Acta Astronauaca Vol. 28, pp. 65-72, 1992 0094-5765/92 $.5.00+0.00 Printed in Great Britain Pergamon Press Ltd

DESIGNING BERTHING MECHANISMS FOR INTERNATIONAL COMPATIBILITY

John Winch* Boeing Missiles and Space Division

Huntsville, Alabama, U.S.A.

Juan Jose Gonzalez-Vallejo** Sencr

Madrid, Spain

Atuama

This paper examines the challenge of designing Space Station Freedom ~ mechanisms compatible with U.S. and international panncrs. The Space Station Freedom berthing

arc critical to all phases of station assembly and operations. The berthing mechani~ra design must provide common interfaces between the U.S., European, and Japanese preuufized modules and between the Space Station Manned Base and the European Man-Tended Free Flyer. Because of the neceui W of prov/ding a conunon inmrface between hardware supplied by three intemational partners, a key aspect of the design is arriving at a common set of requirements. The techrfical challenges involved in arriving at a common berthing mechanism design and the methods used to accomplish the task are described.

The Space Station Freedom configuration (Fig. 1) will include pressurized modules which facilitate crew habitation, laboratory experiments, and logistics operations. These pressurized modules will be brought into low Earth orbit by the Space Shuttle and berthed on orbit. Each Space Station Freedom pressurized module is permanently or temporarily berthed and slructurally attached usiqg passive and active berthing mechanism halves (Figs. 2, 3, and 4). The active half includes the mechanisms necessary to initiate and secure the module-to- module attachment. The berthing mechanism provides a shirt-sleeve passageway between pressurized modules.

ASSURED CREW RETURN VEHICLE

CRYD SUB-CARRIERS (4 PLACES)

GAS CONDI TI ONI NG ASSEMBLY _ _ ( LDCATED I NSI DE TRUSS)

- - A I R L O C K

/-'-ESA MODULE

HA8 MODULE A - -

PRESSURI ZED OOCkl NO ADAPTER

* Deputy Program Manager, Space Station Freedom Program

** Docking/Bc~'thing Mechanisms Project Manager

65

x-'JAPANESE MODULE

RESOURCE NODE (2 PLACES)

CUPOLA

PRESSURI ZEO LOGI s'rl Cs MODULE

USL MODULE A

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~ '- POWERED BOLT (16 PL)

CAPTURE LATCH (4 PL

Fig. 2 Active Rigid Berthing Mechanism

ALIGNMCNT GUIDE (4 PL) -PASSIVE RIGID BERTHING MEClIANISU

Fig. 3 Passive Rigid Berthing Mechanism

ALIGNMENT GUIDE (4 PL)

Fig. 4

/-- BELLOWS PASSIVE FLEXIBL[ ~ BERTHING MECHANISI~

Passive Flexible Rigid Berthing Mechanism

The Joint Develonment Effort

The Space Station Freedom Program is international in scope. The European Space Agency (ESA) provides the Columbus Attached Pressurized Module (APM), which is a laboratory module similar to the U.S. Laboratory Module. ESA

also provides the Man-Tended Free Flyer (MTFF), which is a free-flying laboratory in its own orbit with the capability of berthing to the space station. The National Space Development Agency of Japan (NASDA) provides the Japanese Experiment Module (JEM), an Experiment Logistics Module (ELM), and an exposed experiment facitity. All NASA, ESA, and NASDA pressurized modules are berthed using the space station common berthing mechanism (CBM). An identical CBM to pressurized module interface is necessary for all U.S. and international pressurized modules. ESA requirements for the MTFF are unique to the program and it is, therefore, logical to develop the CBM design jointly between NASA and ESA.'Boeing Missiles and Space Division, a part of the Boeing Defense & Space Group, is the prime contractor to NASA, Marshail Space FLight Center (MSFC), for the development of all CBMs for Space Station Freedom. Sener, headquartered in Madrid, Spain, is an engineering company with experience in decking mechanisms for space applications. Sener is under contract to ESA for the Columbus Program. A joint development effort has been initiated between NASA and ESA and between their contractors, Boeing and Sener, for the CBM design and development.

