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This presentation is distributed to the students of the University of Liege
Satellite Engineering Class November 21, 2011
This presentation is not for further distribution
Spacecraft Loads AnalysisAn Overview
Adriano Calvi, PhD
ESA / ESTEC, Noordwijk, The Netherlands
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Spacecraft Loads Analysis - Contents
1. Introduction and general aspects
2. Mechanical environment
3. Requirements for spacecraft structures
4. Mathematical models and structural analyses5. Spacecraft mechanical testing
6. Mathematical models validation
7. Conclusions
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Example of satellite structural design conceptExample of satellite structural design concept
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What is Loads AnalysisWhat is Loads Analysis
The task of loads analysis
Loads analysis substantially means establishing appropriate
loads for design and testing.
The goal or purpose of loads analysis
Nearly always to support design or to verify requirements fordesigned or built hardware.
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Spacecraft loads analysis processSpacecraft loads analysis process disciplinesdisciplines
It is 3 years that I work in thiscompany. Now, finally I have
understood what I do, butstill I have to understand why.
Legal aspects
Requirements, contracts
Philosophical aspects
General logic, verification approach, criteria
Physics
Structural dynamics, validation of mathematical
models, criteria
Mathematics
Computational models, verification of models
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OrganizationsOrganizations andand Levels of AssemblyLevels of Assembly an examplean example
Launcher Authority
Spacecraft Authority
Spacecraft Prime Contractor
Payload Contractor
Other Contactors
Spacecraft + launcher
Spacecraft
Spacecraft
Instruments/sub-systems
Units/components/parts
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Levels of AssemblyLevels of Assembly
RFFE
RPM
RFFE
RPM
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Design Loads CyclesDesign Loads Cycles
A load cycle is the process of:
Generating and combining math models for a proposed design
Assembling and developing forcing functions, load factors, etc. to
simulate the critical loading environment
Calculating design loads and displacements for all significantground, launch and mission events
Assessing the results to identify design modifications or risks
Then, if necessary, modifying the design accordingly or choosing to
accept the risk
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Design loads cycle processDesign loads cycle process
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Loads and Factors
Expendable launch vehicles,
pressurized hardware and
manned system Test Logic
Common Design LogicSatellites
Test Logic
Limit Loads - LL
Design Limit Loads
DLL
x Coef. A
DYL
x Coef. B
DUL
x Coef. C
x KQ x KA
QLAL
x KQ x KA
QL
AL
Increasing
LoadL
evel
ECSS E-ST-32-10
Protoflight Prototype
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2. Mechanical environment2. Mechanical environment
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Mechanical loads are caused by:Mechanical loads are caused by:
Transportation
Rocket Motor Ignition Overpressure Lift-off Loads
Engine/Motor Generated Acoustic Loads
Engine/Motor Generated Structure-borne Vibration Loads
Engine/Motor Thrust Transients Pogo Instability, Solid Motor Pressure Oscillations
Wind and Turbulence, Aerodynamic Sources
Liquid Sloshing in Tanks
Stage and Fairing Separation Loads Pyrotechnic Induced Loads
Manoeuvring Loads
Flight Operations, Onboard Equipment Operation
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AccelerationsAccelerations some remarkssome remarks
The parameter most commonly used (in the industry) to define themotion of a mechanical system is the acceleration
Good reasons: accelerations are directly related to forces/stresses andeasy to specify and measure
In practice accelerations are used as a measure of the severity of themechanical environment
Some hidden assumptions Criteria for equivalent structural damage (e.g. shock response spectra)
Note: failures usually happen in the largest stress areas, regardless if theyare the largest acceleration areas!
Rigid or static determinate junction (e.g. quasi-static loads) Important consequences
Need for considering the actual (e.g. test or flight) boundary conditions(e.g. for the purpose of notching)
Need for a valid F.E. model (e.g. to be used for force and stress recovery)
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Launch mechanical environmentLaunch mechanical environment
Steady state accelerations
Low frequency vibrations Broad band vibrations
Random vibrations
Acoustic loads
Shocks
Loads (vibrations) are transmitted to the payload (e.g. satellite)
through its mechanical interface
Acoustic loads also directly excite payload surfaces
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SteadySteady--statestate and lowand low--frequency transient accelerationsfrequency transient accelerations
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Acoustic LoadsAcoustic Loads
During the lift off and the early phases ofthe launch an extremely high level of
acoustic noise surrounds the payload
The principal sources of noise are: Engine functioning
Aerodynamic turbulence
Acoustic noise (as pressure waves)impinging on light weight panel-like
structures produce high response
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Broadband and high frequency vibrationsBroadband and high frequency vibrations
Broad band random vibrations are produce by:
Engines functioning Structural response to broad-band acoustic loads
Aerodynamic turbulent boundary layer
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ShocksShocks
Mainly caused by the actuation of pyrotechnic devices:
Release mechanisms for stage and satellite separation Deployable mechanisms for solar arrays etc.
