ISO 21648:2008

Space systems — Flywheel module design and testing

ISO 21648:2008 establishes the design, analysis, material selection and characterization, fabrication, test and inspection of the flywheel module (FM) in a flywheel used for energy storage in space systems. These requirements, when implemented on a flywheel module, will ensure a high level of confidence in achieving safe operation and mission success. With appropriate modifications, ISO 21648:2008 can also be applied to similar devices, such as momentum and reaction wheels and control-moment gyroscopes. The requirements set forth in ISO 21648:2008 are the minimum requirements for flywheel modules in flywheels used in space flight applications. They are specifically applicable to the parts in the flywheel rotor assembly (FRA), including rim, hub and/or shaft and other associated rotating parts, such as the bearings and the motor generator rotor. The requirements are also relevant to the non-rotating parts, such as module housing, main suspension assembly (magnetic or rolling element bearings, superconductor bearings, etc.), motor stator, caging mechanism and sensors within the module housing, and backup bearings, if applicable. However, control and interface electronics are not covered in ISO 21648:2008.

Systèmes spatiaux — Conception et essai du module de volant moteur

General Information

• Status: Published
• Publication Date: 19-Nov-2008
• ICS: 49.140 - Space systems and operations
• Technical Committee: ISO/TC 20/SC 14 - Space systems and operations
• Drafting Committee: ISO/TC 20/SC 14/WG 1 - Design engineering and production
• Current Stage: 9093 - International Standard confirmed
• Completion Date: 05-Dec-2019

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 21648
First edition
2008-12-01

Space systems — Flywheel module
design and testing
Systèmes spatiaux — Conception et essai du module de volant moteur




Reference number
ISO 21648:2008(E)
©
ISO 2008

---------------------- Page: 1 ----------------------
ISO 21648:2008(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.


COPYRIGHT PROTECTED DOCUMENT


©  ISO 2008
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail [e-mail]
Web www.iso.org
Published in Switzerland

ii © ISO 2008 – All rights reserved

---------------------- Page: 2 ----------------------
ISO 21648:2008(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Terms, definitions, symbols and abbreviated terms. 1
2.1 Terms and definitions. 1
2.2 Symbols . 5
2.3 Abbreviated terms . 6
3 Requirements . 6
3.1 General requirements. 6
3.2 Design requirements . 7
3.3 Requirements for materials . 11
3.4 Fabrication and process control. 14
3.5 Quality assurance. 15
3.6 Repair and refurbishment . 16
3.7 Storage requirements. 16
3.8 Transportation requirements. 16
4 Verification requirements . 17
4.1 Design requirements verification. 17
4.2 Qualification tests. 20
4.3 Acceptance tests . 24
Bibliography . 28

© ISO 2008 – All rights reserved iii

---------------------- Page: 3 ----------------------
ISO 21648:2008(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 21648 was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles, Subcommittee
SC 14, Space systems and operations.
iv © ISO 2008 – All rights reserved

---------------------- Page: 4 ----------------------
ISO 21648:2008(E)
Introduction
Flywheels are mechanical devices that store kinetic energy in a rotating mass. A simple example is the
potter’s wheel, which was widely used by people in ancient times. The first use of such devices dates from
between 3500 and 3000 BC. According to archaeological evidence, these early flywheels were built from
wood, stone and clay. One type of potter’s wheel was a rim made from a unidirectional material (bamboo),
wound in the hoop direction and embedded in a matrix (clay). This design option is clearly a foreshadowing of
the later use of composites for their inherent strength and lightweight nature.
It is, however, only since the 1970s that the use of flywheels as energy storage systems has become the
focus of serious attention from energy researchers due to the constant threat of a shortage of fossil fuel
supplies. Today, a typical flywheel energy system consists of a flywheel rotor, a supporting device (magnetic
bearing ball bearings, superconductor bearings or other types of bearings), a charge/discharge device
(motor/generator) and a safety containment (housing). For space applications, due to weight constraints, the
use of a bulky safety containment system is not necessarily a desirable design. Thus, from a safety point of
view, the design of flywheel energy systems needs to concentrate on reliability and longevity.
Current flywheel energy storage technology is made possible by the use of high-strength, carbon-fibre-based
composite materials in the rotor. Flywheel energy storage systems are designed to both control spacecraft
attitude and to store energy — functions which have historically been performed by two separate systems.
The stored energy is needed for the dark portions of the orbit when the Earth’s shadow makes solar power
unavailable for spacecraft. For many spacecraft, flywheels offer the potential to significantly reduce weight and
extend service life. However, the use of composite materials, coupled with variations in design approaches
and demanding operating conditions, combine to present certification challenges for the rotor assemblies.
This International Standard establishes the design, analysis, material selection and characterization,
fabrication, test and inspection of the flywheel module in a flywheel. Many requirements set forth in this
International Standard can also be adapted by similar types of rotating machineries, but for different usage.
The momentum wheels and momentum gyroscopes are typical examples. The implementation of these
requirements will ensure a high level of confidence in achieving safe operation and mission success for these
critical hardware items.
© ISO 2008 – All rights reserved v

