- 1 - Manfred Schmid, Reiner Barho

Transcription

1 - 1 - DEVELOPMENT SUMMARY AND TEST RESULTS OF A 3 METER UNFURLABLE CFRP SKIN ANTENNA REFLECTOR Manfred Schmid, Reiner Barh Astrium GmbH Friedrichshafen (Germany) Tel: / Fax: / manfred.schmid@astrium-space.cm Structure and Mechanism Sectin 1. ABSTRACT By end f 2001 the develpment f an Unfurlable CFRP Skin Antenna Reflectr was started at Astrium GmbH. Majr develpment steps are nw finished, a 3 m Engineering Mdel f the Reflectr was built and the Functinal and Cntur Perfrmance Verificatin at ambient cnditins was cmpleted. The paper summarises the develpment apprach, verificatin test results and experiences gained with the realised design. 2. INTRODUCTION The design cncept cmprises a number f unfurlable reflectr petals, stwed arund a central dish during launch. The basic design aims at X-Band applicatin, hwever als higher frequencies are defined as an applicatin gal. In rder t achieve a minimum stwed envelpe at maximum deplyed diameter, a thin sheet reflectr cncept (CFRP skin) is fllwed. This cncept des nt nly allw strage f the CFRP petals in a gemetrically ptimised way, but als t deflect the individual petals during strage. This feature enables t accmmdate large reflectrs f gd cntur shape in small launcher envelpes and cnical fairings. A grwing number f applicatins is expected in the future fr accurate antenna reflectrs in the 3 t 6 m class (e.g. fr SAR applicatins r fr X-Band / Ku- Band cmmunicatin). Such reflectrs culd clse the gap between small fixed reflectrs and the large and very large mesh reflectrs mainly dedicated t lw frequency applicatins. In a next develpment step nt nly central feed arrangements but als ffset cnfiguratins are being cnsidered. The majr gal f the develpment phase was t verify the mechanical perfrmance f the chsen reflectr cncept by test. The fllwing majr verificatin steps were perfrmed: Functin Deplyed Cntur Accuracy Deplyment Reprducibility Handling Vibratin (still under wrk) The electrical Surface Reflectivity is at present verified n sample level, full size electrical testing and thermal (vacuum) testing is freseen in a later develpment stage, the thermal deflectins and their influence n the reflectr cntur are presently cvered by analysis. 3. TECHNICAL SPECIFICATION The fllwing key requirements frm the develpment basis: Frequency X Band r higher Surface Reflectivity > 97 % f Aluminium Reflectr Type Central Feed Cntur Shape: Symmetric Parablic Z= (X 2 + Y 2 ) /4400 Main reflectr fcal length 1100 mm Minimum Required Accuracy (rms) <0.75mm (required fr X-Band) Main reflectr diameter deplyed 3m (grwth capability up t 6m) Base diameter f stwed Reflectr package 1 m Ttal Mass 15.5 kg Eigenfrequency stwed >25 Hz Eigenfrequency deplyed >5 Hz

2 DESIGN CONEPT AND FEATURES T release the reflectr, the release wire (tensin wire) is pened and the deplyment is initiated by means f The Reflectr design is driven by the requirement fr the leg springs integrated in each individual panel minimum mass and stwed vlume at maximum hinge, munted t the central dish rim. This design achievable deplyed diameter and cntur accuracy. In avids heavy and bulky deplyment mechanisms t be additin the reflectr perfrmance with regard t the arranged underneath the Central Dish and leaves the in-rbit thermal envirnment, as well as functinal space free fr Waveguides and RF electrnics. rbustness and the pssibility fr easy n grund adjustment and testing is f high imprtance. The abve generic requirements are satisfied best by the described Skin Cncept (thin sheet cncept) which makes use f flexible CFRP sheets f parablic shape. The rtatin axes f the deplying panels are nt arranged tangentially w.r.t. the Central Supprt Structure, but at a slight inclinatin such minimising the envelpe in the stwed cnfiguratin. (see Fig. 1). Additinal advantage is taken frm the minimised sheet thickness which allws the CFRP panels t deflect in the stwed cnditin. The CFRP sheets achieve their nminal well defined parablic cntur after deplyment. Each thin sheet panel is supprted by a hllw CFRP rib interfacing the circular Central Supprt Structure. Fig. 1 shws the stwed reflectr. The individual panels are munted t the Central Supprt Structure prviding a structural interface t the satellite and t the Tripd/ Feed assembly as well. The Central Supprt Structure is designed as a stiff, hllw CFRP ring cmprising the end stps fr the individual panel segments as well as the interfaces fr the panel deplyment axes. The Central Dish itself is manufactured as a CFRP membrane and is glued alng its uter