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Hanson Chang MSC.Software Corporation

Rapid Design Iteration Process for Spacecraft Kinematic Mounts Using Automatic Tet Meshing and Global/Local Modeling Techniques. Hanson Chang MSC.Software Corporation. Acknowledgements. Co-author: Chris Luanglat, TRW Stress analyst. Presentation Outline.

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Hanson Chang MSC.Software Corporation

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  1. Rapid Design Iteration Process for Spacecraft Kinematic Mounts UsingAutomatic Tet Meshing and Global/Local Modeling Techniques Hanson Chang MSC.Software Corporation

  2. Acknowledgements • Co-author: Chris Luanglat, TRW Stress analyst

  3. Presentation Outline • Spacecraft program and kinematic mounts • Design challenges for kinematic mounts • Rapid design iteration process • Direct import of CAD solid geometry • Automatic tet meshing with efficient mesh control and convergence techniques • Global/local modeling techniques • Conclusions

  4. Spacecraft Program Overview • EOS Spacecraft Aqua and Aura • Mission: To study the Earth and its changing environment by observing the atmosphere, oceans, and land surface. • Launch dates: Aqua - 4/2002 Aura - 1/2004

  5. Spacecraft Overview • Spacecraft Spec. • Dimensions: 22 ft x 9 ft x 8 ft • Weight: 6,500 lbs • All-composite spacecraft structures

  6. Spacecraft FEM – View 1

  7. Spacecraft FEM – View 2

  8. FEM – Exploded View

  9. Instrument Spacecraft M M Load Sharing During Launch

  10. DT Instrument Spacecraft Load Sharing On Orbit

  11. Load Isolation Concepts • Statically determinant interface (6 DOF) isolates the instruments from the primary structure load path • This type of structural interface is called a Kinematic Interface • The attachment fittings used in this type of structural interface are called Kinematic Mounts

  12. Releasing a Degree of Freedom • Sliding Design: • Ball/socket, cup/cone, pin/slot, V block/groove, etc. • Relies on low and predictable friction • Flexure Design: • Uses flexibility to isolate loads • Selected for TRW kinematic mount design

  13. Stiff Direction Flexible Direction Notched Column One-Axis Kinematic Mount (KM1)

  14. Flexible Direction Stiff Directions Two-Axis Kinematic Mount (KM2)

  15. Three-Axis Kinematic Mount (KM3)

  16. Typical KM Arrangement KM1 KM3 KM2

  17. Practical KM2 Stiffness Matrix T1 T1 T2 T2 T3 T3 R1 R1 R2 R2 R3 R3 Typical Stiffness Matrix Ideal KM2 Stiffness Matrix

  18. Strength/Fatigue Solution Space Size Stiffness Traditional Solution Space

  19. Strength Solution Space Flexibility Stability Size Stiffness Fracture/Fatigue Kinematic Mount Solution Space

  20. CAD PRE/POST PROCESSOR MSC.Nastran Design Iteration Process Strength SOL 101 Flexibility SOL 101 Stiffness SOL 103 Stability SOL 105 Fracture SOL101/FLAGRO

  21. Rapid Design Iteration Process • Speeding up the iteration process • Direct import of CAD solid geometry • Automatic tet meshing with efficient mesh control and convergence techniques • Global/local modeling techniques

  22. Traditional Method 1 CATIA Solid Geometry SDRC I-DEAS IGES File Traditional Method 2 CATIA Solid Geometry SDRC I-DEAS Drawing Geometry Import - Old Process • Clean up surfaces (slivers, tee, etc.) • Create B-rep solid from surfaces • Create solid geometry based on drawing

  23. Geometry Import - New Process CATIA Direct CATIA Solid Geometry MSC.Patran • Solid geometry directly imported into MSC.Patran as solid geometry with high success rate (95+%) • Sliver surfaces and short edges (dirty geometry) are best correct in the CAD package • Conferences held between designers and analysts to discuss how to identify and eliminate problem geometry

  24. Meshing - Old Process • Hex element (8-node brick) is the preferred element • Created by manual meshing • Created by meshing 5 or 6-sided solids (simple solids) or sweeping 2D elements • Typical part must be broken into simple solids first

  25. Meshing - Old Process (cont.) Notched Regions • Hex meshing of above parts is labor intensive • Meshing time for typical KM is several days • Not acceptable the multiple design iteration environment

  26. Meshing - New Process • Automatic tet meshing using TET10 elements • Can mesh arbitrarily-shaped solids • Meshing time for typical KM is 4 hours • Ideal for the multiple design iteration environment

  27. Meshing - New Process (cont.) • Advantage of Tet Meshing • Fast • Quality of TET10 elements (linear strain) is compatible to HEX8 elements • Disadvantage of Tet Meshing • Larger model

  28. Efficient Tet Meshing • Key to efficient tet meshing is mesh density control • Hitting the automatic tet mesh button without any mesh control typically results in excessively large modes • Correct density control puts a lot of elements in the area of interest and coarsens quickly away from this area

  29. Efficient Tet Meshing (cont.) • Typical density control techniques • Surface mesh selected solid faces with TRIA6 first to guide subsequent tet meshing • Curvature-based meshing • Break the part into multiple solids – cookie cutter method

  30. Cookie Cutter Method • Break the solid with planes or surfaces • Critical solid meshed first with a fine mesh • Sounding solids meshed with a coarse mesh

  31. Cookie Cutter Method (cont.)

  32. Cookie Cutter Method (cont.)

  33. How to Achieve Convergence • 4 elements thru the thickness? • 8 elements thru the thickness? • Multi-pass convergence is time consuming • Single-pass convergence is fast but more subjective • Fringe plot with the “difference” option in MSC.Patran • Plots the stress jumps (discontinuities)

  34. How to Achieve Convergence • Use a combination of both methods • For each type of notch geometry (circular, square, rectangular, etc.), a multi-pass convergence test is performed to establish the required number of elements thru the thickness • Each new part is then meshed using this rule of thumb and verified using the single-pass convergence test

  35. X 20 Spacecraft Model 250,000 DOF Kinematic Mount Models 100,000 to 750,000 DOF Each Integrating the Models • Resulting model is unacceptably large

  36. RBE2 18 x 18 stiffness matrix Global-Local Modeling • Use Static Reduction (Guyan Reduction) to reduce tet10 model to small stiffness matrix • Use ASET entry to specify boundary DOF • PARAM,EXTOUT,DMIGPCH to create DMIG entries • Use K2GG entry to assemble the KM matrices into Spacecraft model

  37. Coupled with launch vehicle model to perform Coupled loads Analysis KM boundary node displacements Global-Local Modeling (cont.)

  38. Conclusions • Rapid design iteration process • Direct import of CAD solid geometry • Automatic tet meshing with efficient mesh control and convergence techniques • Global/local modeling techniques • This process resulted in substantial cycle time reduction for the Aqua and Aura kinematic mounts

  39. Conclusions (cont.) • The notched-column kinematic mount design configurations have been incorporated into the TRW Deployables Handbook Merci beaucoup

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