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ShocksShocks
Time [t]Frequency [Hz]
[g][g]
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Static and dynamic environment specification (typical ranges)Static and dynamic environment specification (typical ranges)
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Static and dynamic environment specification (typical ranges)Static and dynamic environment specification (typical ranges)
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QuasiQuasi--Static Loads (accelerations)Static Loads (accelerations)
Loads independent of time or which vary slowly, so that the dynamicresponse of the structure is not significant (ECSS-E-ST-32). Note: this is
the definition of a quasi-static event! Combination of static and low frequency loads into an equivalent static
load specified for design purposes as C.o.G. acceleration (e.g. NASA RP-1403, NASA-HDBK-7004). Note: this definition is fully adequate for thedesign of the spacecraft primary structure. For the design of components
the contribution of the high frequency loads, if relevant, is included as well!
CONCLUSION: quasi static loading means under steady-stateaccelerations (unchanging applied force balanced by inertia loads). Fordesign purposes (e.g. derivation of design limit loads, selection of the
fasteners, etc.), the quasi-static loads are normally calculated bycombining both static and dynamic load contributions. In this context thequasi static loads are equivalent to (or interpreted by the designer as)static loads, typically expressed as equivalent accelerations at the C.o.G.
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3. Requirements for spacecraft structures3. Requirements for spacecraft structures
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Typical Requirements for Spacecraft Structures
Strength
Structural life
Structural response
Stiffness
Damping
Mass Properties
Dynamic Envelope Positional Stability
Mechanical Interface
Basic requirement: the structure shall support the payload andspacecraft subsystems with enough strength and stiffness topreclude any failure (rupture, collapse, or detrimental deformation)that may keep them from working successfully.
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Requirements evolutionRequirements evolution
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Some definitionsSome definitions
Design:
The process used to generate the set information describing theessential characteristics of a product (ECSS-P-001A)
Design means developing requirements, identifying options, doinganalyses and trade studies, and defining a product in enough detail soit can be built (T. P. Sarafin)
Verification: Confirmation by examination and provision of objective evidence thatspecified requirements have been fulfilled (ISO 8402:1994)
Verification means providing confidence through disciplined steps thata product will do what it is supposed to do (T. P. Sarafin)
Note: we can prove that the spacecraft satisfies the measurable criteriawe have defined, but we cannot prove a space mission will be successful
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Design requirements and verificationDesign requirements and verification
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Examples of (Mechanical) Requirements (1)
The satellite shall be compatible with 2 launchers (potential candidates:
VEGA, Soyuz in CSG, Rockot, Dnepr)...
The satellite and all its units shall withstand applied loads due to the
mechanical environments to which they are exposed during the service-life
Design Loads shall be derived by multiplication of the Limit Loads by a design
factor equal to 1.25 (i.e. DL= 1.25 x LL)
The structure shall withstand the worst design loads without failing orexhibiting permanent deformations.
Buckling is not allowed.
The natural frequencies of the structure shall be within adequate bandwidths
to prevent dynamic coupling with major excitation frequencies
The spacecraft structure shall provide the mounting interface to the launch
vehicle and comply with the launcher interface requirements.
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Examples of (Mechanical) Requirements (2)
All the Finite Element Models (FEM) prepared to support the mechanical
verification activities at subsystem and satellite level shall be delivered in
NASTRAN format
The FEM of the spacecraft in its launch configuration shall be detailed enough to
ensure an appropriate derivation and verification of the design loads and of the
modal response of the various structural elements of the satellite up to 140 Hz
A reduced FEM of the entire spacecraft correlated with the detailed FEM shall bedelivered for the Launcher Coupled Loads Analysis (CLA)
The satellite FEMs shall be correlated against the results of modal survey tests
carried out at complete spacecraft level, and at component level for units above
50 kg
The structural model of the satellite shall pass successfully qualification sine
vibration Test.
The flight satellite shall pass successfully acceptance sine vibration test.
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Spacecraft stiffness requirements for different launchersSpacecraft stiffness requirements for different launchers
Launch vehicle manuals specify minimum values for the payload natural(fundamental) frequency of vibration in order to avoid dynamic coupling betweenlow frequency dynamics of the launch vehicle and payload modes
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4. Mathematical models and structural analyses4. Mathematical models and structural analyses
4.1 Dynamic analysis types - Overview
4.2 Effective mass concept
4.3 Launcher/Spacecraft coupled loads analysis
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Dynamic analysis typesDynamic analysis types
Real eigenvalue analysis (undamped free vibrations)
Modal parameter identification, etc.
Linear frequency response analysis (steady-state response oflinear structures to loads that vary as a function of frequency)
Sine test prediction, transfer functions calculation, LV/SC CLA etc.
Linear transient response analysis (response of linear structures to
loads that vary as a function of time). LV/SC CLA, base drive analysis, jitter analysis, etc.