The Joint Development Team

A Boeing/Sener joint CBM development team was formed in October 1990. The senior management board consists of representatives from both companies with the following responsibilities:

Program Management Design and Engineering Manufacturing Integration Test Product Assurance Business Management

A Boeing/Sener CBM design team was formed consisting of the Boeing CBM design group and Sener design engineers on site at Boeing, Huntsville. The Sener designers arrived on site at Boeing in February 1991. Having the design team together at one facility has been an asset to the design effort. Sener designers can develop concepts and design alternatives on the same computer- aided design workstations as the Boeing designers, allowing ease of concept evaluation and modification. The Sener personnel can participate in meetings and technical discussions that are normally internal to Boeing, allowing direct involvement in the design evolution. The Sener engineers at Boeing also serve as the link between the Boeing/Sener joint design team and the Sener project office in Spain.

Technolo~ Transfer

Transfer of U.S. space hardware, software, and related technologies for use in foreign space projects must be consistent with relevant international and bilateral agreements and arrangements. The Space Station Freedom Program is governed by a multilateral intergovernmental agreement (IGA) and three bilateral memoranda of understanding (MOLT). NASA is responsible for overall program coordination and direction. In May 1988, NASA was granted general approval for exchange of technicid data to our intemationEl paFme~s as reqnired to ~dfi l l

#2nd lAF Congress 67

NASA's responsibilities under these agreements. General approval was granted on a programmatic basis for the lifetime of the Space Station Program. The space station general approval is the broadest advanced approval ever issued for a cooperative international scientific program. Technical data required to be exchanged unde, the IGA and MOUs generally fall under the following five categories:

Design to requirements Operate to requirements Standards Plans Training information

General approval does not cover transfer of data related to detailed design, development, production or manufacturing, or similar know-how. NASA must seek approval for such transfers on a case-by.case basis. Such transfers can also proceed on a company-to-company basis, provided licenses are approved. A manufacturing licensing agreement (MLA) between Boeing and Sener was approved by the U.S. Department of State. The MLA identifies data and technical assistance to be provided by the parties, including manufactm'ing know-how and any special processes to be shared between the contractors.

Develooiw, a Common Set of Reouirements

The first challenge in developing a CBM design to be used by three intemationalpartners is to develop a set of requirements common to all partners.

St~cifieation Dei, elonment

Boeing and Sener are required by their primary contracts to prepare specifications to two distinct formats. Sener is constrained to follow Columbus standards (e.g., STD 1213800 001). BoeingisconstrainedtofollowMI~STD-490A. Thetwo formats have some similarity but are not identical. The Boeing/Sener team may develop separate specifications with mutual agreeraents as to which sections must be identical, compatible, or which need not be co-developed.

All U.S. space station elements are launched using the Space Shuttle. The ESA Columbus APM and the NASDA JEM are also launched in the Shuttle. The ESA MTFF is launched using the ARIANE-5, and the NASDA ELM is launched using the Japanese HII launch vehicle. The CBM will be required to operate after exposure to the launch environment of the Space Shuttle, the ARIANE-5, or the HII launch vehicle.

ESA MTF'F-Unioue Doeking/Berthin~ Reouimments

The required operational on-orbit life of the MTFF is 30 years. The MTFF will have the capability to berth to the space station once every 5 years and the capability to dock to the Hermes space plane in nominal intervals of 6 months to 1 year. The MTFF will dock to Hermes approximately 54 times and will berth to the space station 5 times during its operational life. The current CBM design calls for the MTFF to have a passive CBM half and will bexth to a space station resource node which has the active CBM half. The passive CBM half includes an elastomeric berthing seal to seal the active-to-passive CBM interface. Having the seal exposed to the space environment on the MTFF for its 30-year life is a unique scenario for the Space Station Freedom Program.

Hermes is a European project independem of Space Station Freedom. The requirement for the MTFF to berth to Space Station Freedom and to dock to the Hermes space plane ftmher justifies F.,SA's desire to participate in the development of the space station CBM.

Berthin~ Mechanism Functional Descriotinn

A berthing mechanism assembly cormects two Space Station Freedom pressurized elements. Each berthing mechanism assembly is composed of an active and apassive half. The two halves are directed toward each other by a remotely operated manipulator arm called the remote manipulator system (R.MS). The two halves start to oriem themselves when they are I 0 inches apart using the alignment guide assembly, and the R_MS will continue to move them closer together until they enter the capture latch envelope. At that time the R.MS will stop the relative motion between modules and go into limp mode. When activated, 4 capture latch assemblies pull the two halves together and then 16 powered bolts make the structural attachment after the capture latches have cycled. Once the two berthing mechanisms are bolted together, pressure is allowed into the CBM vestibule by apressure equalization valve and verified by a gauge on the hatch. The hatch can then be opened to allow astronauts to move between modules.

