Figure 1. ESA's handbooks and guidelines for structural engineering
ESA's series of structural handbooks provide guidelines to aid in the design process of a flight article, from the conceptual stage through to its qualification. They also provide a central reference source to accumulated project and research and development experience. Activities currently underway to ensure that the books keep pace with the fast evolving knowledge of space structures are also described.
In space applications, the term 'structure' effectively relates to load-bearing components or those with a high mechanical stiffness. Such components may range from the mechanical constituents of a launcher to the parts of a spacecraft antenna dish.
Realising the design of the structure usually involves a process of evolution and iteration in order to achieve the required mass goal and produce a cost-effective product. This involves, from the outset, careful consideration of the purpose and function of the structure in question, as well as of the induced loads and environmental features associated with the mission. These, in turn, influence the choice of the material and the construction format that will be used. In addition, the knowledge base related to the material, the advocated joining techniques, the manufacturing process, and the verification and product assurance tasks must be considered. As the design development and verification programme evolves, the engineer requires progressively more detailed information. The more he can rely on existing experience and information, gained through other, similar projects, the easier the task should be.
ESA has developed a series of Structural Handbooks and Guidelines over the years, to help fulfil such requirements for information (Fig. 1); each handbook brings together the accumulated project and R&D experience into a single source book. The handbooks are intended to ease the interface between design, manufacture and test requirements by highlighting typically important development features. A further aim is to facilitate information exchange within the space community as a means of harmonising approaches to common or similar structural engineering problems. Additionally, research institutes and similar establishments can use them to maintain an awareness of the general state of the art in this field.
Figure 1. ESA's handbooks and guidelines for structural
engineering
At a time when the industry has undergone a period of recession and regrouping, the handbooks also represent an accumulation of knowledge that might otherwise be lost, or require extensive time and effort devoted in costly research to rediscover it.
The information contained within the guidelines is based on many sources, which include:
The collection of such information is not without its difficulties. The size and complexity of space projects means that, whilst important information exists and is documented, it may not be readily accessible. The proprietary nature of part of the information may cause additional complications. If such information is deemed commercially important, access will be carefully controlled. However, much has become available in the interest of enhancing a common knowledge base for the sometimes highly specific needs of the space industry as a whole.
The responsibility for retrieving and synthesising such information has been entrusted in part to ERA Technology (UK) and RJ Technical Consultants (France). Specialists at ESTEC and within industry also participate, depending on the topics of interest. For example, DASA (Bremen, Germany) has contributed extensive information on composites and insert technology; British Aerospace (Bristol, UK) has been largely responsible for the dissemination of information on joints with threaded fasteners; and both British Aerospace and the Institute of Sound and Vibration Research (Southampton University, UK) pioneered the manual on structural acoustics.
The documents are reviewed by an ESA-sponsored Advanced Structural Materials Information Exchange Group (ASMIEG) and by ESTEC staff specialising in materials and structures. The ASMIEG was formed in 1981 and meets three or four times a year. Each ESA member state is entitled to representation. From time to time, the group is supported by other specialists from industry.
Standards and specifications are generally created in response to a specified need.
Unfortunately, in many instances they may have grown independently as national standards, or may even reflect an individual company's needs based on commercial requirements.
It is evident that harmonising standards on a European basis is a challenging task. The case of the historical development of composites, with the inherent anisotropy of such materials as a key factor, is a good example. Their extensive use is accompanied by differing in-house test standards. It is clear that those test standards as practised by companies have often been influenced by commercial requirements. Material portfolios, manufacturing capabilities and techniques, structural design experience, historical data bases and cost of testing all have their influence.
Cooperative programmes appear to be the main incentive for developing common standards as a means of reducing costs and aiding technical collaboration. Compared with programmes the size of Airbus Industrie, for example, it remains to be seen if space programmes are of sufficient commercial longevity to ensure success without additional support. Nevertheless, the handbooks bring together information that 'points-up' any identified need. Continual monitoring of 'who is doing what and how and for what reason', as undertaken for the handbooks, helps to keep such matters in perspective.
As research and development continue to provide new information, the handbooks must also evolve. They are continuously being reviewed and updated. The following is a summary of the status of each book and a discussion of some of the issues affecting the topic or the work now being undertaken in that area.
Aide Memoire on Structural Materials and Space Engineering
The
most recent addition to the series is the Aide Memoire on
Structural Materials and Space Engineering (ESA PSS-03-212). It
brings together in one document all the factors that designers
of space structures must evaluate (Fig. 2). It provides basic
information on the use of metals and fibre-reinforced composite
materials within the whole design development process. The
emphasis is on materials in current use or intended for future
project applications but some consideration is given to emerging
high-temperature materials. The handbook cannot, for reasons of
useable size, present a detailed analysis of each topic. However,
a comprehensive referencing system directs the reader to source
documents, design guides, other manuals and ESA PSS documents,
and verification tools including recognised test specifications
and non-destructive techniques.
