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System Identification of a Nanosatellite Structure
Craig L. Stevens, Jana L. Schwartz, and Christopher D. Hall Aerospace and Ocean Engineering Virginia Tech Blacksburg, Virginia Session 7, Earth and Lunar Missions AAS/AIAA Astrodynamics Conference Quebec City, Canada July 30 – August
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Overview 3 2 Introduction Design Analysis Fabrication Testing
Conclusions 4 5
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AFRL Multi-Satellite Deployment NASA Shuttle Hitchhiker
Introduction Virginia Tech Ionospheric Scintillation Measurement Mission (VTISMM) aka HokieSat Ionospheric Observation Nanosatellite Formation (ION-F) Utah State University University of Washington Virginia Tech University Nanosatellite Program 2 stacks of 3 satellites Sponsors: AFRL, AFOSR, DARPA, NASA GSFC, SDL University Nanosatellites AFRL Multi-Satellite Deployment System (MSDS) NASA Shuttle Hitchhiker Experiment Launch System (SHELS)
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Multiple Satellite Deployment System
Mission Configuration: 3CS ION-F USUSat Dawgstar HokieSat Multiple Satellite Deployment System Scenario:
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Design HokieSat 18.25” major diameter hexagonal prism 12” tall
39 lbs (~18 kg) Isogrid Structure Aluminum 6061 T-651 Composite Side Panels 0.23” isogrid 0.02” skins
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Design External Configuration Crosslink Antenna Solar Cells
GPS Antenna Crosslink Antenna Solar Cells LightBand Pulsed Plasma Thrusters Data Port Camera Downlink Antenna Uplink Antenna Science Patches
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Design Internal Configuration Crosslink Components Cameras Power
Processing Unit Torque Coils (3) Magnetometer Camera Pulsed Plasma Thrusters (2) Camera Battery Enclosure Downlink Transmitter Electronics Enclosure Rate Gyros (3)
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Static Analysis Requirement: Withstand ±11.0 g accelerations (all directions) Margin of Safety 0, where Factor of Safety (FS) Finite Element Analysis Results
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Dynamic Analysis Finite Element Analysis of Isogrid Side Panel (Without Skin) Mode 1 fn = 131 Hz Mode 2 fn = 171 Hz
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Finite Element Analysis of Complete Isogrid Structure (Without Skin)
Dynamic Analysis Finite Element Analysis of Complete Isogrid Structure (Without Skin) Mode 1 fn = 249 Hz
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Finite Element Analysis of Complete Isogrid Structure (Without Skin)
Dynamic Analysis Finite Element Analysis of Complete Isogrid Structure (Without Skin) Mode 2 fn = 263 Hz
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Finite Element Analysis of Complete ION-F Stack
Dynamic Analysis Finite Element Analysis of Complete ION-F Stack Requirement: First mode natural frequency: >100 Hz Results: First mode natural frequency: 74.6 Hz Solution: Stiffen joints around attachment points to raise first mode natural frequency ~100Hz
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Fabrication Composite structure comprised of 0.23” isogrid and 0.02” skin
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Test Requirements Static test
Stiffness test to simulate expected loading conditions during launch Sine sweep test Vibration test to determine free and fixed-base natural frequency Sine burst test Vibration test to verify structural strength at extreme loads Random vibration test Vibration test to verify structural integrity Random Vibe Requirements:
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Static Testing Strength & stiffness test of structure without skin panels Strength & stiffness test of loading fixture
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Static Testing Strength & stiffness test of structure with skin panels
Experiment demonstrated a 32% gain in stiffness in the cantilever mode due to addition of skins Skins added less than 8% to the total mass
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Modal (tap) Testing of Side Panels
Dynamic Testing Modal (tap) Testing of Side Panels Hammer provides impulsive input Accelerometer measures accelerations used to characterize natural frequencies Tap testing with and without skins Verification of predictions of finite element analysis
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Dynamic Testing Modal Testing of Side Panels (Without Skin) Mode 1
fn = 131 Hz (vs 131 Hz predicted) Mode 2 fn = 169 Hz (vs 171 Hz predicted)
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Dynamic Testing Modal Testing of Side Panels (With Skin) Mode 1
fn = 213 Hz (vs 131 Hz without skin) Mode 2 fn = 453 Hz (vs 169 Hz without skin)
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Dynamic Testing Modal Testing of Structure (Without Skins) Mode 1
fn = 245 Hz (vs 249 Hz predicted) Mode 2 fn = 272 Hz (vs 263 Hz predicted)
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Dynamic Testing Accelerometer Placement X-axis control Y-axis control
Z Accelerometer Placement X-axis control Y-axis control Z-axis control Side panel 1 Side panel 2 Zenith panel GPS (3 axis) CPU (3 axis) PPU (3 axis) Battery box (3 axis) Structure survived all tests Determined component locations to raise natural frequencies
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Conclusions Aluminum isogrid increases structural performance at reduced mass Modal testing verifies accuracy of isogrid side panel finite element model within ~1% error Modal testing demonstrates 26% increase in structural stiffness of side panel by adding thin aluminum skins Analyses and experiments verify structure satisfies all Shuttle payload requirements
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Acknowledgements Air Force Research Laboratory
Air Force Office of Scientific Research Defense Advanced Research Projects Agency NASA Goddard Space Flight Center NASA Wallops Flight Facility Test Center University of Washington Utah State University Virginia Tech Professor A. Wicks Professor B. Love Members of ION-F
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