the esa laboratory support equipment for the iss

The ESA Laboratory Support Equipmentfor the ISS

A. PetrivelliLaboratory Support Equipment Section, ESA Directorate of Manned Spaceflight andMicrogravity, ESTEC, Noordwijk, The Netherlands

IntroductionThe LSE programme was started at ESA in1994 following the finalisation of the Agreementin Principle for ISS Early Utilisation, which wassubsequently translated into a Memorandum ofUnderstanding (MOU) in 1997. According tothis Agreement, NASA provides ESA with flightopportunities and resources for accessing theISS before the ESA Columbus module islaunched. In exchange, ESA provides NASA withgoods and services. This includes the develop-ment and delivery of the two pressurisedfacilities – MELFI and MSG – and the externalpayload pointing mechanism – Hexapod.

Later, the LSE programme was extended toinclude the development of another pressur-ised facility – the Cryosystem – as well as thesustaining engineering for MELFI, MSG andCryosystem. Those additional items wereintroduced with the ESA/NASA Arrangementfor the Columbus Laboratory Launch, signed in1997. In the same year, an MOU was signedbetween ESA and NASDA whereby NASDAprovides ESA with twelve ISS StandardPayload Racks (ISPR). In return, ESA providesNASDA with a MELFI unit, identical to those tobe delivered to NASA.

In addition, the LSE programme includes thedevelopment of general payload GroundSupport Equipment (GSE), such as the TestEquipment for Payload Development (TEPAD),originally developed for supporting the LSEfacilities, but now available as ISS interfacesimulators for the development of anypressurised payload (Fig. 1).

From 1994 onwards, the LSE programmeestablished and has maintained programmaticand technical interfaces with the NASA ISSPayload Office and the NASDA PayloadOrganization, as well as with the NASA centresresponsible for the various disciplines, whichinclude Marshall Space Center (MSFC)Microgravity Program Office, the JohnsonSpace Center (JSC) Environmental ControlDepartment, the Ames Research Center (ARC)Life Science Program Office, and the Langley

The Laboratory Support Equipment (LSE) for the International SpaceStation (ISS) is a suite of general-purpose items that will be availableonboard the Station either as self-standing facilities or as equipmentthat can be used at defined locations. Dedicated to supporting systemmaintenance and payload operations, some LSE items are derivedfrom commercial equipment, while others have been specificallydeveloped for the ISS.

ESA is currently engaged in developing three pressurised facilities andone pointing mechanism that will become part of the LSEcomplement, namely:– the Minus Eighty degree centigrade Laboratory Freezer for the ISS

(MELFI)– the Microgravity Science Glovebox (MSG)– the cryogenic storage and quick/snap freezer system (Cryosystem)– the external-payload pointing system (Hexapod).

esa laboratory support equipment for the iss

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MELFI

HEXAPOD

TEPAD & othercommon GSE

MSG

CRYOSYSTEM

Research Center (LaRC) Earth ObservationMissions Office.

As result of this activity, Joint ImplementationPlans for all the projects have been establishedand put under the control of the Multi-lateralPayload Control Board (MPCB).

The LSE development programmeThe four LSE projects are currently at differentstages of implementation: – MELFI and MSG have completed their design

and development phases. MSG was deliveredto NASA in October, and MELFI will be shippedin March 2002.

– Hexapod is in the final stages of development,with delivery to NASA foreseen for mid-2002.

– Cryosystem has been undergoing basicscientific and utilisation requirementassessments, before starting the preliminarydesign phase (Phase-B) at the beginning ofFebruary.

The same general development philosophy hasbeen adopted for the first three projects. ThePhase-B, which lasted about two years, hasincluded the development of breadboards ofthe most critical equipment and assembliesand has demonstrated basic functional perfor-mance. The main development phase (Phase-C/D) was started in October 1996 for MELFIand MSG, and in January 1998 for Hexapod.

Many technical challenges have beenencountered during the development of the firstthree payloads:– they were among the very first to be designed

for the ISS, for an environment and payloadinfrastructure that were not yet settled

– the ten-year-lifetime and in-orbit-maintenancedesign requirements for the two pressurisedfacilities were important novelties, and thedesign was heavily driven by those requirements

– the MELFI development included a significanteffort for the preparation of the two keytechnologies on which the freezer’s designwas based, namely the Brayton turbo-compressor cooler and the super-vacuumthermal insulation; significant breadboardingand qualification effort was accomplishedwithout exceeding the restricted financingavailable

– the MSG, as the first user of the EuropeanStandard Payload Outfitting Equipment(SPOE), had to adjust to the contemporarydevelopment of those items and at the sametime serve as the test bed for their development

– the not yet settled ISS environment for theexternal payload rendered the developmentof the Hexapod external interfaces bothdifficult and risky; only the good technicalcoordination between the NASA and ESA

project teams has allowed progress to bemade in the development activities.

Ground models have been produced for MELFIand MSG to support the science-procedurepreparation and crew-training activities. Inaddition, engineering models for each facilityhave supported the development activities andwill be available for the sustaining engineeringduring the operational phase. With the designand development of MELFI and MSG, the rackstructural design and qualification expertise forthe NASDA-provided ISPR has been fullyacquired by ESA.

Following the Memorandum of Understanding,NASA will assume ownership of the LSEpayloads and will be responsible for theiroperation, but ESA will have to provide sparesand technical support for their maintenance.These activities will be part of the ESAExploitation Programme, once it is initiated andoperational. Due to the time constraints,however, the detailed responsibilities andactivities to be performed within the scope ofthe general LSE programme agreements havealready had to be defined and agreed withNASA. After many meetings and discussions, ithas been possible to baseline Logistic SupportPlans for MELFI, MSG and Cryosystem, whichhave been accepted by all parties.

ConclusionsWithin the ESA LSE programme, the MSG isnow integrated into the MPLM ready for anexpected launch by May 2002. The MELFI andHexapod activities will soon be completed inEurope and these items delivered to NASA. TheCryosystem is entering the design phase.

Significant experience has been accumulatedwith these projects, not least in coping with themany challenges generated by the Inter-Agency Barter Agreement environment inwhich they are taking place. So far, all of thetechnical objectives for the individual facilitieshave been met within the assignedprogrammatic constraints. This allows us toconclude that the ESA LSE programme hasdemonstrated the usefulness and practicality ofthe Inter-Agency Agreements, whereby theparticipating agencies exchange goods andservices without a direct exchange of funds.They have allowed optimal exploitation of theresources available at the various Agencies.Some hard lessons have also been learned, inthat the mechanism only works well if the basicrequirements – technical and programmatic –and the procedures for handling changes,which are unavoidable when dealing with long-duration projects, are firmly establishedtogether with the initial political agreements.

r bulletin 109 — february 2002

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Figure 1. (a) MELFI firstflight unit with one dewar

door open and a traypartially extracted, and

(b) rear of the unit with thecover removed to show thenitrogen piping interfacing

with the cold box (uppercontainer) and the four

dewars

NASA and to provide the necessary spares andsustaining engineering to maintain them for upto 10 years of operations.