---------------------- Page: 5 ----------------------
INTERNATIONAL STANDARD ISO 21648:2008(E)

Space systems — Flywheel module design and testing
1 Scope
This International Standard establishes the design, analysis, material selection and characterization,
fabrication, test and inspection of the flywheel module (FM) in a flywheel used for energy storage in space
systems. These requirements, when implemented on a flywheel module, will ensure a high level of confidence
in achieving safe operation and mission success. With appropriate modifications, this International Standard
can also be applied to similar devices, such as momentum and reaction wheels and control-moment
gyroscopes.
The requirements set forth in this International Standard are the minimum requirements for flywheel modules
in flywheels used in space flight applications. They are specifically applicable to the parts in the flywheel rotor
assembly (FRA), including rim, hub and/or shaft and other associated rotating parts, such as the bearings and
the motor generator rotor. The requirements are also relevant to the non-rotating parts, such as module
housing, main suspension assembly (magnetic or rolling element bearings, superconductor bearings, etc.),
motor stator, caging mechanism and sensors within the module housing, and backup bearings, if applicable.
However, control and interface electronics are not covered in this International Standard.
2 Terms, definitions, symbols and abbreviated terms
2.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
2.1.1
A-basis allowable
mechanical strength value above which at least 99 % of the population of values is expected to fall, with a
confidence level of 95 %
NOTE See also B-basis allowable (2.1.4).
2.1.2
acceptance tests
required formal tests conducted on hardware items to ascertain that the materials, manufacturing processes
and workmanship meet specifications
2.1.3
allowable load
allowable stress
allowable strain
maximum load that can be accommodated by a structure/material without rupture, collapse or detrimental
deformation in a given environment
NOTE Allowable loads commonly correspond to the statistically-based minimum ultimate strength, buckling strength
and yield strength, as applicable.
© ISO 2008 – All rights reserved 1

---------------------- Page: 6 ----------------------
ISO 21648:2008(E)
2.1.4
B-basis allowable
mechanical strength value above which at least 90 % of the population of values is expected to fall, with a
confidence level of 95 %
NOTE See also A-basis allowable (2.1.1).
2.1.5
catastrophic failure
structural failure event due to the rotor separation, or the rupture or collapse, of other flywheel rotor assembly
components or assembly
2.1.6
composite material
combination of materials which differ in composition or form on a macro-scale
NOTE The constituents retain their identities in the composite, i.e. they do not dissolve or otherwise merge
completely into each other, although they act in concert. Normally, the composites can be physically identified and exhibit
an interface between one another.
2.1.7
damage tolerance
ability of structure/material to resist failure due to the presence of flaws for a specified period of unrepaired
usage
2.1.8
damage tolerance life
required period during which a part of a flywheel module, even containing a large undetected crack, is shown
by analysis or testing not to fail catastrophically in the expected service load and environment
2.1.9
damage tolerance analysis
damage tolerance testing
analysis/testing that is used to demonstrate damage tolerance life
NOTE For metallic parts, this type of analysis is also referred to as safe-life analysis.
2.1.10
design safety factor
multiplying factor to be applied to the limit load and/or maximum expected operating speed
2.1.11
fatigue life
number of load cycles experienced in service that a defect-free part in a flywheel module can sustain before
failure of a specified nature could occur
NOTE The number of load cycles experienced in service can be flight loads, ground test loads and charge/discharge
cycles.
2.1.12
flaw
local discontinuity in a structural material
EXAMPLE Crack, delamination, void.
2.1.13
flight-like test article
test article that is built in accordance with a fabrication process identical to the flight hardware
2 © ISO 2008 – All rights reserved