rim t the Central Supprt Structure. At the tp f each panel rib, a Launch Lcking Bracket is munted. All brackets tgether frm a clsed ring in the stwed cnfiguratin (Fig.2). The resulting frm lcking structural ring is preladed by a circumferential release wire cmpressing all ring segments in rder t achieve a stiff tp ring structure, taking mments and frces. The launch lads are guided thrugh the panel ribs dwn t the Central Supprt Structure. The Feed Structure (Tripd) is independent frm the reflectr panels and nt used t prvide supprt t the stwed reflectr during launch. This feature prvides design flexibility and defines a clear interface nly between Tripd base and Reflectr Central Ring. Fig. 1. Stwed Cnfiguratin Fig.2. Launch Lck Device Ring Span During deplyment the mtin is cntrlled by means f a small frictin damping device. A thin string (unladed during launch) is guided thrugh all launch lck brackets and is pulled ff a small cable drum attached t the tp f ne panel rib by means f the deplying panels, s that the release velcity f the reflectr is cntrlled. Synchrnisatin f the deplying petals is achieved inherently by their verlap during deplyment (Fig.3). Only in the final deplyed end psitin are all panels lcated ne besides the ther. (Fig. 4) Fig. 3: Partially deplyed Fig. 4: Fully Deplyed Reflectr After cmplete deplyment abut the spring laded panel rtatin axes, the deplyed end psitin is defined by means f adjustable end stp elements attached t the Central Supprt Structure. This interface mates with the rt f the panel rib carrying an adequate Aluminium End-stp fitting. The residual spring trque still acting abut the deplyment axis after deplyment, suppresses ptential residual backlash in the deplyment hinge and helps t achieve gd deplyment reprducibility. All the 30 individual deplying panels are independent frm each ther after deplyment. This feature guarantees a parablic cntur shape insensitive t any crss cupling effects (manufacturing tlerances, adjustment, thermal effects) between the adjacent panels. Each individual panel prvides a well-defined parablic cntur and can be independently fine-tuned by rtatin abut its deplyment axis in rder t achieve an ptimised glbal cntur either fr 1 g (n grund testing) r fr 0 g (n-rbit) cnfiguratin. By this cncept it is easily pssible t exchange ne panel segment fr maintenance r repair activities withut disturbing the glbal cntur adjustment. Due t the high stiffness f the panel rib and the lw verall panel mass (abut 180 g), lcal deflectins f the panel

3 - 3 - sheet due t 1 g effects can be neglected during initial reflectr adjustment in the vicinity f the rib. The design requires small gaps (f abut 0.5 mm) between the adjacent petals and between the panels and the central dish and is therefre advantageus in view f PIMP effects relevant t cmmunicatin applicatins. The CFRP material used has t fulfil the fllwing requirements: gd stiffness and strength lw mass lw CTE lw Creeping (at deflectin during strage) lw residual stresses after curing gd temperature stability gd electrical reflectivity fr all plarisatin directins ATOX Prtectin In rder t ptimise fr these requirements, a sample manufacturing / test prgram was carried ut at the beginning f the develpment phase. The utcme was t use a high mdulus Carbn fibre tgether with a ht curing resin system s that rbit temperatures up t abut 130 deg C are allwable. As an ATOX prtectin a PVD SiO2 cating is used, its gd adherence t the CFRP surface was checked by pullff tests. 5. MAJOR SUB- UNITS 5.2 Unfurlable Panels Each individual panel is manufactured and cured n a dedicated mld. Each individual rib is manufactured and cured n a separate mld. Bth parts are brught tgether in a third wrking step t frm the cmplete panel. T the panel rt the deplyment hinge axis (Deplyment Mechanism) and Panel Endstp Device are munted in a further wrking step prir t integratin int the Central Dish. Endstp Device Deplyment Mechanism Panel Rib Fig. 6: Unfurlable Panel Segment Panel Sheet The full size panels were submitted t phtgrammetric measurement in rder t verify the achieved manufacturing accuracy and reprducibility n basis f the chsen manufacturing prcess. The measurement yielded gd results with typical values f abut 0.15 mm t 0.3 mm rms fr the single panels. 