ESA selected Astrium-Toulouse (formerly MatraMarconi Space) as its prime contractor; theother main sub-contractors participating in theMELFI industrial consortium are:– Air Liquide (F), for the Brayton subsystem– Linde AG (D), for the MELFI cold chain– Kayser-Threde (D), for the electrical sub-

system and some rack components– ETEL (CH), for the motor and motor-drive

electronics– DAMEC (DK), for the utilisation hardware and

concept.

After a pre-development Phase-B study, themain development phase (Phase-C/D) began inJanuary 1997. The Preliminary Design Review(PDR) was completed by the end of that year,and the Critical Design Review (CDR) wassuccessfully completed in summer 1999.Staggered verification reviews were completedby February 2002. The Qualification/Acceptance

IntroductionThe Minus Eighty degree celsius LaboratoryFreezer for ISS (MELFI) is a rack-sized facilitythat will provide the Space Station with arefrigerated volume for the storage and fastfreezing of life-science and biological samples(Fig. 1a,b). It will also ensure the safe trans-portation of conditioned specimens to and fromthe ISS by flying in fully powered mode in theMulti-Purpose Logistic Module (MPLM)installed in the Shuttle’s cargo bay. Continuousoperation for up to 24 months is foreseen foreach MELFI mission.

MELFI has been developed by the Agencyunder various Barter Agreements, wherebyESA will deliver three MELFI flight units to NASAand one flight unit to NASDA. In addition, ESAhas agreed to deliver certain ground units to


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MELFI

J.A. JiménezLaboratory Support Equipment Section, ESA Directorate of Manned Spaceflight and Microgravity, ESTEC, Noordwijk, The Netherlands

A. BrunschvigAstrium Space, Toulouse, France

Figure 2. The reverseBrayton thermodynamic

cycle

Review in Europe for the first MELFI flight unitwill take place in March 2002. The finalverification process using the ISS and MPLMsimulators located at Kennedy Space Centerwill start in May 2002. Thereafter, the MELFIfirst flight unit will be ready for launch inSeptember 2002. It is planned to launch MELFIto the ISS on the ULF-1 flight in January 2003.

The cooling conceptMany trade-off and technology studies werecarried out before selecting the coolingconcept for MELFI. The key drivers were:– Low electrical power-budget allocation

(900 W) for the size of the cold volume andthe low working temperature.

– High reliability (two years of continuousoperation) and easy maintenance.

– Very low thermal leaks to cope with a power-off time of 8 hours.

– Very low disturbances to the ISS microgravityenvironment.

– Very low acoustic-noise requirements.

From the outset, the technology for a –80°Cfreezer was identified as being very difficult toachieve, involving the development of new andvery challenging technologies and/or the majoradaptation of existing ones.

At the beginning of the 1990’s, and after anextensive trade-off with the Stirling cycle, thereverse Brayton cycle was selected for thecooling thermodynamics. It was chosen mainlyfor its power efficiency in the temperature rangeof interest and for the reduced microgravityperturbation, thanks to its being a rotatingrather than a linear machine. The cold powerproduction relies on a closed thermodynamicloop, in which nitrogen gas is compressed,cooled and expanded to achieve the lowtemperatures required (Fig. 2). Depending onthe electrical power provided to the Braytonmachine, the temperature in the expander (step#4) can reach values as low as –95°C.

The heat is removed from the samples bypassive means, i.e. conduction and radiation

only. Cooling by forced convectionwas eliminated during the earlystages of the project because fansinside the cold cavity are not reliable(easily blocked by ice), increaseelectrical power consumption, andgenerate acoustic noise. Also, it isdifficult to produce predictableconvection flows to the samples,because the flow depends on thefilling status. Natural convection dueto gravity effects occurs when MELFIis on the ground, but it has beendesigned to meet the requirementswithout convection.

The system and its capabilitiesFigure 3 shows the main systemcomponents. The cooling engine is aBrayton turbo-machine, whichprovides the flow of cold nitrogen.The rotating components are the


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Figure 3. MELFI blockdiagram

Figure 4. Close-up of thefront face of MELFI,showing the activeelectronic unit control panel(upper right). The spareelectronic unit is partiallyvisible to the left of theactive one. Below the activeelectronic unit, the coverplate has been removed toshow the Cold Box with theBrayton machine inserted

record the temperature in the dewars whenMELFI is not powered. All of the areas wherethe electronics could dissipate significantamounts of heat are instrumented with thermalswitches that control potentially hazardoussituations within the ISS (fire protection).

The electrical subsystem provides overallcontrol of the MELFI system and powers theBrayton motor and control electronics (Fig. 4).The freezer’s continuous availability is crucial tomission success and it is therefore imperativethat failure of any sensitive component berecoverable within the maximum time for whichMELFI can protect the samples in passivemode, namely 8 hours. Consequently, theElectronics Unit (EU) and the Brayton machinehave been designed as Orbital ReplaceableUnits (ORU), with spares for each availableonboard (Fig. 5).

MELFI is integrated into a six-post aluminiumrack provided by NASDA, and manufacturedby Japan’s Ishikawajima-Harina HeavyIndustries. The IHI rack was selected becauseit was already space-qualified, meets the ISPRmechanical interfaces, and is the only existingrack structure able to carry MELFI’s maximummass of about 800 kg.

Designed for an operational lifetime of 10 years,MELFI has been qualified for 15 launches in theMPLM. It has been basically designed forinstallation in the US Lab module of the ISS, butefforts are underway to interface it with theJapanese Experiment Module (JEM) also.

turbo-expander, the compressor and the motormagnet, all of which are integrated onto asingle shaft supported on gas bearings. Thisbearing technology was selected because of itslong lifetime and the low-perturbationrequirements. Two radial bearings support theshaft close to the compressor and turbinewheels, while one axial bearing carries the axialforces introduced in the shaft by the wheels’aerodynamics. The bearings are made oftungsten carbide to withstand the friction thatoccurs during the starting and stopping of theshaft. The shaft itself can run at speeds inexcess of 90 000 rpm, depending on thecooling energy required. The gap between thestatic and rotating parts of the bearings is only 10microns. It is therefore very important tobalancing the shaft very precisely, and the gascirculating in the cycle must retain very highlevels of cleanliness. The machine is cooled bywater running through the motor heatexchanger that surrounds the cartridge.

The Brayton motor relies on brushless andsensorless technology. Brushes are notsuitable for the high speed and the long-liferequirements, and they generate pollution. Thesensorless technology was selected for itsrobustness in the cold environment and becauseit allows the very high rotor accelerationsrequired to ‘lift’ the shaft on the gas bearingsafter just a few turns. Implementation of thistechnology proved a major challenge,especially in controlling the motor starting phase.

