System Identification of a Nanosatellite Structure

1
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 2 2001
2
Overview
3
2

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

4
5
3
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)
4
Mission
Configuration
Multiple Satellite Deployment System
Scenario
5
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

6
Design
External Configuration
Solar Cells
Crosslink Antenna
GPS Antenna
LightBand
Pulsed Plasma Thrusters
Data Port
Camera
Uplink Antenna
Downlink Antenna
Science Patches
7
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)
8
Static Analysis

• Requirement Withstand 11.0 g accelerations
  (all directions)
• Margin of Safety ? 0, where
• Factor of Safety (FS)
• Finite Element Analysis Results

9
Dynamic Analysis
Finite Element Analysis of Isogrid Side Panel
(Without Skin)
Mode 1 fn 131 Hz
Mode 2 fn 171 Hz
10
Dynamic Analysis
Finite Element Analysis of Complete Isogrid
Structure (Without Skin)
Mode 1 fn 249 Hz
11
Dynamic Analysis
Finite Element Analysis of Complete Isogrid
Structure (Without Skin)
Mode 2 fn 263 Hz
12
Dynamic Analysis
Finite Element Analysis of Complete ION-F Stack

• Requirement First mode natural frequency
  gt100 Hz
• Results First mode natural frequency 74.6
  Hz
• Solution Stiffen joints around attachment
  points to raise first mode natural frequency
  100Hz

13
Fabrication
Composite structure comprised of 0.23 isogrid
and 0.02 skin
14
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

15
Static Testing
Strength stiffness test of structure without
skin panels
Strength stiffness test of loading fixture
16
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

17
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

18
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)
19
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)
20
Dynamic Testing
Modal Testing of Structure (Without Skins)


Mode 2 fn 272 Hz (vs 263 Hzpredicted)
Mode 1 fn 245 Hz (vs 249 Hz predicted)
21
Dynamic Testing
Accelerometer Placement

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)

• Structure survived
• all tests
• Determined component locations to raise natural
  frequencies

22
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

23
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