---------------------- Page: 7 ----------------------
ISO 21648:2008(E)
2.1.14
flywheel module
FM
assembly of mechanical parts which support and spin the flywheel rotor assembly and which house the
appropriate sensors, rotor support systems and motor, which with the appropriate avionics suite and software
can act as a stand-alone functional flywheel unit
NOTE A flywheel module typically includes the housing, main suspension system (magnetic or rolling element
bearing, superconductor bearings), motor stator, caging mechanism, sensors and backup bearings, if applicable.
2.1.15
flywheel rotor assembly
FRA
assembly in a flywheel which consists of rim, shaft and/or hub, bearings, motor generator rotor and other
associated parts that rotate under normal operation
2.1.16
fracture critical part
classification of a part for manned space systems, which assumes that fracture or failure of that part resulting
from occurrence of a crack-like defect would create a catastrophic hazard
NOTE Such classification is required on components unless it can be shown otherwise, i.e. if the part (and
subsequent parts it could fail) can be shown to be contained, or in the case of low released energy, or if the part is failsafe,
or if there is only a remote possibility of significant crack growth on the part to begin with.
2.1.17
fracture control
application of design philosophy, analysis method, manufacturing technology, quality assurance and operating
procedures to prevent premature structural failure caused by the propagation of cracks or crack-like flaws
during fabrication, assembly, testing, transportation and ground-handling and service
2.1.18
fracture mechanics
engineering discipline that describes the behaviour of cracks or crack-like flaws in materials under stress
2.1.19
fracture toughness
generic term for measurements of resistance to extension of a crack
2.1.20
impact damage
damage in a non-metallic part within the flywheel module that is caused by an object striking the part or by the
part striking an object
2.1.21
impact damage tolerance
ability of the fracture critical non-metallic parts in the flywheel module to resist strength degradation due to the
impact damage event
2.1.22
initial flaw size
maximum flaw size, as defined by non-destructive evaluation, that is assumed to exist for the purpose of
performing a damage tolerance (safe-life) analysis or testing
2.1.23
key process parameter
KPP
critical process parameter that affects design and product characteristics
© ISO 2008 – All rights reserved 3

---------------------- Page: 8 ----------------------
ISO 21648:2008(E)
2.1.24
life factor
factor by which the service life is multiplied to obtain total fatigue life or damage tolerance life
NOTE Life factor is often referred to as a scatter factor that is normally used to account for the scatter of a material’s
fatigue or crack growth rate data. It can also account for the dispersion of loading spectra parameters and other
uncertainties, when appropriate.
2.1.25
limit load
maximum expected external load, or combination of loads, that a rotating part can experience during the
performance of a specified mission in specified environments
NOTE When a statistical estimate is applicable, the limit load is that load not expected to be exceeded at 99 %
probability with 90 % confidence.
2.1.26
margin of safety
MS
⎛⎞
τ
allow
margin of safety expressed as −1
⎜⎟
τ ×k
⎝⎠limit safe
where
τ is the allowable load;
allow
τ is the limit load;
limit
k is the design safety factor
safe
NOTE Load can mean stress or strain (see 2.1.3).
2.1.27
maximum expected operating speed
MEOS
maximum spinning speed that a part in a flywheel module is expected to experience during its normal
operation
NOTE Maximum expected operating speed is synonymous with limit speed.
2.1.28
maximum design speed
MDS
highest possible operating speed based on a combination of credible failures
NOTE Maximum design speed is required for some manned systems to accommodate any combination of two
credible failures that will affect speed.
2.1.29
non-destructive evaluation
NDE
process or procedure for determining the quality or characteristics of a material, part or assembly without
permanently altering the subject or its properties
NOTE In this International Standard, non-destructive evaluation is synonymous with non-destructive inspection (NDI)
and non-destructive testing (NDT).
4 © ISO 2008 – All rights reserved

---------------------- Page: 9 ----------------------
ISO 21648:2008(E)
2.1.30
operating environments
all environments experienced during service life of the flywheel module
2.1.31
proof spin test
spin test run on a flight flywheel module at a pre-selected spinning speed that is higher than maximum
expected operating speed
2.1.32
qualification tests
required formal tests used to demonstrate that the design, manufacturing and assembly have resulted in
hardware conforming to specification requirements
NOTE Qualification test is synonymous with certification test.
2.1.33
service life
period of time (or cycles) starting with item inspection after the manufacturing and continuing through all
testing, handling, storage, transportation, normal operation, refurbishment, re-testing and reuse that may be
required or specified for that part
2.1.34
stress-rupture life
time during which the composite maintains structural integrity considering the combined effects of stress
level(s), time at stress level(s) and associated environments
2.1.35
touchdown bearings
bearings required to act as the rotor suspension system in the non-operating mode and/or the backup
suspension system in the operating mode during main suspension system failure
2.1.36
touchdown event
event that can occur with flywheel modules supported on magnetic bearings whereby the rotor is
unexpectedly forced onto its touchdown bearings during normal operation due to malfunction of magnetic
bearings, overload or other anomaly
2.1.37
ultimate load
product of the limit load and the design ultimate safety factor
NOTE The ultimate load is the load that the parts in a flywheel module need to withstand without catastrophic failure
in the expected environment.
2.1.38
visual damage threshold
VDT
impact energy level shown by test(s) which creates an indication that is detectable by a trained inspector
using an unaided visual technique
2.2 Symbols
a crack size
da/dN fatigue crack growth rate
I polar mass moment of inertia
p
© ISO 2008 – All rights reserved 5