The heat exchangers needed to implement theBrayton thermodynamic cycle are integratedinto a closed container called the ColdBox, into which the Brayton machine isinserted to form an integratedassembly called the BraytonSubsystem. A set of tubes (Fig. 1b)distributes the cold nitrogen to fourindependent cold cavities (thedewars). The supply and returnnitrogen flows are in concentric tubes.The nitrogen tubing provides thecooling power to the dewars in aclosed loop (i.e. the nitrogen is not indirect contact with the samples in thedewars), at the so-called ‘cold fingers’that house the load heat exchangers.A valve at the tip of each cold fingerregulates the nitrogen flow. In this way,the temperature in the dewars can becontrolled independently in threeoperating modes (at -80, -26 and+4°C). The dewars are designed toimprove the thermal coupling betweenthe samples and the cold fingers.A battery-driven Temperature DataRecorder (TDR) provides the ability to


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In addition to the four flight units, the projectincludes the following ground units:– Laboratory Ground Model (LGM), to be used

to support scientific sample accommodationin MELFI and the development of sampleintegration procedures. This unit is a flight-like stand-alone dewar that includes all thestandard sample-accommodation hardware.

– The MELFI Training Unit, which will supportactive crew training in a flight-like environment.

– The MELFI Engineering Unit, which is a fullyintegrated rack that will support sustaining-engineering functions.

System utilisationThe utilisation of MELFI requires very close co-ordination between the scientists who willconduct the experiments aboard the ISS andthe ISS ‘cold storage team’. It is important toknow well in advance the type of sample,sample container, and cooling and storagerequirements, so that the integration of allexperiments can be properly planned. Thetime-lining between the associated experimentsand MELFI also has to be correctly co-ordinated.

The utilisation scenario will provide late accessto MELFI whilst the Shuttle is on the launchpad. There will also be the possibility to haveearly access to retrieve the samples once theShuttle has landed. MELFI provides thenecessary ground-support equipment to carryout these operations. In orbit, the samples canbe transferred at facility level by exchanging thecomplete rack, or at tray level by using theMELFI-provided in-orbit transfer bag.

Each dewar includes four trays, each of whichcan be extracted without disturbing thesamples in the other three. In addition, MELFIprovides standard accommodation hardwarefor the insertion of samples of different shapesand sizes (Figs. 6 & 7):– Standard vial card: This card is a flat

aluminium plate (10 x 11cm) that can carryvials ranging from 2 to 10 ml in size, andattached by elastic loops. Each card canaccommodate up to 20 small vials (10 eachside) or 10 large (long) ones (5 each side).


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Figure 6. MELFI tray withthe foam insulation block

attached to the front of thetray. In this configuration,the tray includes two 1/4-

size box modules and one1/2-size box module

Figure 5. The Braytonmachine cartridge (top) and

the machine shaft, withcompressor and turbine

wheels removed to showthe radial and axial gas

bearings

Figure 7. A 1/4-sizestandard box module full ofstandard vial cards withfrozen samples

technologies and integrated them into a newspace freezer that will provide the scientificcommunity with a large permanent cold-storage facility in space for the first time. TheAgency and its MELFI contractors now lookforward to contributing their expertise tokeeping the MELFI system operational, in orderto foster and grow the interest of the scientificcommunity in doing science aboard the ISS inthe years to come.

AcknowledgementThe success of the MELFI project could nothave been achieved without the dedication ofthe industrial teams. Their flexibility in adaptingto the many changes encountered during theearly development phases of the ISSinfrastructure and in resolving the technicaldifficulties faced during the development of thechallenging technologies involved meritsspecial recognition. The support received fromESA’s Technical and Operational SupportDirectorate at ESTEC, which helpedconsiderably in establishing good technicalcommunications between ESA, NASA, NASDAand the MELFI contractors, is also gratefullyacknowledged. Last but not least, thanks alsogo to those in ESA, NASA and NASDA whohave contributed to the complex co-ordinationof the many interfaces involved in theimplementation of the multilateral Agencyagreements. r

– Contact cards: Similar to the standard vialcard, but designed to improve the thermalconductivity between the card and the boxmodule, thereby increasing the cooling rate.This card features elastic loops to restrainvials (4 long or 8 small) on one side, andVelcro straps to accommodate irregularshapes (with one flat side) on the other.

– Standard box modules: The boxes interfacedirectly to the trays and can store a numberof vial cards, vial bags, or any large samplethat fits the module. Elastic bands areprovided to secure the cards.

– Receiving box module 1: This module isidentical in size and shape to the standardone, but provides a dedicated interface tohold the Contact Card to improve the thermalconductivity. It also provides a specialinterface to accommodate ‘bottle holders’.

– Receiving box module 2: Identical in size andshape to the standard one, it features a solidbottom profile instead of a perforated one forbetter thermal coupling between the tray, thebox module and the inserted samples.

It will be user’s responsibility to provide thededicated accommodation hardware neededfor special applications.

In-orbit commissioningThe MELFI system has been fully verified on theground, with the thermal performances inparticular being measured during extensivetests. The cooling performances that will beachieved in orbit have been predicted byanalysis, using thermal models correlated duringthe ground tests. It is necessary to confirm inorbit that those predictions are correct.

In addition to the standard checkout of thegeneral interfacing and system functionality ofthe MELFI rack, the in-orbit commissioning ofthe first MELFI flight unit will include verificationof the actual cooling performances provided tothe samples. For this, ESA has developed theMELFI On-Orbit Commissioning Experiment(MOOCE), which provides additional instru-mentation in the dewar cold cavity that holdsthe scientific samples. MOOCE’s 24 thermo-couples provide comprehensive temperaturemapping of the tray, the box modules and thesamples. During the test, the MOOCE’sexternal data-acquisition unit will providecontinuous recording and de-multiplexing ofthe temperature data, which will subsequentlybe retrieved via the ISS Laptop and the testsresults sent to ground using the ISS downlinkcommunication services.

ConclusionsWith the MELFI project, ESA and Europeanspace industry have jointly developed novel


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Figure 1. The MicrogravityScience Glovebox (MSG)

ready for integration into theMPLM

IntroductionThe Microgravity Science Glovebox (MSG) forthe ISS completed its development andverification in Europe in October 2001. Aftershipment to NASA KSC, it underwent a lastseries of ISS interface tests before beingintegrated into the MPLM on 1 March 2002.From May onwards, it will equip the ISS withunique and multidisciplinary laboratory supportcapabilities.

The MSG has been designed as a modularmulti-user facility for performing a wide varietyof materials, combustion, fluids and bio-technology investigations in the microgravityenvironment (Fig. 1). Primarily it provides an

enclosed and sealed Work Volume (WV)equipped with lighting, mechanical, electrical,data, gas and vacuum connections, andthermal control. The WV is provided with built-in glove ports for safe handling by the crew andisolates the item under investigation from theoperator area and the general ISS environment.An attached Airlock (AL) allows specimens andtools to be inserted or removed during MSGoperations with limited environmental exchangebetween the WV and the ISS cabin. The MSGfacility will also accommodate minor repair/servicing of hardware requiring a clean and/oran encapsulated working environment (e.g. the Fluid Science Laboratory’s investigationcontainers).

The design and development of MSG hasevolved substantially from the former Spacelaband Mid-deck Gloveboxes that have beenflown on numerous Space Shuttle missionsand on Mir. Significant enhancements include asubstantially larger working volume to housebigger experiments, increased poweravailability, enhanced diagnostics and datacontrol, and temperature and humidity control.