Berthin~ Mechanism Component Description

The following paragraphs contain conflgurational and functional descriptions of the active rigid berthing mechanism assembly, passive rigid berthing mechanism assembly, passive flexible berthing mechanism assembly, and multiplexing motor controller.

Active Rigid Berthing Mechanism Assembly

The active rigid berthing mechanism assembly (Fig. 2) is composed of the structural ring, the debris cover, 4 capture latch assemblies, 8 alignmem guide assemblies, 16 powered bolts, 3 interface seals, and differential pressure transducers to monitor berthing seal integrity.

Structural Rin~L Active Pallid

The structural ring of the active rigid berthing mechanism assembly is machined from a 2219-T852 forging having a design envelope of 79.24 inches maximum outside diameter, an inside diameter of 71.375 inches and a length of 7.5 inches. The ring is attached to a pressurized module by 64 bolts. Three O-ring interface seals allow the berthing vestibule to hold pressure. The ring supports the capture latch and powered bolt mechanisms as well as providing a seal surface when it is mated to the passive berthing mechanism. Differential pressure transducers are attached to this ring to monitor berthing seal integrity. The ring can survive an impact from the passive berthing mechanism ring caused by a 0.16 fl/s RMS runaway velocity.

Debris Cover

The debris cover (Fig. 5) provides thermal insulation and prevents meteoroid damage to the berthing mechanism flange, seals, and to the hatch before station assembly. This cover is attached to the berthing mechanism at lannch and is removed by the RMS and stored before berthing two modules together.

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DEBRIS SHIELD COVER

. . . . . . . . . . ' ~ M I I

Fig. 5 Active Rigid Berthing Mechanism

Camm'e Latch A~ernbly

The capture latch assembly (Figs. 6 and 7) is composed of four me, chanisms located on the active rigid berthing mechanism assembly with four capture latch fittings located on the passive berthing mechanisms. Each latch is bolted to the active berthing mechanism assembly r ~ and is powered by an actuator located inside the resource node. The vision system indicates when the passive berthing mechanism assembly arrives in the capture latch envelope. The capture latch mechanisms arc then activated re.aching out to engage the capture latch fittings. Should a capture latch actuator fsil, manual backup is possible with hand tools. Berthing is possible with three capture latch assemblies.

The actuator clutch will allow it to be back driven opposite to the capture direction if the torque on the drive shaft exceeds 43 in-lb. The actuator is designed to supply 43 in-lb torque in the drive direction into a gearbox that increases the torque three times. The actuator rotates 660 deg during one complete capture cycle. The maximum strength capability of the arm is 2100 Ib when it is fxdly extended and it is capable of providing a capture force to the module of 33 Ib minimum at the least mechanical advantage position.

Alimament Guide Assembly

The alignment guide assembly (Fig. 8) is used to ensure proper orientation of the berthing halves relative to each other allowing the powered bolts to engage. There are eight aligranent guides on the active berthing mechanism assembly and four on the passive berthin 8 mechanism assembly. Therefore, each alignment guide blade on the passive assembly slides between two alignment guide blades on the active assembly. Each alignment guide assembly is launched stowed beneath the debris COVCI'.

Powered Boll

After the capture latch assembly pulls the two berthing

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Fig. 6 Capture Latch Mecha_n.ism

42nd IAF Consren 69

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Fig. 7 Capmrc Latch Motion

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halves together, 16 powered bolt mechanism actuators (Figs. 9, 10, and 11) loc, lcd in the resource node are activated m engage the bolts into 16 powered bolt nuts located on the passive beaching ~ assembly. Manual backup is possible fxom inside the resource node should the acummr fail The boltis made from Inccc~1718 with a 0.625-18 UNYF thread and drivun by an ~mm_r t l ~ can l~ovide up to 900 in-lb torque. This torque Imdom~ the joint to 6500 lb/bolt after pulling the two berthing

to metal-to-metal contact. The bolt also has a taper befox~ the threads to help align it with the powered bolt nut discussed further in a later section.

Fig. 8

./

Al~t Guide Assembly

Differential Pressure Transducers

The seals between the active rigid and the passive berthing mechanism are critical to maintain pressure of 14.7 psig within the berddng vestibule area. Thea~ore, the diffemmdM pressure trm~lucers (Fig. 12) along with a mon~oring system are necessary to monixor the performance of these seals and alert the crew should seals failpremamrely. Four u'msducers are needed with a wire bundle to connect it to a firmware corm'oiler.

