Rapid Design Iteration Process for Spacecraft Kinematic Mounts

UsingAutomatic Tet Meshing and Global/Local

Modeling Techniques

Hanson ChangMSC.Software Corporation

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Acknowledgements

• Co-author: Chris Luanglat, TRW Stress analyst

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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

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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

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Spacecraft Overview

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

structures

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Spacecraft FEM – View 1

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Spacecraft FEM – View 2

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FEM – Exploded View

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Spacecraft

Instrument

Load Sharing During Launch

M M

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Load Sharing On Orbit

Spacecraft

Instrument

T

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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

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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

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One-Axis Kinematic Mount (KM1)

Notched Column

Stiff Direction

Flexible Direction

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Two-Axis Kinematic Mount (KM2)

Stiff DirectionsFlexible Direction

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Three-Axis Kinematic Mount (KM3)

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Typical KM Arrangement

KM1

KM3

KM2

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Typical Stiffness Matrix

Ideal KM2 Stiffness Matrix

T1 T2 T3 R1 R2 R3

Practical KM2 Stiffness Matrix

T1 T2 T3 R1 R2 R3

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Traditional Solution Space

Strength/Fatigue

StiffnessSize

Solution Space

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Kinematic Mount Solution Space

Strength

StiffnessSize

FlexibilityStability

Fracture/Fatigue

Solution Space

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CAD

Design Iteration Process

Strength SOL 101 Flexibility SOL 101 Stiffness SOL 103 Stability SOL 105 Fracture SOL101/FLAGRO

MSC.Nastran

PRE/POST PROCESSOR

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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

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Geometry Import - Old Process

• Clean up surfaces (slivers, tee, etc.)

• Create B-rep solid from surfaces

CATIA Solid Geometry

SDRC I-DEASIGES File

Traditional Method 1

CATIA Solid Geometry

SDRC I-DEASDrawing

Traditional Method 2

• Create solid geometry based on drawing

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Geometry Import - New Process

CATIA Solid Geometry

MSC.Patran

CATIA Direct

• 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

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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

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Meshing - Old Process (cont.)

• Hex meshing of above parts is labor intensive• Meshing time for typical KM is several days• Not acceptable the multiple design iteration

environment

Notched Regions

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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

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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

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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

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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

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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

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Cookie Cutter Method (cont.)

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Cookie Cutter Method (cont.)

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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)

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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

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Integrating the Models

Spacecraft Model250,000 DOF

X 20

Kinematic Mount Models100,000 to 750,000 DOF

Each

• Resulting model is unacceptably large

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Global-Local ModelingRBE2

18 x 18

stiffness matrix

• 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

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Global-Local Modeling (cont.)

Coupled with launch vehicle model to perform Coupled loads Analysis

KM boundary node displacements

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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

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Conclusions (cont.)

• The notched-column kinematic mount design configurations have been incorporated into the TRW Deployables Handbook

Merci beaucoup