The facility has been developed by ESA for theNASA/MSFC Microgravity Program Office andincludes both flight and ground units. The threeground units – Ground Laboratory Unit, TrainingUnit and Engineering Unit – have all beendelivered to the relevant NASA entres and havealready been used extensively by the MSFCIntegration Team for experiment developmentand by the crew for training (Fig. 2).

The development contract was awarded to anindustrial team composed of:– Astrium GmbH (D), Prime Contractor,

responsible for System Engineering, Integrationand Verification


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MSG

M.N. De ParolisLaboratory Support Equipment Section, ESA Directorate of Manned Spaceflight andMicrogravity, ESTEC, Noordwijk, The Netherlands

M. Cole, C. CokerMicrogravity Science and Applications Department, NASA Marshall Space FlightCenter, Huntsville, Alabama, USA

M. Zell, A. SchuetteSpace Infrastructure, Orbital Systems, Payload & Missions/Operations Centres,Astrium GmbH, Bremen, Germany

Figure 2. The MSG TrainingUnit

physical barrier of the wall and the secondthrough an under-pressure in the WVcompared with the surrounding environment,i.e. in the event of a leak, any airflow will alwaysgo into and be confined within the WV. Thecontinuous air circulation inside the WV andAirlock that maintains the under-pressure isfiltered and cooled, providing both a clean-room environment and the possibility to removeup to 200 W of thermal energy from the itemunder investigation.

– Bradford Engineering (NL), responsible forthe Core Facility and the Video Drawer

– Verhaert Design and Development (B),responsible for the Airlock and the Outfitting Equipment.

At a later stage, the team was joined by Laben(I), responsible for the Analogue Video InterfaceBoard,0 and Atos-Origin (NL), responsible forthe MSG Application Software.

System architecture and characteristicsThe MSG has been developed following thescience requirements defined by the MSFCMicrogravity Science Team, and the functionaland interfaces requirements for ISS payloads,defined by NASA Johnson Space Centre (JSC).It is integrated into an ISS Standard PayloadRack (ISPR).

The MSG system architecture (Fig. 3) is builtaround the Core Facility (CF), hosting theinvestigations in the WV and providing themwith all the resources needed. The other mainparts of the facility are:– the rack infrastructure with ESA’s Standard

Payload Outfitting Equipment (SPOE),consisting of the Remote Power DistributionAssembly (RPDA), the Standard PayloadComputer (SPLC), the Avionics Air Assembly(AAA)

– three ISIS stowage drawers with supportingequipment for payload operations andconsumables for facility operations

– the ISIS-based Video Drawer Assembly (VD),supporting optical investigation diagnostics.

Figure 4 shows the Flight Unit (FU) rack duringElectro-Magnetic Compatibility (EMC) testing atAstrium.

The MSG facility has been built for a projectedten years of operational use in orbit. It will beinitially accommodated in the United StatesLaboratory (USLab), but could be moved at alater stage to the European ColumbusLaboratory.

Core-Facility capabilitiesThe Core Facility includes the large WorkingVolume (WV), the Airlock and electronics forcontrol, housekeeping and investigationresources and it occupies the upper half of theoverall rack (Fig. 4). The Command andMonitoring Panel (CMP), located on top of the WVfor ease of crew operation, monitors the facility’sstatus and performance and provides all meansfor the manual operation of MSG by the crew.

The WV is a large confined volume of 255 litresoffering two levels of containment forinvestigations. The first level is achieved by the


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Figure 3. The MSG systemarchitecture

Figure 4. The MSG flight-unit rack during EMC testing

at Astrium

The WV is provided with front and side ports foraccess/loading, all of which can be equippedwith gloves. The WV can be slid in and out ofthe rack assembly to afford access to the sideports for the initial introduction and manipulationof investigations, and to fix accessories such asthe cameras to selected mounting points. Theentire front surface of the WV is a Lexan window,which provides the operator with a wideviewing angle. For investigations needing adarkened environment, the window and thelateral panels of the WV can be covered withspecial stray-light covers.

The large Work Volume and the large accessports make it possible to perform large andcomplex investigations, and two investigationscan be accommodated simultaneously. Otherexperiments can be bolted to the bottom of theWV and to the left part of the back wall (Fig. 5).The WV’s internal lighting can be dimmed, andthere is also a spotlight to provide illumination inrestricted areas. An Investigation Cold Plateembedded in the floor of the WV allowsremoval of up to 800 W of heat from thebaseplate of the investigations.

The Airlock (Fig. 6) provides access through asealed and tiltable top lid and has its own WV-independent internal lighting.

Video Assembly characteristics The Video Assembly (VA) has been contributedto the MSG project by the Dutch Agency for

Space (NIVR). It is integrated into adedicated active International SubrackInterface Standard (ISIS) drawer,provided with a sliding top lid for easyaccess (Fig. 7). It is a self-standingsubsystem including 4 colour cameras,2 monitors, 2 analogue and 2 digitalrecorders, a touch pad, powerdistribution, a power and data line and acontroller board. Thermal control for thepowered components inside the VideoDrawer is provided by a rear suction fan for air circulation to transfer amaximum of 125 W to the MSG internalair-cooling loop.

The VA has a dedicated controller thatallows the user to command the videosystem via an input device connected tothe RS-422 serial interface (normallyconnected to the MSG SPLC) or via thetouch pad. The functionality providedthrough these digital interfaces is inaddition to the fully hardwired commandpossibility via the drawer front-panelswitches and the individual localcommand functions on the recorder,monitor and camera units themselves.

Three dedicated, sealed feed-through ports inthe front corners of the Work Volume allow theuse of all external MSG video components(video cameras, monitors, touch pad, etc.)within the WV.

Command and control capabilityThe MSG facility will allow both unattended andcrew-attended investigations, and thereforemany different command and controlcapabilities are offered. In the local mode, thecrew can introduce commands via the Controland Monitoring Panel (CMP) and, in some


44

Figure 5. The MSG WorkVolume (WV)

Figure 6. The MSG Airlock

Figure 7. The MSG VideoAssembly, contributed byNIVR (NL)

facility and will therefore pre-screen theEuropean-proposed investigations and thenpresent them to NASA for final approval.

Both NASA and ESA periodically announcemicrogravity research opportunities via NASAResearch Announcements (NRAs) and ESAAnnouncements of Opportunity (AOs),respectively. More details can be found at:http://floyd.msfc.nasa.gov/msg, and http://www.esa.int/export/esaHS/research.html.

To assist potential Principal Investigators (PIs),there is a MSG Investigation Integration Teamlocated at NASA’s Marshall Space Flight Center(MSFC), composed of a core group ofmanagers, engineers, and support personnel.This Team oversees the interface and safetyrequirements, the schedule for meeting ISStemplate milestones, the administrative supportfor documenting interfaces to the MSG facility,and the investigation manifesting, analyticalintegration, and flight operations. It alsosupports the investigation development teamsin ensuring that the engineering interfaces tothe MSG and the ISS are met. The investigationintegration process and schedule are defined inthe Microgravity Science Glovebox InvestigationIntegration Plan (MSFC-PLAN-3052), whichalso addresses the implementation activitiesand the development of data products.

