System Identification of a Nanosatellite Structure Craig L. Stevens, Jana L. Schwartz, and Christopher D. Hall Aerospace and Ocean Engineering Virginia Tech Blacksburg, Virginia 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 Session 7, Earth and Lunar Missions AAS/AIAA Astrodynamics Conference Quebec City, Canada July 30 – August

Overview 1.Introduction 2.Design 3.Analysis 4.Fabrication 5.Testing 6.Conclusions

NASA Shuttle Hitchhiker Experiment Launch System (SHELS) AFRL Multi- Satellite Deployment System (MSDS) University Nanosatellites 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

3CS ION-F USUSat Dawgstar HokieSat Multiple Satellite Deployment System Mission Configuration: Scenario:

Isogrid Structure Aluminum 6061 T-651 Composite Side Panels –0.23” isogrid –0.02” skins Isogrid Structure Aluminum 6061 T-651 Composite Side Panels –0.23” isogrid –0.02” skins HokieSat 18.25” major diameter hexagonal prism 12” tall 39 lbs (~18 kg) Design

Data Port Crosslink Antenna Uplink Antenna Downlink Antenna Science Patches LightBand GPS Antenna Pulsed Plasma Thrusters Solar Cells Camera External ConfigurationDesign

Torque Coils (3) Rate Gyros (3) Downlink Transmitter Cameras Camera Electronics Enclosure Battery Enclosure Magnetometer Camera Power Processing Unit Crosslink Components Internal ConfigurationDesign Pulsed Plasma Thrusters (2)

 Requirement: Withstand ±11.0 g accelerations (all directions)  Margin of Safety  0, where  Factor of Safety (FS)  Finite Element Analysis Results Static Analysis

Mode 1 f n = 131 Hz Dynamic Analysis Mode 2 f n = 171 Hz Finite Element Analysis of Isogrid Side Panel (Without Skin)

Dynamic Analysis Mode 1 f n = 249 Hz Finite Element Analysis of Complete Isogrid Structure (Without Skin)

Dynamic Analysis Mode 2 f n = 263 Hz Finite Element Analysis of Complete Isogrid Structure (Without Skin)

 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 Dynamic Analysis Finite Element Analysis of Complete ION-F Stack

Fabrication Composite structure comprised of 0.23” isogrid and 0.02” skin

 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 Test Requirements  Random Vibe Requirements:

Strength & stiffness test of structure without skin panels Strength & stiffness test of loading fixture Static Testing

Strength & stiffness test of structure with skin panels Static Testing 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

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

Mode 1 f n = 131 Hz (vs 131 Hz predicted) Mode 2 f n = 169 Hz (vs 171 Hz predicted) Dynamic Testing Modal Testing of Side Panels (Without Skin)

Dynamic Testing Mode 1 f n = 213 Hz (vs 131 Hz without skin) Mode 2 f n = 453 Hz (vs 169 Hz without skin) Modal Testing of Side Panels (With Skin)

Modal Testing of Structure (Without Skins) Dynamic Testing Mode 1 f n = 245 Hz (vs 249 Hz predicted) Mode 2 f n = 272 Hz (vs 263 Hz predicted)

1. X -axis control 2. Y -axis control 3. Z -axis control 4.Side panel 1 5.Side panel 2 6.Zenith panel 7.GPS (3 axis) 8.CPU (3 axis) 9.PPU (3 axis) 10.Battery box (3 axis) Accelerometer Placement X Y Z Dynamic Testing Structure survived all tests Determined component locations to raise natural frequencies

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

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