Determinate Space Frame Telescope Structures for SNAP Bruce C. Bigelow University of Michigan Department of Physics 7/28/04

2 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

3 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…

4 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

5 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”)

6 SNAP Space Frames prtruss3 – initial concept design

7 Baffles fully enclose optical system, FPA

8 Lower baffles removed

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

10 All baffles removed

11 Structure is self-supporting without spacecraft

12

13 View from FPA side

14 View from tertiary side

15 Bottom view

16 Top view

17 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)

18 Static FEA Elements

19 Static FEA Gz, z-axis deflections, in meters

20 Static FEA Gz, deflected shape

21 Static FEA Gz, x-axis deflections, in meters

22 Static FEA Gz, y-axis deflections, in meters

23 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

24 Static FEA First ground mode, 72 Hz

25 Static FEA Second ground mode, 74 Hz

26 Static FEA Third ground mode, 108 Hz

27 Static FEA First free mode, 106 Hz

28 Static FEA Second free mode, 110 Hz

29 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

30 SNAP Space Frames prtruss1 – symmetric mounts for tertiary, FPA

31 SNAP Space Frames prtruss2 – hexapod tube for tertiary, FPA

32 SNAP Space Frames prtruss4 – 3 stacked hexapods, interferes with PM

33 SNAP Space Frames prtruss5 – 3 stacked hexapods, mid-level elements intersect

34 SNAP Space Frames prtruss6 – alternate support for secondary hexapod