AcknowledgementsThe Microgravity Science Glovebox’sdevelopment, integration and testing haveinvolved many different individuals andorganizations, who have contributed greatly tothe success of this facility. Due acknowledge-ments are made to the MSG Industrial Team,the ESTEC Technical and Operational SupportDirectorate team, as well as the MSG Teams at NASA/MSFC. We all look forward to many years of successful MSG scientificutilization! r

cases and for a limited set of commands, alsothrough the Internal Control Panel (ICP) insidethe Working Volume. In the remote mode,commanding is possible via:– the MLC, which can be hooked up to the

MSG internal MIL 1553 Bus at the MSG frontpanel. A subset of non-safety relatedcommands is available.

– the MLC connected to the ISS MIL 1553Bus, from the MSG

– the US Lab System via the payload MDM– by ground commands, which are accepted in

parallel with CMP controls; this mode wouldalso allow unattended operations for non-hazardous investigations or unattendedstandby control.

Facility operationsThe MSG rack will be launched in passivemode in the MPLM on ISS Utilisation FlightNo.2 (UF-2), foreseen for May 2002. It will thenbe moved to its location in US Lab, for in-orbitcommissioning. After the successfulcompletion of this phase, it will be ready to startits operational life.

The experiments will be launched/retrievedwithin stowage drawers (Express Racks orResupply/Stowage Racks) or Shuttle Mid-deckLockers. During certain MSG operations (suchas cleaning of the WV, filter change outs) theycan be stored temporarily within other MSGracks and stowage drawers.

The MSG can operate in an open mode, withair circulating from the WV to the MSG rackinterior, and in a closed mode with aircirculating only within the Work Volume. Thereis also the possibility to maintain in an inert dry-nitrogen atmosphere such that the oxygenvolume is kept to 10% or less.

The MSG will also accommodate ISS LaboratorySupport Equipment (LSE), such as general-purpose tools, fluid-handling tools, cleaningequipment, mass-measurement devices, a pHmeter, a dissecting microscope and supplies,digital multi-meters and a compoundmicroscope. Apart from the equipment requiredfor the general upkeep of the WV, a significantamount of resources and outfitting equipment isavailable for MSG science investigations.

Science and information interfaceGlovebox investigations cover four majordisciplines: material science, biotechnology,fluid science and combustion science. A similarpeer-review process to that used for earlierSpace Shuttle missions is being applied toselect the investigations to be flown in the MSGby NASA’s Microgravity Research ProgramOffice (MRPO). ESA has utilisation rights for this


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IntroductionThe Cryosystem is an ultra-low-temperaturefacility for supporting life-sciences payloads inspace. It brings together a unique set offacilities for the optimal preparation, preservationand storage of biological samples and proteincrystals at cryogenic temperatures. Thanks toits ultra-rapid cooling capability and its relativelylarge cold volume, it will provide a greatimprovement in the quality and quantity ofscience investigations in the fields of lifesciences, physiology and biotechnology.

The Cryosystem will complete in the ultra-lowtemperature field (–180˚C) the range of freezersprovided by ESA to NASA for use on board theInternational Space Station (ISS), the other twosystems being MELFI working in thetemperature range from +4 to –80˚C, and theCrew Refrigerator Freezer covering the rangefrom +4 to –26˚C.

During the Cryosystem’s preliminary designphase (Phase-B), which has started in February2002, prototypes of the complete freezers aswell as some in-orbit support equipment will bedeveloped to prove the feasibility of theproposed concepts. The design phase istherefore foreseen to last about 18 months,concluding with a Preliminary Design Review(PDR). The subsequent main developmentphase (Phase-C/D) will be concluded with thedelivery of the last freezer unit, in 2008.

Since the ISS assembly sequence is currentlyunder review, the first launch of a Cryosystemelement is not yet fixed, but the On-OrbitPreservation Rack (OPAR) could be launchedin the Centrifuge Accom-modation Module(CAM) not earlier than January 2008.

The Cryosystem industrial development teamselected consists of: – Astrium (Germany), Prime Contractor,

responsible for systems engineering, integrationand verification, as well as for development of

the rack infrastructure– L’Air Liquide (France), responsible for the

cryogenic cubsystem, including the freezerdrawers and the associated orbital andground-support equipment

– Thales Cryogenics (The Netherlands), whichwill be responsible for the cryocooler’sdevelopment

– Damec (Denmark), responsible for the scienceinterfaces and utilisation hardware.

System architecture and operational scenarioThe Cryosystem is being developed to meetthe science requirements defined by the NASAScience Working Group, and the functional andinterface requirements for ISS payloads definedby NASA Johnson Space Center (JSC).

The heart of the system is the CryogenicStorage and Quick/Snap Combo Freezer.Contained within one drawer, it will support thefollowing functions: – storage and preservation of already frozen

biological samples and supplies contained invials

– ultra-rapid cooling and ‘snap freezing’ ofvarious specimens, such as tissues, eggsand cells

– transportation to and from orbit of specimensand supplies

– transportation of specimens to/from otherISS racks, such as the Life Science Glovebox(LSG) and X-Ray Diffraction Facility (XCF).

The Orbital Support Equipment (OSE),consisting of tools and ancillaries needed forthe freezer operations (including maintenance)in orbit, will be stowed in a dedicated drawer.

The Cryorack is an ISS Standard Payload Rack(ISPR) outfitted with a liner (mechanicalinfrastructure) and subsystems to supportoperation and transportation of the freezers,and the transportation of passive payloads inthe Mini Pressurised Logistic Module (MPLM).


46

Cryosystem

M.N. De ParolisLaboratory Support Equipment Section, ESA Directorate of Manned Spaceflight andMicrogravity, ESTEC, Noordwijk, The Netherlands

W. RuemmeleCrew and Thermal Systems Division, NASA Johnson Space Center, Houston, USA

Figure 1. The Cryosystem operating scenario (courtesy of Astrium)

Figure 2. Cryorack basic outfitting accommodation (courtesy of Astrium)

NASA and ESA are also consideringdeveloping a second type of combo freezer toallow the ultra-rapid freezing and storage ofspecimens contained in bags, such as thoseused for tissue cultures.

The system will have a flexible and variablearchitecture, depending on the utilization needsof the various freezers. The first Cryorack will betransported to orbit within the CentrifugeAccommodation Module (CAM), and willremain there for its projected design lifetime often years. The other two Cryoracks will ensurethe cyclical up- and down-loading of thefreezers and the specimens therein. Both willsupport multiple missions, including installationin the MPLM, installation of freezers,transportation to and from orbit, removal fromthe MPLM, and refurbishment before the nextmission (Fig. 1).