Shock response spectrum analysis
Specification of equivalent environments (e.g. equivalent sine input),
Shock test specifications, etc. Vibro-acoustics (FEM/BEM, SEA) & Random vibration analysis
Vibro-acoustic test prediction & random vibration environment definition
Loads analysis for base-driven random vibration
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Reasons to compute normal modes (real eigenvalue analysis)Reasons to compute normal modes (real eigenvalue analysis)
To verify stiffness requirements
To assess the dynamic interaction between a component and itssupporting structure
To guide experiments (e.g. modal survey test)
To validate computational models (e.g. test/analysis correlation)
As pre-requisite for subsequent dynamic analyses To evaluate design changes
Mathematical model quality check (model verification)
Numerical methods: Lanczos,
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Real eigenvalue analysisReal eigenvalue analysis
Note: mode shape normalizationScaling is arbitraryConvention: Mass, Max or Point
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Mode shapesMode shapes
Cantilever beam Simply supported beam
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Satellite Normal Modes Analysis
Mode 1: 16.2 Hz Mode 2: 18.3 Hz
INTEGRAL Satellite (FEM size 120000 DOFs)
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Frequency Response AnalysisFrequency Response Analysis
Used to compute structural response to steady-state harmonic
excitation
The excitation is explicitly defined in the frequency domain
Forces can be in the form of applied forces and/or enforced
motions
Two different numerical methods: direct and modal Damped forced vibration equation of motion with harmonic
excitation:
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Frequency response considerationsFrequency response considerations
If the maximum excitation frequency is much less than the lowestresonant frequency of the system, a static analysis is probably
sufficient
Undamped or very lightly damped structures exhibit large dynamic
responses for excitation frequencies near natural frequencies
(resonant frequencies)
Use a fine enough frequency step size (f) to adequately predict
peak response.
Smaller frequency spacing should be used in regions near resonant
frequencies, and larger frequency step sizes should be used in
regions away from resonant frequencies
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Harmonic forced response with dampingHarmonic forced response with damping
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Transient Response AnalysisTransient Response Analysis
Purpose is to compute the behaviour of a structure subjected to time-
varying excitation
The transient excitation is explicitly defined in the time domain
Forces can be in the form of applied forces and/or enforced motions
The important results obtained from a transient analysis are typically
displacements, velocities, and accelerations of grid points, andforces and stresses in elements
Two different numerical methods: direct (e.g. Newmark) and modal
(e.g. Lanczos + Duhamels integral or Newmark)
Dynamic equation of motion:
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Modal Transient Response AnalysisModal Transient Response Analysis
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Transient response considerationsTransient response considerations
The integration time step must be small enough to represent accuratelythe variation in the loading
The integration time step must also be small enough to represent the
maximum frequency of interest (cut-off frequency)
The cost of integration is directly proportional to the number of time steps Very sharp spikes in a loading function induce a high-frequency transient
response. If the high-frequency transient response is of primary
importance in an analysis, a very small integration time step must be
used
The loading function must accurately describe the spatial and temporal
distribution of the dynamic load
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ShockShock response spectrum (and analysis)response spectrum (and analysis)
Response spectrum analysis is an approximate method ofcomputing the peak response of a transient excitation applied to a
structure or component There are two parts to response spectrum analysis: (1) generation
of the spectrum and (2) use of the spectrum for dynamic responsesuch as stress analysis
Note 1: the part (2) of the response spectrum analysis has alimited use in structural dynamics of spacecraft (e.g. preliminarydesign) since the accuracy of the method may be questionable
Note 2: the term shock can be misleading (not always a physicalshock, i.e. an environment of a short duration, is involved. Itwould be better to use response spectrum)
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Generation of a response spectrum (1)Generation of a response spectrum (1)
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Generation of a response spectrum (2)Generation of a response spectrum (2)
the peak response for one oscillator does not necessarily occur at the
same time as the peak response for another oscillator
there is no phase information since only the magnitude of peak response iscomputed
It is assumed in this process that each oscillator mass is very small relative
to the base structural mass so that the oscillator does not influence the
dynamic behaviour of the base structure
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Shock Response Spectrum. Some remarksShock Response Spectrum. Some remarks
The 1-DOF system is used as reference structure (since thesimplest) for the characterization of environments (i.e.
quantification of the severity equivalent environments can bespecified)
In practice, the criterion used for the severity is the maximumresponse which occurs on the structure (note: another criterionrelates to the concept of fatigue damage)
A risk in comparing two excitations of different nature is in theinfluence of damping on the results (e.g. maxima are proportionalto Q for sine excitation and variable for transient excitation!)
The absolute acceleration spectrum is used, which provides
information about the maximum internal forces and stresses The shock spectrum is a transformation of the time history which is
not reversible (contrary to Fourier transform)
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Shock Response SpectrumShock Response Spectrum
(A) is the shock spectrum
of a terminal peaksawtooth (B) of 500 Gpeak amplitude and 0.4millisecond duration
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Random vibration (analysis)Random vibration (analysis)
Random vibration is vibration that can be described only in astatistical sense
The instantaneous magnitude is not known at any given time;rather, the magnitude is expressed in terms of its statisticalproperties (such as mean value, standard deviation, and probabilityof exceeding a certain value)
Examples of random vibration include earthquake ground motion,wind pressure fluctuations on aircraft, and acoustic excitation dueto rocket and jet engine noise
These random excitations are usually described in terms of apower spectral density (PSD) function
Note: in structural dynamics of spacecraft, the random vibrationanalysis is often performed with simplified techniques (e.g. basedon Miles equation + effective modal mass models)
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Random noise with normal amplitude distributionRandom noise with normal amplitude distribution
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Power Spectral Density (conceptual model)Power Spectral Density (conceptual model)
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Sound Pressure Level (conceptual model)Sound Pressure Level (conceptual model)
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VibroVibro acoustic analysis at spacecraft levelacoustic analysis at spacecraft level
Detailed analysis using Finite Elements (FE), Boundary Elements
(BEM) and Statistical Energy Analysis (SEA)
Random levels on units and instruments can be compared to
specifications or qualification levels
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4. Mathematical models and structural analyses4. Mathematical models and structural analyses
4.1 Dynamic analysis types - Overview
4.2 Effective mass concept
4.3 Launcher/Spacecraft coupled loads analysis
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Modal effective mass (1)Modal effective mass (1)
It may be defined as the mass terms in a modal expansion of the
drive point apparent mass of a kinematically supported system
Note: driving-point FRF: the DOF response is the same as the excitation
This concept applies to structure with base excitation
Important particular case: rigid or statically determinate junction
It provides an estimate of the participation of a vibration mode, interms of the load it will cause in the structure, when excited
Note: avoid using: it is the mass which participates to the mode!
