Space Frame Structures for SNAP

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Space Frame Structures for SNAP

• Bruce C. Bigelow
• University of Michigan
• Department of Physics
• 11/04/04

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Space Frames for SNAP

• SNAP already has baseline primary and secondary
  structures. Why look at others?
• Minimizing structure mass mission flexibility
• Higher resonant frequencies are (almost) always
  better
• Minimizing carbon fiber mass reduces H2O dry-out
  issues
• Open structures provide maximum access to
  payloads
• Space frame structures are prevalent in space
  (heritage)

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Space Frames

• Features
• Loads carried axially (ideally)
• Joints/nodes carry some moments (not space
  truss)
• Deflections scale linearly with length
• d PL/AE loads carried in tension/comp. (SF)
• Versus
• d PL/nAG loads carried in shear (monocoque)
• d PL3/nEI loads carried in bending
• Fast and easy to model with FEA
• Facilitate test and integration
• Space frames are ideal for supporting discrete
  loads
• Space frames make poor fuel tanks and fuselages

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Space Frames for SNAP

• Status of space frames for SNAP
• (PPT presentations in BSCW PS1300/Weekly)
• Space frame spectrograph mount 05/14/04
• Athermal (constant length) strut
  designs 06/04/04
• Det. space frame designs for TMA-63 07/29/04
• Indet. Space frame designs for TMA-65 08/26/04
• Node/joint design concepts 09/02/04
• Survey of space heritage structures 09/02/04
• Minimum obscuration SMA structure 09/16/04
• TMA 65, fold mirror, and lower baffle 10/28/04

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Spectrograph mount

• Design features
• Hexapod space frame to carry 10Kg spectrograph
• 21 hexapod geometry gt horizontal deflections,
  no tilts
• Attaches to common focal plane mounting points
• Essentially no loads carried by focal plane
  assembly
• Simple interface to spectrograph
• 3 discrete support points, or round flange
• Supports spectrograph load near center of mass
• Minimizes moment loads
• Simple interface to FP (mount points,
  cylindrical volumes)
• Spectrograph and mount easily separate from FPA
• Invar, CF, or athermal struts
• Simple control of spectrograph thermal defocus

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FEA Model

• SNAP Baseline design
• Moly, Invar, Ti flexures
• Attaches to FPA baseplate
• Loads carried near detect.
• Natural frequencies for spectrograph, mount, and
  flexures 116, 121, 164 Hz.
• Mass ?

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Spectrograph mounting structure
Ease of access to detector connections
FP assembly with spectrograph included (note
redundant str.)
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Dynamic FEA

• First 6 freq
• 413 Hz
• 415 Hz
• 416 Hz
• 470 Hz
• 478 Hz
• 490 Hz

f1 413 Hz, transverse mode, 25 x 2 mm Invar
struts, 2.5 Kg, f1 675 Hz for carbon fiber
(MJ55), 25 x 2 mm struts, 0.5Kg
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Athermal Struts

• Design features
• Thermally compensated or controlled length
  struts
• 3 materials to provide varying
  expansion/contraction
• Avoid high stresses due to CTE mismatches
• Provide integral flexures for kinematic
  constraints
• Provide features for length adjustments
  (alignment)
• Application details required for FEA

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Athermal Struts
Blue Ti CP Grade 1 --- 17 PPM/K Light Grey
Invar --- 1.26 PPM/K Dark Grey Ti 6Al 4V ---
6.7 PPM/K
L1 156mm, L2 78mm, L3 222mm(x2), 600mm long
strut
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Athermal Struts
21 truss geometry on focal plane assy, 600mm
long struts
EDM cross-flexure
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OTA Space Frames

• Motivations
• Minimize telescope structure deflections under
  gravity
• Maximize resonant frequencies on ground and in
  orbit
• Minimize structure mass, CF outgassing, etc.
• Maximum access to optical elements (assembly,
  test)
• Explore parameter space for SNAP structure

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OTA Space Frames TMA 63

• Design objectives
• 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 nodes and 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 carbon fiber composite used for all
  struts
• CF can be optimized for cross section, thermal
  expansion

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OTA Space Frames TMA 63
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TMA-63 structure FEA
Elements
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Dynamic FEA

• Dynamic analyses
• Telescope mass 360kg payload, 96kg structures
• 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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Nodes for space frames

• Design features
• Nodes connect the struts in a space frame
• Accommodate diameters of struts (constant
  diameter, wall)
• Minimize mass (often a large fraction of the
  mass in a SF)
• Maximize ease of fabrication and assembly
• Provide attachment points for secondary
  structures

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Nodes for space frames

• Molded node, 22mm x 2mm tubes, V 13103 mm3
• Invar 0.1 Kg, Ti 0.06 Kg, CC 0.02 Kg

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Nodes for space frames

• Machined node, 22mm OD tubes, V 58561 mm3
• Invar .47 Kg, Ti 0.26 Kg, CC 0.09 Kg

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Secondary Mirror Structure

• Design features
• Minimize pupil obscuration by SMA structures
• Minimize structure mass
• Maintain high first resonance
• Secondary support vanes
• 25 mm diameter x 2 mm wall
• Requires revisions to current outer baffle
  design

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Secondary Structure
Blue/green hexapod struts are outside of CA
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Secondary Structure
Trial 9, ring at 2.85m elev.
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Space frame developments

• Latest work
• TMA 65 structure with nodes
• Fold mirror sub-frame
• Lower baffle structure (Al) and close-outs
• Rings have 50 x 50 x 3 mm sections
• Struts have 50 x 50 x 6mm sections
• Upper baffle mass 190 Kg
• Baffle structure (38 Kg) close-outs (27 Kg)
  65 Kg
• f1 33 Hz
• CF baffle structure 20Kg, 40Hz

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TMA-65 structure with nodes
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Fold mirror sub-structure
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Lower baffle structure
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Lower baffle, structure clearance
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Deformation in 1g held by GSE(baffle
displacement2.6mm)
Baseline mass 79 Kg
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Lower baffle structure
mass 65 Kg
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Baffle/OTA Assembly Mode 1, 20Hz
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Lower baffle structure
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Space frames for SNAP

• Conclusions
• Space frames are applicable to most SNAP
  structures
• Space frame structures offer significant mass
  reductions over current baseline designs
• Space frame structures provide higher
  frequencies/mass compared to baseline designs
• Space craft structure heritage is well
  established
• Space frame structures will readily scale to
  larger apertures
• Space frames for SNAP Ready for prime time!