The Cryorack may accommodate up to threefreezer drawers (for vials or for bags) and anumber of International Subrack InterfaceStandard (ISIS) drawers, containing either thefreezer orbital support equipment or additionalpassive payloads (Fig. 2). The ISS ColdStowage Working Group will be responsible fordefining the configuration needed for eachincrement, depending on the experimentsplanned and on the availability of resources onthe MPLM and ISS.


47

Figure 3. Schematic of vialfreezer dewar

Figure 4. The Cryocooler’sdisplacer and flexurebearing (courtesy ofThales/L’Air Liquide)

The Cryorack basic structure will be an ISSStandard Payload Rack (ISPR), provided byNASA but suitably modified to allow an optimaldesign for the freezers and their infrastructure.The rack will be outfitted to accommodate thefreezer drawers, the electrical subsystem (ESS)drawer, and the additional ISIS drawers for thepassive payloads and the Orbital SupportEquipment. A water loop will ensure optimalcooling of the freezers and the subsystemsneeded for their correct operation. Cryorackwill provide the necessary power, thermalcontrol, and command and data handling. Inaddition, OPAR may include a system forsupplying gaseous nitrogen for humidity controlwithin the freezers’ cold volumes.

Cryogenic storage and quick/snap combofreezerThe freezer will be integrated into a drawer 14panel units high (about 650 mm) and half a rackin width (about 450 mm). The freezer itself willbe a cylindrical vessel (dewar), vacuum-

insulated, and provided with a complexsystem of small ports to avoid heatleakage, whilst still allowing for theloading/unloading of the specimencontainers (Fig. 3).

Cooling will be provided by a Stirlingcooler, connected via a cold finger tothe dewar’s internal structure. Given theseverity of the cooling requirementsand the ten-year lifetime requirement,adaptation of an existing cooler – usedfor military applications – has alreadybeen initiated by the companiesinvolved (Thales and L’Air Liquide).Figure 4 shows the development-modelcooler displacer provided with a flexurebearing.

The dewar will hold about 800 2-ml or 400 5-ml vials, or a combination of the two. Thecontainers will be arranged as a concentricarray of tubes providing: maintenance of therequired temperature, support and restraintduring critical orbital phases, and the possibilityof indexing every container. In particular, acarrousel mechanism integrated inside thedewar allows three-axis identification (angular,radial and in-depth) of each container. Both thedewar structure and each container willtherefore be suitably labelled and codedaccording to the ISS Inventory ManagementSystem.

The freezer will have four basic modes ofoperation: storage and transportation, quickfreezing, snap freezing, and collection ofalready processed and frozen protein crystals.To accomplish this, it will be able to interfacewith and to operate in different racks: theCryorack, which is its ‘home rack’, the LifeScience Glovebox (LSG), and the X-Ray

Crystallography Facility (XCF)/CrystalPreparation Prime Item (CPPI).

Transportation of specimens andsupplies to/from orbitThe Investigators’ specimens andsupplies will be transported from theirlaboratories to the Kennedy SpaceCenter (KSC) processing facilitiesusing dedicated ground dewarsprovided by the Cryosystem developer.The specimens can be stored in thefreezer either before its installation inthe MPLM, after its installation in theMPLM whilst still at the processingfacility, or during the late-accessoperations with the MPLM/STSalready on the launch pad.


48

Figure 5. (a) The snap-freezing tool. (b) Operatingconfiguration at the vialfreezer port (courtesy ofL’Air Liquide)

freezing tools will be contained inside thefreezer. The crew will remove the tools from thefreezer, transfer the sample from the surgicaltable to the dedicated tool and then re-insertthe specimen container into the freezer’s quick-freeze zone. The vial freezer will be able to cool-down a 5-ml vial filled with standard salinesolution from ambient temperature to –160˚C inless than 10 minutes. The freezer will be able tosupport multiple quick-freezing operations,according to defined scenarios, before it needsto be transferred back to OPAR for recoveryand storage.

Snap freezingSnap freezing is a process whereby exposedplant or animal tissue is rapidly frozen so thatsub-cellular structure is preserved during thefreezing process. During ordinary specimenfreezing, the water contained within the cellforms large ice crystals that destroy sub-cellularstructures. With snap-freezing, the surface ofthe material is frozen almost instantaneously, sothat amorphous ice or very small ice crystals,which do not destroy sub-cellular structure, areformed.

Since the cooling is delivered to the specimensurface and then propagates through thematerial, the practical limitations of this processlimit ‘good freezing’ to a thin region (about 15microns thick) near the specimen’s surface. The vial freezer will be able to snap-freezesamples with an area of about 36 mm2 whilehaving at the least 10% of this area free fromcellular damage. These snap-freezing operationswill take place with the freezer installed at theLSG.

A prototype snap-freezing device (Fig. 5) hasdemonstrated a success rate of about 80%,which is comparable with the performance ofsimilar equipment used in ground laboratories.

The freezer will receive all necessary power,cooling and data-handling resources from theMPLM, except for certain periods during thelaunch and landing phases. It is thereforedesigned to have sufficient thermal inertia tomaintain the required temperature inside thecold volume (i.e. -180 ± 5˚C) during thoseperiods. The cold-volume temperature will bemonitored throughout the specimens’ lifetimein order to provide the Investigators with data toconfirm the ‘suitability’ of their specimens fortheir investigations.

After the docking of the MPLM to the ISS, thefreezer will be removed from Cryorack/MPLMand transported and installed by the crew intothe Cryorack location in CAM. The reverseoperation will be performed after the freezerhas completed its mission.

Transportation of specimens to other ISS racksA challenging requirement for the vial freezer isthe ability to withstand multiple cycles ofinstallation/de-installation in various ISS racks,as it continuously ‘travels’ from its home rack(Cryorack) to either the LSG or the XCF-CPPIracks and back. Wear problems are to beexpected for all mechanically interfaced partssuch as connectors and drawer slides.

Quick-freezing functionQuick freezing occurs when a biologicalspecimen (plant tissue, animal tissue, animalbody fluid, etc.) is removed surgically (orequivalent) from the host, inserted into aspecimen container and subsequently cooled-down to below prescribed temperature limitsover a period of several seconds to minutes.

These quick-freezing operations will occur withthe freezer installed at the LSG. The surgicaloperations will be performed on the table insidethe LSG working volume, while the quick-


49

Vial Freezer

external surface

Vial Freezer

Port

Figure 6. Cryovial, and aCryoloop with a ‘hanging’

drop of protein crystalsolution (photos courtesy of

Hampton Research andLawrence LivermoreNational Laboratory,

respectively)

Protein crystal storage The XCF is a NASA-integrated rack hostingvarious sub-facilities for the growth,preparation, mounting and freezing of proteincrystals. In particular, the CPPI (CrystalPreparation Prime Item) will take care of the lasttwo operations and will move the frozencontainers robotically to a location where thefreezer transfer port can be installed. The CPPIequipment will ‘fish’ the crystal from the growthsolution, mount it on a ‘cryoloop’ and insert thelatter into a suitable vial (Fig. 6).

