SPACE STRUCTURES Prof. Alessandro Airoldi INTRODUCTION TO THE COURSE

SPACE STRUCTURESProf. Alessandro Airoldi

INTRODUCTION TO THE COURSE

2Space Structures - Prof. Alessandro Airoldi

Introduction to the course

Objectives of the course

Stressed skin constructions in aircraft structures

Loads in space structures

Examples of space structures

Contents and organisation of the course



Approaches to the analysis of structures in aerospace

constructions




Aerospace constructions Aerospace structures

THRUST

INE

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IAL

LO

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AERODYNAMIC LOADS AERODYNAMIC

LOADS

INERTIAL LOAD

Structure works to transfer the applied

loads

Force equilibrium (D’Alembert principle)

Such considerations apply to all type of structures (not only aerospace structure)




Stiffness

THRUST

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AERODYNAMIC LOADS AERODYNAMIC

LOADS

INERTIAL LOAD


Requirements:Limitation to the relative displacements due to functional requirements (e.g. aerodynamics)

Avoid frequency coupling (resonance)

StrengthAvoid permanent deformation and the collapse of the structures under operative load

Shape ConstraintsAERODYNAMICS, INTERNAL VOLUMES FOR PAYLOADS

OBJECTIVE: perform structural functions, fullfilling requirements, respect constraints with

MINIMUM WEIGHT




Why analysis ?

Analysis of existing structures helps understanding the functions of structural elements, critical issues in design, the available solution for design (synthesis)

Design is an iterative process, which involve analysis of design hypothesis at different level of detail




Enhance the capability to apply the approaches of structural mechanics to the structural types that are employed in aerospace structure. Given the applied loads:

-methods for the evaluation of internal stress and strain states

-methods for the evaluation of stiffness, displacements, natural frequencies

Learn the main features of aerospace structures: comprehension of structural roles, capability to critically analyse a structure

Achieve the bases for a proper use of structural calculation software: knowledge of principles, technologies, limitations



Stressed skin constructions in aircraft structuresThe peculiar and severe requirements for aircraft structures led to the development of a very effective structural typology:

Thin load bearing skin (stressed skin), reinforced by longitudinal stringers and internal frames

SEMI-MONOCOQUE STRUCTURES

They still represent the basic structural concept in aerospace structures, together with

TRUSS STRUCTURES




Motivations for the development of stressed skin constructions can be traced to the beginning of flight

TRUSS STRUCTURE

INTERNAL FRAMES (RIBS) AND LONGITUDINAL REINFORCEMENT (SPARS)

FABRIC COVER

Biplane (1916)




2

2

1VCS

Qp

High pitch angle

Low pitch angle

WING TORSION

Torsional stiffness: critical issue in wing design




Torsional stiffness of biplane wings



Stressed skin constructions in aircraft structuresHurricane had originally a fabric cover (1935)

All metal stressed skin provided in 1939

Load bearing skin provides a closed high-stiffness path for shear stress, contribute to bending stress and stiffness




Semi-monocoque structure (1943)

Thin load bearing skin (stressed skin), reinforced by longitudinal stringers and internal frames




Modern airliner structure

COMPOSITE VERTICAL TAIL WITH SEMI-MONOCOQUE MORPHOLOGY

CLOSELY SPACED FRAME AND RIBS (internal diaphragm in fuselage and wing



Stressed skin constructions in aircraft structuresWing box and ribs

Integrally stiffened composite skin

REAR SPAR

FWD SPAR




Type of diaphragms and

instability

TRUSS STRUCTURES

C – SHAPED BEAMS WITH VARIABLE SECTION AND CUTOUTS

CLOSED LOOP OF BEAMS WITH L, C, Z or other shape SECTIONS




Tail structure and bulkheads

AIRBUS A 300 TAILBULKHEAD WORKING UNDER PRESSURE LOADS

A380 COMPOSITE BULKHEAD




Supersonic fighters

WINGS BECOME VERY THIN AND STRINGERS ARE MERGED IN A SERIES OF SPARS

FUSELAGE FRAMES ARE MORE SPACED DUE TO NEED OF LARGE CUT-OUTS (Cockpit, cut-out inspections of engines, air inlets)




Natural environment: climatic, thermal, chemical and vacuum conditions, required cleanliness, levels of radiation and the meteoroid and space debris environment.

ground handling launch manoeuvres and disturbances re-entry descent and landing Additional induced loads, which include static pressure within the payload

volume, temperature and thermal flux variations

Mechanical and thermal loads that are induced by operation of spacecraft:




Ground handling and transportation loads




Ground handling and transportation loads

<<

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-la

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g

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Launch

3.7g

7 g

4.2 g

THRUST

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Launch phase corresponds to the most severe loads

Launcher structure is heavily compressed

SATURN V

ARIANE V

MODIFIED ICBM LAUNCHER




Loads are typically transient loads, vibration loads

When load are applied for an extended period of time they can be considered static (g loading / steady state accelerations)

The design load factors are represented by Quasi Static Loads, which are the most sever combination of dynamic and steady state acceleration

Ariane V QSL

COMPRESSIVE LOADS ARE THE HIGHESTLATERAL LOADS BEND THE STRUCTURE AND CAN NOT BE NEGLECTED

Axi

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Late

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Loads in space structuresInternal forces in a launcher (first stage for the application of a beam model )

Generation of oscillatory axial tensile at engine shut down



Loads in space structuresLaunch loads also act on spacecraft, which represent the launcher payload. Spacecraft are constrained to launchers by means of structures called adapter (or dispenser).

