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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

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System identification of a nanosatellite structure

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


Overview

3

2

  • Introduction

  • Design

  • Analysis

  • Fabrication

  • Testing

  • Conclusions

4

5


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)


Mission

3CS

ION-F

USUSat

Dawgstar

HokieSat

Configuration:

Multiple Satellite Deployment System

Scenario:


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


Design

External Configuration

Solar Cells

Crosslink Antenna

GPS Antenna

LightBand

Pulsed Plasma

Thrusters

Data Port

Camera

Uplink Antenna

Downlink Antenna

Science

Patches


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)


Static Analysis

  • Requirement: Withstand ±11.0 g accelerations (all directions)

  • Margin of Safety  0, where

    • Factor of Safety (FS)

  • Finite Element Analysis Results


Dynamic Analysis

Finite Element Analysis of Isogrid Side Panel (Without Skin)

Mode 1

fn = 131 Hz

Mode 2

fn = 171 Hz


Dynamic Analysis

Finite Element Analysis of Complete Isogrid Structure (Without Skin)

Mode 1

fn = 249 Hz


Dynamic Analysis

Finite Element Analysis of Complete Isogrid Structure (Without Skin)

Mode 2

fn = 263 Hz


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


Fabrication

Composite structure comprised of 0.23” isogrid and 0.02” skin


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:


Static Testing

Strength & stiffness test of structure without skin panels

Strength & stiffness test of loading fixture


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


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


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)


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)


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)


Dynamic Testing

Z

Y

X

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


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


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