Analysis and ground testing for validation of the Inflatable Sunshield In Space (ISIS) experiment

EXPERIMENTSebastien Lienard, 1John Johnston 2Mike Adams, 2 Diane Stanley, 2 Jean-Pierre Alfano, 1Paolo Romanacci 1(1) Universities Space Research Association, Greenbelt, MD(2) NASA Goddard Space Flight Center, Greenbelt, MDABSTRACTThe Next Generation Space Telescope(NGST) design requires a large sunshield toprotect the large aperture mirror and instrumentmodule from constant solar exposure at its L2orbit. The structural dynamics of the sunshieldmust be modeled in order to predict disturbancesto the observatory attitude control system andgauge effects on the line of site jitter. Models oflarge, non-linear membrane systems are not wellunderstood and have not been successfullydemonstrated. To answer questions aboutsunshield dynamic behavior and demonstratecontrolled deployment, the NGST project is flyinga Pathfinder experiment, the Inflatable Sunshiel_,_in Space (ISIS). This paper discusses in details_)the modeling and ground-testing efforts performe_"at the Goddard Space Flight Center to: validateanalytical tools for characterizing the dynamicbehavior of the deployed sunshield, qualify theexperiment for the Space Shuttle, and verify thefunctionality of the system. Included in thediscussion will be test parameters, test setups,problems encountered, and test results.INTRODUCTIONSpace inflatable technology developmentstarted in the 60's with the ECHO balloonreflectors (30m in diameter) that were put in orbitas a relay to broadcast television across theocean. Several projects were then developed butthe technology was put on hold later for materialcapability reasons. With the incredible progressesin material development, the technology is comingback now and it is competing with othermechanical type of structures. NASA and otherprivate industries are putting significant resourcesinto the development of this technology becauseof its wide range of applications: large reflectors,support structures for solar arrays, large antennas,sunshields, solar sails, etc. This technology isapplicable and advantageous for large structuresthat must be lightweight and packaged in a smallvolume.The government team developing thereference concepts for the Next Generation SpaceTelescope (NGST) has baselined an inflatablesunshield to passively cool optics and instruments.The reasons are simple: a very large aperture(32mx14m) sunshield has to be packaged in arelatively small volume (1.5xl .5x0.5m). In order tovalidate the concepts and reduce the risks ofsingle point failure of sub-system, a flightexperiment of a subscale of NGST sunshield (3times smaller) has been implemented in thetechnology roadmap: the Inflatable Sunshield InSpace (ISIS) flight experiment. 1"2The ISIS sunshield, in its deployed state,consists of two sub-systems: (1) the supportstructure and (2) the thermal shield. The supportstructure is composed of a container and fourinflatable booms arranged in a cruciform shape.The thermal shield is composed - for ISIS - offour layers of thin film membranes (13 micronsthick) made of VDA coated Kapton® attached atthe root to the container and at the ends to theinflatable booms. Constant force springs at theinterface between the booms and the membraneskeep the shield under constant tension.References provide a detailed description of thedesign of the ISIS flight experiment. 3Constant thickness scaling laws 4 have beendeveloped and used to dimension the ISISsunshield keeping the thickness of the membranesconstant. Besides validating technologicalconcepts (packaging, deployment methods, filmhandling, rigidization process, etc), another