spacecraft loads analysis

Spacecraft Loads Analysis - A. Calvi 1

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 Analysis An Overview

Adriano Calvi, PhDESA / ESTEC, Noordwijk, The Netherlands


Spacecraft Loads Analysis - Contents

1. Introduction and general aspects2. Mechanical environment3. Requirements for spacecraft structures4. Mathematical models and structural analyses5. Spacecraft mechanical testing6. Mathematical models validation7. Conclusions


Example of satellite structural design conceptExample of satellite structural design concept


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 for designed or built hardware.


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…

• PhysicsStructural dynamics, validation of mathematical models, criteria…

• MathematicsComputational models, verification of models…


““OrganizationsOrganizations”” and and ““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


Levels of AssemblyLevels of Assembly

RFFE

RPM

RFFE

RPM


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 significant

ground, 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


Design loads cycle processDesign loads cycle process


Loads and Factors

Expendable launch vehicles,pressurized hardware and manned system Test Logic

Common Design LogicSatellitesTest Logic

Limit Loads - LL

Design Limit LoadsDLL

x Coef. A

DYL

x Coef. B

DUL

x Coef. C

x KQ x KA

QLAL

x KQ x KA

QL

AL

Incr

easi

ng L

oad

Leve

lECSS E-ST-32-10

“Protoflight” “Prototype”


2. Mechanical environment2. Mechanical environment


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


AccelerationsAccelerations…… some remarkssome remarks

• The parameter most commonly used (in the industry) to “define the motion of a mechanical system” is the acceleration

• Good reasons: accelerations are directly related to forces/stresses and “easy” to specify and measure

• In practice accelerations are used as a measure of the severity of the mechanical 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 they are 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)


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


““SteadySteady--statestate”” and lowand low--frequency transient accelerationsfrequency transient accelerations


Acoustic LoadsAcoustic Loads

• During the lift off and the early phases of the 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


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


ShocksShocks

Mainly caused by the actuation of pyrotechnic devices:• Release mechanisms for stage and satellite separation• Deployable mechanisms for solar arrays etc.


ShocksShocks

Time [t] Frequency [Hz]

[g][g]


Static and dynamic environment specification (typical ranges)Static and dynamic environment specification (typical ranges)


QuasiQuasi--Static Loads (accelerations)Static Loads (accelerations)• “Loads independent of time or which vary slowly, so that the dynamic

response 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 the design 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-state accelerations (unchanging applied force balanced by inertia loads). For design purposes (e.g. derivation of design limit loads, selection of the fasteners, etc.), the quasi-static loads are normally calculated by combining both static and dynamic load contributions. In this context the quasi static loads are equivalent to (or interpreted by the designer as) static loads, typically expressed as equivalent accelerations at the C.o.G.


3. Requirements for spacecraft structures3. Requirements for spacecraft structures


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 and spacecraft subsystems with enough strength and stiffness to preclude any failure (rupture, collapse, or detrimental deformation) that may keep them from working successfully.


Requirements evolutionRequirements evolution


Some definitionsSome definitions• Design:

– The process used to generate the set information describing the essential characteristics of a product (ECSS-P-001A)

– Design means developing requirements, identifying options, doinganalyses and trade studies, and defining a product in enough detail so it can be built (T. P. Sarafin)

• Verification:– Confirmation by examination and provision of objective evidence that

specified requirements have been fulfilled (ISO 8402:1994)– Verification means providing confidence through disciplined steps that

a product will do what it is supposed to do (T. P. Sarafin)

• Note: we can “prove” that the spacecraft satisfies the measurable criteria we have defined, but we cannot “prove” a space mission will be successful


Design requirements and verificationDesign requirements and verification


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 or

exhibiting 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.


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 be delivered 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.


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 between low frequency dynamics of the launch vehicle and payload modes


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


Dynamic analysis typesDynamic analysis types• Real eigenvalue analysis (undamped free vibrations)

– Modal parameter identification, etc.• Linear frequency response analysis (steady-state response of

linear 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


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 its

supporting 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,…


Real eigenvalue analysisReal eigenvalue analysis

Note: mode shape normalizationScaling is arbitrary

Convention: “Mass”, “Max” or “Point”


Mode shapesMode shapes

Cantilever beam Simply supported beam


Satellite Normal Modes Analysis

Mode 1: 16.2 Hz Mode 2: 18.3 Hz

INTEGRAL Satellite (FEM size 120000 DOF’s)


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:


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 dynamicresponses for excitation frequencies near natural frequencies (resonant frequencies)

• Use a fine enough frequency step size (Δf) 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


Harmonic forced response with dampingHarmonic forced response with damping


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 + Duhamel’s integral or Newmark)

• Dynamic equation of motion:


Modal Transient Response AnalysisModal Transient Response Analysis


Transient response considerationsTransient response considerations

• The integration time step must be small enough to represent accurately the 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


““ShockShock”” response spectrum (and analysis)response spectrum (and analysis)• Response spectrum analysis is an approximate method of

computing 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 a limited use in structural dynamics of spacecraft (e.g. preliminary design) since the accuracy of the method may be questionable