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QSL are the design loads for launcher structure, adapters and spacecrafts. Strength requirements imply a margin of safety > 1

THE TYPE OF LAUNCHER DEFINES THE LOAD CONDITIONS FOR THE SPACECRAFT

Ariane IV QSL AT PAYLOAD



Loads in space structuresFor spacecraft a fundamental (stiffness) requirement is avoiding resonant coupling with vibrations by launcher

Dynamic decoupling must be attained: spacecraft structure must exhibit natural frequency higher than the ones of launcher induced vibrations

THE TYPE OF LAUNCHER DEFINES THE STIFFNESS REQUIREMENTS FOR THE

SPACECRAFT



Loads in space structuresOther loads: Pressure fluctuation due to engine operations and unsteady aeordynamic

phenomena Shock (e.g. during jettison of stages) Thermo-elastic actions due to thermal flux Collision with meteoroids and space debris



Examples of space structures: SPACE SHUTTLE

SRB External tank

Orbiter

Unique combination of semi-monocoque, pressure vessels, truss structures structural concepts

Many different materials used:

-Aluminium alloy

- high strength steel

-Titanium

-Boron/aluminium composite

-Carbon/epoxy composites

-Fibreglass

-Ceramics



Solid bustersMAIN STRUCTURE:

SEGMENTED STRUCTURE (11 SEGMENT)HIGH STRENGTH STEEL 13 mm THICK

JOINED BY STEEL PINS

JUNCTIONS WRAPPED BY FIBERGLASS

SEALED WITH RUBBER BANDS

SUCH MAIN STRUCTURE IS CLOSED BY THE FWD AND AFT SEGMENT DOMES

IT IS THE EXTERNAL STRUCTURE BETWEEN THE FORWARD AND THE AFT SKIRT

EXTERNAL COVER, SUCH AS NOSE CAP AND SKIRTS ARE MADE OF WELDED ALUMINUN




PRE-FORMED ALUMINUM ELEMENTS (PANELS, MACHINED THICK ELEMENTS)

PRESENCE OF INTEGRALLY MACHINED STRINGERS AND RING FRAMES

RING FRAMES STABILIZE THE TANK AT HIGH COMPRESSIVE LOADS

External TankTWO TANKS: OXYGEN AND HYDROGEN

INTERTANK STRUCTURE IS A MORE CONVENTIONAL SEMIMONOCOQUE STRUCTURE (PANELS-SKIN-FRAMES MECHANICALLY JOINTED)




Orbiter


BASED ON SEMIMONOCOQUE PRINCIPLES

LARGE PARTS MADE OF ALUMINUM ALLOY

PECULIAR ASPECTS

CONVENTIONAL FORWARD FUSELAGE STRUCTURE HOSTS WELDED PRESSURISED CREW MODULE

CENTRAL SECTION FRAMES MADE OF BORON/ALUMINUM TRUSS STRUCTURE

THRUST BEARING TRUSS STRUCTURE WITH BORON/EPOXY REINFORCEMENTS

WINGS WITH HONEYCOMB SKIN COVER



Orbiter


FORWARD FUSELAGE: EXTERNAL SHELL STRUCTURE (SEMIMONOCOQUE CONCEPT)

INTERNAL PRESSURIZED VESSEL




Orbiter

CENTRAL SECTION

LONGHERON CARRY BENDING LOADS

HIGH STIFFNESS- STRENGTH REQUIRMENT FOR FRAMES: TRUSS WITH BORON/ALUMINUM TUBES

CONCEPTS OF STRESSED SKIN CONSTRUCTION LARGELY EMPLOYED




Orbiter

GRAPHITE\EPOXY PAYLOAD BAY DOORS

REINFORCED BY FRAMES AND END TORQUE BOXES

HIGH STRENGTH 3D TRUSS STRUCTURE TO SUSTAIN THE THRUST LOAD OF MAIN ENGINES




Orbiter

CONVENTIONAL ALUMINUM STRUCTURE WITH MULTI SPAR AND RIB ARRANGEMENT

HONEYCOMB SKIN REINFORCED BY ALUMINUM HAT-SHAPED STRINGERS



SEMI-MONOCOQUE AL 7075 INTERTANKS AND SKIRTS

AL 2219 – T87 TANK WITH ANTI-SLOSH BAFFLES (diaphragms that reduces fuel movements)

