanism that is only used once.  The SSAS is only required to connect once and therefore are not designed to accommodate docking and mobility such as Common Berthing Mechanisms (CBM).  The importance of mobility, reconfiguration and changeability is important for safety, usability and cost of the ISS.

14.3-2 Hatches & Operations

ISS hatches are pressurized structural openings which allow crewmembers to transfer themselves and miscellaneous objects between two pressurized segments.  There are three main types of hatches aboard ISS: U.S. Common, APAS and Probe/Drogue.

U.S. Common Hatch

The U.S. Common hatch is 1.3 m (51.2 in) by 1.3 m (51.2 in) (S&MTM, 2.1-2).  The 1.3 m (51.2 in) square shape allows for maximum cross-sectional area, required for rack translation.  The square shape is complemented with rounded corners in order to reduce stress concentrations (S&MTM, 2.1-2).  The U.S. Common hatch has two sides.  “The Intravehicular Activity (IVA) side of the hatch is also referred to as the dome side, due to the smooth, clean appearance” (S&MTM, 2.1-2).  The Extravehicular Activity (EVA) side is the second side of the hatch and is referred to as the ribbed side.  The ribbed side houses the pushrods, pinion gear, and latches (S&MTM, 2.1-2).  Both the Intravehicular Activity side and the Extravehicular Activity side are equipped with separate handles, and a Manual Pressure Equalization valve.  However, the IVA and the EVA side share a common view port, which is complemented with a latch position indicator.  Figure 14.3-1.1 illustrates both the IVA and EVA side of the U.S. Common Hatch.

Figure 14.3-1.1 U.S. Common Hatch (S&MTM, 2.1-3)

U.S. Common Hatch Components

The latch position indicator allows ISS crewmembers to evaluate the hatch’s security and position.  The latch position indicator indicates whither the latch is in the Latched, Equalized, Test, or Unlatched position.  When the Latched position is indicated the hatch’s position is completely closed and the latches are fully engaged—the hatch is secure. (S&MTM, 2.1-2)

“The Equalize position indicates that a detent in the pinion gear has been reached which prevents further opening of the hatch if pressure is not equalized.  The Test position is the maximum activation of the latch mechanisms without disengaging the latches.” (S&MTM, 2.1-2)  The Unlatched position informs the operating ISS crewmember that the hatch’s latches are fully retracted.  Once the latches are fully retracted the hatch can be opened.  The hatch opens by swinging out and locking in an overhead stowage position perpendicular its latched position.

Overhead hatch stowage is possible by way of a stowage handle that the crewmembers use to swing the hatch into placement.  Placement of overhead stowage is mechanically activated by way of a stowage latch located on the IVA side of the U.S Common Hatch.  The stowage latch is responsible for securing the hatch once in the overhead stowage position.

U.S. Pressure Equalization Valves

In order to maintain safety of the crew and ISS the ability to regulate pressures on opposite sides of each U.S. Common Hatch is required.  Before opening a U.S. Common Hatch the pressure on each side must be within equalization equivalence of each other.  Regulating the pressure difference is done so through Pressure Equalization Valves (PEV). (S&MTM, 2.1-2)  There are three types of Pressure Equalization Valves used by the U.S. Common Hatch: Positive Pressure Relief Valve (PPRV), Manual Pressure Equalization Valve (MPEV), and Negative Pressure Relief Valve (NPRV).

Positive Pressure Relief Valve (PPRV)

“Positive Pressure Relief Valve (PPRV) is a pneumatically-actuated valve, which opens to prevent a module from being over-pressurized.” (S&MTM, 2.1-2)  PPRVs are located in the starboard and port sections of Node 1.  PPRVs are also located in the Lab forward and aft positions.  Activation of the PPRV occurs when the vessel pressure is 5350.4 millimeters of mercury (mmHg) (14.8 Pounds per square inch (psi)). (S&MTM, 2.1-3)  The valve is in full operation (i.e. fully opened), when the pressure of the vessel reaches 5365.6 mmHg (15.1 psi). (S&MTM, 2.1-3)  For the safety of the crew and ISS the activation pressures are mechanically manufactured into the design and construction.  The activation pressures are not adjustable.  However, the crew is able to switch the valves for automatic to open by way of a manual override switch. (S&MTM, 2.1-3)  The PPRV are not as flexible and controllable as the Manual Pressure Equalization Valves (MPEVs).  For this reason, over time the PPRV will be replaced with MPEVs.  Below is Figure 14.3.1-2, which illustrates a three dimensional isometric view of the PPRV.

Figure 14.3.1-2 PPRV isometric/PPRV cross-sectional view
(S&MTM, 2.1-3)

The Manual Pressure Equalization Valve (MPEV)

The second Pressure Equalization Valve is the Manual Pressure Equalization Valve.  “The Manual Pressure Equalization Valve (MPEV) is a manually-actuated, two position, rotary ball valve which can be operated from either side of a U.S. Common Hatch.”(S&MTM, 2.1-3)  The MPEV is accessible from either side of the U.S. Common Hatch.  Thirteen MPEVs will be launched on ISS.  The first section of ISS assemble to use MPEVs is Node 1.  Node 1 has four MPEVs installed, each with a manual handle that allows ISS crew to open by turning counterclockwise and close by turning clockwise.  The handle is protected from ISS crewmember accidental disturbance with a handle guard (S&MTM, 2.1-3).  Continuous valve position insight is possible by way of the MPEV’s visual indicator. (S&MTM, 2.1-3)

Negative Pressure Relief Valve (NPRV)

The third Pressure Equalization Valve is the Negative Pressure Relief Valve (NPRV).  “The Negative Pressure Relief Valve (NPRV) is designed to activated when the pressure is 0.1 pounds per square inch-differential (psid) external to the module”(S&MTM, 2.1-3).  Lack of pressure within a/multiple segment(s) of ISS could cause ISS’ segments to implode.  Over expansion or implosion is highly dangerous to the ISS.  Both over expansion and implosion will cause the structural integrity to fail by fracturing segment connections and seals.

Once the seals are fractured or broken, the segment in question would prove unlivable for crewmembers and a hazard for the ISS.  Each segment of ISS has a maximum pressure and minimum pressure limit, which governs ISS’s atmospheric capabilities.  In order to prevent ISS structural integrity damage, pressure differential control is highly important to the ISS.  The NPRV is fully open at 10.64 mmHg (0.2 psid).  The NPREV will remain fully open until the pressure differential is 5.32 mmHg (0.1 psid), at which it will reset itself (S&MTM, 2.1-3).  Figure 14.3.1-2 illustrates a cross-sectional view of the NPRV.

Figure 14.3.1-3 NPRV cross-sectional view
(S&MTM, 2.1-4)

14.3-3 Vestibule Connections: Hardware & Jumpers

Non-structural connectivity between ISS modules is provided through multiple vestibule connections.  Structurally, ISS modules are connected by way of berthing mechanisms.  However, data, power, communications, commands, fluids, avionics, air ducts and atmospheric control between ISS modules are connected trough vestibule connections.  There are two major components of vestibule connections: hardware and jumpers (S&MTM, 2.1-5).


The vestibule hardware is located on the berthing connecting mechanisms.  The vestibule hardware is rigid and responsible for supporting the connections between ISS modules.  Jumpers are hoses and wires that carry actual connection material, fluids, data, and voltages from module to module.  The Environmental Control and Life Support System (ECLSS) uses jumpers to transport regulated ISS atmosphere from module-to-module.