Dynamic amplification factor
Modal reaction forces
Base (junction) excitation
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Modal effective mass (2)Modal effective mass (2)
The effective mass matrix can be calculated either by the modal
participation factors or by using the modal interface forces Normally only the values on the leading diagonal of the modal
effective mass matrix are considered and expressed in percentageof the structure rigid body properties (total mass and second
moments of inertia) The effective mass characterises the mode and it is independent
from the eigenvector normalisation
Gen. mass
Resultant of modal interface forces
i-th mode Rigid body modes
Modal participation factorsEffective mass
Eigenvector max value
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Modal effective mass (3)Modal effective mass (3)
For the complete set of modes the summation of the modal
effective mass is equal to the rigid body mass
Contributions of each individual mode to the total effective mass
can be used as a criterion to classify the modes (global or local)
and an indicator of the importance of that mode, i.e. an indication of
the magnitude of participation in the loads analysis
It can be used to construct a list of important modes for the
test/analysis correlation and it is a significant correlation parameter
It can be used to create simplified mathematical models (equivalent
models with respect to the junction)
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Example of Effective Mass table
(MPLM test and FE model)
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4. Mathematical models and structural analyses4. Mathematical models and structural analyses
4.1 Dynamic analysis types - Overview
4.2 Effective mass concept
4.3 Launcher/Spacecraft coupled loads analysis
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A5 Typical Sequence of eventsA5 Typical Sequence of events
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A5 Typical Longitudinal Static AccelerationA5 Typical Longitudinal Static Acceleration
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Sources of Structural Loadings (Launch)Sources of Structural Loadings (Launch)
Axial-Acceleration Profile for the Rockot Launch Vehicle
g
8
7
6
5
4
3
2
1
0t,150 200 250 300100500
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-20
-10
0
10
20
30
40
50
60
70
80
acceleration[m/s2]
t [s]
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Axial Acceleration at Launcher/Satellite Interface (Engines Cut-off)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-20
-10
0
10
20
30
40
50
60
70
80
acceleration[m/s2]
t [s]
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Load Factors for Preliminary Design (Ariane 5)Load Factors for Preliminary Design (Ariane 5)
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QuasiQuasi--Static Flight Limit loads for Dnepr and SoyuzStatic Flight Limit loads for Dnepr and Soyuz
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Launcher / Satellite C.L.A.
Mode 18: 2.93 Hz Mode 53: 16.9 Hz
A5 / Satellite Recovered System Mode shapes
CLA: simulation of the structural response tolow frequency mechanical environment
Main Objective: to calculate the loads on thesatellite caused by the launch transients (lift-off, transonic, aerodynamic gust, separation ofSRBs)
Loads (in this context): set of internal forces,displacements and accelerations thatcharacterise structural response to the appliedforces
Effects included in the forcing functions :thrust built-up, engine shut-down/burnout,gravity, aerodynamic loads (gust), separationof boosters, etc.
UPPER
COMPOSITE
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Ariane-5 Dynamic Mathematical Model
PAYLOAD
EAP+EAP-
EPC
Dynamic effects up to about 100 Hz
3D FE models of EPC, EAP, UC
Dynamic Reduction using Craig-Bampton formulation
Incompressible or compressible fluids models for liquid
propellants
Structure/fluid interaction Nearly incompressible SRB solid propellant modeling
Pressure and stress effects on launcher stiffness
SRB propellant and DIAS structural damping
Non-linear launch table effects
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Sizing flight events (CLA with VEGA Launcher)Sizing flight events (CLA with VEGA Launcher)
1. Lift-off (P80 Ignition and Blastwave)
2. Mach1/QMAX Gust
3. P80 Pressure Oscillations
4. Z23 Ignition
5. Z23 Pressure Oscillations
6. Z9 Ignition
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CLA OutputCLA Output
LV-SC interface accelerations
Equivalent sine spectrum
LV-SC interface forces
Equivalent accelerations at CoG
Internal responses Accelerations,
Displacements
Forces
Stresses
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Payload / STS CLAPayload / STS CLA
Lift-off Force Resultant in X [lbf]
Lift-offMain Fitting
I/F ForceX Dir. [N]
Lift-offMain Fitting
I/F Force
Z Dir. [N]
Lift-off
Keel FittingI/F ForceY Dir. [N]
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Shock Response Spectrum and Equivalent Sine Input
A shock response spectrum is a
plot of maximum response (e.g.displacement, stress, acceleration)
of single degree-of-freedom
(SDOF) systems to a given input
versus some system parameter,generally the undamped natural
frequency.