Figure 2. Basic contents of the Aide Memoire on Structural
Materials and Space Engineering (PSS-03-212)
Given that project requirements vary with each mission, the first chapter provides an overview of the design-development process for ESA space projects (Fig. 3). Using a systems approach, it describes how the structure is part of the overall project and how the various subsystems interface with each other. The terminology and typical project phases used within the rest of the handbook are defined and illustrated by example. The remaining chapters are organised into 'factors to be considered' at each stage of the project, from concept through to, in some cases, the recovery, refurbishment and re-use of structures.
Figure 3. The design-development process for ESA space projects
Structural Materials Handbook
The Structural Materials Handbook
(ESA PSS-03-203) has been enlarged from the original, only
polymer-based composites design handbook (now incorporated into
Volume 1) to contain information on metal-and ceramic-based
materials, non-polymer composites, coatings, smart materials and
textiles (Volume 2). All aspects of their technology and actual
or potential uses in space structures are described (Fig. 4).
Figure 4. Basic contents of the Structural Materials Handbook
(PSS-03-203)
A revision to the handbook, expected to be issued in 1996, will reflect the growing interest in cyanate ester systems in the field of reinforced plastics. This interest is due, in part, to the need to replace older, discontinued epoxy systems together with the better resistance of cyanate esters to moisture take-up and possibly microcracking.
An investigation, involving five European companies, examined eight cyanate ester or epoxy Ultra High Modulus (UHM) Carbon Fibre Reinforced Plastic (CFRP) systems. In some cases, excellent resistance to micro-cracking was exhibited. The study did not reject any system on the basis of its susceptibility to microcracking. However, the study did demonstrate that 'prepreg' acceptance for space use is determined by a number of parameters, including: availability of specific fibre/resin combinations; confidence in prepreg supply; satisfaction of procurement acceptance criteria; and production of good-quality laminates by optimised processing conditions.
The handbook will also incorporate recent improvements to information on non-destructive testing.
A survey of mechanical test methods for composites as used by industry has demonstrated the strong historical allegiances to specific test methods. However these differ in detail between organisations. Often, their material data bases are built up by consistently using the same method and retaining confidence in the results. There is also evidence to indicate that results from mechanical tests can be operator-dependent, giving further inconsistencies when comparing results from different organisations. In fact, different organisations can place different emphases on the expectations placed on the test methods. Test-method selection criteria will include: comparison of different materials, generation of design data, quality control procedures, damage tolerance investigations and collaborative data exchange.
Such a scenario presents a somewhat daunting prospect in terms of any attempts to impose common standards. However, the section on this topic is being extensively updated to place the situation in proper perspective and to provide clear guidelines on all aspects of the test methods.
Other intentions are to include information on hot structures as developed for the Hermes programme and more general information from Ariane-5 developments.
Guidelines for Carbon and Other Advanced Fibre Prepreg
Procurement Specifications
The Prepreg Procurement Guideline (ESA
PSS-03-207) is intended as a general guide for organisations
preparing specifications for the procurement of particular
thermo-setting resin impregnated reinforcing fibre systems
(prepreg) (Fig. 5). These are primarily epoxy-, bismaleimide-,
polyimide- and cyanate ester-matrix based materials. Many of the
features also relate to thermoplastic-matrix prepregs. The
parameters that may be necessary to control and/or monitor for
qualification and batch control of a prepreg are described.
Figure 5. Basic contents of the Prepreg Procurement Guideline
(PSS-03-207)
Adhesive Bonding Handbook
The Adhesive Bonding Handbook (ESA PSS-
03-210) has recently been updated and reissued (Fig. 6). It now
contains new and extended information on adhesives suited for
space and factors influencing the successful design and
manufacture of bonded joints, both as an assembly technique and
used in repair. Bonding in sandwich panel construction is also
described. Test and inspection methods are covered, along with
application examples in the form of case studies.
Figure 6. Basic contents of the Adhesive Bonding Handbook (PSS-
03-210)
Insert Design Handbook
The wide use of sandwich panels, normally
with a honeycomb core, in load-bearing space structures has meant
that a reliable method is required to join them together and
attach other items. Inserts are used for this purpose. The Insert
Design Handbook (ESA PSS-03-1202) is currently under revision to
expand the content to include the need for information on
subjects highlighted in a comprehensive industry study conducted
in 1995. It is intended to include a simplified mathematical
design approach, test-prediction corroboration studies for real
space structures and a section on the design approach for
composite-skinned sandwich panels rather than the metal-skinned
variety (Fig. 7).