The Communication and Tracking (C&T) system and the Command and Data Handling (CDH) system use jumpers to transmit communicated data pulses and signals from assorted sensors and central processing units (CPUs) from module-to-module throughout the Station.  The Electrical Power System (EPS) uses jumpers to supply, transmit and archive electrical power throughout the ISS.  Figure14.3-3.1 is a diagram of a Flex-hose connection stretched between two connections.  The diagram illustrates both the Flex-hose and the berthing connecting hardware (S&MTM, 2.1-6).

Figure 14.3-3.1 CBM Flex-hose (S&MTM, Figure 2.1-7)

14.3-4 ISS Racks

ISS Rack Types

There are three primary types of racks on ISS: System, Payload and Stowage. (S&MTM)  ISS is a system of integrated trusses, pressurized vessels, projects and Photovoltaic Arrays (PVAs).  Due to the mobility and need for experiments, equipment and tool storage required, ISS racks are constructed to be light-weight, durable, mobile, and most importantly, an inexpensive means of reliable storage.  The three rack types relatively are similar; they differ only slightly due to their structural loads, capabilities and requirements.  “The Italian built Multi-Purpose Logistics Module (MPLM) is used to bring racks and other cargo to and from the ISS” (S&MTM, 2.1-7).  Due to the architecture of the racks and similarities, removal and replacement of large experiments and logistics is easily accomplished. (S&MTM)

The structural components, like their uses, are also relatively similar.  Depending on the mass and size of load, ISS racks have either four or six load-bearing structural posts.  Both the four and six post racks have an outer shell composed of graphite epoxy.  This shell allows for a lightweight structural durability for the rack’s longevity.

For stability and uniform displacement, structural posts are located on each of the four corners on a four-post rack.  For heavier loads, two additional posts are placed in the front (center) and the rear (center) of the rack.  The four-post rack’s maximum weight capacity is 400 kg (881.8 lbs). (S&MTM, 2.1-7)  The six-post rack’s maximum weight capacity is 600 kg (1322.8 lbs). (S&MTM, 2.1-7)  The two additional structural posts increase the rack’s structural integrity by 200 kg (441.0 lbs).  The Integrated rack design limit is 804.2 kg (1773 lbs). (S&MTM, 2.1-7)

System Racks

System racks, first of three primary types of racks on the ISS,  are designed and used to provide support/mounting for critical ISS systems.  System racks are responsible for the majority of U.S. systems hardware as well as housing pumps, Multiplexers/Demultiplexers (MDMs), Remote Power Control Mechanisms (RPCMs) and DC-to-DC Converter Units (DDCUs). (S&MTM)  Lab System racks include Air Revitalization System (ARS), Mobile Servicing System (MSS)/Avionics and the Internal Thermal Control System (ITCS). (S&MTM)

Payload Racks

Payload racks, second of three primary types of racks on ISS are designed to provide support/mounting for IVA experiments and testing equipment.  Due to the structural similarities between System racks and Payload racks, according to the International Space Station Structures and Mechanisms Training Manual, it is possible to install a Systems rack in a Payload location.

Unlike the Payload utility cables, which are permanently affixed to the rack and connected to the Utility Interface Panel (UIP) or “Z-Panel (located on the module standoff”), the utility cables for System racks are located within the standoff and connect to the UIP. (S&MTM)  Therefore, the Systems rack could physically be placed in the location of the Payload racks, but because the utility cables are unable to mate, interchangeability could not take place.  There are three common types of Payload racks on ISS: International Standard Payload Rack (ISPR), EXpedite the PRocessing of Experiments to the Space Station (EXPRESS) rack, and the Active Rack Isolation System (ARIS).

Stowage Racks

Stowage racks on ISS are responsible for storing loose equipment, supplies, and tools.  Stowage racks are quite popular on the ISS.  They can be used freely for storing any number of loose items that ISS crew members may need to remove from their confined work area.  Stowage racks are less complex than Payload racks or System racks.  They do not require power, nor do they have any utility cables.  In short, Stowage racks are similar to a house’s garage; an accessible affixed extension of protected area primarily used to store items for later usage.  “There are several types of stowage hardware: Zero-G Stowage Platforms, Re-Supply Stowage Platforms, and Aisle Stowage Containers.”  (S&MTM, 2.1-8)

Rack Components

“On-Orbit mechanical interfaces between a rack and a module consist of the Upper Attach Mechanism, Knee Brace Assembly, Lower Attach Mechanism, Rack Pivot Mechanism, and Utility Interface Panel.”  (S&MTM, 2.1-9)  However, the Lower Attach Mechanism is only required during ascent/entry therefore it is not used on orbit. (S&MTM)

Upper Attachment Mechanism

The Upper Attachment Mechanism (UAM) is mounted to the module wall, and thereby is responsible for providing attachment of the rack to the module.  The UAM also provides an attachment point for the rack knee braces. (S&MTM)

Knee Braces

Knee braces are struts.  Knee braces uses captive pit-pins—which are tethered to the knee brace to attach to fittings on the module longeron and the MPLM. (S&MTM)  In order to accommodate all ISS crewmembers, the knee brace’s length is adjustable by way of a rotating center turnbuckle.

Lower Attach Mechanism

Used only for ascent/entry, the Lower Attach Mechanism (LAM) is similar to the UAM.  They are both used as restraint tie-downs.  The LAM mates the bottom of the rack to the module wall’s longeron fitting, thereby forcing the LAM to aid in controlling the rack’s mass and force distribution by damping movement and transferring energy to the higher structural integrity of the modules walls (S&MTM).

Pivot Mechanisms

Pivot Mechanisms are located between the rack and the Pivot Pin fitting.  Pivot Mechanisms aid ISS crewmembers, by reducing complications, such as requiring more personnel and excessive equipment, during rack installation.  During installation, Pivot Mechanisms allow ISS crewmembers to rotate racks on Pivot Pins into their required mounting locations.  The majority of the mass is shifted from the crewmembers to the Pivot Pins, therefore allowing crewmembers to focus their efforts and strengths towards mounting the racks, once held in the correctly aligned position.

Examples of Pivot Mechanisms can be found in common every-day applications.  One commonly used passive pivot mechanism, in the shape of an open hook, is located on the mounting plate of most household ceiling fans.  Once mounted onto the ceiling mount, the fan’s pivot mount can be used to hold the fan’s weight by way of a passive loop located on the fan’s housing plate.  The pivot mount holds the fan, allowing accessibility for an electrician during wiring and installation.  This comparison illustrates how the similarly designed Pivot Mechanisms on ISS, function as aids for the installing racks within modules of ISS.

Pivot Pin Fitting

The Pivot Pin Fitting (PPF) is installed on orbit.  The Pivot Pin Fitting can not structurally support the loads during ascent/entry. (S&MTM)  The PPF allows the Pivot Pin to rotate— mechanically similar to a bushing.  Between the Pivot Pin Fitting alignment, the Pivot Pin, and the Pivot Mechanism connection to the rack, the rack’s alignment error are minimized.

Utility Interface Panel Connectors

Utility Interface Panel Connectors are ISS’s means for providing power, data, and coolant fluids to requiring rack components (S&MTM).  Visually, the UIP is a conglomerate of varied connections, aligned in mutually spaced distance and order from one another, resembling a control panel.  The UIP in the U.S. Lab can supply low or moderate temperature coolants.  However, according to the International Space Station Structures and Mechanisms Training Manual, simultaneous supply is not possible.

