《纺织复合材料》课程参考文献（Textile Composites and Inflatable Structures）16 Recent Advances in the Rigidization of Gossamer Structures

Recent Advances in the Rigidization of Gossamer Structures B.Defoortl,V.Peypoudatl,M.C.Bernasconi2,K.Chuda3 and X.Coqueret3 1 EADS SPACE Transportation BP11-33165 Saint Medard en Jalles Cedex-France brigitte.defoort@space.eads.net 2 MCB Consultants 8953 Dietikon Switzerland 3 Laboratoire de Chimie Organique et Macromoleculaire UMR CNRS 8009 USTL -59655 Villeneuve d'Ascq Cedex-France 1 Introduction The interest of using inflatable and rigidizable structures for space equipment (such as solar arrays,antenna reflectors ...)has been identified for many years,but this has not yet been implemented on operational space equipment in Europe,due to the lack of adapted materials and technologies.Recent improvements in these fields allow today the development of such projects and solar arrays have been identified as one of the most promising applica- tion.This paper focuses on inflatable and rigidizable lightly loaded structures. Typical driving requirements are high packaging efficiency,very low specific mass and large size.As with other flexible-wall structures,they exploit gas pressure for their deployment ("inflatable structures").But,inflated struc- tures unavoidably loose the gas that pressures them,and therefore require a pressure control apparatus and a gas supply to replenish the losses.This disadvantage is acceptable only for items that have to last for very short pe- riods of time or for items where pressurization is a basic function (as in the case of habitats).It is commonly admitted that all inflated structures shall be rigidized in space as soon as their life time exceeds one week.As a result, the use of rigidizable materials that enable an inflated structure to become permanently rigid without relying on inflation is obviously a key technology in the field of Gossamer structures.For a given architecture,various kinds of rigidization techniques can be proposed:chemical rigidization [use of UV radiation (solar or with integrated light sources),thermal curing (using so- lar radiation and/or active heating),physical rigidization removal of volatile components in vacuum (solvent boil-off)],or mechanical rigidization [metal 259 E.Onate and B.Kroplin (eds.).Textile Composites and Inflatable Structures,259-283. 2005 Springer.Printed in the Netherlands

Recent Advances in the Rigidization of Gossamer Structures B. Defoort1, V. Peypoudat1, M.C. Bernasconi2, K. Chuda3 and X. Coqueret3 1 EADS SPACE Transportation BP11 - 33165 Saint M´edard en Jalles Cedex - France brigitte.defoort@space.eads.net 2 MCB Consultants 8953 Dietikon - Switzerland 3 Laboratoire de Chimie Organique et Macromol´eculaire UMR CNRS 8009 USTL - 59655 Villeneuve d’Ascq Cedex – France 1 Introduction The interest of using inflatable and rigidizable structures for space equipment (such as solar arrays, antenna reflectors · · ·) has been identified for many years, but this has not yet been implemented on operational space equipment in Europe, due to the lack of adapted materials and technologies. Recent improvements in these fields allow today the development of such projects and solar arrays have been identified as one of the most promising applica￾tion. This paper focuses on inflatable and rigidizable lightly loaded structures. Typical driving requirements are high packaging efficiency, very low specific mass and large size. As with other flexible-wall structures, they exploit gas pressure for their deployment (“inflatable structures”). But, inflated struc￾tures unavoidably loose the gas that pressures them, and therefore require a pressure control apparatus and a gas supply to replenish the losses. This disadvantage is acceptable only for items that have to last for very short pe￾riods of time or for items where pressurization is a basic function (as in the case of habitats). It is commonly admitted that all inflated structures shall be rigidized in space as soon as their life time exceeds one week. As a result, the use of rigidizable materials that enable an inflated structure to become permanently rigid without relying on inflation is obviously a key technology in the field of Gossamer structures. For a given architecture, various kinds of rigidization techniques can be proposed: chemical rigidization [use of UV radiation (solar or with integrated light sources), thermal curing (using so￾lar radiation and/or active heating)], physical rigidization [removal of volatile components in vacuum (solvent boil-off)], or mechanical rigidization [metal 259 E. Oñate and B. Kröplin (eds.), Textile Composites and Inflatable Structures, 259–283. © 2005 Springer. Printed in the Netherlands

