Space Frame Structures for SNAP Bruce C. Bigelow University of Michigan Department of Physics 11/04/04

2 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 H 2 O dry-out issues  Open structures provide maximum access to payloads  Space frame structures are prevalent in space (heritage)

3 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/nAGloads carried in shear (monocoque)  d = PL^3/nEIloads 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…

4 Space Frames for SNAP Status of space frames for SNAP (PPT presentations in BSCW PS1300/Weekly):  Space frame spectrograph mount05/14/04  Athermal (constant length) strut designs06/04/04  Det. space frame designs for TMA-6307/29/04  Indet. Space frame designs for TMA-6508/26/04  Node/joint design concepts09/02/04  Survey of space heritage structures09/02/04  Minimum obscuration SMA structure09/16/04  TMA 65, fold mirror, and lower baffle10/28/04

5 Spectrograph 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

6 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: ?

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

8 Dynamic 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: Hz Hz Hz Hz Hz Hz

9 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

10 Athermal Struts Blue = Ti CP Grade PPM/K Light Grey = Invar PPM/K Dark Grey = Ti 6Al 4V PPM/K L1 = 156mm, L2 = 78mm, L3 =222mm(x2), 600mm long strut

11 Athermal Struts EDM cross-flexure 2:1 truss geometry on focal plane assy, 600mm long struts

12 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

13 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

14 OTA Space Frames – TMA 63

15 TMA-63 structure FEA Elements

16 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

17 Static FEA First ground mode, 72 Hz

18 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

19 Nodes for space frames Molded node, 22mm x 2mm tubes, V = mm^3 Invar = 0.1 Kg, Ti = 0.06 Kg, CC = 0.02 Kg

20 Nodes for space frames Machined node, 22mm OD tubes, V = mm^3 Invar =.47 Kg, Ti = 0.26 Kg, CC = 0.09 Kg

21 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

22 Secondary Structure Blue/green hexapod struts are outside of CA

23 Secondary Structure Trial 9, ring at 2.85m elev.

24 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

25 TMA-65 structure with nodes

26 Fold mirror sub-structure

27 Lower baffle structure

28 Lower baffle, structure clearance

29 Deformation in 1g held by GSE (baffle displacement~2.6mm) Baseline: mass = 79 Kg

30 Lower baffle structure mass = 65 Kg

31 Baffle/OTA Assembly Mode 1, 20Hz

32 Lower baffle structure

33 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!