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SRS/ESI of the following transient acceleration:
Q
SRS
ESI
ESI
ESI
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ESI for Spacecraft
CLA (Coupled Load Analysis)
0 10 20 30 40 50 60 70 800
50
100
150
200
250
SRS[m
/s2]
frequency [Hz]
SRS
Q
SRSESI
ESI
12
Q
SRSESI
SRS
Difference is negligible for small damping ratios
2DOF
SDOF
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/[ sm
][s
2.41
[Hz
]/[ sm2.46
2DOF2DOF]/[ sm
][s
01.0Hz23frequencynatural
[Hz
Hz23frequencynatural
1.97 01.0]/[ sm
Transient responseTransient response
Frequency response at ESI levelFrequency response at ESI level
SDOFSDOF
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5. Spacecraft testing5. Spacecraft testing
5.1 Introduction and general aspects
5.1 Mechanical tests
Modal survey test
Sinusoidal vibration testAcoustic noise test
Shock test
Random vibration test
5.2 Over-testing and under-testing
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Testing techniquesTesting techniques Introduction (1)Introduction (1)
Without testing, an analysis can give completely incorrect results
Without the analysis, the tests can represent only a very limited reality
Two types of tests according to the objectives to be reached:
Simulation tests for structure qualification or acceptance
Identification tests (a.k.a. analysis-validation tests) for structureidentification (the objective is to determine the dynamic characteristics ofthe tested structure in order to update the mathematical model)
Note: identification and simulation tests are generally completely
dissociated. In certain cases (e.g. spacecraft sine test) it is technicallypossible to perform them using the same test facility
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Testing techniquesTesting techniques Introduction (2)Introduction (2)
Generation of mechanical environment
Small shakers (with flexible rod; electrodynamic)
Large shakers (generally used to impose motion at the base)
Electrodynamic shaker
Hydraulic jack shaker
Shock machines (pyrotechnic generators and impact machines) Noise generators + reverberant acoustic chamber (homogeneous and
diffuse field)
Measurements
Force sensors, calibrated strain gauges Accelerometers, velocity or displacement sensors
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Classes of tests used to verify requirements (purposes)Classes of tests used to verify requirements (purposes)
Development test
Demonstrate design concepts and acquire necessary information for
design
Qualification test
Show a design is adequate by testing a single article
Acceptance test
Show a product is adequate (test each flight article)
Analysis validation test
Provide data which enable to confirm critical analyses or to change
(update/validate) mathematical models and redo analyses
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Tests for verifying mechanical requirements (purposes)Tests for verifying mechanical requirements (purposes)
Acoustic test
Verify strength and structural life by introducing random vibration
through acoustic pressure (vibrating air molecules) Note: acoustic tests at spacecraft level are used to verify adequacy of
electrical connections and validate the random vibration environments
used to qualify components
(Pyrotechnic) shock test Verify resistance to high-frequency shock waves caused by separation
explosives (introduction of high-energy vibration up to 10,000 Hz)
System-level tests are used to verify levels used for component testing
Random vibration test Verify strength and structural life by introducing random vibration
through the mechanical interface (typically up to 2000 Hz )
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Tests for verifying mechanical requirements (purposes)Tests for verifying mechanical requirements (purposes)
Sinusoidal vibration test
Verify strength for structures that would not be adequately tested in
random vibration or acoustic testing Note 1: cyclic loads at varying frequencies are applied to excite the
structure modes of vibration
Note 2: sinusoidal vibration testing at low levels are performed to verify
natural frequencies Note 3: the acquired data can be used for further processing (e.g.
experimental modal analysis)
Note 4: this may seem like an environmental test, but it is not.
Responses are monitored and input forces are reduced as necessary
(notching) to make sure the target responses or member loads arenot exceeded.