Passive Rigid Berthing Mechanism Assembly

Whereas there is only one type of active rigid berthing mechanism assembly throughout the space station, there are two different types of passive berthing mechanism assemblies: flexible and rigid. The passive rigid berthing mechanism assembly (Fig. 3) is used to male modules together outside of the module loop such as the airlock, the presmuiz~ logistics modules (PLM), the Columbus APM, and the NASDA YEM. It is composed of the su~cmral ring, the debris cover, 4 capture latch filzings, 4 alignment guide assemblies, 16 nuts, 3 elas~omeric berthing seals, and 3 O-ring interface seals.

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POWCRF.0 ROI T NUT ]NSTt

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RI gl O BERTItl NG ,,L_. _ I I ~ MECHRNI SM ASSYJ [ ~ BEVEL WASHEn ~R PARSI vE LEXIBLE BERT)lING

MECHANI SM ASBY

BOLT

,:!ifi! I, ,:!ifi!I, ~ BUSHI NG

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POWERED BOLT I NSTL

Fig. 9 Powered Bolt/Nut Assembly

Smmtural Rin¢,. Pmmive Rieicl

The structural ring of the passive rigid berthing mecharusm meamably is machined fi'om a 2219-3"852 forging having a design envelope of 79.24 inches maximum outside diameter, an inside diameter of 71.375 inches and is 13.5 inches long. The ring is attached to a module by 64 bolts. Three O-ring interface seals allow the berthing vestibule to hold pressure. The ring supports the capture latch fittings, nuts for the powered bolts, and retains the beahing and interface seals.

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Fig. I0 Powered Bolt/Nut Assembly

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Fig. 11 Powered Bolt Operation

PRESSURE.

O-RING SEALS

Fig. 12 CBM Seal Detection

BERTI'IffiO kR:CHANISM FLANGES

calm~t.lal~J~m~

The capture latch fitting allows the capture latch assembly a hook to grab onto and is bolted onto the ring in four places with two removable bolts (Fig. 6).

Powered Bolt Nut

After the capture latch putts the modules to within approximately 0.08 in of each other, the powered bolts are activated. Each bolt engages a nut located in a recess of the su-uctural ring called the powered bolt nut (Fig. 10). This nut floats :L-0.06 inches as well as rotates 1.0 (leg on the bevel washer to permit rnisalignment between mating berthing flanges.

Them arc three berthing seals that are retained by the passive rigid berthing ring. These seals provide two-fanh tolerance

42nd IAF Congress 71

redundancy across a gap calculated to be 0.01-0.02 inches. The seals development test is exploring other options for seat materials and configuration. Differential pressure transducers will monitor the performance of these seals throughout the station's ~e,

Passive Flexible Berthing Mechanism Assembly

The passive flexible berthing mechanism assembly (Fig. 4) uses an aluminum bellows in its weldment that is capable of 1.5 deg angulation or a reduction of length of 2.0 inches when assembling the space station in space. After the station is assembled, eight powered struts located inside the pressurized area ensure that the bellows is held rigid. The passive flexible berthing mechanism assembly contains the same components as the passive rigid berthing mechanism assembly plus the powered struts.

The weldment is composed of two forged flanges of 2219-T852 aluminum welded to a two-ply bellows of 2219-T62 aluminum 0.032 inches thick per ply. Its maximum outside diameter is 79.90 inches, with a minimum inside diameter of 71.3"75 inches, and is 13.5 inches long. The weldment is attached to a module by 64 bolts. Three O-ring interface sea.Is allow the berthing vestibule to hold pressure. The weldment supports the 4 capture fittings, 16 powered bolt nuts, 8 powered struts, and retains the berthing seals.

Powered Strut Assemblv

The powered strut assembly (Fig. 13) is actuator driven and is designed to move a total of 2.0 inches causing the bellows to angulate 1.5 deg. The actuation system cannot operate when the volume enclosed by the passive and active berthing mechanisms (berthing vestibule) is pressurized. The strut is designed to take tensile loads although it also has limited compression load capability. The current design does not include load cells to monitor the on-orbit loads in each strut. The powered struts are located coincidental with the powered bolt nuts to minimize flange deflection as loads are transferred between berthing mechanisms.

Multinlexin~ Motor Controller

The active rigid, passive rigid, and passive flexible berthing mechanism assemblies are mounted to the berthing bulk_head of their respective modules. Actuators inside the module drive the be~hing mechanisms with control through a multiplexing motor controller.