Unlike U.S. Lab, JEM Life Sciences Racks can support both low and moderate temperature coolants (S&MTM).  Moreover, the Attached Pressurized Module (APM) can only provide moderate temperature coolant for the International Standard Payload Racks (ISPRs) (S&MTM).

14.3-5 Umbilical Connectors & Operations

Umbilical connectors and their operations are essential to the ISS.  Permanent connectivity of fluids, power and data connections, are accomplished through umbilical connectors.  There are four primary types of umbilical connectors on the ISS (S&MTM, 2.1-12).

Connection Types

The four umbilical connection types include the NASA Breech Lock Coupling (NBLC), the NASA Threaded Coupling (NATC), the NASA Zero-G Lever (NZGL), and the NASA Zero-G Wing (NZGW) (S&MTM).  The NASA Breech Lock Coupling (NBLC) is responsible for remote or inoperative IVA or EVA access.   The NASA Threaded Coupling (NATC) is also an umbilical connection designed for little or no-orbit astronaut interface (S&MTM, 2.1-12).  Astronauts on EVAs or IVAs are strictly limited because of their suits and therefore umbilical connections that require astronaut interface are designed to be user-friendly.  There are two types of user-friendly umbilical connections. The NASA Zero-G Lever (NZGL) and NASA Zero-G Wing (NZGW) compose the third and fourth primary umbilical connections.

Umbilical connectors do not mate and unmate in the same manner.  There are two distinct manners in which ISS Umbilical connectors mate and unmate (S&MTM).  NASA Breech Lock Coupling, NASA Threaded Coupling and NASA Zero-g Wing connectors have two keys and two keyways that align each connector for mating.  To mate the connectors, the Main Key is to be aligned with the Main Keyway (the second key and keyway are must also line up).  Secondly, crewmembers manually push the connectors together until they reach the activation point.

The activation point is located determined when the Main Keyway stops the Main Key’s motion.  The keys and keyways prevent this operation if they are not properly aligned.  Once at the activation point, activation occurs when the coupling mechanisms are manually rotated (S&MTM).

Rotation is allowed by way of the keys sliding within the keyways.  After twisting the connectors, another operation—a slide backwards towards their entering direction (as if one were attempting to pull the connection apart) locks the receptacle and plug together.   The umbilical connection is then locked.  However, in order to assure successful connection, a visual activation confirmation tool is on every umbilical connection.

Proper activation can be visually confirmed.  There are two colored bands that wrap around the receptacle.  The band color closest to the opposite connector is red.  This band, when properly mated is covered by the Plug.  The second colored band is blue and is located on the receptacle just out of the plug’s covering ability (this so when properly connected, only the blue band will show).

The plug has only a single-colored band, which is blue.  After twisting the connectors the connectors, lock together by way of the keyways that are cut back towards the receptacle’s opening and only two blue bands (meaning proper connection) can be seen.  The keyways are cut similar to the shape of a “J.”  (WWW-1)   The Main Keyway’s J design allows the key to enter, twist, and pop back into locking position.  Figure 14.3-5.1 illustrates this style of connector’s unmated and mated position.

Figure 14.3-5.1 Unmated and Mated NBLC, NATC, and NZGW type connectors (S&MTM, 2.1-12 Figure 2.1-11)

NASA Zero-G Lever

The NASA Zero-G Lever connectors use the second manner of connection.  The NZGL connection is accomplished through two corresponding flat faces that are slid into locking position by way of a manually operated lever (S&MTM).   When first jointed together, the connections are not axially aligned.  The two round connection ends meet in the form of two half cylinders misaligned by exactly the distance of their radii (a reference is illustrated in Figure 14.3-2).  The following procedure is a sliding motion, perpendicular to the faces.  At this point the connection is in pre-activation.

The NZGL features a lever-actuated coupling mechanism, mounted on the plug connector, enables a connection lock. The lever-actuated mechanism is mechanically joined to the NZGL, but is a simplistic mechanism in itself.

The lever-actuated mechanism order of electrical engagement is (1) Shells, (2) Electromagnetic Interference (EMI) provision, (3) Contacts.” (S&MTM, 2.1-13)  The perpendicular face slide and the lever locking are relatively one motion.  They are mechanically jointed together and require the other’s proper operation in order to successfully mate or unmate. Figure 14.3-5.2 is an illustration of how the NASA Zero-G Lever connector mated.  To unmate, the operation is reversed.

Figure 14.3-5.2 NZGL Electrical mate/locking sequence
(S&MTM, 2.1-13 Figure 2.1-12)

14.3-6 Common Berthing Mechanism (CBM)

CBM Introduction

The Common Berthing Mechanism (CBM) is two-halved mechanism used to connect International Space Station modules together during the assembly process.  CBM’s are designed to provide access for ISS crew between modules, and maintain structural integrity between ISS modules.  The two halves of a CBM are the Active Common Berthing Mechanism (ACBM) and the Passive Common Berthing Mechanism (PCBM) (S&MTM).

The ACBM has electrical operated mechanisms, remotely operated bolts and latches that lock onto the PCBM.  Due to the electrical requirements and higher mechanism complexity, the CBM’s ACBM half is always on the ISS side of the berthing interface mounted to the Station (S&MTM).  The ACBM uses the PCBM’s structural ring for attachment.  The PCBM is always mounted to the mating module or vessel.

Figure 14.3-6.1 Active and Passive CBM rings (S&MTM, 2.2-1 Figure 2.2-1)

Due to the fact that the PCBM is symmetric and of general form, the attachment by the Manual Berthing Mechanism (MBM) is relatively easy.  However, the ACBM can only attach to the PCBM.  CBMs are scheduled to be used from flight 2A –8A (S&MTM).  CBMs can be mated and unmated according to ISS needs.  Table 14.3-6-I outlines the berthing and unberthing CBM schedule.

Elements Berthed and Unberthed
PMA 1 to Node 1 
(ACBM components are removedprior to launch)
PMA 2 to Node 1 Forward
Z1 Truss to Node 1 Zenith
PMA 3 to Node 1 Nadir
PMA 2 form Node 1 Forward to Z1 Forward
(Using Manual Berthing Mechanism (MBM))
U.S. Lab to Node 1 Forward
PMA 2 to U.S. Lab Forward
Multi-Purpose Pressurized Logistic Module (MPLM) to Node 1 Nadir—(only for the duration of this flight)
PMA 3 from Node 1 Nadir to Node 1 Port
Multi-Purpose Pressurized Logistic Module (MPLM)to Node 1 Nadir—(only for the duration of this flight)
Airlock to Node 1 Starboard
Multi-Purpose Pressurized Logistic Module (MPLM) to Node 1 Nadir—(only for the duration of this flight)
Multi-Purpose Pressurized Logistic Module (MPLM) to Node 1 Nadir—(only for the duration of this flight)
Table 14.3-6-I CBM mate/demate schedule form 2A-8A (S&MTM, 2.2-5 Table 2.2-1)

Active Common Berthing Mechanism (ACBM)

The Active Common Berthing Mechanism (ACBM) is an aluminum alloy structure that is always mounted to the ISS.  The ACBM is the primary structure for the CBM.  The ACBM houses the majority of the CBM components and is responsible for allowing access between ISS crew and the Master Control Panels—controlling panels used for controlling berthing and unberthing.