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RockotRockot Dynamic SpecificationDynamic Specification
Marketed by: Eurockot
Actually flight qualified
Manufactured by: Khrunichev
Capability: 950 kg @ 500 km
Launch site: Plesetsk
Environment Level
Sine vibration
Longitudinal= 1 g on [5-10] Hz
1.5 g at 20 Hz
1 g on [40-100] Hz
Lateral = 0.625 g on [5-100] Hz
Acoustic
31.5 Hz = 130.5 dB
63 Hz = 133.5 dB
125 Hz = 135.5 dB
250 Hz = 135.7 dB
500 Hz = 130.8 dB
1000 Hz = 126.4 dB
2000 Hz = 120.3 dB
Shock
100 Hz = 50 g
700 Hz = 800 g
1000 Hz 1500 Hz = 2000 g
4000 Hz 5000 Hz = 4000 g
10000 Hz = 2000 g
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5. Spacecraft testing5. Spacecraft testing
5.1 Introduction and general aspects
5.2 Mechanical tests
Modal survey test
Sinusoidal vibration testAcoustic noise test
Shock test
Random vibration test
5.3 Over-testing and under-testing
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Modal survey test (identification test)Modal survey test (identification test)
Purpose: provide data for dynamic mathematical model validation
Note: the normal modes are the most appropriate dynamic
characteristics for the identification of the structure
Usually performed on structural models (SM or STM) in flight
representative configurations
Modal parameters (natural frequencies, mode shapes, damping,
effective masses) can be determined in two ways:
by a method with appropriation of modes, sometimes called phase
resonance, which consists of successively isolating each mode by an
appropriate excitation and measuring its parameters directly
by a method without appropriation of modes, sometimes called phase
separation, which consists of exciting a group of modes whoseparameters are then determined by processing the measurements
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Different ways to get modal data from testsDifferent ways to get modal data from tests
Hammer test
Vibration test data analysis
Dedicated FRF measurement & modal analysis
Full scale modal survey with mode tuningIncreasi
ngeffort
Dataconsistency
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Modal Survey Test vs. Modal Data extracted from the Sine Vibration Test
Modal Survey:
requires more effort (financial and time)
provides results with higher quality
Modal Data from Sine Vibration:
easy access / no additional test necessary
less quality due to negative effects from vibration
fixtures / facility tables not indefinitely stiff
higher sweep rate (brings along effects like beating or control instabilities)
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Ariane 5Ariane 5 -- Sine excitation at spacecraft base (Sine excitation at spacecraft base (sine-equivalent dynamics)
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Test Set-up for Satellite Vibration Tests
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Herschel on Hydra
A ti t t ( bj ti )A ti t t ( bj ti )
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Acoustic test (objectives)Acoustic test (objectives)
Demonstrate the ability of a specimen to
withstand the acoustic environment duringlaunch
Validation of analytical models
System level tests verify equipment
qualification loads
Acceptance test for S/C flight models
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Ariane 5Ariane 5 Acoustic noise spectrum under the fairingAcoustic noise spectrum under the fairing
Shock test Objecti es and remarksShock test Objecti es and remarks
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Shock test. Objectives and remarksShock test. Objectives and remarks
Demonstrate the ability of a specimen to
withstand the shock loads during launchand operation
Verify equipment qualification loadsduring system level tests
System level shock tests are generallyperformed with the actual shockgenerating equipment (e.g. clamp bandrelease)
or by using of a sophisticated pyro-shock generating system (SHOGUN forARIANE 5 payloads)
Shock machine (metalShock machine (metal metal pendulum impact machine)metal pendulum impact machine)
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Shock machine (metalShock machine (metal--metal pendulum impact machine)metal pendulum impact machine)
Random Vibration Test (vs Acoustic Test)Random Vibration Test (vs Acoustic Test)
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Random Vibration Test (vs. Acoustic Test)Random Vibration Test (vs. Acoustic Test)
Purpose: verify strength and structural life by introducing random
vibration through the mechanical interface
Random Vibration
base driven excitation
better suited for Subsystem / Equipment tests
limited for large shaker systems
Acoustic
air pressure excitation
better suited for S/C and large Subsystems with low mass / area
density
Random vibration test with slide tableRandom vibration test with slide table
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Random vibration test with slide tableRandom vibration test with slide table
5 Spacecraft testing5 Spacecraft testing
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5. Spacecraft testing5. Spacecraft testing
5.1 Introduction and general aspects
5.2 Mechanical tests
Modal survey test
Sinusoidal vibration test
Acoustic noise test
Shock test
Random vibration test
5.3 Over-testing and under-testing
Overtesting:Overtesting:
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Overtesting:Overtesting:an introductionan introduction(vibration absorber effect)(vibration absorber effect)
Introduction to overtesting and notching
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Introduction to overtesting and notching
The qualification of the satellite to lowfrequency transient is normallyachieved by a base-shake test
The input spectrum specifies theacceleration input that should excitethe satellite, for each axis
This input is definitively different fromthe mission loads, which are transient
Notching: Reduction of acceleration
input spectrum in narrow frequencybands, usually where test item hasresonances (NASA-HDBK-7004)
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GOCE on ESTEC Large Slip Table Herschel on ESTEC Large Slip Table
The overtesting problem (causes)
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The overtesting problem (causes)
Difference in boundary conditions between test and flight
configurations
during a vibration test, the structure is excited with a specified input
acceleration that is the envelope of the flight interface acceleration,
despite the amplitude at certain frequencies drops in the flight
configuration (there is a feedback from the launcher to the spacecraft in