• Note 2: the term “shock” can be misleading (not always a “physical shock”, i.e. an environment of a “short duration”, is involved. It would be better to use “response spectrum”)


Generation of a response spectrum (1)Generation of a response spectrum (1)


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 is

computed• 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


Shock Response Spectrum. Some remarksShock Response Spectrum. Some remarks• The 1-DOF system is used as reference structure (since the

simplest) for the characterization of environments (i.e. quantification of the severity → equivalent environments can be specified)

• In practice, the criterion used for the severity is the maximum response which occurs on the structure (note: another criterion relates to the concept of fatigue damage…)

• A risk in comparing two excitations of different nature is in the influence of damping on the results (e.g. maxima are proportional to 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)


Shock Response SpectrumShock Response Spectrum

(A) is the shock spectrum of a terminal peak sawtooth (B) of 500 G peak amplitude and 0.4 millisecond duration


Random vibration (analysis)Random vibration (analysis)• Random vibration is vibration that can be described only in a

statistical sense• The instantaneous magnitude is not known at any given time;

rather, the magnitude is expressed in terms of its statistical properties (such as mean value, standard deviation, and probability of exceeding a certain value)

• Examples of random vibration include earthquake ground motion, wind pressure fluctuations on aircraft, and acoustic excitation due to rocket and jet engine noise

• These random excitations are usually described in terms of a power spectral density (PSD) function

• Note: in structural dynamics of spacecraft, the random vibration analysis is often performed with simplified techniques (e.g. based on “Miles’ equation” + effective modal mass models)


Random noise with normal amplitude distributionRandom noise with normal amplitude distribution


Power Spectral Density (conceptual model)Power Spectral Density (conceptual model)


Sound Pressure Level (conceptual model)Sound Pressure Level (conceptual model)


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


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, in

terms 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



• 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



• 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)


Example of Effective Mass table (MPLM test and FE model)


A5 Typical Sequence of eventsA5 Typical Sequence of events


A5 Typical Longitudinal Static AccelerationA5 Typical Longitudinal Static Acceleration


Sources of Structural Loadings (Launch)Sources of Structural Loadings (Launch)

Axial-Acceleration Profile for the Rockot Launch Vehicle

g8

7

6

5

4

32

1

0t, s150 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

acce

lera

tion

[m/s

2 ]

t [s]


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

acce

lera

tion

[m/s

2 ]

t [s]


Load Factors for Preliminary Design (Ariane 5)Load Factors for Preliminary Design (Ariane 5)


QuasiQuasi--Static Flight Limit loads for Dnepr and SoyuzStatic Flight Limit loads for Dnepr and Soyuz


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 to low frequency mechanical environment

• Main Objective: to calculate the loads on the satellite caused by the launch transients (lift-off, transonic, aerodynamic gust, separation of SRBs…)

• Loads (in this context): set of internal forces, displacements and accelerations that characterise structural response to the applied forces

• Effects included in the forcing functions : thrust built-up, engine shut-down/burnout, gravity, aerodynamic loads (gust), separation of boosters, etc.


Ariane-5 Dynamic Mathematical ModelPAYLOAD

UPPER COMPOSITE

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


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


CLA OutputCLA Output• LV-SC interface accelerations

– Equivalent sine spectrum

• LV-SC interface forces– Equivalent accelerations at CoG

• Internal responses– Accelerations,– Displacements– Forces– Stresses– …


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 ForceZ Dir. [N]

Lift-offKeel Fitting

I/F ForceY Dir. [N]


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.


SRS/ESI of the following transient acceleration:

QSRSESI

ESI

ESI


ESI for Spacecraft

CLA (Coupled Load Analysis)

0 10 20 30 40 50 60 70 800

50

100

150

200

250

SR

S [m

/s2 ]

frequency [Hz]

SRSQSRSESI

ESI

12

Q

SRSESI

SRS

Difference is negligible for small damping ratios


/[ sm

][s

2.41

[Hz

]/[ sm 2.46

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


5. Spacecraft testing5. Spacecraft testing

5.1 Introduction and general aspects

5.1 Mechanical testsModal survey testSinusoidal vibration testAcoustic noise testShock testRandom vibration test

5.2 Over-testing and… under-testing


Testing techniques Testing 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 structure

identification (the objective is to determine the dynamic characteristics of the 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 technically possible to perform them using the same test facility


Testing techniques Testing 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


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


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 )


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 are not exceeded.


