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Finite Element Analysis and Testing Correlation of the Mercury Laser Altimeter Craig L. Stevens Craig.Stevens@nasa.gov Mechanical Systems Analysis & Simulation Branch NASA Goddard Space Flight Center Greenbelt, Maryland Craig L. Stevens Craig.Stevens@nasa.gov Mechanical Systems Analysis & Simulation Branch NASA Goddard Space Flight Center Greenbelt, Maryland PIP Level 2 Presentation June 11, 2003 PIP Level 2 Presentation June 11, 2003

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Overview 1.Introduction 2.Mission Overview 3.Engineering Model (EM) Finite Element Analysis (FEA) Vibration Test Test and Analysis Correlation 4.Flight Model (FM) Finite Element Analysis Vibration Test 5.Conclusions 6.Accomplishments

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Introduction MLA Assigned Tasks Update flight model (FM) finite element model (FEM) Perform optical component sag and alignment analyses Create engineering model (EM) FEM –Perform frequency and random response analyses using MSC NASTRAN –Participate in EM workmanship test Correlate EM FEM with test data Recreate FM FEM using correlated EM FEM –Perform frequency and random response analyses using MSC NASTRAN –Generate notching criteria to ensure survivability in protoflight random vibration test environment Participate in FM MLA instrument protoflight vibration test

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Mercury Laser Altimeter (MLA) on Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft MLA produces accurate measurements of topography and measures Mercury's wobble (due to the planet's libration) MESSENGER Objectives: –Determine the structure of Mercury’s mantle and crust –Investigate Mercury's polar caps –Determine the state of Mercury’s core (fluid or solid?) MESSENGER developed by Johns Hopkins University Applied Physics Laboratory (APL) MLA developed by NASA Goddard Space Flight Center (Deliver to APL June 2003) Launch –March or May 2004 –Delta II 2925H-9-5 Mission Overview

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MESSENGER Configuration Plasma Spectrometer Neutron Spectrometer Gamma-Ray Spectrometer X-Ray Spectrometer Dual Imager on Pivot Platform Atmospheric and Surface Spectrometer Energetic Particles Spectrometer MLA

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Mission Overview Receiver tubes [4] (Be) Laser beam expander (Be) Laser bench (Be) Main housing (Be) Mounting flexures [3] (Ti) Aft optics (Be) Power Converter Assy. Housing (Mg) Y Z X MLA Configuration

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Engineering Model FEA EM manufactured to verify optical and mechanical designs EM constructed using aluminum to minimize time and cost Performed vibration test to subject flight- like optical design to dynamic environment and provide data for model correlation EM FEM created using existing FM FEM – EM titanium flexure dimensions based on preliminary analysis – No electronics – No PCA Normal modes analysis using MSC NASTRAN – Lateral modes (1 & 2) shown to impart highest loads

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Engineering Model FEA Mode 1: 57 Hz (High mass participation in Y-direction) X Y Z

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Engineering Model FEA Mode 2: 66 Hz (High mass participation in X-direction) X Y Z

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Aluminum engineering model tested at GEVS minimum workmanship Accelerometers placed at key locations Results show FEM exhibits 30% frequency error EM Test

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EM Correlation Chose to correlate lateral modes 1 & 2 –High mass participation –Low frequencies (Z-mode @ 443 Hz) Signature sine sweeps used for correlation Several modifications made to correlate model: –Damping estimated using half-power method (Quality Factor, Q = 40, 67 instead of ~25) 10 2 10 3 10 -3 10 -2 10 -1 10 0 10 1 Beam Expander Top Lens Sine Response Input: Y-Axis Response: Y-Axis Frequency, Hz Peak Acceleration, G Test Analysis

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EM Correlation Several modifications made to correlate model (cont’): –Alter housing geometry to more accurately represent load paths –Add solid elements to represent bosses –Offset plate elements Uncorrelated Correlated

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EM Correlation Several modifications made to correlate model (cont’): –Stiffen joints in assembly –Modify analog cover FEM –Change mass distribution based on weighed values –Correlate flexure bend test data with flexure FEM –Massless bars added at orthonormal corners of plate element arrays UncorrelatedCorrelated

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EM Correlation – Massless bars added at orthonormal corners of plate element arrays (cont’) Learned technique in MSC NASTRAN training x1x1 uz1uz1 Quadratic Linear Plate 2 provides no stiffness in normal (Z 2 ) direction, therefore, corner bending stiffness is deficient Plate 1 Plate 2 uz1uz1 ux2ux2 ux2ux2 Y1Y1 Z1Z1 X1X1 Y2Y2 X2X2 Z2Z2 x1x1 Node A Node B

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10 2 10 3 10 -3 10 -2 10 -1 10 0 10 1 Beam Expander Top Lens FRF Input: Y-Axis Response: Y-Axis Frequency, Hz H(f) Test Analysis EM Correlation CorrelatedUncorrelated 10 2 10 3 10 -3 10 -2 10 -1 10 0 10 1 Beam Expander Top Lens Sine Response Input: Y-Axis Response: Y-Axis Frequency, Hz Peak Acceleration, G Test Analysis Results correlate to approximately 0.2% error for first mode and 2% error for second mode

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Flight Model FEA Flight Model FEM recreated using correlated EM FEM EM FEM modified –Beryllium instrument –Titanium flexures dimensions –Electronics boards –Laser components Normal modes analysis performed using MSC NASTRAN Results show 40% increase in Y-axis mode frequency and 28% increase in X-axis mode frequency

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Flight Model FEA Mode 1: 124 HzMode 2: 142 Hz

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FM Test Flight model tested at APL required protoflight levels Accelerometers placed at similar locations Results exhibit an error of less than 5%

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FM Test Results X-axis frequency response function correlation Mode 1

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FM Test Results Y-axis frequency response function correlation Mode 2

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FM Test Results Mode 1 Shape Comparison FEM: 124 Hz Test: 119 Hz

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FM Test Results Mode 2 Shape Comparison FEM: 142 Hz Test: 138 Hz

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Conclusions Modeling bosses and fillets was critical for accurately representing the MLA housing stiffness Plate element orthagonal corners exhibited insufficient stiffness for the MLA model which significantly lowered the overall FEM stiffness Application of beryllium significantly increased lateral modal frequencies of MLA structure Vibration testing and FEM correlation of aluminum EM proved to be an effective and time efficient means of correlating the beryllium FM

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Accomplishments MLA Accomplishments Created and correlated engineering model (EM) FEM –Performed frequency response and random vibration analyses using MSC NASTRAN –Participated in EM workmanship test Recreated flight model FEM using correlated EM FEM –Performed frequency response and random vibration analyses using MSC NASTRAN –Generated notching criteria to ensure survivability in protoflight random environment Participated in MLA instrument protoflight vibration test Analyzed optical issues –Sag test fixturing –Alignment error due to integration

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Accomplishments Professional Accomplishments Gained a working knowledge of optical systems Participated in protoflight vibration test Learned FEMAP and NASTRAN Completed random vibration course at UMD Generated Individual Development Plan Participated in FEMCI workshop coordination Participated in electrostatic discharge certification class

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Ryan Simmons MLA Team Jeff Bolognese Daniel Kaufman Scott Gordon Jim Loughlin Debbie Wheeler Code 542 Dan Worth Don BakerAcknowledgements

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Acronym List

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Z EM Correlation Backup Slides

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