260 B.Defoort,V.Peypoudat,M.C.Bernasconi,K.Chuda and X.Coqueret layer stretch/aluminum laminates.One of the most promising rigidization techniques envisioned by EADS-ST is in-orbit UV curing of a composite struc- ture.After a brief overview of Gossamer structures,exemplifying applications and potentialities of the technique by showing impressive realizations,we will detail a technology trade off related to the many rigidization processes that may apply to inflatable structures.Finally,we will focus on radiation initiated polymerization as a versatile tool to rigidize backbone structures. 2 An Overview of Gossamer Structures 2.1 Definition Since the beginning of space flight,researchers and experimenters have been confronted with the problem of packaging into the restricted volumes avail- able on the carrier vehicles items that they actually wanted to become much greater;and since those beginnings "inflatable"elements were among those proposed to master this challenge [1].In recent years,NASA has introduced the "gossamer"expression to label those forms of spacecraft exceptionally low in mass and suitable for packaging into very small volumes,compared to conventional spacecraft:in general,it applies to inflatable and membrane structures for space use. A more descriptive term,especially as it relates to the theme of the present book,is that of flexible-wall,expandable structures.The (initial)compliance of the walls allows the compact packaging and also enables the geometric ef ficiency of the materials that leads to the low mass,reinforced by the fact that one can design such structures for the space environment properly not mainly to survive the launch phase.And,we refer to all those struc- tures that are completely assembled at their manufacture site,then folded, stowed,packaged,or otherwise compacted for transport to their operational location,where they are deployed and installed for functional use.The instal- lation sequence may include a rigidization procedure(mechanical,physical, or chemical-as discussed in the next Section),spinning the spacecraft (for a rotationally-stabilized object),pressurization (for continuously-inflated ob- jects),etc.Expandable structures with flexible walls have been flown but only in a small number of cases.To date,the greatest majority of "large" structures deployed in space belong to the rigid-component,variable-geometry (RCVG)kind,that rely on actuation mechanisms to perform the transition from packaged to deployed state. The range of technological approaches to the implementation of flexible- wall expandable structures is just as vast as that of the applications for which such structures can be used.One can organize the field using different dis- crimination criteria,deriving them from application-oriented considerations (e.g.,the type of loads or geometrical requirements that drive a design),from characteristics of the structural elements'build-up (e.g.,whether thin-walled

260 B. Defoort, V. Peypoudat, M.C. Bernasconi, K. Chuda and X. Coqueret layer stretch/aluminum laminates]. One of the most promising rigidization techniques envisioned by EADS-ST is in-orbit UV curing of a composite struc￾ture. After a brief overview of Gossamer structures, exemplifying applications and potentialities of the technique by showing impressive realizations, we will detail a technology trade off related to the many rigidization processes that may apply to inflatable structures. Finally, we will focus on radiation initiated polymerization as a versatile tool to rigidize backbone structures. 2 An Overview of Gossamer Structures 2.1 Definition Since the beginning of space flight, researchers and experimenters have been confronted with the problem of packaging into the restricted volumes avail￾able on the carrier vehicles items that they actually wanted to become much greater; and since those beginnings “inflatable” elements were among those proposed to master this challenge [1]. In recent years, NASA has introduced the “gossamer” expression to label those forms of spacecraft exceptionally low in mass and suitable for packaging into very small volumes, compared to conventional spacecraft: in general, it applies to inflatable and membrane structures for space use. A more descriptive term, especially as it relates to the theme of the present book, is that of flexible-wall, expandable structures. The (initial) compliance of the walls allows the compact packaging and also enables the geometric ef- ficiency of the materials that leads to the low mass, reinforced by the fact that one can design such structures for the space environment properly – not mainly to survive the launch phase. And, we refer to all those struc￾tures that are completely assembled at their manufacture site, then folded, stowed, packaged, or otherwise compacted for transport to their operational location, where they are deployed and installed for functional use. The instal￾lation sequence may include a rigidization procedure (mechanical, physical, or chemical – as discussed in the next Section), spinning the spacecraft (for a rotationally-stabilized object), pressurization (for continuously-inflated ob￾jects), etc. Expandable structures with flexible walls have been flown but only in a small number of cases. To date, the greatest majority of “ large” structures deployed in space belong to the rigid-component, variable-geometry (RCVG) kind, that rely on actuation mechanisms to perform the transition from packaged to deployed state. The range of technological approaches to the implementation of flexible￾wall expandable structures is just as vast as that of the applications for which such structures can be used. One can organize the field using different dis￾crimination criteria, deriving them from application-oriented considerations (e.g., the type of loads or geometrical requirements that drive a design), from characteristics of the structural elements’ build-up (e.g., whether thin-walled