If the vial freezer collects a specimen inunpowered status, the collection time will belimited to one hour to maintain the very stricttemperature requirement for the proteincrystals (-180 ± 5˚C). The interfaces betweenthe freezer and the CPPI, as well as the numberof containers that can be transferred in onehour, have to be defined during the detaileddesign phases for both the freezer and theCPPI.

Combo Storage Freezer for bags At the end of the Cryosystem design phase(Phase-A), it became evident that 2 and 5-mlvials would not be the only containers needed

for life-science experiments. In particular, tissuecultures would require use of bigger and morecomplex ‘bags’ (Fig. 7), where the tissue cangrow without any intervention from the crewother than the injection of the growth solution.These samples also need to be stored atcryogenic temperatures after growth. However,the vial freezer defined above cannot be usedfor this because mixing of the two containertypes and the various sizes would greatlyreduce either the available capacity or thequick/snap-freezing performance of the freezer,or both. For this reason, NASA required ESA topropose an additional combo freezer to hostthose particular containers. NASA will take adecision regarding the implementation of thebag freezer in the second quarter of 2002.

If NASA decides to fund this option, the bagfreezer will be very similar to the previous one.The only differences will be in the internaloutfitting of the dewar, since the bags arebigger and have a more complex configurationthan the vials. The foreseen dewar capacitycould be around 150 10-ml or 80 30-ml bags,or a combination of the two sizes. In addition, itwill be possible to store about 40 sealedsyringes (or other cylindrically shapedcontainers) of 10-ml capacity. This will allow theuploading of growth solutions at very lowtemperatures, to be injected into bags or vialsfor life-science experiments.

The quick-freezing capability will beimplemented using dedicated outfitting. Snap-freezing is not possible because the specimenis not directly accessible, being confined withinthe bag. The bag freezer will be used only atthe Cryorack, because the specimen will becontained within the bags and will not require aclosed and sealed environment for itsdevelopment.

The supplies and specimens will betransported to/from orbit using the samescenario as for the vial freezer.

AcknowledgementsThe authors wish to thank the industrialcontractors for providing preliminary drawingsand descriptions of the hardware, and the ESAand NASA Cryosystem teams for their hardwork during the requirement consolidationphase. r


50

Figure 7. Biotechnologybags inserted into a metalframe (courtesy of NASA)

Figure 1. The SAGE IIIinstrument’s key elements

this could be modified by revisions in the ISSassembly sequence.

The development of SAGE III and the Hexapodwere both approved and funded in 1994. Thecontract for the Hexapod design phase (Phase-B) was awarded by ESA to Alenia Spazio (I) in1995, with Carlo Gavazzi Space (I) and ADSItalia as subcontractors. The main developmentphase (Phase-C/D) was initiated at thebeginning of 1998, with Alenia Spazio (PrimeContractor) and Carlo Gavazzi Space. TheCritical Design Review (CDR) was completed inNovember 2000, and flight-unit completion isscheduled by mid-2002. LaRC will then beresponsible for the overall Hexapod and SAGEIII payload integration.

IntroductionSAGE III (Stratospheric Aerosol and GasExperiment), an Earth-observation instrumentdeveloped by NASA’s Langley Research Center(LaRC), was one of the first scientific externalpayloads selected for the International SpaceStation (Fig. 1). It was conceived to fly on aspacecraft able to provide ± 1 degree pointingaccuracy. Since the ISS’s attitude can vary byseveral degrees over a long period, it wastherefore necessary to provide a dedicatednadir-pointing system. For this task, NASAselected the hexapod-based pointing system(‘Hexapod’ for short) included by ESA in the listof proposed European contributions to the ISSearly utilisation phase. Launch is currentlyscheduled with assembly flight UF-3, although


51

Hexapod

P.C. GaleoneSpace Station Utilisation Division, ESA Directorate of Manned Spaceflight andMicrogravity, ESTEC, Noordwijk, The Netherlands

L. Szatkowski, O.H. BradleyNASA Langley Research Centre, Hampton, Virginia, USA

R. Trucco, B. MusettiAlenia Spazio, Turin, Italy

Elevation Scan Mirror(0 - 300km, 1km resolution)

Azimuth Gimbal (±185°)

Telescope/Spectrometer(290 - 1550nm)

Figure 2. The Hexapodsystem integrated with

SAGE III (together with theelectronics boxes for both)

on the Express PalletAdapter

Mission overviewThe Express Pallet System (ExPS) is both aShuttle carrier and an in-orbit payloadaccommodation facility, which can berobotically installed on the truss of the ISS. Itaccommodates payloads mounted on ExpressPallet Adapters (ExPA). The ExPA hostingHexapod and SAGE III is integrated onto theExPS on the ground, and is used as thepayload carrier at launch. Hexapod will belaunched unpowered, with the Orbiterproviding power for its ‘stay-alive’ heaters afterthe payload-bay doors are opened in orbit.

The complete ExPS will be moved roboticallyfrom the Shuttle and transported by using theISS Mobile Base System (MBS) to its nominalin-orbit accommodation site. The locationassigned is the nadir-starboard outer payloadattachment site of ISS truss segment S3 (closeto the thermal radiator), with Hexapod andSAGE III accommodated on the ExPA installedat the ExPS corner location in the ram-outboard direction.

Hexapod and SAGE III will be activated andstart their science mission only after the ExPSis connected at its attachment site. Both aredesigned for five years of in-orbit operationwithout maintenance. Users will, however, beable to up-link Hexapod flight-software updates.

At the end of their mission, Hexapod and SAGEIII will be removed from the ExPS along withtheir ExPA. They will remain unpowered duringthe transfer to the Shuttle Orbiter and duringEarth re-entry. Fail-safe brakes in the Hexapodlinear actuators ensure that accelerationsduring reentry do not cause payloaddisplacement in the cargo bay.

Hexapod design characteristicsThe key performance requirements forHexapod consist of achieving a pointingaccuracy of ±90 arcsec in the nadir direction –with a pointing stability of 0.0025 deg/sec, apointing range equivalent to an 8 deg cone,and with an angular pointing rate of at least 1.2 deg/sec – and accommodating a 35 kgSAGE III sensor assembly.

Hexapod determines attitude based on theISS-provided attitude state vector (attitudequaternions and GPS data), and applies anattitude correction matrix to take into accountthe local attitude deviations actually experiencedat the mounting location. The matrix is definedon the ground, based on the actual SAGE IIImeasurements and uplinked to Hexapod (viaSAGE III) along with the telecommand data forthe pointing manoeuvre.

The Hexapod flight unit, including theelectronics, weighs 116 kg. The overallpayload, including the SAGE III electronicsboxes, harness and other system-leveloutfitting, will be close to the 225 kg allowancefor the ExPA. The accommodation of the SAGEIII sensor assembly in the volume between theHexapod’s legs takes best advantage of thespecific geometry of this type of mechanism(Fig. 2).

Hexapod’s power consumption in orbit variesaccording to the operating mode. It rangesbetween the power needed to survive ExPA


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The SAGE III Scientific Instrument

SAGE III is designed for the global monitoring of the vertical distributionsof stratospheric aerosols, ozone, water vapour, nitrogen dioxide andtrioxide, chlorine dioxide, and temperature from Earth orbit. It usesspectrographic techniques based on light-source occultation, whichtypically offer the capability for self-calibration, high vertical resolution,high signal-to-noise ratio, and excellent inversion accuracy.