---------------------- Page: 10 ----------------------
ISO 21648:2008(E)
I transverse mass moment of inertia
t
K plane strain fracture toughness
IC
K stress intensity threshold for stress-corrosion cracking
Iscc
N number of cycle
T glass transition temperature
g
2.3 Abbreviated terms
ATP Acceptance Test Programme
CT Computer Tomography
FM Flywheel Module
FMECA Failure Mode, Effects and Criticality Analysis
FRA Flywheel Rotor Assembly
KPP Key Process Parameter
MDS Maximum Design Speed
MEOS Maximum Expected Operating Speed
MPE Maximum Predicted Environment
MRB Material Review Board
MS Margin of Safety
NDE Non-Destructive Evaluation
NDI Non-Destructive Inspection
NDT Non-Destructive Testing
SP Specification Performance
S-N Stress versus Life
ε-N Strain versus Life
VDT Visual Damage Threshold
3 Requirements
3.1 General requirements
This clause presents the general requirements for the parts in the FM for
⎯ design,
⎯ material selection and characterization,
6 © ISO 2008 – All rights reserved

---------------------- Page: 11 ----------------------
ISO 21648:2008(E)
⎯ fabrication and process control,
⎯ quality assurance,
⎯ repair and refurbishment, and
⎯ storage.
Most of the requirements are specified for both manned and unmanned space systems.
NOTE Requirements primarily applicable to manned space systems are specifically stated and identified by an
asterisk (*) which follows the clause/subclause title or which precedes the paragraph(s) concerned.
3.2 Design requirements
3.2.1 General design requirements
The general design requirements for the parts in the FM are delineated in 3.2.2 to 3.2.14.
3.2.2 System analysis
3.2.2.1 General
A thorough system analysis of the flywheel shall be used to establish design parameters for the FM.
3.2.2.2 System impact threat analysis*
For fracture critical non-metallic parts in the FM used in manned space systems, a system threat analysis
shall be conducted to provide information for preparing the damage control plan. The threat analysis shall
document the conditions (source and magnitude of threat) under which impact damage can occur.
3.2.2.3 Failure Mode, Effects and Criticality Analysis (FMECA)
A Failure Mode, Effects and Criticality Analysis (FMECA) shall be conducted. This analysis is used to
systematically evaluate and document, by item failure mode analysis, the potential effect of each functional or
hardware failure on mission success, personnel and system safety, system performance, maintainability and
maintenance requirements. Each potential failure is ranked by the severity of its effect, in order that
appropriate corrective actions may be taken to eliminate or control the high-risk items.
3.2.3 Loads, speeds and environments
The anticipated load, speed and associated temperatures throughout the service life of the flywheel shall be
used to define the design load/temperature profile for the parts in the FM. Other environmental effects
(radiation, corrosive atmosphere, vacuum, etc.) pertinent to the structural strength and life of these parts shall
be considered, as appropriate.
Throughout this International Standard, limit load and maximum expected operating speed (MEOS) are used
as the baseline load and speed.
NOTE The term maximum design speed (MDS) can be used for the design and testing of FMs. The basic difference
between MDS and MEOS is the degree of consideration of potential credible failure within a FM, and the resultant effects
on the speed of the FM during system operation. MDS is associated with manned systems and is based on the worst-case
combination of two credible system failures. The criteria to be used for the determination of speed for a given design and
application need to be clearly established by the contracting parties.
© ISO 2008 – All rights reserved 7