5.1 Central Dish During the develpment it was fund that the panel accuracy is very sensitive against changes in material and manufacturing prcess. Even the use f a different batch f the identical CFRP material resulted in a different quality f the cured panels. Such effects are ccurring due t the lw thickness f the CFRP sheets f nly 0.4 mm, increasing the sensitivity against built in residual stresses which cannt be cunteracted by the design inherent lw panel stiffness. The Central Dish Structure prvides the stiff reflectr cre interfacing the satellite and accmmdates the Tripd Structure. The Central Dish is frmed by a ring structure fully manufactured in CFRP, stiffened by shear walls and framewrk bars. The Ring Structure carries all Hinge Brackets used t munt the individual reflectr panels and the end-stps defining the deplyed panel psitin. T the rim f the ring structure the central dish reflectr is glued. The central dish reflectr is manufactured, as the deplying panels are, frm a 0.4 mm thick CFRP sheet. Access Therefre at the beginning manufacturing prcess ptimisatin and creeping tests n different material samples were carried thrugh in rder t select the ptimum material and prcess cmbinatin under cnsideratin f lw manufacturing csts. Launcher I/F Bracket Unfurlable Central Ring Structure Fig. 5:Central Ring Structure Deplyment 5.3 Deplyment Mechanisms Each individual panel (rib) carries at its rt an I/F fr the Deplyment Hinge Axis. Each Hinge Axis is equipped with a set f tw (redundant) deplyment springs prviding the reflectr deplyment trque fr this individual panel. Due t the cncept inherent deplyment synchrnizatin between the adjacent panels, trque variatins in the springs are cmpensated (i.e. a ptentially lwer trque f ne panel is supprted by the higher trque prvided by

4 - 4 - the adjacent ne). The deplyed psitin f the panels is individually adjustable abut their rtatin axis by means f a mechanic end stp device. In additin a (limited) lateral and tilt adjustment pssibility is prvided in the panel rtatin axis. Redundant Leg Spring Axis Prelad Damper Reel Panel Rib I/F Fig. 8: Deplyment Damping Device Damper Bracket Damper Crd Launch Lck Tensin Wire and Wire Supprt stud Axis Lateral Screw Sping Trque Device Fig. 7: Panel Deplyment Mechanism 5.4 Deplyment Damper T ne f the Launch Lcking Brackets (the s called Damping Bracket) a small frictin damping device is added. The damper cnsists f a rtating shaft munted int a husing. By rtatin f the shaft tw small brass masses are accelerated radially s that frictin between the husing and the mass is prduced. T ne end f the shaft a small cable reel is munted nt which a thin crd is wund up frm tw sides. The tw ends f the crd are guided thrugh hles n each side f the Damping Bracket and frm there thrugh small guidance penings in each individual Launch Lcking Bracket. The ends f the crd are fixed t the Launch Lcking Bracket n the ppsite side f the Damping Bracket. If the reflectr deplyment is initiated, the crd is pulled ff the reel, the frictin damper is accelerated and a velcity dependent frictin frce is prduced t cntrl the panel deplyment. In rder t avid inadvertent rtatin f the damping reel during launch, a frm lcking interface between the reel and the Launch Lcking Device tensin wire is prvided (see Fig.8). 5.5 Launch Lcking System In parallel t the damping crd which is nt laded during launch, tw steel tensin wires each running arund half the Launch Lck ring, prvide the prelad t the lcking system in stwed cnfiguratin. The tensin wires are hked tgether and secured by a pin in the Release Bracket that lcks them in place. After that, the wire is tensined by a screw prelading a set f belville washers used t ensure that the tensin wires maintain their adjusted frce f abut 600 N during vibratin. The Launch Lcking System as such is fully flight representative, hwever at present n flight standard release actuatr initiating the release prcess was included. Such an actuatr culd be a pyrtechnic device, nn-explsive actuatr r electr-magnetic pin puller. In the present EM nly manual release f the Launch Lcking Device is freseen. If a manual release pin is turned, the Launch Lcking Device is actuated s that the reflectr panels can deply. Due t the spring frce inherent t the steel lcking wire, the tw wire segments mve back t their initial straight psitin after reflectr release and are psitined behind the deplyed reflectr surface utside the reflectr aperture. This psitin is assured by an additinal spring element, frcing the wire t a defined psitin after release, s that interfere with the deplying r deplyed reflectr segments are avided. Frm Lcking I/F t adjacent bracket The cmplete Reflectr deplyment time using the damper unit, under 0-g cnditins but with additinal air damping effect, is between 5 and 6 s. Panel Rib I/F Steel Tensin Wire Supprt Stud Fig. 9: Launch Lcking Brackets (Tp Ring Segment)