the main modes of the spacecraft)
The excitation during the flight is not a steady-state sine function andneither a sine sweep but a transient excitation with some cycles in a
few significant resonance frequencies
The objective of notching of the specified input levels is to take into
account the real dynamic response for the different flight events. Inpractice the notching simulates the antiresonances in the coupled
configuration
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Notching
ESI(equivalent sine)
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Random vibration test: notching of test specificationRandom vibration test: notching of test specification
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g pg p
Illustration of notching of random vibration test specification,at the frequencies of strong test item resonances
6. Mathematical models validation6. Mathematical models validation
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Validation of Finite Element Models(with emphasis on Structural Dynamics)
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(with emphasis on Structural Dynamics)
Everyone believes the test data except for the
experimentalist, and no one believes the finite
element model except for the analyst
All models are wrong, but some are still useful
Verification and Validation Definitions(ASME Standards Committee: V & V in Computational Solid Mechanics)
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(ASME Standards Committee: V & V in Computational Solid Mechanics )
Verification (of codes, calculations): Process of determining that a
model implementation accurately represents the developers
conceptual description of the model and the solution to the model Math issue: Solving the equations right
Validation: Process of determining the degree to which a model is
an accurate representation of the real world from the perspective ofthe intended uses of the model
Physics issue: Solving the right equations
Note: objective of the validation is to maximise confidence inthe predictive capability of the model
Terminology: Correlation, Updating and Validation
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Correlation:
the process of quantifying the degree of similarity and dissimilarity betweentwo models (e.g. FE analysis vs. test)
Error Localization:
the process of determining which areas of the model need to be modified
Updating: mathematical model improvement using data obtained from an associated
experimental model (it can be consistent or inconsistent)
Valid model:
model which predicts the required dynamic behaviour of the subjectstructure with an acceptable degree of accuracy, or correctness
Some remarks on the validation of critical analyses
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Loads analysis is probably the single most influential task indesigning a space structure
Loads analysis is doubly important because it is the basis for statictest loads as well as the basis for identifying the target responsesand notching criteria in sine tests
A single mistake in the loads analysis can mean that we design andtest the structure to the wrong loads
We must be very confident in our loads analysis, which means wemust check the sensitivity of our assumptions and validate theloads analysis that will be the basis of strength analysis and statictesting
Note: Vibro-acoustic, random and shock analyses are usually notcritical in the sense that we normally use environmental tests toverify mechanical requirements
Targets of the correlation(features of interest for quantitative comparison)
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(features of interest for quantitative comparison)
Characteristics that most affect the structure response to applied forces
Natural frequencies
Mode shapes
Modal effective masses
Modal damping
Total mass, mass distribution
Centre of Gravity, inertia
Static stiffness Interface forces
Correlation of mode shapesCorrelation of mode shapes
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Spacehab FEM coupled to the test rig model & Silhouette
GOCE modal analysis and survey testGOCE modal analysis and survey test
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CrossCross--Orthogonality Check (COC) and Modal Assurance Criterion (MAC)Orthogonality Check (COC) and Modal Assurance Criterion (MAC)
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The cross-orthogonality between the analysis and test mode
shapes with respect to the mass matrix is given by:
The MAC between a measured mode and an analytical mode is
defined as:
aTm MC
as
Tasmr
Tmr
asTmr
rsMAC 2
Note: COC and MAC do not give a useful measure of the error!
Columbus: Cross-Orthogonality Check up to 35 Hz (target modes)TEST
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
FEM Err.% [Hz] 13.78 15.80 17.20 23.81 24.23 24.65 25.36 25.59 26.59 27.19 27.53 28.87 30.19 30.55 32.73 33.15 33.86 34.57 35.21 36.16
1 -2.94 13.37 1.00
2 -0.95 15.65 1.00
3 -1.73 16.90 0.99
4 -3.26 23.03 0.93 0.35
5 -1.16 23.95 0.34 0.93
6 -2.00 24.16 0.95
7 -1.98 24.86 0.95 0.27
8 -0.12 25.56 0.86
9 -0.95 26.34 0.22 0.90
10 -2.65 26.47 0.95
11 -0.40 27.42 0.26 0.96
12 -3.65 27.82 0.82 0.27
13 -6.00 28.38 0.46 0.89
15 1.19 30.91 0.26 0.95
17 1.63 33.26 0.94 0.21 0.32
18 - 33.71 0.64 0.34 0.62
19 -4.72 34.45 0.95
20 1.21 34.99 0.57 0.81
Soho SVM Cross-Orthogonality CheckF E M
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F.E.M.
1 2 3 4 8 9 10 15 21 26 29 30 31
TEST Err. % Freq. Hz 34.83 37.24 44.07 45.19 51.51 52.68 55.46 62.18 70.92 77.99 81.53 82.25 84.42
1 2.87 35.86 0.87 0.46
2 0.00 37.24 0.47 0.87
3 4.17 45.99 0.87
4 4.78 47.46 0.77 0.245 -3.39 49.82 -0.33 0.76
6 0.96 53.19 0.75 0.22
7 2.10 56.65 0.79 0.21
8 58.67 -0.28 -0.35 -0.22
9 60.24 -0.30 -0.22 0.46 0.46
10 3.30 64.30 0.61
11 66.40 0.21 0.32
12 67.50 0.45 -0.43
13 68.73-0.38
14 69.68
15 71.69
16 72.71 0.37 - 0.33
17 3.30 73.34 0.21 0.85
18 74.78
19 75.63
20 78.77 -0.24
21 82.12
22 7.72 84.51 0.76
23 5.54 86.31 0.87 -0.23
24 7.21 88.64 0.64
25 5.45 89.29 -0.33 0.63
26 94.44
27 97.15
28 99.56
GOCEGOCE -- MAC and Effective MassMAC and Effective Mass
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Lack of Matching between F.E. Model and Test
(
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Modelling uncertainties and errors (model is not completely physicallyrepresentative)
Approximation of boundary conditions
Inadequate modelling of joints and couplings Lack or inappropriate damping representation
The linear assumption of the model versus test non-linearities
Mistakes (input errors, oversights, etc.)