ACBM Components

The Active Common Berthing Mechanisms major components are: structural and sealing support, capture equipment, alignment aids, bolt/nut assemblies, motor controllers, thermal striker plates, Ready-to-Latch (RTL) indicators, alignment pins, and covers (S&MTM).  Each component is either mounted to the ring or to the hatchway of the ACBM (S&MTM).

The ACBM’s structural ring is made of an aluminum alloy and measures 203 cm (80 in) in diameter and 13 cm (5 in) high (S&MTM).  In order to maximize the structural load capabilities of the CBM, the ACBM components are symmetrically spaced (S&MTM).  The ACBM has four latches spaced 90° apart (S&MTM).  There are 16 bolts that are symmetrically spaced 22.5° apart from one another (S&MTM).

ACBM Structural & Sealing Support

The structural sealing support components of the ACBM are actually a collective group of smaller mechanisms that jointly work together in order to successfully mate/demate with PCBM.  The ACBM’s structural ring’s mating face is a flat and smooth aluminum surface.  The surface is required to be flat and smooth due the PCBM sealing beads which will be discussed in Passive Structural & Sealing Support section.

ACBM Capture Equipment

The ACBM capture equipment includes four Ready-to-Latch (RTL) mechanisms (S&MTM).  The RTLs are positioned symmetrically 90° apart.  The RTLs are the first contact points between the ACBM and the PCBM.  They are mounted with sensors that inform the crew of connection status.  Connection is indicated by the RTLs when the PCBM and the ACBM capture envelope is 11.4 cm (4.5 in) (S&MTM).

“According to flight rules, three operational capture latches are required to berth the PCBM” (S&MTM, 2.2-9).  Once the contact is established, the capture latches are operated manually by way of an ISS crew operation switch.  They capture latches have small hooked end that latches onto the PCBM.  The capture latch motor revolves the latch and thereby brings the PCBM towards the ACBM.  An illustration of this procedure can be viewed in Figure 14.3-6.2.

Figure 14.3-6.2 RTL Connection –Capture and Deploy Latch Sequence
(S&MTM, 2.2-9 Figure 2.2-4)

The Capture Switch, mounted to the bottom of the Capture Latch, determines capture completion (S&MTM). The determination is based on the angle that the Capture Latch Motor’s shaft has turned—illustrated in Table 14.3-6-II.  The motor’s shaft is connect to the latch through a linkage box.  The motor’s revolutions, calculated against the linkage gear reduction, yield an exact latch position.  Figure 14.3-6.3 illustrates a side view of a capture latch’s components: linkage housing, hinge, capture switch and motor.  Figure 14.3-6.3 is a side view of the latch and motor operations.  Figure 14.3-6.4 also illustrates how the PCBM is pulled towards the ACBM.

Figure 14.3-6.3 Side view of an Active Common Berthing Mechanism Capture Latch (S&MTM, 2.2-9 Figure 2.2-5)


Latch Position 

(Output Motor Shaft Angle)—in Degrees (°)

Capture Switch State
Closed (from fully deployed position)
Closed (from fully deployed position)


Table 14.3-6-II Capture Switch Relationship to Latch Position  (S&MTM, 2.2-10 Table 2.2-2)

Figure 14.3-6.4 Capture and Deploy Latch Sequence (S&MTM, 2.2-10 Figure 2.2-6)

ACBM Alignment Aids

Alignment aids are used to help orient attitude between ACBM and PCBM (S&MTM).  While the latches bring the PCBM towards the ACBM, the importance for proper alignment increases.  There are two major components of ACBM alignment aids.  The first and larger of the alignment aids is the alignment guides.  Made of aluminum alloy with nitronic-60 contact edges, the alignment guides are primarily pyramid shaped guide-runners.

There are four alignment guides in each system.  Two alignment guides are on the ACBM and two on the PCBM.  The system is similar to pushing two kitchen forks together—the motion is restricted and follows the openings between the tines.  The ACBM alignment guides are close together and act as one large alignment guide.  Figure 14.396.5 is an illustration of the ACBM alignment guides.

Figure 14.3-6.5 ACBM Alignment Guide (S&MTM, 2.2-17,Figure 2.2-14)

The ACBM has four alignment pins located 90° apart from one another.  The four pins are relativity small in size, but large in importance.  The alignment pins are used for finer alignment—towards the end of the attachment process.  The alignment pins slide into the CBM’s passive alignment aid—an alignment socket. (S&MTM)

ACBM Bolt Assembly

The ACBM bolt assemblies are responsible for the permanent attachment between ACBM and PCBM.  There are 16 Powered Bolt Assemblies on ACBM (S&MTM).  As a built-in passive redundancy aid, the bolt assembles are located 22.5° apart around the ACBM structural ring (S&MTM).  For secure pressurized sealing, flight rules mandate that fifteen bolt assemblies are successfully executed (S&MTM).  However, only eight successfully attached bolts are required to accommodate non-pressurized structural loads (S&MTM).  All 16 bolt assemblies house one tapered-end bolt each (S&MTM).  The tapered-end is to help guide each bolt into the CBM nut assembly.  When completely tightened, each bolt’s static load is 85.4 kN (19,300 lbs) (S&MTM).  Figure 14.3-6.7 is a cross-sectional view of a CBM Bolt and Nut Assembly—both ACBM and PCBM.

Figure 14.3-6.7 CBM Bolt and Nut Assembly (S&MTM, 2.2-12 Figure2.2-9)

ACBM Motor Controllers

There are four Controller Panel Assemblies (CPAs) on each ACBM (S&MTM).  Each CPA contains five firmware controllers (S&MTM).  The first controller is designatedas the Master Latch Controller.  The second through fifth are designated Bolt Controller 1,2,3,4.  Each of the four Master/Latch Controllers (MLCs) communicate with the Multiplexers/Demultiplexers (MDM) as a Master Controller (S&MTM).  The MLC controls each bolt using information gathered for an Electromagnetic Interface Module (EMI) and the 1553/485 bus interface (S&MTM).  The Master/Latch also communicates with the other firmware controller on the RS485 bus (S&MTM).  Only One Master/Latch has the capability of acting as the Master Controller at a time.  However, each MLC has the capability of operating Latch Controllers at all times (S&MTM).

Figure 14.3-6.8 Controller Panel Assembly (S&MTM, 2.2-13 Figure 2.2-10)

ACBM Thermal Striker Plates

The active Thermal Striker Plates are actually a passive mounting plate located on the outer edge of the ACBM structural ring.  The active Thermal Striker Plates are relatively small sized platforms for the PCBM’s Thermal Plungers.  The strike plate can be seen in Figure 14.3-6.9.  The strike plate is responsible for maintain plunger distance, allowing radiant interchange for thermal equalization, and keep an even loading to seal during Intermediate Bolt command (Ibolt) (S&MTM).  There are 16 strike plates, located outside the ACBM structural ring ever 22.5°; co-linear to a radial line through each bolt assembly.

Figure 14.3-6.9 ACBM Strike Plate (after S&MTM, 2.2-20 Figure 2.2-16)

ACBM Covers

Due to the harsh environment of space, both the ACBM and PCBM require coverings.  Space damage can result due to temperature extremes, space particles and debris, chemical reactions.  The ACBM has five covers four petal covers and one center cover (S&MTM).  However, axial CBMs, such as Node 1 Forward, do not  have covers because the CBMs are protected by other structural attached elements (S&MTM).