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


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 whose parameters are then determined by processing the measurements


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 tuningIncr

easi

ng e

ffor

t

Dat

a co

nsis

tenc

y


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)


Ariane 5 Ariane 5 -- Sine excitation at spacecraft base (Sine excitation at spacecraft base (sine-equivalent dynamics)


Test Set-up for Satellite Vibration Tests


Herschel on Hydra


Acoustic test (objectives)Acoustic test (objectives)

• Demonstrate the ability of a specimen to withstand the acoustic environment during launch

• Validation of analytical models

• System level tests verify equipment qualification loads

• Acceptance test for S/C flight models


Ariane 5 Ariane 5 –– Acoustic noise spectrum under the fairingAcoustic noise spectrum under the fairing


Shock test. Objectives and remarksShock test. Objectives and remarks

• Demonstrate the ability of a specimen to withstand the shock loads during launch and operation

• Verify equipment qualification loads during system level tests

• System level shock tests are generally performed with the actual shock generating equipment (e.g. clamp band release)

• or by using of a sophisticated pyro-shock generating system (SHOGUN for ARIANE 5 payloads)


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)

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


Overtesting:Overtesting:an introductionan introduction(vibration absorber effect)(vibration absorber effect)


Introduction to overtesting and notching• The qualification of the satellite to low

frequency transient is normally achieved by a base-shake test

• The input spectrum specifies the acceleration input that should excite the satellite, for each axis

• This input is definitively different from the mission loads, which are transient

• Notching: “Reduction of acceleration input spectrum in narrow frequency bands, usually where test item has resonances” (NASA-HDBK-7004)


GOCE on ESTEC Large Slip Table Herschel on ESTEC Large Slip Table


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 flightconfiguration (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 and neither 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. In practice the notching simulates the antiresonances in the coupled configuration


Notching

ESI (equivalent sine)


Random vibration test: notching of test specificationRandom vibration test: notching of test specification

Illustration of notching of random vibration test specification,at the frequencies of strong test item resonances


6. Mathematical models validation6. Mathematical models validation


Validation of Finite Element Models(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”)

• Verification (of codes, calculations): Process of determining that a model implementation accurately represents the developer’s 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 of the intended uses of the model

– Physics issue: “Solving the right equations”

Note: objective of the validation is to maximise confidence in the predictive capability of the model


Terminology: Correlation, Updating and Validation• Correlation:

– the process of quantifying the degree of similarity and dissimilarity between two 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 subject

structure with an acceptable degree of accuracy, or “correctness”


Some remarks on the validation of “critical analyses”

• Loads analysis is probably the single most influential task in designing a space structure

• Loads analysis is doubly important because it is the basis for static test loads as well as the basis for identifying the target responses and “notching criteria” in sine tests

• A single mistake in the loads analysis can mean that we design and test the structure to the wrong loads

• We must be very confident in our loads analysis, which means we must check the sensitivity of our assumptions and validate the loads analysis that will be the basis of strength analysis and static testing

• Note: Vibro-acoustic, random and shock analyses are usually “not critical” in the sense that we normally use environmental tests to verify mechanical requirements


Targets of the correlation(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

• Spacehab FEM coupled to the test rig model & Silhouette


GOCE modal analysis and survey testGOCE modal analysis and survey test


CrossCross--Orthogonality Check (COC) and Modal Assurance Criterion (MAC) Orthogonality Check (COC) and Modal Assurance Criterion (MAC)

• 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

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.

1 2 3 4 8 9 10 15 21 26 29 30 31TEST 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 0.462 0.00 37.24 0.47 0.873 4.17 45.99 0.874 4.78 47.46 0.77 0.245 -3.39 49.82 -0.33 0.766 0.96 53.19 0.75 0.227 2.10 56.65 0.79 0.218 58.67 -0.28 -0.35 -0.229 60.24 -0.30 -0.22 0.46 0.4610 3.30 64.30 0.6111 66.40 0.21 0.3212 67.50 0.45 -0.4313 68.73 -0.3814 69.6815 71.6916 72.71 0.37 -0.3317 3.30 73.34 0.21 0.8518 74.7819 75.6320 78.77 -0.2421 82.1222 7.72 84.51 0.7623 5.54 86.31 0.87 -0.2324 7.21 88.64 0.6425 5.45 89.29 -0.33 0.6326 94.4427 97.1528 99.56


GOCE GOCE -- MAC and Effective MassMAC and Effective Mass


Lack of Matching between F.E. Model and Test• Modelling uncertainties and errors (model is not completely physically

representative)– 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 correlation between measured and predicted values is acceptable

ECSS-E-ST-32-11 Proposed Test / Analysis Correlation Criteria


7. Conclusions7. Conclusions


Spacecraft Loads Analysis - Contents

1. Introduction and general aspects2. Mechanical environment3. Requirements for spacecraft structures4. Mathematical models and structural analyses5. Spacecraft mechanical testing6. Mathematical models validation7. Conclusions


Final VerificationFinal Verification

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

• 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 and tested to predicted loads

– Finite element models correlation with the results of modal and static 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””

• Uni-axial vibration and shock test facilities• Low frequency transient often simulated at the subsystem and

system 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

• 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• 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 User’s Manual, Arianespace, http://www.arianespace.com/


Bibliography Bibliography -- ECSS DocumentsECSS Documents• ECSS-E-HB-32-26 Spacecraft Loads Analysis (to be published)• 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!Acknowledgements:

TAS France for the data concerning the project SENTINEL-3ALENIA SPAZIO, Italy, for the data concerning the projects GOCE, COLUMBUS,

MPLM and SOHOEADS ASTRIUM, UK, for the data concerning the project AEOLUS and

EarthCAREESA/ESTEC, Structures Section, NL, for the data concerning ARIANE 5 FE

model and LV/SC CLA