Recent Advances in the Rigidization of Gossamer Structures 261 membranes or thicker,more plate-like layouts),or from the methods used to stabilize shape of the object at installation.A first attempt classifies the structures according to four use and requirements criteria as follows: "Lightly-loaded",flexible-wall,expandable space structures-sized for the orbital environment (generally against buckling loads);typical require- ments are:high packaging efficiency,very low specific mass,large size;the tension within an element's wall is of the order of 0.1 kN/m;a further subdivision distinguishes between: support structures in general ("backbones"),in which a small-to- moderate integration between structure and system function occurs, and precision structures,where the structural element and its shape have a direct system function impact "Heavy-duty"flexible-wall,expandable structures-sized to carry (inter- nal)loads(generally,pressurization forces);typical requirements are:large enclosed volumes,compatibility with crew presence,moderate packaging efficiency and/or specific mass;the tension within an element's wall is of the order of 100 kN/m. "High-temperature"flexible-wall,expandable structures-sized to sustain significant temperature levels,as generated during planetary entry (al- though reduced thanks to the lower area loading such structures enable). This article focuses mainly on lightly-loaded and heavy duty flexible-walls and not on high temperature elements which differ from the two first classes in technological terms even if some synergies exist and are used.Wilde and colleagues [2 give a summary of recent work in Western Europe on high temperature flexible walls. 2.2 Applications Historical Background Early Work:Inflatable Satellites After the pioneering suggestions by Gatland and co-workers [1],the idea of in- flatable spacecraft -in particular,to create optically observable orbital bodies- was developed by John O'Sullivan and his colleagues at Langley Research Cen- ter [3].Soon,they prepared 12-ft (3.66-m),mechanically rigidized spheres, launched as Explorer IX and Explorer XIX for contributing to the measure- ment of the high atmosphere's density [4].The Explorer spheres were sturdy enough to support themselves unpressurized under 1-g acceleration(Fig.1). From this work evolved the concept and the technology for the passive com- munications satellites (Echo I II,[5]),that eventually enabled the 40-m diameter PAGEOS (PAssive GEOdetic Satellite)6.Advanced concepts stud- ied to achieve better mass efficiency than using spherical reflectors involved inflated lens/torus configurations and the wire grid sphere satellites,using photolyzable wall materials for the deployment and leaving eventually only a

Recent Advances in the Rigidization of Gossamer Structures 261 membranes or thicker, more plate-like layouts), or from the methods used to stabilize shape of the object at installation. A first attempt classifies the structures according to four use and requirements criteria as follows: • “Lightly-loaded”, flexible-wall, expandable space structures - sized for the orbital environment (generally against buckling loads); typical require￾ments are: high packaging efficiency, very low specific mass, large size; the tension within an element’s wall is of the order of 0.1 kN/m; a further subdivision distinguishes between: – support structures in general (“backbones”), in which a small-to￾moderate integration between structure and system function occurs, and – precision structures, where the structural element and its shape have a direct system function impact • “Heavy-duty” flexible-wall, expandable structures - sized to carry (inter￾nal) loads (generally, pressurization forces); typical requirements are: large enclosed volumes, compatibility with crew presence, moderate packaging efficiency and/or specific mass; the tension within an element’s wall is of the order of 100 kN/m. • “High-temperature” flexible-wall, expandable structures - sized to sustain significant temperature levels, as generated during planetary entry (al￾though reduced thanks to the lower area loading such structures enable). This article focuses mainly on lightly-loaded and heavy duty flexible-walls and not on high temperature elements which differ from the two first classes in technological terms even if some synergies exist and are used.Wilde and colleagues [2] give a summary of recent work in Western Europe on high temperature flexible walls. 2.2 Applications & Historical Background Early Work: Inflatable Satellites After the pioneering suggestions by Gatland and co-workers [1], the idea of in- flatable spacecraft –in particular, to create optically observable orbital bodies– was developed by John O’Sullivan and his colleagues at Langley Research Cen￾ter [3]. Soon, they prepared 12-ft (3.66-m), mechanically rigidized spheres, launched as Explorer IX and Explorer XIX for contributing to the measure￾ment of the high atmosphere’s density [4]. The Explorer spheres were sturdy enough to support themselves unpressurized under 1-g acceleration (Fig. 1). From this work evolved the concept and the technology for the passive com￾munications satellites (Echo I & II, [5]), that eventually enabled the 40- m diameter PAGEOS (PAssive GEOdetic Satellite) [6]. Advanced concepts stud￾ied to achieve better mass efficiency than using spherical reflectors involved inflated lens/torus configurations and the wire grid sphere satellites, using photolyzable wall materials for the deployment and leaving eventually only a