Petal Cover
Petal covers protect radial Node CBMs (S&MTM).  Petal covers are deployed/opened and closed using the CBM capture latches (S&&MTM).  When deployed/opened the petal covers cover a small area that is uncovered by the center dick cover.  The position of the covers is highly important to ISS crew.  “Berthing operations cannot take place with the petals in the closed position” (S&MTM).

Center Disk Cover
The center disk cover is a crucial protection aid to ACBM.  The ACBM center disk cover is shaped similar to a plus sign (+) with rounded inner corners.  The ACBM center disk cover protects the hatch face and electronic equipment (S&MTM).  The electrical equipment is damageable by space debris and extreme thermal exposures.  Unlike the petal covers, the center cover detachment or movement is not necessary for berthing operations (S&MTM).  According the Structures and Mechanisms Training Manual, ISS crews typically remove the center disk cover form inside.  However, an EVA removal of the center disk cover is possible.

PCBM Components

The Passive Common Berthing Mechanism major components are the structural and sealing support, capture equipment, alignment aids, bolt/nut assemblies, motor controllers, thermal striker plates, Ready-to-Latch (RTL) indicators, alignment pins, and covers (S&MTM).  The PCBM structural ring is 203 cm (80 in) in diameter and 28 cm (11 in) high (S&MTM).  Similar to ACBM in size, the PCBM structural components are located differently.  The PCBM’s components are only attached to its structural ring and not to the hatchway—like the ACBM (S&MTM).

Passive Structural & Sealing Support

The passive structural and sealing supports differ from the active structural and sealing supports.  The PCBM has two major components that aid in structural sealing and support: Silicon Seal Beads and Thermal Plungers.  Each component is vital to the ISS CBM berthing and deberthing.  As do most ACBM components the silicon seal beads and thermal plungers work together with ACBM components, to secure proper berthing and deberthing.

Silicon Seal Beads
Unlike the ACBM smooth flat-faced sealing interface, the PCBM sealing interface is lined with three silicon seal beads.  When completely mated,  the PCBM silicon seal beads are compressed against the ACBM interface, thereby creating secure pressurized seal (S&MTM).  The temperature extremities are hazardous to berthing (S&MTM).  Due to the ACBM/PCBM’s aluminum alloys expansion capability and the low tolerance for bolt alignment, the ACBM and PCBM have to be within 16.7° C (30° F) of one another—directly prior to mating (S&MTM).  Working in such a demanding environment requires the CBM berthing tolerance to be extremely high.

 Thermal Plunger
There are 16 thermal plungers located on the PCBM structural ring.  The thermal plungers are relatively positioned in the same location as the ACBM thermal strike plates.  The PCBM thermal plungers use the ACBM strike plates to regulate distance for thermal equalization and supply even disbursement of the CBM’s structural loadings to seal during Intermediate Bolt command (IBolt) (S&MTM).  Figure 14.3-6.9 illustrates a PCBM Thermal plunger and its components Thermal Stand-Off, Shim, and Plunger.

Figure 14.3-6.9 PCBM Thermal Plunger (after S&MTM, 2.2-20 Figure 2.2-16)

Passive Capture Equipment

There are no moving components to the passive capture equipment.  However, there are four capture fittings on PCBM.  The capture fittings located 90° apart and are used by the capture latches to pull the PCBM towards the ACBM (S&MTM).  The capture fittings are centered directly between every two passive alignment guides.

Passive Alignment Aids

The PCBM alignment aids work with the ACBM alignment aids.  The PCBM alignment aids include two major components: eight alignment guides, and four fine alignment sockets.  Rotational alignment is extremely important to berthing.  The rotational attitude alignment determines if successful bolt attachment is possible.

Alignment Guides

The eight guides are grouped in pairs of two, and located every 90° around the outside of the PCBM structural ring (S&MTM).  The passive alignment guides are composed of the same aluminum alloy as the ACBM, they are also provided with nitronic-60 contact edges (S&MTM).  The alignment guides are pyramid shaped and force the PCBM alignment guides to be wedged into place—sliding occurs on the nitronic-60 edges.  Figure 14.3-6.11 illustrates a set of PCBM alignment guides.   Figure 14.3-6.12 illustrates an ideal alignment by ACBM and PCBM alignment guides.

Figure 14.3-6.11 PCBM Alignment Guide (after S&MTM, 2.2-20 Figure 2.2-17)

Alignment Pin Socket
There are four PCBM alignment pin sockets.  Each alignment pin mounted on the ACBM structural ring requires an PCBM alignment pin socket to adjust rotational attitude.  The four alignment pin sockets are located on the PCBM corresponding to the same location on the ACBM alignment pins—at 90° apart from one another (S&MTM).  Due to the roundness and smooth surface of the ACBM alignment pins, the PCBM alignment pin sockets force the alignment pins to exact berthing position.

PCBM Nut Assembly
In correlation with the 16 bolt assemblies on the ACBM, the PCBM has 16 nut assemblies.  The PCBM nut assembly is responsible for with standing the static 85.4 kN (19,300 lbs) force exerted by the bolts on the PCBM.  The necessary force is required to structurally hold ISS CBM modules together.  The fight requirements stated that 15 of 16 bolt-to-nut attachments must be fully attached (S&MTM).  The requirements for the bolt assembly are also followed by the PCBM nut assembly, when matted they act as one system (S&MTM).

The nut assembly houses a threaded interface for the ACBM bolt assembly (S&MTM).  The nut assembly contains an encapsulated nut, spring washer, floating washer, spherical washer, holding nut, and nut plate (S&MTM).  The location and order to interfacing can be visualized in Figure 14.3-6.7.  Figure 14.3-6.14 also illustrates how the ACBM bolt assembly threads into the PCBM nut assembly.

Figure 14.3-6.7 CBM Bolt & Nut Assembly (S&MTM, 2.2-21 Figure 2.2-19)

PCBM Covers

The PCBM has two covers.  The first cover is a thermal cover which is used to maintain temperature control (S&MTM).  The second cover of PCBM is a contamination cover (S&MTM).  Before berthing is possible, the ISS crew must make an EVA to remove both covers (S&MTM).  Both covers are only used once, and after completed berthing the covers are sent back to Earth for reuse (S&MTM).

Thermal Cover
Due to the restrictions of differential temperature differences between ACBM and PCBM (16.7° C (30° F)) the PCBM thermal cover is the PCBM’s most important protection element.  “This is particularly important because an unattached element does not generate its own heat; therefore it can be much affected by the extreme coldness of space” (S&MTM, 2.2-22).  The thermal cover is divided into two half-circles.  Each half is composed of a Multilayer Insulation (MLI) (S&MTM).  Velcro straps are used to hold the two half-circle thermal covers together (after S&MTM).  Figure 14.3-6.14 is an illustration of the PCBM’s two half-circled thermal protection covers.

Figure 14.3-6.14 PCBM Thermal Cover (S&MTM, 2.2-22 Figure 2.2-20)

Contamination Cover
 The silicon seal beads are required to be contaminant free in order to successful compress with ACBM and allow pressurization.  The PCBM contamination covers protect the silicon seal beads (S&MTM).  The contamination cover is positioned and removed by way of snaps (S&MTM).  Contaminate covers are required during Space Shuttle launch and while on orbit, when the probability of the silicon seal beads becoming contaminated is moderately high (when compared to available tolerance requirements per-square-inch) (S&MTM).  Figure 14.3-6.16 is a cross-sectional view of the PCBM structural ring, silicon seal beads and the contamination

Figure 14.3-6.16 Seal Contamination Cover (S&MTM, 2.2-22 Figure 2.2-21)

CBM Flight Rules and Response Operations

All ISS operations are designed to run with the least amount of errors and failures as possible.  However, the foresight to plan for possible errors and failures helps assures CBM success.  Table 14.3-6-III is a list of the CBM Flight Rules which list the possible responses to failures during the steps of CBM berthing and unberthing.