First Stage

Examples of space structures: SATURN V

Separate serial tanks within semimonocque structure



INTEGRALLY STIFFENED TANKS MADE OF DIFFUSION-WELDED AL 2014 PARTS

Second Stage


COMMON BULKHEAD: AL 2014 SHEET + FIBERGLASS/ PHENOLIC HONEYCOMB CORE

Integral serial tanks with common bulkhead

SKIRTS, INTERSTAGES, THRUST STRUCTURE: AL7075 SEMIMONOCOQUE



Third Stage

Serial tanks with common bulkhead

VERY SIMILAR TO 2° STAGE STRUCTURE


INTEGRALLY STIFFENED TANKS AND SEMIMONOCOQUE STRUCTURES



Examples of space structures: SPACECRAFTS

PRIMARY STRUCTURES

IN SPACECRAFTS THE DISTINCTION BETWEEN PRIMARY AND SECONDARY STRUCTURES IS IMPORTANT:

Experimental spacecraft designed at John Hopkins University: multisensor platform including a Spatial Infrared Telescope

TRANSMIT LOADS TO THE BASE OF THE SATELLITE THROUGH SPECIFICALLY DESIGN COMPONENTS (CENTRAL TUBE, HONEYCOMB PLATFORM, BAR TRUSS, ETC.).

PROVIDE THE ATTACHEMENT POINTS FOR THE PAYLOAD AND THE ASSOCIATED EQUIPMENTS.

FAILURE OF THE PRIMARY STRUCTURE LEADS TO COLLAPSE OF SATELLITE

SECONDARY STRUCTURES

BAFFLE, THERMAL BLANKET SUPPORT AND SOLAR PANELS MUST ONLY SUPPORT THEMSELVES AND ARE ATTACHED TO THE PRIMARY STRUCTURE WHICH GUARANTEE THE OVERALL STRUCTURAL INTEGRITY.




SEVERAL DIFFERENT STRUCTURAL TYPES:-TRUSS (ALSO IN HIGH STIFFNESS/STRENGTH COMPOSITE MATERIAL)- HONEYCOMB PANELS (OFTEN USED FOR ELECTRONIC SUPPORT AND SOLAR CELL SUPPORT)-MACHINED BEAMS AND PLATES




MANNED SPACECRAF INCLUDES -TRUSS STRUCTURES-SEMI-MONOCOQUE CONCEPTS (THIN WALLED STRUCTURES WITH STIFFENERS AND FRAMES)-STIFFENED PRESSURE VESSELS


Introductory lessons

• Central role of two different but very effective structural types: semi-monocoque and truss structures

• pressure vessels design concepts and honeycomb structures

Beam models can be applied at level of the vehicle structure, for the analyses of truss systems, for the analyses of ribs and frames

Plate theory is required to understand the behavior of panels and covers

Extensive knowledge of

- different structural typologies and engineering solutions

- different theoretical models and analysis approaches to meet severe requirements in a multiplicity of conditions

Examples of space structures:Structural concepts

Basic structural elements (models)

Required skills in aerospace structures



1. MECHANICS OF DEFORMABLE BODIES

2. BEAM MODELS AND BEAM SYSTEMS

3. SEMI-MONOCOQUE STRUCTURES

4. DISPLACEMENT APPROACHES AND APPROXIMATE METHODS

5. STRUCTURAL INSTABILITY

6. PLATES

7. FE METHOD

Course Content and Organisation:Lectures (theory)



Course Content and Organisation:

Course Material & Textbooks

Slides of the lectures will be provided during the course

MALVERN, MECHANICS OF CONTINUOUS MEDIUM

T.H. MEGSON, AIRCRAFT STRUCTURES FOR ENGINEERING STUDENTS, BUTTERWORTH-HEINEMANN, 1972

J.N. REDDY, ENERGY PRINCIPLES AND VARIATIONAL METHODS IN APPLIED MECHANICS, WILEY 2002

K.J. BATHE, FINITE ELEMENT PROCEDURES, PRENTICE HALL 1982

Continuum mechanics, general principles

Semi-monocoque structures, force and displacement approach to beam systems

V. GIAVOTTO, STRUTTURE AERONATICHE CITTA’ STUDI

Energy methods, Ritz Method, Plate Theory

Finite elements

Covers several parts of the course



Course Content and Organisation:Exercise classes

1. MECHANICS OF DEFORMABLE BODIES

2. BEAM MODELS AND BEAM SYSTEMS

3. SEMIMONOCOQUE STRUCTURES

4. PLATES (THIN & THICK PLATES, LAMINATES, PRESSURE VESSELS HONEYCOMB)

5. STIFFNESS APPROACH AND APPROXIMATE METHODS

6. FE METHOD

7. NON-LINEAR PROBLEMS AND INSTABILITY

1. STRESS AND STRAIN MEASURES

2. 3D BEAM SYSTEMS

3. SEMIMONOCOQUE STRUCTURES

4. DISPLACEMENT APPROACHES AND APPROXIMATE METHODS

The exercises that will be proposed during the classes will be all available on-line



Course Content and Organisation:Written Examination

Based on the same type of exercises that have been presented, solved and discussed during classes

Capability to critically apply concepts as well as to organize and carry out calculations

Admission to oral examination is possible only if the written text will obtain a positive mark

Oral Examination

Comprehension of structural concepts, analytical and numerical approach

Will include proofs of main theorems and formulation development

Documents

SPACE STRUCTURES Prof. Alessandro Airoldi INTRODUCTION TO THE COURSE