Figure 7. Contents of the current Insert Design Handbook (PSS-03-
1202), and topics under revision
Guidelines for Threaded Fasteners
Threaded fasteners are
frequently used for assembling structural components. The
Guidelines for Threaded Fasteners (ESA PSS-03-206) provides
detailed descriptions of joint design using threaded fasteners,
calculation steps and worked examples. It also provides guidance
on preloading methods, embedding and relaxation, fatigue and
fracture mechanics, allowable lubrication for space use, and
other issues.
Further studies, including friction effects and vibration loosening, have produced information that will be included in a revision (Fig. 8).
Figure 8. Contents of Guidelines for Threaded Fasteners (PSS-03-
210), and topics under revision
Assessments of under-the-head and thread friction components of the torque tension relationship are not well established. After earlier development problems, a satisfactory load cell has now been produced and a 'ruggedised' version will be used to provide data this year. For some critical joints, differences between friction conditions can lead to preloading problems. For example, when the bolt is tightened from the head, when underhead friction is high, insufficient preload in the bolt can result. Conversely, if underhead friction is too low, too high a preload can result and failure of the bolt can ensue, particularly in such materials as titanium.
Work investigating the effect of reusing fasteners is ongoing using the same load cell facilities. The facility does in fact offer assessments of different bolt configurations, not previously available to stress engineers.
DASA and NLR have recently completed a detailed study into the damage tolerance characteristics of threaded fasteners including non-destructive testing techniques. A synopsis of this work will be included in the next release of the Guideline.
Structural Acoustics Design Manual
A new issue of the Structural
Acoustics Design Manual (ESA PSS-03-204) will be released in 1996
(Fig. 9). The previous version focused on the use of statistical
energy analysis (SEA) for the treatment of acoustically-induced
equipment vibration problems. Both the reverberant sound field
used to simulate the payload launch environment and most classes
of structures at frequencies above around 150 Hz have a high
modal density. It is therefore necessary to treat the dynamical
systems involved on the basis of statistical populations having
known distributions of their dynamical parameters. The vibratory
energy is the primary variable of interest. The more familiar
parameters, such as pressure and acceleration, can be derived
from considerations of this energy of vibration.
Figure 9. Contents of the Structural Acoustics Design Manual
(PSS-03-204), and topics under revision
In conjunction with the new handbook, the associated software program has been reissued, as GENSTEP 3, and includes a PC version. It can now also treat the transmission of high frequency vibrations due to local 'point' loadings as required, in particular, for microvibration analysis.
The new issue has also been extended to incorporate reference to other prediction tools such as finite element (FEM) and boundary elements together with design aids for simpler models using classical modal analysis. This facilitates the examination of the individual modes and their behaviour. It may be needed at frequency regimes of low structural modal density and where SEA does not apply.
Details of a means of predicting noise reduction for cylinders using classical modal interaction analysis in conjunction with the program 'Proxmode' are also included. This has been used successfully for initial estimations of Ariane fairing internal noise prior to the production of large-finite element models.
Another cause for concern is the generally acknowledged gap in the frequency range covered by FEM (low) and SEA (high). Alternative means of predicting the response behaviours for both mechanical and acoustic excitation are the subject of current investigation by both MATRA and ISVR. The manual will be updated accordingly.
Work is continuing on improving both vibration-level predictions for platform-mounted equipment and their representation in unit- level tests. At the moment, projects rely heavily on tools based solely on statistical syntheses of past test data.
Work on Artemis, Polar Platform, Olympus and other programmes has highlighted the importance of microvibrations as a source of unwanted 'jitter' for sensitive payloads. The prediction tools used at high frequencies relate closely to those used for structural acoustics investigations and the representation of the structural elements is the same. Work is ongoing to augment the existing manual with design information that has been gleaned from investigations to date, which is quite extensive.
Longer term plans include the incorporation of information on design aspects for habitation acoustics which relate to onboard generation of noise and its transmission via air circulating ducts, for example. Means of noise estimation and methods for passive and active means of noise control will be incorporated.
The series of ESA handbooks are a vehicle to promote information exchange across the European space community. Each handbook represents a single-source of accumulated knowledge directly appropriate to space projects, which may be otherwise difficult and time consuming to locate. They are a valuable resource by providing extensive information and guidelines to aid structural space engineering. They are appropriate to a readership of many disciplines working on space projects, and their role is complementary to that of standards and specifications.
As space structures continue to evolve, so will the handbooks, thus assuring accessibility to state-of-the-art information for the space community.
Thanks are due to many people for their continued support and for providing valuable contributions to the various hand-books: the members of ASMIEG, David Bashford of ERA Technology Ltd, Jorg Bolz of DASA, David Light of British Aerospace, and Neil Pinder of ISVR Consultancy Services (Southampton University, UK).
The Guidelines and Manuals described here can be obtained, at a nominal charge, from ESA Publications Division. See Order Form etc. inside back cover of this issue for further details.
ESA Bulletin Nr. 86.