The core sensor is a spectrometer able to measure the extinction of bothsolar and lunar radiation through the Earth’s atmosphere duringoccultation events, between 290 and 1550 nm with 1 nm spectralresolution and 1 km vertical altitude resolution. The instrumentincorporates a unique charge-coupled detector (CCD) array and a singlephotodiode detector in the focal plane to perform the sciencemeasurements. The sensor assembly provides spatial resolution over analtitude range from cloud-top (or the Earth’s surface on cloud-free days)to 300 km. Science data are taken up to an altitude of 150 km. Datasequences are also taken between 150 and 300 km altitude on someorbits to radiometrically and spectrally calibrate the instrument.

Figure 3. The electro-mechanical linear actuator

the separation device to be simplifiedconsiderably.

Hexapod-based positioning/pointing systemsare able to achieve pointing in a particulardirection via a very large set of possiblecombinations of the six linear actuator lengths.This provides the ability to recover partially fromlinear actuator failures by adjusting the pointingalgorithms to take into account the actuallength of the faulty leg(s), and to continue toexecute pointing manoeuvres using only thefully functional legs.

The electromechanical linear actuator (Fig. 3) isa key element in the hexapod mechanism. Ithas to guarantee a positioning accuracy ofbetter than ±25 micron over the full pointingstroke, with a minimum resolution of 10 micronand a positioning repeatability of better than ±5 micron. The lengths of the linear actuators can range from the fully-retracted minimum of471 mm, including the two cardan joints, to themaximum stroke length of 568 mm. Theirnominal length, corresponding to the ‘zero’reference position of the upper platform, is 528 mm.

A DC three-phase brushless motor is installedin direct-drive frameless configuration insidethe linear actuator. The stator is installed insidethe motor cage, and the rotor is installed on thesatellite screw shaft. The motor is doublewound for cold redundancy, with the phasecommutation provided by two sets of three Hallsensors (one set for each winding). The motorcan provide a continuous torque of 0.7 Nm upto 150 rpm, and a peak torque of 1.4 Nm.

cold phases (the in-orbit stay-alive heatersrequire around 100 W, with thermostaticcontrol), and a maximum operating power of435 W during the execution of pointingmanoeuvres when all legs are movingsimultaneously (not required in all cases). Theestimated average power consumption isabout 120 W, given that the execution of acomplete pointing manoeuvre takes just a fewseconds.

Hexapod consists of two main parts: theElectronic Unit (HEU), and the MechanicalAssembly (HMA). The HEU is the integratedpower and control unit that handles powerdistribution, telemetry and telecommandmanagement, data processing, and commandand control. It provides the computer controlfor the co-ordinated movement of the six linearactuators. Hexapod flight software resides onthe Standard Payload Computer (SPLC).

The HMA includes six electromechanical linearactuators, arranged as three trapezoids andconnected by means of 12 universal joints to abottom flange and to an upper platform. Thelatter’s attitude and position are determined insix degrees of freedom (translation along androtation about all orthogonal axes in 3D space)by the combination of the lengths of the sixlinear actuators. The resulting configuration is astatically determined structure that enables thestroke of each individual linear actuator to bechanged without causing internal stresses inthe mechanism. The bottom flange is fixed tothe ExPA and the upper platform accom-modates the SAGE III sensor assembly. TheHexapod’s bottom flange includes an offset-wedge function, suitably shaped toprovide static compensation of theestimated average ISS pitch bias(tilted 7 deg) and maximise the SAGE IIIfield of view.

The other important constituent of theHMA is the off-loading device installedaround the hexapod mechanism tointerconnect the upper platform andthe ExPA during launch. It enables thelinear actuators to be protected fromlaunch loads, thereby preserving theirhigh accuracy for supporting thescientific mission. The off-loadingdevice will only be disconnected oncethe Hexapod is in place on the ISStruss, after which the simultaneousactivation of all six linear actuators willput the upper platform into its nominaloperating position. The Hexapod’sability to control linear translationsaccording to pre-defined trajectoriesenables the design and operation of


53

Figure 4. The Hexapod high-fidelity mechanical-interface

simulator

A brake is used to lock the rotor, preventing thesatellite roller screw back-driving during the re-entry phase. Based on two steel-toothed rings(each with 200 teeth), the brake has twoseparate solenoid windings (main andredundant), each of which when energised isable to disengage the toothed rings (they areautomatically engaged when power is off). Thebrake system, the materials of which have beenspecially selected to avoid debris productionduring engagement/disengagement, isdimensioned to resist a torque of at least 6.3 Nm.

Hexapod ground modelsThree Hexapod ground models are deliverableitems to support NASA LaRC in its role asintegrator of the payload system. The High-Fidelity Avionics Interface Simulator reproducesthe hardware and software serial interface toSAGE III. The High-Fidelity MechanicalInterface Simulator (Fig. 4) is an exact mock-upof the essential items of the flight unit as far asoverall dimensions, placement of hardware onthe lower and upper platforms, and holelocations are concerned. This simulator is usedfor verifying that the SAGE III sensor assemblymounts properly to Hexapod, and to supportpayload system integration. The EngineeringUnit (Fig. 5) supports sustaining-engineeringfunctions such as anomaly resolution (hardwareand software), software modifications, and on-ground verification of system changes duringin-orbit operation.

Future perspectivesESA’s development of the Hexapod marks theupgrading of hexapod-based positioning/pointing systems for space application.Although tailored to meet the SAGE III nadir-pointing requirements, it can be adapted tosupport other ISS external payloads, orpayloads to be flown on different spacecraftcarriers. Feasibility studies conducted beforestarting the Hexapod design phase indicatedthe possibility of using them for spaceapplications requiring pointing within about±30º cones. A target-tracking capability (notrequested by SAGE III) can also be provided.The possibility to control payload attitude/position in six degrees of freedom is anotherattractive feature that could serve a number ofspace applications in which the relativedisplacement of two items has to be controlledwith high accuracy.

Other possible applications include active jitterstabilisation at the payload interface, operatingHexapod as an anti-vibration platform. Ademonstration test performed in 1996 on theHexapod Phase-B development modelindicated that it was able to react ‘as-built’ to asimulated disturbance by producing an in-phasedynamic response suitable for compensatinglow-frequency jitter of up to about 2 Hz.

AcknowledgementsThe authors wish to acknowledge the efforts ofall members of the ESA, NASA and Industry(Alenia Spazio and Carlo Gavazzi Space) teamswho have contributed to the Hexapod project’ssuccess through their dedication and profes-sionalism and in a friendly spirit. Last but notleast, very special thanks are due to the late W. Gallieni (ADS Italia), whose innovative ideasand specialist skills were fundamental to thepreliminary design-definition phases of theproject. r


54

Figure 5. Functional testingof the Hexapod Engineering

Unit

the esa laboratory support equipment for the iss

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