CBM Flight Rules:
¨Response to a failure is determined by taking not account the extent of the loss and at what berthing phase the failure has occurred 
¨Power to CBM RPC is inhibited during Non-operation for CBM Firmware Controllers to prevent inadvertent actuation
¨CBM’s Check is performed prior to a CBM removing the passive element form the payload bay
¨CBM Cover Deployment must be visually verified prior to Mating
¨Shuttle and Station CBM Thruster is Inhibited prior to CBM berthing and deberthing 
¨A minimum of three out of four RTL indicators is required before CBM capture
¨CBM Capture Latch Position is determined by either Latching command, Deploy command or Visual verification
¨SRMS or SSRMS will remain grappled to the element until eight bolts have acquired the passive element to a pre-determined preload
CBM Failure Impact Matrix:
¨Pre-berthing –Continue unless you are zero fault tolerant for CDH and EPS
¨Berthing—Continue no matter what failure occurs
¨Pre-deberthing—Delay to maintain a single fault tolerance
¨Deberthing—Continue unless a bolt or latch fails.If a bolt or latch failure, reberth and do maintenance.

Table 14.3-6-III CBM Flight Rules and Failure Response Matrix (After S&MTM 2.2-27)

14.3-7  Manual Berthing Mechanism (MBM)

The Manual Berthing Mechanism (MBM) is constructed into two circular-halves, the Active Berthing Mechanism and Passive Berthing Mechanism.  The MBM is designed to mechanically attach on-pressurized modules to the ISS for temporary stowage (S&MTM).  Structurally, the MBM and CBM are very similar.  The difference between the MBM and CBM is an absence of any CDH and EPS interfaces (S&MTM).  The foundation of the MBM is its manual drive shaft that is operated on EVA by ISS crew with a specialized EVA-power tool (S&MTM).

There are two MBMs, the MBM 1 and the MBM 2.  The MBM structural ring measures 203 cm (80 in) in diameter (S&MTM).  The alignment aids and capturing latches are virtually identical (S&MTM).  MBM’s life span is designed to accommodate a minimum of one year, and at least five berthing/deberthing cycles (S&MTM).

MBM Components

The MBM structural ring is composed of aluminum alloy and provides housing for all of the MBM components (S&MTM).  The main components of an MBM are the Manual Bolt Assembly, Drive Screw and Crank Assemblies, latch tie rod assembly, capture latch, striker plate, alignment guides/pins, and MBM Covers (S&MTM).  Figure 14.3-7.1 is a visual aid for all of the component locations on a MBM.

Figure 14.3-7.1 MBM 1 Components (S&MTM, 2.3-3 Figure 2.3-3)

MBM Bolt Assembly

Like the CBM, the MBM has 16 bolt assemblies located in the same positions around its structural ring (22.5° apart from one another) (S&MTM).  The bolt assemblies are only on MBM 1 (S&MTM).  MBM 2 does not have a manually bolt assembly system, therefore MBM 2 is not capable of withstanding the load factors of reboost or EVA loads (S&MTM).

MBM Drive Screw Assembly

The drive screw assembly provides an interface for an ISS crew on EVA.  The MBM requires an EVA-power tool that is specialized to rotate the drive screw assembly, which is connected to the drive strut.  As the EVA-power tool rotates the drive screw assembly while the drive strut rotates the crank assembly.  The crank assembly is located in the center of the MBM and is a housed crankcase used to transfer one axis of rotation into four linear latch tie rod movements.  There are four latch tie rods on the MBM.  Each latch tie rod is located 90°apart from one another.  Each is responsible for transferring energy from the crank assembly into the four capture latches.  The tie rods become the drive for the capture latches (S&MTM).  Figure 14.3-7.3 illustrates how the rotational motion is transfer into four linear motions.

Figure 14.3-7.3 Crank Assembly (S&MTM, 2.3-3 Figure 2.3-4)

MBM Alignment Aids

MBM alignment aids are identical to those on the CBM.  The primary alignment aids are the alignment guides used to wedge alignment corrections during berthing.  The secondary alignment aids are the four alignment pins.

MBM Alignment Pins

The MBM alignment pins are identical to the CBM, because they are both required to mate PCBM (S&MTM).  There are four pins positioned equidistant around the MBM structural ring.  The pins are used for finer rotational alignment form 3 mm (0.125 in) to almost zero (S&MTM).

MBM Alignment guides

MBM alignment guides are identical to ACBM except for a proximity indicator stripe (S&MTM).  The stripe is used by EVA crewmembers to determining whether the PCBM is in the MBM capture envelope (S&MTM).  Each alignment guide is 15 cm (6 in) tall (S&MTM).  By using the MBM alignment guides the rotational alignment between the MBM and PCBM is within 3 mm (0.1325 in) (S7MTM).

Latch Assembly

MBM 1 and MBM 2 have the same latch assembly (S&MTM).  There are four capture latches on the MBM (S&MTM).  The capture latches are similar to the ACBM capture latches.  However, the MBM capture latch works on a series of mechanical linkages and the ready-to-latch position is towards the center of the MBM ring (S&MTM).  Since the MBM is designed to mate with any PCBM, the overall operations of the capturing latch must be similar the ACBM capture latch operations.  Figure 14.3-7.6 MBM is a diagram in which the tie rod horizontal motion transfer motion into the capture latch linkage hosing can be seen.

Figure 14.3-7.6 MBM Capture Latch (S&MTM, 2.3-2 Figure 2.3-2)

MBM Covers

There are five major MBM covers: one MBM tie rod covers and four MBM latch tie rod covers (S&MTM).  The protection against Micro-Meteoroid Orbiting Debris (MMOD) is extremely important to the life span of both MBM 1 and MBM 2.  Unlike the CBM, where the covers are solely to protect the CBM, the MBM covers are also designed to protect ISS crewmembers from moving mechanisms (S&MTM).  Figure 14.3-7.7 is a diagram of the MBM cover types and their location.

Figure 14.3-7.7 MBM Covers (S&MTM, 2.3-5  Figure 2.3-6)

14.3-8  Segment-to-Segment Attachment System (SSAS)

Segment-to-Segment Attachment System (SSAS) are designed to remotely attach two truss segments together (S&MTM).  The SSAS uses a passive and active half to make a complete truss-to-truss interface.  The active half of the SSAS contains the bulk of latching mechanisms, buses and attachment assemblies.  The passive half of the SSAS is ridged with coarse alignment mechanism and attachment bars.  Collectively, the two halves work as one greatly important attachment mechanism. Figure 14.3-8.1 is an illustration of the SSAS active and passive halves.  The location of various components on both the active and passive SSAS can be seen in Figure 14.3-8.1.

Figure 14.3-8.1 Segment-to-Segment Attachment System (SSAS) (S&MTM, 2.6-1 Figure 2.6-1)

Segment-to-Segment Attachment System Components

“The SSAS is a collection of active and passive components mounted to the end bulkheads of adjoining truss segments” (S&MTM, 2.6-2).  Three are five categories of components: support structure, alignment aids, capture equipment, bolt/nut assemblies, and motor controllers (S&MTM).

