Transcript Presentation Title Arial 36

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
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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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Load Sharing During Launch
Instrument
M
Spacecraft
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M
Load Sharing On Orbit
Instrument
Spacecraft
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DT
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)
Stiff Direction
Flexible Direction
Notched
Column
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Two-Axis Kinematic Mount (KM2)
Flexible
Direction
Stiff Directions
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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
Practical KM2
Stiffness Matrix
R3
T1
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T2
T3
R1
R2
R3
Traditional Solution Space
Strength/Fatigue
Size
Solution
Space
Stiffness
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Kinematic Mount Solution Space
Strength
Solution
Space
Flexibility
Stability
Size
Stiffness
Fracture/Fatigue
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Design Iteration Process
CAD
PRE/POST
PROCESSOR
MSC.Nastran
Strength SOL 101
Flexibility SOL 101
Stiffness SOL 103
Stability SOL 105
Fracture SOL101/FLAGRO
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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
Traditional Method 1
CATIA Solid
Geometry
IGES File
SDRC I-DEAS
• Clean up surfaces
(slivers, tee, etc.)
• Create B-rep solid from
surfaces
Traditional Method 2
CATIA Solid
Geometry
Drawing
SDRC I-DEAS
• Create solid geometry
based on drawing
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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
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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.)
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
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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
X 20
Spacecraft Model
250,000 DOF
Kinematic Mount Models
100,000 to 750,000 DOF
Each
• Resulting model is unacceptably large
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Global-Local Modeling
RBE2
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
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