Transcript Det. Space Frames - University of California Observatories

Determinate Space Frame
Telescope Structures for SNAP
Bruce C. Bigelow
University of Michigan
Department of Physics
7/28/04
Determinate 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 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 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 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 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 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 FEA
Elements
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Static FEA
Gz, z-axis deflections, in meters
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Static FEA
Gz, deflected shape
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Static FEA
Gz, x-axis deflections, in meters
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Static FEA
Gz, y-axis deflections, in meters
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Dynamic 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 FEA
First ground mode, 72 Hz
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Static FEA
Second ground mode, 74 Hz
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Static FEA
Third ground mode, 108 Hz
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Static FEA
First free mode, 106 Hz
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Static FEA
Second free mode, 110 Hz
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Determinate 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 Frames
prtruss1 – symmetric mounts for tertiary, FPA
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SNAP Space Frames
prtruss2 – hexapod tube for tertiary, FPA
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SNAP Space Frames
prtruss4 – 3 stacked hexapods, interferes with PM
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SNAP Space Frames
prtruss5 – 3 stacked hexapods, mid-level elements intersect
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SNAP Space Frames
prtruss6 – alternate support for secondary hexapod
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