Support Structure

“The end cross-section of the truss segments on which SSAS is used is a half-hexagon measuring 4.34 by 1.78 meters (171 by 70 in)” (S&MTM, 2.6-2).  There are four bolts located on the corner of each SSAS system (S&MTM).  The load on each bolt is 62.3 kN (14,000 lbs) (S&MTM).  The capture latch for the SSAS is located in the center of the SSAS structure.  The capture latch can pull with 53.4 kN (12,000 lbs) (S&MTM).

Alignment Aids

There are three coarse alignment guides on SSAS (S&MTM).  The three alignment guides orient the two bulkheads during the segment pull.  Each of the three alignment cones is accompanied by one RTL strike plate.  Fine alignment is provide by four shear cups.  The shear cups are located within the nut assemblies (S&MTM).  The capture latch can only be opened after all four bolts are tightened (S&MTM).

Capture Equipment

Artificial Vision Unit (AVU) targets are located on each side of the SSAS.  The AVUs are responsible for assisting the robotic operator in alignment of the incoming elements (S&MTM).  When the elements are 8-10 cm (3-4 in) apart from one another, the RTL indicators are activated.

The RTLs are located on the active SSAS.  All RTLs are equipped with two limit switches for redundancy (S&MTM).  The robotic operator then receives positive indication that is within the capture envelope and therefore commands the capture latches to activate to active phase (S&MTM).  The SSAS sensor on the active half determines fine alignment and coarse adjustments.  The sensor hits the strike plate and relays information back to the latch claw.

Figure 143-8.2 RTL sensor and striker plate (S&MTM, 2.6-3 Figure 2.6-3)

The capture latch assemble is design to work similar to a claw.  The capture latch assembly is designed to remain common among USOS-powered attachment mechanism (S&MTM).   The capture latch has two latch claws.   The claws are designed to close around the passive SSAS capture bar (Segment-to-Segment Attachment of Capture Bar).  Each capture latch is equipped with an EVA override switch (S&MTM).

The switch is to be used to release the claw in case the IMCA becomes inoperative (S&MTM).  The IMCA is a worm gear box actuator that is electronically operated (S&MTM).  Figure 14.3-8.4 illustrates the Capture Latch Assembly (CLA) and its components  latch claw, EVA release pin, IMCA, Manual EVA override and Gearbox (after S&MTM).

Figure 14.3-8.3 Segment-to-Segment Attachment System--Capture Claw
(S&MTM, 2.6-5 Figure 2.6-5)

 Figure 14.3-8.4 Capture Latch Assembly (S&MTM, 2.6-3 Figure 2.6-4)

Bolt & Nut Assembly

The SSAS is equipped with four Motorized Bolt Assemblies (MBAs).  Each MBA is a motor assembly and a bolt.  Each of the four bolts have tapered ends to help with the alignment bolts properly interface with the nut assemblies (S&MTM).  Although the four bolt assemblies are identical, the nut assemblies are different.

There are three Flexure Nut Assemblies (S&MTM).  These three bolts supply moderate compliance in assembly.  The fourth bolt assembly is a single degree of freedom bolt assembly (S&MTM).  The reason is to provide a greater accuracy of alignment orientation.  The fourth bolt assembly is called the Fixed Cup Assembly.  Unlike most bolts that move, the Fixed Cup Assembly does not (S&MTM).  The necessity of exact orientation and alignment is crucial in the last stages of SSAS attachments (S7MTM).  Due the inability of the fourth bolt’s movement, SSAS attachment is possible.

Motor Controllers

There are two Bolt Bus Controllers (BBCs) on each SSAS (S&MTM).  The BBCs provide bolting process monitoring and control (S&MTM).  Only one BBC can operate at a time (S&MTM).  Due to this, the second BBC is mandated to provide redundancy.  Each BBC’s switch panel allows for on or off selectability.  Each BBC is power feed through a single input from a separate RPCM (S&MTM).  Once sent the RPCM, data is then relayed to the MBA in control, which then sends on the signal via a 1553 bus to the MDMs (S&MTM).

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External Forces of Space

14.4-1 External Materials of Space

ISS’s orbit strongly influences it’s performance and lifetime (Wertz & Larson).  Size, weight, and the cost of the ISS are affected due to the natural space environment exposures on the ISS’s operational systems (Wertz & Larson).  The natural space environment is characterized by the matter, energy, and nearby forces (such as the Sun, the Earth, interplanetary matter, neutral atmosphere, charged particles and plasmas, cosmic particles, gravitational fields, radiation fields, and magnetic fields (Mess. & Bertrand).  ISS structures and mechanisms exposed to space environments are engineered with higher precession and durability.

Particle Debris

Operational systems require protection against space environmental interactions which occur when objects are subjected to vacuum conditions; one example is organic outgassing.  Organic materials outgas when depressurized.  Outgassing is the process by which organic materials generate spurious molecules due to rapid ambient atmospheric depressurization (Wertz & Larson).  The process can be responsible for contaminating surrounding surfaces such as mirrors, antennas, optics, and collection devices(Wertz & Larson).

Pressurized modules of the ISS are constructed with longerons, ring frames, and shell paneling.  Because the longerons and ring frames support the majority of the structural load, the shell paneling is very thin.  Composed of aluminum alloys, like the majority of the ISS, the shell paneling is subject to particle and debris hazards.  In LEO, spacecraft are subject to approximately 8-km/hr-particle bombardment (Wertz & Larson).  Depending on mass, size and velocity, orbiting particles and debris with differential velocities are an extreme concern for ISS survival.  The energy (E) of a particle can be calculated by:

Variable Relationships:
E = (m / 2)*w2
Energy of a single particle
Relative Mass
Particle’s Velocity
Where:m = mR / [ {1-(w/c) 2 }1/2 ]
Rest Mass
Speed of Light in Vacuum
Table 14-4.1-I Energy of a Particle Equation (after Mess. & Bertrand, 66)

An estimation of the required energy to separate two aluminum alloy molecules is given by:  F=(Gm1m2/r2)*k (Mess. & Bertrand).  “k” is an alloy molecular bond strength proportionality that justifies the left-hand-side and the right-hand-side (Mess. & Bertrand).  The ISS is equipped with two different types of particle and debris protection systems.

Particle Debris Protection for the ISS

The Micro-Meteoroid Orbital Debris (MM/OD) is a passive subsystem designed to protect ISS against particle and space debris collisions.  ISS has two MM/OD concepts; the U.S. MM/OD and the Russian MM/OD (S&MTM).

U.S. MMOD System

The U.S. MM/OD concept consists of a 1.27mm (.05 in) thick sheet of aluminum that envelops the pressurized vessels with a 101.6 mm (4in) of clearance (S&MTM).  The intent of the U.S. design is to reduce the mass and velocity of a on-path collision particle by breaking up the particle into smaller and slower fragments (S&MTM).

The particles first interaction with to the MM/OD is to the outer 1.27mm aluminum shell (S&MTM).  The particle is forced into a particle cloud created by the particles own molecular destruction.  The 101.6 mm (4 in) gap allows the particle cloud to dissipate its energy over a larger area, thereby reducing the potential of the particle to penetrate the pressurized shell of the vessel (S&MTM).  “This design approach allows for minimal damage to Station.”  (ISS S&M TM 1.1-5)  Figure 14.4-1.1 illustrates the ideal effects of a particle or space debris collision with ISS.