262 B.Defoort,V.Peypoudat,M.C.Bernasconi,K.Chuda and X.Coqueret structure of stretched metal wires,making a radio reflector less sensitive to the solar pressure.Several such wire-grid spheres actually flew,e.g.the USAF OV1-8 satellite. Fig.1.An example of a balloon satellite:a mechanically-rigidized,12-ft Explorer IX inflatable sphere under full-gravity testing.(NASA picture) Precision Structures Solar concentrators (for thermodynamic power generation)represent a fur- ther application that received extensive treatment using different gossamer technology approaches(with inflated membranes,with various foam-in-place techniques,with chemically-rigidized composites-both in form of membranes and of expandable-honeycomb structures),and with most designs adopting the lens-torus layout.Throughout the 1960s,they were studied in the US [7]. but also in Germany where,around 1965,Bolkow investigated an inflatable foam-rigidized solar-thermal power collector 8].Eventually,early in the 1970s, MBB built a 1-m inflatable and rigidized antenna reflector using glass-fibre- reinforced gelatin for the torus and the reflector shells,and a polymer-film radome [9]. In 1979,ESA began sponsoring a series of development contracts at Con- traves(Zurich,Switzerland)that one of the authors (MCB)had the privilege

262 B. Defoort, V. Peypoudat, M.C. Bernasconi, K. Chuda and X. Coqueret structure of stretched metal wires, making a radio reflector less sensitive to the solar pressure. Several such wire-grid spheres actually flew, e.g. the USAF OV1-8 satellite. Fig. 1. An example of a balloon satellite: a mechanically-rigidized, 12-ft Explorer IX inflatable sphere under full-gravity testing. (NASA picture) Precision Structures Solar concentrators (for thermodynamic power generation) represent a fur￾ther application that received extensive treatment using different gossamer technology approaches (with inflated membranes, with various foam-in-place techniques, with chemically-rigidized composites – both in form of membranes and of expandable-honeycomb structures), and with most designs adopting the lens-torus layout. Throughout the 1960s, they were studied in the US [7], but also in Germany where, around 1965, Bolkow investigated an inflatable ¨ foam-rigidized solar-thermal power collector [8]. Eventually, early in the 1970s, MBB built a 1-m inflatable and rigidized antenna reflector using glass- fibre￾reinforced gelatin for the torus and the reflector shells, and a polymer-film radome [9]. In 1979, ESA began sponsoring a series of development contracts at Con￾traves (Zurich, Switzerland) that one of the authors (MCB) had the privilege

Recent Advances in the Rigidization of Gossamer Structures 263 to execute,lead,and participate in.For historical reasons,those development activities concentrated on microwave antenna reflectors,exemplified by the re- alization of the first rigidized offset reflector,but work was done in all classes of objects but for the "high-temperature"one. Work on this technology -identified as Inflatable Space-Rigidized Struc- tures(ISRS)-included a series of experimental activities using objects in the size range from 1-to 10-m aperture.First came three small models of a sym- metric (center-fed)reflector to gauge issues such as folding and deployment, manufacture processes,and initially achievable accuracy.In successive phases, three 2.8-m reflectors (called LOAD-3 and designed for operation at 3.6 GHz) allowed the execution of following tests [10]: accuracy-improved manufacturing procedures adopted during that devel- opment phase allowed a reduction of the RMS error from 0.9 to 0.7 mm, while identifying the main sources of the remaining inaccuracies; packaging efficiency-were verified using the object that was successively subjected to electrical measurements;without degradation of surface qual- ity as consequence of the folding and deployment exercises; controlled deployment in vacuum-a test within ESTEC's Dynamic Test Chamber demonstrated the quality of the residual air control procedures, the correctness of the deployment sequence,and the controlled deployment of the structure; electrical performance -measures were performed on the first complete object,after a full cycle of pressurization tests,folding,packaging,deploy- ment,and cure; cure under(simulated)space conditions-a thermal-vacuum chamber solar simulation test demonstrated the correctness of the reflector's thermal design. Further reflectors were manufactured and tested (under clean-room con- ditions): a 5.7-m diameter Test Article for the QUASAT radio telescope reflector, a center-fed layout [11,and a 10-m aperture offset-fed reflector (LOAD-10,Fig.2),designed for oper- ation at 1.6 GHz;after the folding and deployment cycle,the surface error had grown from 2.15 mm to 2.66 mm RMS,still yielding a gain of 42.6 dB and a sidelobe level of-33.8 dB [12]. The ISRS developments in Europe apparently found a resonance in Japan. Around the mid 1980s,a team formed around ISAS and began work on a modular,hybrid antenna reflector concept [13-a variable-geometry truss backbone carrying ISRS reflector facets-as an unsuccessful candidate for the VSOP mission (the Japanese equivalent of QUASAT that eventually flew as HALCA)

