Determinate Space Frame Telescope Structures for SNAP

1
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. PL3/nEI
• No redundant members
• Free-body strut to node ratio S 3N 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
(No Transcript)
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