Hanson Chang MSC.Software Corporation

Rapid Design Iteration Process for Spacecraft Kinematic Mounts

UsingAutomatic Tet Meshing and Global/Local

Modeling Techniques

Hanson ChangMSC.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

Spacecraft

Instrument

Load Sharing During Launch

M M

10

Load Sharing On Orbit

Spacecraft

Instrument

T

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

One-Axis Kinematic Mount (KM1)

Notched Column

Stiff Direction

Flexible Direction

14

Two-Axis Kinematic Mount (KM2)

Stiff DirectionsFlexible Direction

15

Three-Axis Kinematic Mount (KM3)

16

Typical KM Arrangement

KM1

KM3

KM2

17

Typical Stiffness Matrix

Ideal KM2 Stiffness Matrix

T1 T2 T3 R1 R2 R3

Practical KM2 Stiffness Matrix

T1 T2 T3 R1 R2 R3

18

Traditional Solution Space

Strength/Fatigue

StiffnessSize

Solution Space

19

Kinematic Mount Solution Space

Strength

StiffnessSize

FlexibilityStability

Fracture/Fatigue

Solution Space

20

CAD

Design Iteration Process

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

MSC.Nastran

PRE/POST PROCESSOR

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

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

23

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

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.)

• 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

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

Integrating the Models

Spacecraft Model250,000 DOF

X 20

Kinematic Mount Models100,000 to 750,000 DOF

Each

• Resulting model is unacceptably large

36

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

37

Global-Local Modeling (cont.)

Coupled with launch vehicle model to perform Coupled loads Analysis

KM boundary node displacements

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