Space Frame Structures for SNAPSpace Frame Structures for SNAP

Bruce C. Bigelow

University of Michigan

Department of Physics

11/04/04

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Space Frames for SNAPSpace 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 FramesSpace 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 = PL^3/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 SNAPSpace 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 mountSpectrograph mount

Design features: Hexapod space frame to carry 10Kg spectrograph 2:1 hexapod geometry => 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 ModelFEA 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 structureSpectrograph mounting structure

Ease of access to detector connectionsFP assembly with spectrograph included (note redundant str.)

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Dynamic FEADynamic FEA

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

First 6 freq:

1. 413 Hz

2. 415 Hz

3. 416 Hz

4. 470 Hz

5. 478 Hz

6. 490 Hz

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Athermal StrutsAthermal 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 StrutsAthermal 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 StrutsAthermal Struts

EDM cross-flexure2:1 truss geometry on focal plane assy, 600mm long struts

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OTA Space FramesOTA 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 63OTA 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 63OTA Space Frames – TMA 63

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TMA-63 structure FEATMA-63 structure FEA

Elements

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Dynamic FEADynamic 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 FEAStatic FEA

First ground mode, 72 Hz

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Nodes for space framesNodes 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 framesNodes for space frames

• Molded node, 22mm x 2mm tubes, V = 13103 mm^3

• Invar = 0.1 Kg, Ti = 0.06 Kg, CC = 0.02 Kg

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Nodes for space framesNodes for space frames

• Machined node, 22mm OD tubes, V = 58561 mm^3

• Invar = .47 Kg, Ti = 0.26 Kg, CC = 0.09 Kg

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Secondary Mirror StructureSecondary 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 StructureSecondary Structure

Blue/green hexapod struts are outside of CA

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Secondary StructureSecondary Structure

Trial 9, ring at 2.85m elev.

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Space frame developmentsSpace 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 nodesTMA-65 structure with nodes

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Fold mirror sub-structureFold mirror sub-structure

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Lower baffle structureLower baffle structure

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Lower baffle, structure clearanceLower baffle, structure clearance

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Deformation in 1g held by GSEDeformation in 1g held by GSE(baffle displacement~2.6mm)(baffle displacement~2.6mm)

Baseline: mass = 79 Kg

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Lower baffle structureLower baffle structure

mass = 65 Kg

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Baffle/OTA Assembly Mode 1, 20HzBaffle/OTA Assembly Mode 1, 20Hz

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Lower baffle structureLower baffle structure

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Space frames for SNAPSpace 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!