Scatter in manufacturing Uncertainties in physical properties (geometry, tolerances, material properties)
Uncertainties and errors in testing
Measured data or parameters contain levels of errors
Uncertainties in the test set-up, input loads, boundary conditions etc.
Mistakes (oversights, cabling errors, etc.)
Test-Analysis Correlation CriteriaThe degree of similarity or dissimilarity establishing that the correlationbetween measured and predicted values is acceptable
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between measured and predicted values is acceptable
ECSS-E-ST-32-11 Proposed Test / Analysis Correlation Criteria
7. Conclusions7. Conclusions
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Spacecraft Loads Analysis - Contents
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1. Introduction and general aspects
2. Mechanical environment
3. Requirements for spacecraft structures
4. Mathematical models and structural analyses
5. Spacecraft mechanical testing
6. Mathematical models validation
7. Conclusions
Final VerificationFinal Verification
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Consist of:
Making sure all requirements are satisfied (compliance)
Validating the methods and assumptions used to satisfy
requirements
Assessing risks
Criteria for Assessing Verification Loads (strength)Criteria for Assessing Verification Loads (strength)
Analysis: margins of safety must me greater that or equal to zero
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Analysis: margins of safety must me greater that or equal to zero
Test: Structures qualified by static or sinusoidal testing Test loads or stresses as predicted (test-verified math model and test
conditions) are compared with the total predicted loads during the
mission (including flying transients, acoustics, random vibration,
pressure, thermal effects and preloads)
Test: Structures qualified by acoustic or random vibration testing
Test environments are compared with random-vibration environments
derived from system-level acoustic testing
Final Verification (crucial points)Final Verification (crucial points)
To perform a Verification Loads Cycle for structures designed
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To perform a Verification Loads Cycle for structures designed
and tested to predicted loads
Finite element models correlation with the results of modal andstatic testing
Loads prediction with the current forcing functions
Compliance with analysis criteria (e.g. MOS>0)
To make sure the random-vibration environments used to
qualify components were high enough (based on data
collected during the spacecraft acoustic test)
Note: in the verification loads cycle instead of identifying required
design changes (design loads cycle) the adequacy of the structure
that has already been built and tested is assessed
Technical inconsistenciesTechnical inconsistencies
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Uni-axial vibration and shock test facilities
Low frequency transient often simulated at the subsystem andsystem assembly level using a swept-sine vibration test
Infinite mechanical impedance of the shaker (and the standard
practice of controlling the input acceleration to the frequency
envelope of the flight data) Vibro-acoustic environment often simulated at the subsystem
and units assembly level using a random vibration test
Test levels largely based on computational analyses (we must
validate critical load analyses!)
NEW TRENDSNEW TRENDS
Protoflight vs prototype
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Protoflight vs prototype
Testing (force limited vibration testing)
Computational mechanics
Vibro-acoustic analysis
Random vibration analysis
Complexity of the Industrial organization
BibliographyBibliography
Sarafin T.P. Spacecraft Structures and Mechanisms, Kluwer, 1995
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p , ,
Craig R.R., Structural Dynamics An introduction to computer methods, J. Wiley
and Sons, 1981
Clough R.W., Penzien J., Dynamics of Structures, McGraw-Hill, 1993
Ewins D.J., Modal Testing Theory, practice and applications, Research Studies
Press, Second Edition, 2000
Wijker J., Mechanical Vibrations in Spacecraft Design, Springer, 2004
Girard A., Roy N., Structural Dynamics in Industry, J. Wiley and Sons, 2008
Steinberg D.S., Vibration Analysis for Electronic Equipment, J. Wiley and Sons,
2000
Friswell M.I., Mottershead J.E., Finite Element Model Updating in Structural
Dynamics, Kluwer 1995 Ariane 5 Users Manual, Arianespace, http://www.arianespace.com/
BibliographyBibliography -- ECSS DocumentsECSS Documents
ECSS-E-HB-32-26 Spacecraft Loads Analysis (to be published)
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p y ( p )
ECSS-E-ST-32 Space Project Engineering - Structural
ECSS-E-ST-32-03 Structural finite element models ECSS-E-ST-32-10 Structural factors of safety for spaceflight
hardware
ECSS-E-ST-32-02 Structural design and verification of
pressurized hardware ECSS-E-ST-32-11 Modal survey assessment
ECSS-E-ST-32-01 Fracture control
ECSS-E-10-02 Space Engineering - Verification
ECSS-E-10-03 Space Engineering - Testing
THE END!
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THE END!
Acknowledgements:
TAS France for the data concerning the project SENTINEL-3
ALENIA SPAZIO, Italy, for the data concerning the projects GOCE, COLUMBUS,MPLM and SOHO
EADS ASTRIUM, UK, for the data concerning the project AEOLUS and
EarthCARE
ESA/ESTEC, Structures Section, NL, for the data concerning ARIANE 5 FE
model and LV/SC CLA
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