Recent Advances in the Rigidization of Gossamer Structures 263 to execute, lead, and participate in. For historical reasons, those development activities concentrated on microwave antenna reflectors, exemplified by the re￾alization of the first rigidized offset reflector, but work was done in all classes of objects but for the “high-temperature” one. Work on this technology –identified as Inflatable Space-Rigidized Struc￾tures (ISRS)– included a series of experimental activities using objects in the size range from 1- to 10-m aperture. First came three small models of a sym￾metric (center-fed) reflector to gauge issues such as folding and deployment, manufacture processes, and initially achievable accuracy. In successive phases, three 2.8-m reflectors (called LOAD-3 and designed for operation at 3.6 GHz) allowed the execution of following tests [10]: - accuracy - improved manufacturing procedures adopted during that devel￾opment phase allowed a reduction of the RMS error from 0.9 to 0.7 mm, while identifying the main sources of the remaining inaccuracies; - packaging efficiency - were verified using the object that was successively subjected to electrical measurements; without degradation of surface qual￾ity as consequence of the folding and deployment exercises; - controlled deployment in vacuum - a test within ESTEC’s Dynamic Test Chamber demonstrated the quality of the residual air control procedures, the correctness of the deployment sequence, and the controlled deployment of the structure; - electrical performance – measures were performed on the first complete object, after a full cycle of pressurization tests, folding, packaging, deploy￾ment, and cure; - cure under (simulated) space conditions - a thermal-vacuum chamber solar simulation test demonstrated the correctness of the reflector’s thermal design. Further reflectors were manufactured and tested (under clean-room con￾ditions): - a 5.7-m diameter Test Article for the QUASAT radio telescope reflector, a center-fed layout [11], and - a 10-m aperture offset-fed reflector (LOAD-10, Fig. 2), designed for oper￾ation at 1.6 GHz; after the folding and deployment cycle, the surface error had grown from 2.15 mm to 2.66 mm RMS, still yielding a gain of 42.6 dB and a sidelobe level of -33.8 dB [12]. The ISRS developments in Europe apparently found a resonance in Japan. Around the mid 1980s, a team formed around ISAS and began work on a modular, hybrid antenna reflector concept [13] – a variable-geometry truss backbone carrying ISRS reflector facets – as an unsuccessful candidate for the VSOP mission (the Japanese equivalent of QUASAT that eventually flew as HALCA)

264 B.Defoort,V.Peypoudat,M.C.Bernasconi,K.Chuda and X.Coqueret Fig.2.The LOAD-10 offset reflector While the ESA initiative originally had but vague relations to previous US work,it contributed to the renewed interest there,when ESTEC person- nel introduced the work done at Contraves to several JPL science projects teams.On the other hand,in 1980,L'Garde had proposed new approaches to continuously inflated antenna reflectors [14]and,after a number of develop- ment activities,in 1996 they finally achieved a test flight for a 15-m object, deployed from the Shuttle Orbiter 15.Work on those inflatable reflectors continues [16]. Finally,under the USAF leadership,the inflatable solar concentrator was born again,this time to support the development of solar-thermal propul- sion 17,a concept originally introduced by Ehricke [18.While most designs foresee two offset parabolic reflectors,alternative configurations have investi- gated the use of flexible Fresnel lenses,also supported by gossamer elements. ESA has also sponsored studies for applying solar-thermal propulsion to upper stages for geocentric transportation [19](Fig.3). Backbones Concepts,type of applications,and study and development activities have been too numerous to attempt even a brief summary as done for the preci- sion structures above.Many backbone structures (but not all by any means) involve skeletons,assembled from tubular components.Indeed,such a "one- dimensional"element forms the simplest backbone morphology.Following evolutionary considerations,one may discuss morphology and applications of backbones in the following order:

264 B. Defoort, V. Peypoudat, M.C. Bernasconi, K. Chuda and X. Coqueret Fig. 2. The LOAD-10 offset reflector While the ESA initiative originally had but vague relations to previous US work, it contributed to the renewed interest there, when ESTEC person￾nel introduced the work done at Contraves to several JPL science projects teams. On the other hand, in 1980, L’Garde had proposed new approaches to continuously inflated antenna reflectors [14] and, after a number of develop￾ment activities, in 1996 they finally achieved a test flight for a 15-m object, deployed from the Shuttle Orbiter [15]. Work on those inflatable reflectors continues [16]. Finally, under the USAF leadership, the inflatable solar concentrator was born again, this time to support the development of solar-thermal propul￾sion [17], a concept originally introduced by Ehricke [18]. While most designs foresee two offset parabolic reflectors, alternative configurations have investi￾gated the use of flexible Fresnel lenses, also supported by gossamer elements. ESA has also sponsored studies for applying solar-thermal propulsion to upper stages for geocentric transportation [19] (Fig. 3). Backbones Concepts, type of applications, and study and development activities have been too numerous to attempt even a brief summary as done for the preci￾sion structures above. Many backbone structures (but not all by any means) involve skeletons, assembled from tubular components. Indeed, such a ”one￾dimensional” element forms the simplest backbone morphology. Following evolutionary considerations, one may discuss morphology and applications of backbones in the following order:

Recent Advances in the Rigidization of Gossamer Structures 265 Fig.3.European solar-thermal upper stage concept,with inflatable offset concen- trators (EADS-ST image) Planar Frames:two-dimensional support for items such as,e.g.,flat shields, solar sails [20],solar reflectors,and photovoltaic arrays [21](Fig.4),RF devices (reflectarrays,rectennae,lens,..[22]),or arrays of sensors. Single-Tier Structures:prismatic backbones (tripod,tetrapod,etc)for other functions,e.g.for light aerobraking [23],lens positioning,etc. Two-Tier Structures:Three-dimensional elements for telescopes tubes, cryogenic shield,hangars,and other unpressurized enclosures.The Con- traves FIRST ISRS thermal shield concept belongs to this category:a complete 3.5-m skeleton [24](Fig.5),was manufactured and used for pack- aging,deployment,cure,and geometric tests. Special Configurations:Mast and booms,other (mostly)planar structures -for low-gain aerial structures (helix,Yagi),radiators. Trussworks:generic support structures,e.g.backbone structures both for Michelson [25]and Fizeau interferometers [26]; Polyhedral Skeletons other,more complex forms:Modified two-tier de- signs (e.g.for greenhouses),more complex lattice structures,spheres and spherical approximations. Heavy-Duty Elements for Manned Flight Gossamer structures hold the promise to provide significant capabilities in support of manned missions:throughout the 1960s,NASA and USAF stud- ied and developed relatively small crew transfer tunnels and airlocks,orbital and surface shelters in support of exploration missions,full space stations

Recent Advances in the Rigidization of Gossamer Structures 265 Fig. 3. European solar-thermal upper stage concept, with inflatable offset concen￾trators (EADS-ST image) - Planar Frames: two-dimensional support for items such as, e.g., flat shields, solar sails [20], solar reflectors, and photovoltaic arrays [21] (Fig. 4), RF devices (reflectarrays, rectennae, lens,... [22]), or arrays of sensors. - Single-Tier Structures: prismatic backbones (tripod, tetrapod, etc) for other functions, e.g. for light aerobraking [23], lens positioning, etc. - Two-Tier Structures: Three-dimensional elements for telescopes tubes, cryogenic shield, hangars, and other unpressurized enclosures. The Con￾traves FIRST ISRS thermal shield concept belongs to this category: a complete 3.5-m skeleton [24] (Fig. 5), was manufactured and used for pack￾aging, deployment, cure, and geometric tests. - Special Configurations: Mast and booms, other (mostly) planar structures – for low-gain aerial structures (helix, Yagi), radiators. - Trussworks: generic support structures, e.g. backbone structures both for Michelson [25] and Fizeau interferometers [26]; - Polyhedral Skeletons & other, more complex forms: Modified two-tier de￾signs (e.g. for greenhouses), more complex lattice structures, spheres and spherical approximations. Heavy-Duty Elements for Manned Flight Gossamer structures hold the promise to provide significant capabilities in support of manned missions: throughout the 1960s, NASA and USAF stud￾ied and developed relatively small crew transfer tunnels and airlocks, orbital and surface shelters in support of exploration missions, full space stations

266 B.Defoort,V.Peypoudat,M.C.Bernasconi,K.Chuda and X.Coqueret Fig.4.Concept for a solar sailing spacecraft with four 2,500-m2 saillets.(left);the "Sun Tower"solar power station builds on gossamer structures:supporting tori and flexible Fresnel-lens concentrators (right)(NASA picture) Fig.5.The 1/3-scale model of the ISRS skeleton for the FIRST's thermal shield (right)deployed out of an annular stowage volume around a simulated spacecraft central cylinder (left) and pressurized hangar enclosures capable of holding entire spacecraft during scheduled maintenance/repair activities.The latest US entry in this class in the TransHab concept for a multi-storied habitat [27,28].Activities along this direction have also been started in Europe [29,30

266 B. Defoort, V. Peypoudat, M.C. Bernasconi, K. Chuda and X. Coqueret Fig. 4. Concept for a solar sailing spacecraft with four 2,500-m2 saillets.(left); the “Sun Tower” solar power station builds on gossamer structures: supporting tori and flexible Fresnel-lens concentrators (right) (NASA picture) Fig. 5. The 1/3-scale model of the ISRS skeleton for the FIRST’s thermal shield (right) deployed out of an annular stowage volume around a simulated spacecraft central cylinder (left) and pressurized hangar enclosures capable of holding entire spacecraft during scheduled maintenance/repair activities. The latest US entry in this class in the TransHab concept for a multi-storied habitat [27,28]. Activities along this direction have also been started in Europe [29,30]