---------------------- Page: 12 ----------------------
ISO 21648:2008(E)
3.2.4 Strength
3.2.4.1 General
All parts in the FM shall possess adequate strength to preclude detrimental deformation at corresponding limit
loads in the expected test and operating environments throughout their respective service lives. All parts in the
FM shall also possess adequate strength to preclude catastrophic failure at design ultimate load.
3.2.4.2 Local yielding
Local yielding in an FM part due to secondary or peak stresses shall be acceptable only if all of the following
are satisfied:
a) the structural integrity of the part shall be demonstrated by adequate analysis and/or test;
b) there shall be no detrimental deformation that affects FM function or stability; and
c) the service life requirements are met.
3.2.5 Static stiffness
All parts in the FM shall possess adequate stiffness to preclude detrimental deformation at corresponding limit
loads in the expected test and operating environments throughout their respective service lives. All parts shall
also possess adequate stiffness to preclude collapse at design ultimate load.
3.2.6 Rotor dynamics
This set of requirements shall be applied to FRAs in FMs. Natural frequencies and critical speeds shall not be
of a type or of a frequency response that would be deleterious to the safety and operation of the flywheel
system. Established and proven methods shall be followed to minimize the number of lateral critical speeds
and natural frequencies within the operating range, and particularly those that are characterized by a higher
level of strain energy within the rotor (e.g. bending modes of operation). Shaft-bending critical speeds in the
operating speed range shall be avoided, if possible, but if they are present, it shall be demonstrated by test to
be safe to operate at or through the critical speeds. Stability of the rotor to torsion, axial and lateral excitations
(e.g. imbalance, motor torsion oscillations, seismic events, touchdown bearing impact) shall be maximized
through the application of isolation, damping and/or related means to permit stable rotor performance.
3.2.7 Thermal
Thermal effects, including temperatures, thermal gradient, thermal stresses and deformations, and changes in
the physical, mechanical, and the glass transition temperature (T ) of the composite materials of construction,
g
shall be considered in the design of all parts in the FM. These effects shall be based on temperature extremes
in accordance with 3.2.3.
3.2.8 Static strength margin of safety
3.2.8.1 General
For all parts in the FM, the margin of safety (MS) shall be calculated by using material allowable strengths and
the design safety factor. For metallic and composite parts, the minimum design ultimate safety factor shall be
1,5. To allow for local yielding in metallic parts, the yield design safety factor shall be 1,1. For bonded
interfaces and ceramics parts, the minimum design ultimate safety factor shall be 2,0. All MSs shall be
positive for all applicable loading conditions, such as launch, landing and touchdown event.
8 © ISO 2008 – All rights reserved

---------------------- Page: 13 ----------------------
ISO 21648:2008(E)
3.2.8.2 Manned space systems*
For manned space systems, MS calculations for fracture critical parts shall use A-basis allowables. Otherwise,
B-basis allowables may be used. These allowables shall be derived from samples of size and shape
representative of manufacturing to the greatest degree practical. Where properties are not attained from
specimens tied closely to the FM geometry and manufacturing processes, a smaller number of specimens at
the subcomponent level shall be tested to anchor these properties and account for size, shape and residual
stress effects. The speed range used in this analysis shall encompass the MEOS, unless otherwise specified.
3.2.8.3 Unmanned space systems
For unmanned space systems, MS calculations may use B-basis allowables. These allowables shall also be
derived from samples of size and shape representative of manufacturing to the greatest degree practical.
Where properties are not attained from specimens tied closely to the FM geometry and manufacturing
processes, a smaller number of specimens at the subcomponent level shall be tested to anchor these
properties. The speed range used in this analysis shall encompass the MEOS, unless otherwise specified.
3.2.9 Fracture control*
3.2.9.1 Fracture critical selection criteria
Unless otherwise specified, rotating parts that meet containment requirements specified in 3.2.9.2 are not
considered to be fracture critical.
Parts are fracture critical if it is credible that cracks in the part could lead to a catastrophic failure. For
composite materials, the term crack also includes delamination, defects due to manufacturing, impact damage
and in-service mechanical damage.
All fracture critical parts shall meet the damage-tolerance life requirements of 3.2.9.3. For composite fracture
critical parts, the impact damage requirements of 3.2.9.4 shall also be satisfied.
3.2.9.2 Containment
For a rotating part to be classified as a contained part, it shall meet the following requirements:
a) it is clearly contained in associated housing/enclosure, etc.; and
b) it can be shown that the failure of the part will not cause a catastrophic hazard.
Analyses or tests shall be performed where there is uncertainty regarding containment of fragmented pieces.
3.2.9.3 Damage-tolerance life
In a manned space system, all fracture
...