Figure 14.4-1.1 U.S MM/OD design concept (S&MTM, 1.1-5, Figure 1.1-4)

Russian MMOD System

The Russian MM/OD concept consists of seven sections.  The section ordering is with respect to particle penetration order.  The first section is a glass cloth that lines the outer-most surface.  The glass cloth’s internal construction is composed of a three-layered screen-vacuum thermo-insulation barrier.  The glass cloth is attached to an aluminum honeycomb sub-structure.

The aluminum honeycomb substructure is the second section of the Russian MM/OD concept.  The honeycomb substructures purpose is reduce the velocity at the same time dissipating particle energy (S&MTM).

The third section of the Russian MM/OD is an open gap between the second and fourth sections.  The gap on the Russian MM/OD serves the same purpose as the gap on the U.S. MM/OD.  The gap allows the particle destruction freedom to further destruct into smaller and smaller fragments.  This process is called diffusion—the act of a body to reduce its specific mass or intensity as function of time and distance  (Mess. & Bertrand).

The fourth section of the Russian MM/OD is a thin sheet of carbon plastic.  The carbon plastic molecular bond allows for a higher stress fracture resistance than the aluminum shell.  This is why plastics bend multiple times before fracturing like metals.  The carbon plastic applies a higher resistance force to incoming particles over the aluminum shell (S&MTM).

Just beneath the carbon plastic layer is a honeycomb shaped screen (S&MTM).  This screen is the fifth section of the Russian MM/OD.  Effective in the same manner as the honeycombed aluminum section, the screen section is constructed with tighter-webbed beam structure to allow greater particle penetration protection (S&MTM).

The sixth section of the Russian MM/OD is another layer of carbon plastic lining the under side of the honeycombed screen.

The last, the seventh section, is another particle dissipation gap.  This last section is to allow maximum dissipation of a particle’s energy before it strikes the pressurized shell of a structure (S&MTM).

Figure 14.4-1.1 Russian MM/OD design concept (after S&MTM, 1.1-5, Figure 1.1-4)

NASA estimates there are 20,000 objects greater than 50 mm (2 in) in Low Earth Orbit  (S&MTM).  They also report that the probability of an object traveling 7 cm/sec (15,000 mph) penetrating the U.S. MM/OD is 7.5 percent (S&MTM).  The same object has only a five percent (5%) probability of penetration the pressurized shell a Russian module (S&MTM).

The U.S. MM/OD and the Russian MM/OD concepts structurally differ, but perform the same task using different forms of the same physical properties of particle collisions.  Both the U.S and Russian MM/OD concept’s outer layers are designed to slow the objects down and diffuse the particle’s structural integrity.  They also both contain particle diffusion gaps, in order to allow the particles diffused fragments to spread out their energy of a larger area of aluminum shell.

The U.S MM/OD and the Russian MM/OD differ in their structural repair ability.  The Russian MM/OD, due to its complex seven-section design is extremely costly and difficult to repair (S&MTM).  Entire sections of the MM/OD would have to be removed in order to repair an area on the Russian pressurized vessels.  However, the U.S. MM/OD concept, because of its simplicity is repairable by crewmembers during an EVA (S&MTM).  During an EVA, the outer shielding would be removed and replaced with its replacement shielding.  This method is less expensive and can be repaired in an emergency situation—if such need arises (S&MTM).

Neither the Russian nor the U.S. MM/OD have spare components onboard the Station such as shields, shells, screens, or screen- vacuum thermo-insulation layers.  If the need arose for a MM/OD repair, the assembly of the desired section would be fabricated on Earth and brought to the ISS on the next Space Shuttle mission.

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ISS Structures and Mechanisms Conclusion

14.5-1 Structures Summary

The International Space Station structures are responsible for providing protection to crewmembers (S&MTM).  Structures are responsible for withstanding and supporting the ISS form multiple/simultaneous acting loads (S&MTM).  Structures are generally composed of aluminum alloys due to their lightweight, easy machine ability, and relatively low material cost (Wertz & Larson).

14.5-2 Mechanisms Summary

The International Space Station mechanisms allow “Orbital, Progress and Soyuz” (S&MTM, 1.1-7) to dock with Station.  Mechanisms are also responsible for executing “temporary attachment for external payloads” (S&MTM, 1.1-7).  Crewmembers rely on mechanisms to aid in berthing, moving payloads, and providing temporary stowage.  Mechanisms allow large-task capabilities to much restricted crewmember EVA/IVA operations.  Mechanisms provide Shuttle-to-Station docking, module-to-module docking, truss-to-truss docking (S&MTM).

14.5-3 External Forces of Space Summary

The International Space Station is in constant contact with the environment of space.  External forces place loads directly, indirectly, and by combination of either of the two combining.  External forces include: temperature variations, atmospheric drag, gravity gradient torques, energy radiations, and highly corrosive atomic oxygen (Wertz & Larson).  The key to the ISS’s survival against external forces is the ISS’s structural management—(absorption control, oscillation control, and structural integrity control) (S&MTM).

14.5-4 Collective Summary of “ISS STRUCTURES AND MECHANANISMS”

Upon completion, the International Space Station will become the largest zero gravity test facility constructed by mankind (ISSUG).  Station will provide 46,000 cubic feet of pressurized atmosphere for crewmembers (WWW-1).  The International Space Station is considered a marvel of mankind, an achievement of uniting-counties in the advancement of technology and manufacturing.  The ISS stands for more than a scientific platform, it symbolizes teamwork, collective technological advancements, and how human dedication can surpass the physical restrictions of a new frontier.

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(Mess. & Bertratnd)    Dr. Messerschmid, Ernest and Bertrand, Reinhold;
                                        Space Stations: System and Utilization.
                                        Springer-Verlag Berlin Heidelberg 1999, translated and printed in Germany.

(S&MTM)                  International Space Station: Structures and Mechanisms Training Manual,
                                        Clark Toni, Lindner Dan, Todd Keith, Dr. Camarda Charles.
                                        National Aeronautics and Space Administration (NASA), January 22, 1999

(Wertz & Larson)       Wertz, James R. & Larson, Wiley J.
                                        Space Mission Analysis And Design—Third Edition
                                        Jointly published by: {Microcosm Press El Segundo, California} &
                                        {Kluwer Academic Publishers; Dordrecht / Boston / London.}
                                        1999—Second Printing.

Online References:

(AAEC)        Asia Aluminum Extrusion Concil
                        Information archived on (11-17-00)

(AML)         Advanced Materials Laboratory, Inc
                        Last updated April 15, 2000 Information archived on (11-17-00)

(ISSUG)     International Space Station User’s Guide
                       Available :
                       Information archived on (11-01-00)

(SSFATD)   Space Station Fracture Assessment Technology Development
                        Information archived on (11-17-00)

(STS92)        STS 92 Payload: A1 Integrated Truss Segment
                        Information archived on (11-18-00)

WWW-1     Welcome to the Boeing web page for the International Space Station,
                       Information archived on (11-15-00)

WWW-2     SP 300, Chapter 11—Spacecraft Structures,
                       Information archived on (11-15-00)

WWW-3     Unity and Zarya: Assembly and Performance,
                       Information archived on (11-16-00)

WWW-4     Boeing Components & Structures of the ISS--U.S. Laboratory,
                       Information archived on (11-17-00)

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Author: Mark A. Newby Jr.
Dec 2000