Recent Advances in the Rigidization of Gossamer Structures 267 3 Review of Rigidization Techniques The use of rigidizable materials that enable an inflated structure to become rigid is a key technology in the field of Gossamer structures.The term"rigid" needs however to be clarified when discussing lightweight structures.For ex- ample,the 155 microns thick chemically rigidized material used for ISRS 31] is 39 times less rigid than a 100 microns thick steal foil in term of membrane stiffness and 280 000 times less rigid than a 10 cm thick foam plate in term of beam stiffness.Those ratio drop to 6.4 and 10500 respectively,once one considers the stiffness to surfacic weight ratio.This illustrates the fact that the weight and packed volume are the concepts that drive the development of thin flexible rigidizable walls. Many technologies are identified for in orbit rigidization of Gossamer struc- tures [32.We firstly present a discussion of rigidization technologies,begin- ning with the identification and review of the different techniques and finally up to an evaluation of the existing technology.A set of evaluation criteria is defined and used to select the best candidates for a tubular solar array struc- ture,to be suitable for Gossamer structures.The selection criteria include the material's ability to be folded,rigidization conditions (including power needs),thermal and mechanical properties,outgassing,durability in space environment,costs,rigidization reversibility...Discussions of specific materi- als for the different technologies are covered incidentally,to exemplify options and to assist the designer in his evaluation activity. 3.1 Rigidization Techniques and Associated Materials Rigidization technologies can be classified depending on the nature of the phenomena that induces rigidization: Mechanical rigidization is obtained by stretching a polymer/aluminum laminate above its yield strain, Physical rigidization is obtained by phase transition (cooling a material below its glass transition temperature),using shape memory materials or by plasticizer or solvent evaporation, Chemically based rigidization is obtained either by thermally or UV in- duced polymerization.In orbit curing can be triggered or accelerated by gaseous catalysts carried by the inflation gas. The different rigidization techniques are described below. Mechanical Rigidization This is class of structures deployed by inflation and rigidized by inducing through the pressure forces a stress higher than yield stress in a wall's metal- lic layer.Once the pressure is removed,the stressed aluminum maintains the

Recent Advances in the Rigidization of Gossamer Structures 267 3 Review of Rigidization Techniques The use of rigidizable materials that enable an inflated structure to become rigid is a key technology in the field of Gossamer structures. The term “rigid” needs however to be clarified when discussing lightweight structures. For ex￾ample, the 155 microns thick chemically rigidized material used for ISRS [31] is 39 times less rigid than a 100 microns thick steal foil in term of membrane stiffness and 280 000 times less rigid than a 10 cm thick foam plate in term of beam stiffness. Those ratio drop to 6.4 and 10500 respectively, once one considers the stiffness to surfacic weight ratio. This illustrates the fact that the weight and packed volume are the concepts that drive the development of thin flexible rigidizable walls. Many technologies are identified for in orbit rigidization of Gossamer struc￾tures [32]. We firstly present a discussion of rigidization technologies, begin￾ning with the identification and review of the different techniques and finally up to an evaluation of the existing technology. A set of evaluation criteria is defined and used to select the best candidates for a tubular solar array struc￾ture, to be suitable for Gossamer structures. The selection criteria include the material’s ability to be folded, rigidization conditions (including power needs), thermal and mechanical properties, outgassing, durability in space environment, costs, rigidization reversibility... Discussions of specific materi￾als for the different technologies are covered incidentally, to exemplify options and to assist the designer in his evaluation activity. 3.1 Rigidization Techniques and Associated Materials Rigidization technologies can be classified depending on the nature of the phenomena that induces rigidization: - Mechanical rigidization is obtained by stretching a polymer/aluminum laminate above its yield strain, - Physical rigidization is obtained by phase transition (cooling a material below its glass transition temperature), using shape memory materials or by plasticizer or solvent evaporation, - Chemically based rigidization is obtained either by thermally or UV in￾duced polymerization. In orbit curing can be triggered or accelerated by gaseous catalysts carried by the inflation gas. The different rigidization techniques are described below. Mechanical Rigidization This is class of structures deployed by inflation and rigidized by inducing through the pressure forces a stress higher than yield stress in a wall’s metal￾lic layer. Once the pressure is removed, the stressed aluminum maintains the

