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Design, Analysis, Fabrication, and Testing of a Nanosatellite Structure

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### Design, Analysis, Fabrication, and Testing of a Nanosatellite Structure

Craig L. Stevens

Aerospace and Ocean Engineering

Virginia Tech

Blacksburg, Virginia

Thesis Defense

May 28, 2002

Satellites

- Thousands of satellite designs
- Structural design depends upon:
- Mission
- Orbit
- Launch vehicle
- Technology

- NASA Satellite History

NASA Spacecraft Mass History

5

10

4

10

3

10

Mass, kg

2

10

1

10

0

10

1950

1960

1970

1980

1990

2000

2010

Launch Year

- Commerical Satellite History

Commercial Spacecraft Mass History

3000

2500

2000

Mass, kg

1500

1000

500

0

1965

1970

1975

1980

1985

1990

1995

2000

Launch Year

Previous Missions

Explorer 1: Launched January 31, 1958

Size: 6 ft long

Mass: 31 lbs

First US satellite

Discovered Van Allen Belts

Compton Gamma-Ray Observatory:

Launched April 5, 1991

Size: 12.5 ft diameter

25 ft long

Mass: 34371 lbs

Gathered data on galactic radiation

Previous Missions

Solar, Anomalous and Magnetic Particle Explorer (SAMPEX):

Launched July 3, 1992

Size: 2.8 ft diameter

4.9 ft long

Mass: 348 lbs

Began NASA “faster, better, cheaper” program

Measured galactic charged particles

ORBCOMM:

Constellation of 35 spacecraft

Launched between 1995 and 2000

Size: 40” diameter

6” height

Mass: 99 lbs

Provide global two-way messaging

- Virginia Tech Ionospheric Scintillation Measurement Mission (VTISMM) aka HokieSat
- Ionospheric Observation Nanosatellite Formation (ION-F)
- Utah State University
- University of Washington
- Virginia Tech
- AFRL Multi-Satellite Deployment System (MSDS)
- NASA Shuttle Hitchhiker Experiment Launch System (SHELS)
- Sponsors: AFRL, AFOSR, DARPA, NASA GSFC, SDL

Ionospheric Observation Nanosatellite Formation (ION-F)

AFRL Multi-Satellite Deployment

System (MSDS)

NASA Shuttle Hitchhiker

Experiment Launch

System (SHELS)

Design Process:

Initial Criteria

- Configuration
- Stack of 3 spacecraft
- HokieSat at base of stack
- Lightband separation system
- Hexagonal
- Stiffness
- SHELS Users Guide: payload natural frequency > 35 Hz
- Mass
- SHELS Users Guide: payload mass < 400 lbs
- Cost
- Minimize cost
- Student program

Objective Function:

- Previous fabrication materials and methods investigated
- List of criteria created
- Criteria score, Sj, based on literature review and correspondence
- Criteria weighting factors, Wj, selected for program

Objective Function:

- Three weighting factor conditions:
- Structural engineer
- Chief engineer
- Student
- Results: Metallic panels optimum choice for design

Preliminary Design

- Hexagonal prism
- 18” major diameter
- 11.5” height
- Separation Systems
- Lightband
- Starsys
- Isogrid construction
- Manufacture using computer numerical controlled (CNC) milling machines
- 200% increase in structural efficiency
- Al 6061-T651
- High efficiency
- Inexpensive
- Good workability

Final Design

- 18.25” major diameter hexagonal prism
- 11.725” tall
- 39 lbs total mass
- 13.5 lbs structural mass

- Isogrid structure
- Aluminum 6061 T-651
- Isogrid end panels
- 0.25” isogrid
- Composite side panels
- 0.23” isogrid
- 0.02” skins

Hardware

- Isogrid panels manufactured using CNC milling machine
- End panels machined from 0.25” aluminum plate
- Side panels machined from 1” aluminum plate
- Separation system flatness requirements verified
- 0.0005” per inch tolerance
- Final verification during assembly
- Skin panels machined from 0.02” aluminum
- Brackets machined from 0.063” and 0.25” aluminum
- Treated with chromate conversion coating per MIL-C-5541C
- #10-32 fasteners

Epoxy Process

- Composite structure comprised of 0.23” isogrid and 0.02” skin
- Used 3M 2216 Gray
- Spaceflight heritage
- Simple lay-up
- Procedure:
- Surfaces prepared
- Scoured using steel wool
- Methyl ethyl ketone
- Isopropyl alcohol
- Seven 0.005” monofilament lines placed across isogrid surface
- Epoxy applied
- Isogrid
- Skin
- Spatula used to evenly distribute
- Cured for 120 minutes at 80° C

