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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SNAP Space FramesSNAP Space Frames

prtruss3 – initial concept design

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Baffles fully enclose optical system, FPA

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Lower baffles removed

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Radiator removed, FPA clears 12 element (rotated) baffle structure

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All baffles removed

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Structure is self-supporting without spacecraft

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View from FPA side

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View from tertiary side

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Bottom view

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Top view

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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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Static FEAStatic FEA

Elements

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Static FEAStatic FEA

Gz, z-axis deflections, in meters

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Static FEAStatic FEA

Gz, deflected shape

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Static FEAStatic FEA

Gz, x-axis deflections, in meters

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Static FEAStatic FEA

Gz, y-axis deflections, in meters

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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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Static FEAStatic FEA

First ground mode, 72 Hz

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Static FEAStatic FEA

Second ground mode, 74 Hz

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Static FEAStatic FEA

Third ground mode, 108 Hz

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Static FEAStatic FEA

First free mode, 106 Hz

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Static FEAStatic FEA

Second free mode, 110 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

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SNAP Space FramesSNAP Space Frames

prtruss1 – symmetric mounts for tertiary, FPA

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SNAP Space FramesSNAP Space Frames

prtruss2 – hexapod tube for tertiary, FPA

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SNAP Space FramesSNAP Space Frames

prtruss4 – 3 stacked hexapods, interferes with PM

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SNAP Space FramesSNAP Space Frames

prtruss5 – 3 stacked hexapods, mid-level elements intersect

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SNAP Space FramesSNAP Space Frames

prtruss6 – alternate support for secondary hexapod