268 B.Defoort,V.Peypoudat,M.C.Bernasconi,K.Chuda and X.Coqueret structure's rigidity and shape.This concept is very attractive,and was used in flight on the Echo-2 satellites in the sixties [33],as well as on the Optical Calibration Sphere in 2000.The main advantages of this rigidization process are its reversibility,simplicity,predictability and rapidity.Furthermore it does not require additional power,has good space durability and no specific stor- age constraints.However,the anisotropy of the stresses and the need for an accurate control of the pressurization levels are issues that affect this tech- nique for its application to cylindrical or toroidal objects.L'Garde proposes a solution to that problem which is based on fibers winding around a tubular structure [34.Two main issues remain with regard to this technology:the first one is the different thermally induced dilatation of the constituting ma- terials (polymer and aluminum)and the second is the compatibility of this technology with rolled-up storage. Physically induced rigidization:cold rigidization,shape memory and solvent evaporation The cold rigidization process relies on the exposure of originally flexible plastic layers-typically elastomers [35-to the deep space thermal sink to cool them below their glass-transition temperature,rigidizing the structure essentially by freezing the matrix.This concept appears particularly indicated for shield- ing applications outside Earth's orbit,and was studied for shadowing shields of cryogenic stages for Mars flights.More recently,ILC Dover and L'Garde presented structures rigidized using this technique,respectively a hexapod structure [36 and the Space Solar Power Truss [37.This technique is attrac- tive mostly because of its reversibility,simplicity and low energy requirements compare to thermal curing.However,the need for temperature control and the coefficient of thermal expansion of the resins are serious drawbacks. Recently,a number of studies have been conducted on shape memory composites,materials that mimic the behaviour of metallic shape-memory alloys 38,39,40.The structure is completed on ground and consolidated at an elevated temperature,to set the material's geometric shape.The material will return to its original shape when heated above its glass transition temperature. For packaging,the structure is softened by heating it above Tg,taking care to keep it below its set temperature.After cooling,it is kept stowed.Prior to deployment,the stowed structure is again heated above Tg to make it flexible enough to be deployed by inflation.This is quite a complex process that limits the overall size of an object.The deployment in space requires a fair amount of power and control functions,as the heating must be rather uniform overall; also,presumably,the temperature should not drop below Tg. Rigidization of a structure can also be obtained using evaporation of a solvent or a plasticizer in the material.The major issue of this solution is the large amount of solvent or plasticizer involved (e.g.between 13-50%for the Ciba polyimide tested during the Contraves ISRS program [41]).During the 1960s,a fairly large effort was dedicated to the study of rigidizable structures

268 B. Defoort, V. Peypoudat, M.C. Bernasconi, K. Chuda and X. Coqueret structure’s rigidity and shape. This concept is very attractive, and was used in flight on the Echo-2 satellites in the sixties [33], as well as on the Optical Calibration Sphere in 2000. The main advantages of this rigidization process are its reversibility, simplicity, predictability and rapidity. Furthermore it does not require additional power, has good space durability and no specific stor￾age constraints. However, the anisotropy of the stresses and the need for an accurate control of the pressurization levels are issues that affect this tech￾nique for its application to cylindrical or toroidal objects. L’Garde proposes a solution to that problem which is based on fibers winding around a tubular structure [34]. Two main issues remain with regard to this technology: the first one is the different thermally induced dilatation of the constituting ma￾terials (polymer and aluminum) and the second is the compatibility of this technology with rolled-up storage. Physically induced rigidization: cold rigidization, shape memory and solvent evaporation The cold rigidization process relies on the exposure of originally flexible plastic layers – typically elastomers [35] – to the deep space thermal sink to cool them below their glass-transition temperature, rigidizing the structure essentially by freezing the matrix. This concept appears particularly indicated for shield￾ing applications outside Earth’s orbit, and was studied for shadowing shields of cryogenic stages for Mars flights. More recently, ILC Dover and L’Garde presented structures rigidized using this technique, respectively a hexapod structure [36] and the Space Solar Power Truss [37]. This technique is attrac￾tive mostly because of its reversibility, simplicity and low energy requirements compare to thermal curing. However, the need for temperature control and the coefficient of thermal expansion of the resins are serious drawbacks. Recently, a number of studies have been conducted on shape memory composites, materials that mimic the behaviour of metallic shape-memory alloys [38,39,40]. The structure is completed on ground and consolidated at an elevated temperature, to set the material’s geometric shape. The material will return to its original shape when heated above its glass transition temperature. For packaging, the structure is softened by heating it above Tg , taking care to keep it below its set temperature. After cooling, it is kept stowed. Prior to deployment, the stowed structure is again heated above Tg to make it flexible enough to be deployed by inflation. This is quite a complex process that limits the overall size of an object. The deployment in space requires a fair amount of power and control functions, as the heating must be rather uniform overall; also, presumably, the temperature should not drop below Tg. Rigidization of a structure can also be obtained using evaporation of a solvent or a plasticizer in the material. The major issue of this solution is the large amount of solvent or plasticizer involved (e.g. between 13-50% for the Ciba polyimide tested during the Contraves ISRS program [41]). During the 1960s, a fairly large effort was dedicated to the study of rigidizable structures