5 - 5 - Spring laded Release Blt and Manual Release Pin (t be substituted in a Flight Mdel by actuatr TBD) I/F t Panel Rib Lcking Clamp Tensin Device One f Tw Engaging Release Clamps Fixed by the Reflectr Release Device During Launch Fig. 10: Reflectr Release Device (LLD) 7. REFLECTOR CONTOUR ADJUSTMENT Abve the integratin and test rig carrying the integrated reflectr, a zer g- rig equipped with a set f pulleys is psitined by means f a crane in alignment with the (vertical) bresight directin. Over each f the pulleys a string fastened t the tp f ne reflectr panel is guided. The panel weight is cunteracted by a small mass at the ther end f each string. The nly frces acting t the individual panels after deplyment are the residual spring frces abut the rtatin axes. T adjust and verify the verall surface accuracy and reprducibility three different verificatin methds are used: In a first step all individual panel segments are adjusted by means f a mechanical measurement jig frmed by a CFRP frame, referenced in the centre f the central dish and n its rim as well. The CFRP frame allws t detect the z- psitin f each panel rib at a well defined psitin n the reflectr radius by turning the frame abut the reflectr bresight axis panel by panel. 3 T align the crrect reflectr cntur, the deplyed 1 Tensin Wire 2 Tensin Wire Terminal panels are adjusted in height via adjustment screws munted at the panel rt underneath the central dish. These screws interface metallic cunterparts n the Central Ring Structure. 3 Release Clamp 11 Tensin Wire Terminal 12 Prelad Device 18 Belville Washers In a secnd alignment step aluminium targets (tapes) Fig. 11: Launch Lck System Tensining Device 6. REFLECTOR INTEGRATION AND ALIGNMENT T ensure full accessibility during the integratin, the antenna is assembled n an Integratin and Test Rig which can be turned abut tw axes, namely abut the reflectr bresight directin and abut ne hrizntal axis. In rder t get a satisfying radial run-ut during rtatin abve the bre sight directin, the test rig is mechanically aligned befre start f integratin. are attached t the uter rim f each deplying segment s that a cntact free capacitive sensr can measure the distance t the Al-target (see Fig. 12). This measurement result is cnverted int the distance in mm between the sensr and each panel. During measurement the sensr is fixed t the test rig and the reflectr is turned abut its bre-sight directin panel by panel t measure the distance between sensr and target, s that a quick and easy glbal Reflectr alignment and deplyment reprducibility checks can be perfrmed. In a first integratin step the Reflectr Central Ring Structure is attached via its launcher I/F pints t the Integratin and Test Rig. In a secnd step the reflectr panels are integrated t the central dish rim turned t a tp-dwn cnfiguratin. In a last integratin step, the pre-integrated set f launch lcking brackets (launch lcking ring) is glued in ne sht t the tp f the panel ribs. After finishing this activity, the clsed reflectr can be turned with its bresight directin vertical by means f the Integratin and Test Rig and the first manual deplyment can be perfrmed fr visual inspectin. Nw the deplyment springs are laded and adjusted t the crrect trque value. Zer G Device Inductive Sensr Alignment and Test Rig Fig. 12: Deplyment Test Cnfiguratin

6 - 6 - In rder t measure the verall reflectr (rms) cntur accuracy, a third step is perfrmed. This is dne by phtgrammetric measurement f the cmplete reflectr shape. A high reslutin digital camera equipped with a wide angle lens is used t take phts frm different wrking psitins. Fr this activity a set f circular targets is glued nt the Reflectr. Reference bars with knwn target distances are used as a reference. The phts taken frm different viewing angles are evaluated by a special sftware which crrelates the measured target pints and calculates the verall rms value and best fit parabla. Phtgrammetric measurements are made befre and after deplyment and after the vibratin test in rder t ensure the reprducibility f the reflectr surface shape under ambient cnditins. The results f the abve described three measurement methds are evaluated and based n these results individual segments can be re-adjusted r fine-adjusted as apprpriate, in rder t ptimise fr the verall cntur accuracy. 