Structural Verification Procedure

- Establish structural requirements
- Perform preliminary analysis
- Isogrid
- Modal analysis and testing of panels
- Modal analysis and testing of assembly
- Composite
- Modal analysis and testing of side panels
- Three-point-bend testing of side panels
- Environmental testing of assembly
- ION-F stack configuration
- Strength and stiffness testing
- Modal analysis of stack
- Stress analyses

Requirements

- Withstand all inertial loading with limit load factors:

(simultaneous, all permutations)

- Margin of Safety (MS) 0, where
- Factor of Safety (FS)
- Fundamental frequency > 35 Hz

Preliminary Analysis

- Isogrid geometry

b: width of web

d: depth of web

h: height of triangle

a: length of web

- Equivalent monocoque panel
- Equivalent Young’s modulus,
- Equivalent panel thickness = d
- Stress analysis using open isogrid theory

where Nx, Ny, Nxy are membrane stress resultants

Preliminary Analysis

- Finite element analysis to calculate stress resultants
- Analysis demonstrates that 0.200” thick isogrid panels sufficient
- HOWEVER, forced to increase panel thickness to 0.250”
- Stiffness requirements
- Model deficiencies
- Integration

Shell Elements

thickness = d

Finite Element Analysis of Isogrid Structure

Shell

Elements

- Linear beam elements:
- 0.25” ×0.08”
- 0.23” × 0.08”
- Linear quadrilateral and triangular shell elements:
- 0.25” thick
- 0.23” thick
- Separation system attachment points modeled
- Thruster holes neglected
- Flanges and overhangs
- Side panel model
- Neglected in assembly

Beam

Elements

Attachment

Points

Finite Element Analysis of Isogrid Side Panel

Mode 1

fn = 131 Hz

Mode 2

fn = 171 Hz

Finite Element Analysis of Isogrid End Panel

Mode 1

fn = 105 Hz

Mode 2

fn = 182 Hz

Modal (tap) Testing of Panels

- Panels tethered using bungee cords and tape
- Hammer provides impulsive input at several points
- Accelerometer measures accelerations at fixed point

- Frequency response function magnitudes and phases examined
- Verification of predictions of finite element analysis

Modal Testing of Isogrid Side Panels

Mode 1

fn = 131 Hz

(vs 131 Hz predicted)

Mode 2

fn = 169 Hz

(vs 171 Hz predicted)

Modal Testing of Isogrid End Panels

Mode 1

fn = 111 Hz

(vs 105 Hz predicted)

Mode 2

fn = 193 Hz

(vs 182 Hz predicted)

Modal Testing of Isogrid Structural Assembly

Mode 2

fn = 272 Hz

(vs 263 Hzpredicted)

Mode 1

fn = 245 Hz

(vs 249 Hz predicted)

Finite Element Analysis of Composite Side Panel

- Offset neutral axis nodes of isogrid panels
- Linear shell elements created
- 0.02” quadrilateral
- 0.02” triangular
- Rigid elements connect neutral axis nodes

Beam Element

Neutral Axis

Rigid Element

Shell Element

Neutral Axis

Finite Element Analysis of Composite Side Panel

Mode 1

fn = 159 Hz

Mode 2

fn = 219 Hz

Modal Testing of Composite Side Panels

Chladni Patterns:

Mode 2

fn = 220 Hz

(vs 219 Hz predicted)

Mode 1

fn = 159 Hz

(vs 159 Hz predicted)

Mode 1:

fn = 159 Hz

Mode 2:

fn = 220 Hz

Results demonstrate 22% gain in efficiency using skins

Composite Panel Strength Test Results

600

Side 1

Side 2

Side 3

500

Side 4

Side 5

Side 6

400

Load, lbs

300

200

100

0

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Displacement, in

Three-Point-Bend Testing of Composite Side Panels

- Stiffness curves lie within 5% of mean
- Verify bond strength
- Verify assumption to neglect

thruster holes

- Supported on all edges
- Load applied at center web
- First loaded prototype panel to localized failure
- Loaded flight panels to 70% failure load

ION-F Random Vibration Spectrum

0

10

-1

10

ASD, G2/Hz

-2

10

-3

10

1

2

3

4

10

10

10

10

Frequency, Hz

Composite Structure Environmental Testing

- Sine sweep test
- Determines restrained fundamental frequency
- 20-2000 Hz, 0.5 g
- Sine burst test
- Quasi-static strength test at less than one-third fundamental frequency
- 23.8 g’s
- Random vibration test
- Verifies structural integrity
- 9 g RMS, 1 minute duration
- Power spectrum:

Z

X

Y

Prototype Environmental Testing

Accelerometer Placement

- Side panel 1
- Side panel 2
- Zenith panel
- GPS (3 axis)
- CEE (3 axis)
- PPU (3 axis)
- Battery box (3 axis)