Two controllers and two switches are located in each module. Each controller is capable of driving 32 actuators, with any 8 being controlled simultaneously. A switch selects 16 to 20 actuators of the more than 100 berthing actuators in a resource node. Two controllers and switches will be needed to meet redundancy requirements. They will be packaged together with either controller being capable of using either switch and either

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Fig. 13 Powered Strut Assembly

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switch being capable of directing control and power to any of the berthing mechanisms within that element.

Interfaces

Many support systems are needed to help the berthing mechanism perform properly. These include depressurization of the vestibule, power and actuator control, a vision system, and the space station data management system (DMS), which interfaces with a fLrmware conlroller to monitor seal leakage.

Berthin= Mechanism DeDressurization

The berthing vestibule is pressurized after two modules are berthed and the power bolts are engaged. To remove this pressure, air is vented through the equalization valve to the node vent valve with either permanent or temporary pressure hoses.

Berthin~ Mechanism Power and Control

The 120V dc power required to run the multiplexing motor controller and the berthing mechanism actuators is provided through the space station power distribution network.

V~mion System

Cameras located behind the hatch windows provide the astronauts an axial view of the berth. This view, with associated visual cues, provides the additional information needed to accurately position an element with a RMS. The visual cues wiU consist of a target mounted over the hatch of the element being berthed and an overlay of four vertical lines over the RMS operator's video screen (the lines represent approximate distance when compared to the target's video image).

The camera or vision system will transmit six degrees-of- freedom data to the RMS operator. This information will be used to detemaine when the capture latch fittings are within the capture latch envelope and will provide the relative position of the modules during the racetrack closure. It is also fed into a computer which selects the correct powered strut movement when closing the racetrack.

One possible vision system uses laser diodes which bounce light at two alternating frequencies off reflectors on the target. Filte~ on the target and the two alternating frequencies of light provide two alternating images to the video camera. The video images are passed through the video network to a computer which digitizes the video images. These digitized images are then used to calculate relative orientation of the two berthing mechanisms (one active and one passive).

According to the NASA/ESA joint development plan, Boeing will have the lead responsibility for qualification of the CBM design and associated interfaces. Sener will participate in the qualification process by supporting the development of qualifieatiun test requirements and procedures.

For MTFF-unique CBM modifications designed and developed by Sener, Sener will be responsible for the qualification, and NASA and Boeing will review the qualification test results for assessment of impacts on CBM qualification.

Boeing will have the responsibility for fabrication and assembly of berthing mechanisms for NASA pressurized modules, while Sener will have the responsibility for fabrication and assembly of berthing mechanism passive halves for use on Columbus elements. Boeing will supply all necessary technical documentation for CBM hardware and will provide, if needed, all tooling drawings and ground support equipment drawings to Sener.

Development and transfer of manufacturing technologies and processes will re.quire in-depth coordination of the intemational partners to ensure consistent quality and interface compatibility of the hardware. Included in this transfer might be precision machining and welding t~hniques, electrical systems assembly, and master gauge tooling development.

Mandatory inspection points during the manufacturing process are derived primarily from the Failure Mode and Effects Analysis/Critical Items List. These inspections are complemented by experience on similar hardware. Critical interfaces will be verified with check fixtures to ensure mate-up. All inspections and tests will be documented, thus providing a clear audit trail.

Conclusion

Designing Space Station hardware with a common interface to international partners has proven to be very challenging. The Boeing/Sener joint development team has proven its value to the contractors ha fulf'flling their responsibilities under NASA and ESA contracts. The lessons learned in this joint development effort should influence Boeing and Sener in future Space Station Freedom activities as well as future contracts that are international in scope.

Acronyms

ACRV APM CBM DMS ELM ESA HAB IGA JEM MLA MOU MSFC MTFF NASA NASDA PL PLM R_MS STS USL V dc

Assured Crew Return Vehicle Attached Pressurized Module Common Be~b.ing Mechanism Data Management System Experiment Logistics Module European Space Agency Habitation Module Intergovemmental Agreement Japanese Experiment Module Manufacturing Licensing Agreement Memorandum of Understanding Marshall Space Flight Center Man-Tended Free Flyer National Aeronautics and Space Administration National Space Development Agency of Japan Places Pressurized Logistics Module Remote Manipulator System Space Transportation System U.S. Laboratory Module Volts direct current