8. FUCTION AND PERFORMANCE TEST Segment Radius in Deviatin frm rated value in mm 2 1,5 1 0,5 0-0,5-1 -1,5-2 TECSAR Reflectr: Segment Accuracy left rib middle right Segment-Nr.: 3 left rib middle right 0,316 RMS Segment Radius in mm Fig. 14: Target pints n each panel Fig. 14 shws the psitins f the target pints n each panel (n the left, n the rib, in the panel middle and at the right side f the panel) and alng the panel radius. These pints are crrelated n the bttm part f the diagram with the measured results f a typical panel. One can see that the typical peak t peak deviatins n the panel are in the range f 0.6 mm mm, ne can als see that the maximum lcal deflectin n the unsupprted extreme right panel edge (radius 1500 mm) is at abut 0.9 mm. Fig. 15: Overall Panel by Panel Cntur deviatin In Fig. 15 the maximum deviatins f each panel (1 t 30) are pltted fr sme typical pints n a large reflectr radius (1250 and 1495 mm). The evaluatin f all target pints f the whle reflectr leads t an rms value fr the reflectr f abut This value can still be significantly imprved by fine tuning f 1,5 TECSAR Reflectr Evaluatin - Best Fit Central Dish R450 Segment Rib R1495 Segment Rib R1250 0,65 RMS Segment Middle R1495 Fig. 13: Deplyed Reflectr during Cntur Measurement. In Fig. 13 the deplyed reflectr is shwn, equipped with a number f circular target spts n its surface. Reference bars are shwn n tp and bttm. By means f this arrangement the glbal cntur accuracy and reprducibility was measured by means f a phtgrammetric equipment. Deviatin 1,0 0,5 0, ,5-1,0-1,5-2,0-2,5 Segment N. (starting frm -X)

7 - 7 - the reflectr panels and fr a flight mdel by panel screening and imprvement f the panel adjustment pssibility n the hinge axes s that a glbal rms value f abut 0.3 t 0.4 mm is expected. 9. FURTHER VERIFICATION STEPS AND OUTLOOK At present the Reflectr is develped t flight standard. Only minr design adaptatins r imprvements are still t be included int a flight design (e.g titanium hinge brackets n the central dish instead f Al-brackets, r ptimisatin f the panel adjustment capability alng different DF fr imprvement f the integratin prcess). Sme items such as the flight standard Launch Lcking Actuatr have t be selected, ther details might be imprved n basis f the experience gained during the develpment phase. A Vibratin test is under preparatin. Electrical testing was perfrmed n Material sample level and is still pending fr the cmplete reflectr. The same is valid fr vacuum testing. central dish. Therefre at present the trque value applied t each deplying panel is nt identical and has t be harmnised fr a flight cnfiguratin. Hwever the effect n the deplyment kinematics is quite lw due t the cncept inherent panel synchrnisatin during deplyment. Only at the end f deplyment when the individual panels get independent frm each ther, the residual trque difference between the panels yields sme nnunifrmity in the deplyment prcess. The Damping Unit Detailed Design has t be imprved. It was fund during deplyment testing that the winding prcess f the damping string nt the reel is critical w.r.t. the deplyment functin. The string wund frm the left and right side simultaneusly nt the reel tended t jam n the reel if the winding prcess was nt ttally cntrlled. Therefre the reel was separated int tw parts, ne taking the string end cming frm the left side and ne taking the ther end cming frm the right side t the reel. By this measure bth stings are cmpletely separated frm each ther and a prper winding and unwinding prcess can be achieved. In a further develpment step, design cncepts fr central Feed and Offset Cnfiguratins in the diameter class f 5 t 6 m are planned t be investigated. 10. LESSONS LEARNT It was fund during the develpment that the manufacturing prcess f the thin sheet reflectr cmpnents is sensitive against changes in material batches and prcess variatins and tlerances. Therefre the material used fr manufacturing f the cmplete reflectr has t be prcured in ne batch and in sufficient quantity in rder t allw t prduce a sufficient number f additinal reflectr panels used fr selectin and perfrmance ptimisatin purpses. The Adjustability f deplyment axes has t be imprved. At present full adjustment capability is prvided fr each panel abut its deplyment axis. Only a limited adjustability is prvided laterally alng the deplyment axis and in tilt. This capability has t be imprved by mdificatin/ptimisatin f the deplyment mechanism. By this measure a much better glbal reflectr cntur can be adjusted than presently pssible. The design f the Deplyment Spring trque adjustment allws nly t adjust the trque in discrete steps. The zer 0 trque psitin is different fr each panel axis, since the panel axes achieve different rtatinal psitins during panel integratin int the