Zenith Panel FRF: Hzz(f)

1

10

Log H(f)

0

10

-1

10

2

3

10

10

Log Frequency, Hz

Testing Results:

- Structure survived all tests
- Fundamental frequency:
- 78 Hz
- Zenith panel
- Torque coil damaged
- Modified integration scheme
- Raise fundamental frequency
- Prevent damage

Z

X

Y

Flight Environmental Testing

Accelerometer Placement

- Side panel 1
- Side panel 2
- Zenith panel
- Honeycomb
- GPS
- GPS Preamp
- CEE
- PPT (3 axis)
- Fuel bar support (3 axis)
- Battery box

Zenith Panel FRF: Hzz(f)

1

10

0

10

Log H(f)

-1

10

-2

10

2

3

10

10

Log Frequency, Hz

CEE FRF: Hzz(f)

1

10

Log H(f)

0

10

-1

10

2

3

10

10

Log Frequency, Hz

Testing Results:

- Structure survived all tests
- Fundamental frequency:
- 105 Hz
- Nadir panel
- Raised fundamental frequency 35%
- Epoxied honeycomb
- Relocation of GPS components

Mass Properties Testing

- Measured
- Center of mass
- Moments of inertia
- Oriented in seven configurations to calculate principal moments of inertia
- No data recorded for products of inertia Ixz and Iyz
- Assumed z-axis is principal axis

x

y

z

Finite Element Analysis of Complete ION-F Stack

- USUSat:
- 0.25” thick linear shell
- Non-structural point masses
- Dawgstar
- 0.12” thick linear quadrilateral shell elements
- Linear beam elements
- Nonstructural mass
- Lightband
- 0.15” thick linear quadrilateral shell elements
- HokieSat
- Nonstructural mass

USUSat

Lightband

Dawgstar

Lightband

HokieSat

Strength and Stiffness Test

Truss loading fixture

- Three cantilever tests
- Truss
- Isogrid
- Composite
- Evaluate gain in efficiency using composite structure
- Determine boundary conditions

Isogrid and composite structures

Strength and Stiffness Test

Load vs Displacement Plot

- 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
- Overall 22% gain in structural efficiency for cantilever mode

Truss

Isogrid & Truss

300

Composite & Truss

250

200

Load, p (lb)

150

100

50

0

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

Displacement, u (in)

Boundary Condition Correlation

- Model of truss fixture
- 0.15” linear shell elements
- Hexagonal protrusion
- Attached at nodes simulating
- Lightband attachment points

Load vs Displacement of Truss Fixture

300

Test

Analysis

250

200

- Correlation of truss data
- Lightband attachment points fixed on end panel
- Load applied at end
- Young’s modulus modified
- Stiffness curves correlate within 1%

Load, p (lb)

150

100

50

0

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Displacement, u (in)

Truss and Composite Structure Data

300

Test

Analysis

250

200

Load, p (lb)

150

100

50

0

0

0.02

0.04

0.06

0.08

0.1

0.12

Displacement, u (in)

Boundary Condition Correlation

- Correlation of truss and composite data
- Nadir Starsys attachment point node translations fixed (fixed base)
- Flanges modeled using solid elements
- End panels attach to flanges using rigid elements

- Stiffness curves of model and test data correlate within 5%

Modal Analysis of Complete ION-F Stack

Mode 1

fn = 47 Hz

Mode 2

fn = 48 Hz

- Majority of strain energy concentrated in Lightband
- Possible stiffness problems revealed

Stress Analysis of Complete ION-F Stack

- Apply uniform acceleration
- Fixed base boundary conditions
- Required design criteria:
- Minimum MS = 0.094 > 0
- Sine burst stress analysis results
- No yielding or buckling

- Aluminum isogrid increases structural performance at reduced mass
- Modal testing verifies accuracy of isogrid and composite side panel finite element models within ~1% error
- Modal testing demonstrates 22% increase in structural efficiency of side panel by adding thin aluminum skins
- Three-point bend testing validates assumption to neglect thruster hole cutouts in model and verifies bond strength
- Sine sweep testing demonstrates a fundamental frequency of 105 Hz for the restrained composite assembly
- Strength and stiffness testing demonstrates 22% gain in structural stiffness of assembly by adding thin aluminum skins
- Analyses and experiments verify structure survives Shuttle payload environment

- Professor C. Hall
- Professor W. Hallauer
- Professor E. Johnson
- 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 structures team
- Members of ION-F

External Configuration

Solar Cells

Crosslink Antenna

GPS Antenna

LightBand

Pulsed Plasma

Thrusters

Data Port

Camera

Uplink Antenna

Downlink Antenna

Science

Patches

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)

- Previous Designs:
- Materials
- Bus Designs

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