Post on 28-Dec-2015
Determinate Space Frame Determinate Space Frame Telescope Structures for SNAPTelescope Structures for SNAP
Bruce C. Bigelow
University of Michigan
Department of Physics
7/28/04
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Determinate Space FramesDeterminate Space Frames
Motivations: Minimize telescope structure deflections under gravity Maximize resonant frequencies on ground and orbit Minimize structure mass, CF outgassing, etc. Maximum access to optical elements (assembly, test) Explore parameter space for SNAP structure
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Determinate Space FramesDeterminate Space Frames
Determinate space frames: Loads carried axially (ideally) Deflections scale linearly with length:
d = PL/AE vs. PL^3/nEI No redundant members Free-body strut to node ratio: S = 3*N – 6 Fast and easy to analyze with FEA May ease assembly (vs. indeterminate structures) Truss structures are “optimal” for supporting discrete loads Truss structures make poor fuel tanks and fuselages…
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SNAP Space FramesSNAP Space Frames
Design considerations: Maintain symmetry to extent possible Locate nodes for access to primary loads
3 nodes above secondary mirror for hexapod mount 3 nodes above primary for secondary support 3 nodes behind primary for mirror, attach to SC 3 nodes below tertiary axis to stabilize secondary supp.
Locate struts to avoid optical path Size struts to minimize mass and deflections Round struts used for constant stiffness vs. orientation Non-tapered struts used – easy for first cut designs COI M55J CF used for all struts CF can be optimized for cross section, thermal expansion
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SNAP Space FramesSNAP Space Frames
Design and analysis: Still using TMA 63 optics, but results are “portable” 6 structure variants considered 1 selected for analysis Telescope mass: 360kg loads, 96kg structures Static FEA
Zenith pointing, gravity-release Dynamic FEA
Ground test On-orbit, unconstrained (“free-free”)
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Static FEAStatic FEA
Static analysis: Telescope pointed at zenith Parametric solid and FEA models, run in batch mode Optics, FPA modeled with 6 DOF solid elements Struts modeled with 6 DOF pipe elements Optics, FPA structures ignored except for mass effects Densities varied to match current design masses
Primary = ULE, 205 kg Secondary = ULE, 9.7 kg, + 10kg for actuators Fold = Zerodur, 19 kg Tertiary = ULE, 17 kg FPA = MZT, 100 kg (no spectrograph)
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Dynamic FEADynamic FEA
Dynamic analysis: Model and loads from static analysis Modal analysis for ground, launch
f1 = 72 Hz f2 = 74 Hz f3 = 107 Hz f4 = 114 Hz f5 = 131 Hz
Modal analysis for on-orbit (unconstrained) f7 = 106 Hz f8 = 107 Hz
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Determinate Space FramesDeterminate Space Frames
Conclusions: Space frames are viable alternatives to plate/shell structures An space frame design for SNAP was shown and analyzed Many other alternatives, and combinations, exist The final telescope structure design will probably result from a
trade-off of multiple requirements: Weight Stiffness Ease of modification (additional loads) Ease of fabrication (cost and duration) Ease of assembly, integration, and test