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 personnel 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 development 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 propulsion [17], a concept originally introduced by Ehricke [18]. While most designs foresee two offset parabolic reflectors, alternative configurations have investigated 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 precision structures above. Many backbone structures (but not all by any means) involve skeletons, assembled from tubular components. Indeed, such a ”onedimensional” 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 concentrators (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 Contraves FIRST ISRS thermal shield concept belongs to this category: a complete 3.5-m skeleton [24] (Fig. 5), was manufactured and used for packaging, 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 designs (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 studied 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 example, 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 structures [32]. We firstly present a discussion of rigidization technologies, beginning 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 structure, 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 materials 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 induced 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 metallic layer. Once the pressure is removed, the stressed aluminum maintains the