Aerospace Structures — Present Practices and Future aspirations in launch vehicles

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Aerospace Structures — Present Practices and Future Aspirations in Launch Vehicles Lalmoni, Non–member S K D Rachel, Non–member C Bhaskaran, Non–member B P Shastry, Fellow Design of structures for space missions is a formidable challenge to design community due to contradictory requirements of low margin, low mass and cheaper but high reliability. These requirements are presently met by design and / construction concepts of externally stiffened construction and integrally stiffened construction, such as, closely stiffened structures, ring stiffened structures, waffle, isogrid structures, sandwich, composite structures etc. The innovations in design and structures have parallely led to advancements in materials and manufacturing technologies and have substantially contributed to satisfy the specified requirements of the present Indian space mission applications. With increasing competitions in achieving faster, reliable, frequent and cheaper missions at still lower costs, demand is for fully reusable structures with little refurbishing and practically no maintenance to save time and cost. Future space missions have to encounter impact of debris and threat of space debris is alarmingly increasing day by day. Structures for future reusable launch vehicles and other space / interplanetary missions thus need to be essentially designed as self–sensing and self–healing type to tolerate damages and save the mission. Health monitoring of structures assisted by real time reinforcement ensure safety of structure and avoid catastrophic failures. Presently, structural testing is very expensive and time consuming. Alternate methods have to be explored in future to carryout virtual testing. Hot structures and deployable structures are already drawing great attention in space missions and for immediate requirements. The paper initially summarises the challenges in design of structures presently utilised particularly in launch vehicles and later highlights some of those frontier structural technologies which need to be pursued in design/analysis/testing areas for future aerospace missions. Keywords : Aerospace structures; Launch vehicles; Elastic memory composite; Shape memory alloy; Smart key

LAUNCH VEHICLE — TRANSPORTER TO ACE SP SPACE India has an array of achievements in the development of space technology and its application to vital areas like telecommunications, television broadcast and meteorology including disaster warning, resources survey and management. The Indian Space Research Organisation (ISRO) plays a key role in planning and execution of national space programme. The Indian space programme was formally organised in 1972 with the setting up the Space Commission and the Department of Space (DOS) by the Government of India. For a satellite to remain in orbit and to be of service to the nation, it has to travel at great speed of 7 km/s (25000 km/h). The satellite has to leave the earth’s surface Lalmoni, S K D Rachel, C Bhaskaran and B P Shastry are with the Structural Design and Engineering Group, Vikram Sarabhai Space Centre, Thiruvananthapuram 695 022. This paper was presented and discussed at the Twentieth National Convention of Aerospace Engineers held at Thiruvananthapuram during October 29–30, 2006.

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at the required altitude and with sufficient tangential velocity. This is achieved by taking the satellite in a launch vehicle to the pre–determined altitude and releasing it gently in the required orbit. The satellite is packed inside the payload fairing of the vehicle and carried into space, powered by a rocket engine. During launching, the launch vehicle’s rockets lift the satellite off the launch pad and carry it into space, where it circles the earth in a temporary orbit. Then the spent rockets and the launch vehicle drop away and one or more motors attached to the satellite move it into its permanent orbit. A motor is fired up for a certain amount of time, sometimes just one or two minutes, to push the satellite into place. When one of these motors is started, it is called a ‘burn’. It may take many burns, over a period of several days, to move the satellite into its assigned orbital position. A satellite launch vehicle is a complex transportation system which is tailored and engineered to function as a flying machine. This vehicle is made capable of transcending the harsh atmospheric regime and beyond near earth space and deliver the encapsulated satellite in the orbit; thus fulfilling the mission. High level of reliability and quality assurance have to be ensured at all the levels

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of its production, right from the initial stages of mission planning, design, development, fabrication and assembly, at both sub–system and system level for the successful completion of the mission. Launch vehicle development is a multi–disciplinary, complex and challenging activity involving almost all branches of engineering and sciences, both physical and chemical. The vehicle being a costly affair and more so the mission safety, it is crucial for the launch vehicle to successfully achieve the mission objectives. The philosophy of LV systems testing is generally quoted as ‘Test as you fly and fly as you test’. The cost of a mission is so expensive that it is very essential to test, evaluate and ensure its performance before actual launch. Hence, design and testing of all systems have to be proven and critically reviewed before the final test at flight. Launch vehicles are built as an assemblage of several stages which may be solid, liquid, cryo/semi cryo, etc depending on the propulsion systems employed. In multi– stage launch vehicle, the inert stages, which have performed their function, are jettisoned sequentially as flight progresses. This is enabled by staging/separation systems which are essentially pyro/explosive based systems. Launch vehicle structures play vital roles, namely : · Provide necessary external shape as directed by the aerodynamic considerations; · Provide housing for the payload, propulsion, guidance and control systems, etc; · Withstand the load and environmental conditions; and · Shall be of lightweight and cost–effective in construction, ensuring structural integrity while being reliable. Design of structures for launch vehicles is an important task and involves various design expertise and iterations to arrive at good and near optimum design. LAUNCH VEHICLE STRUCTURES Launch vehicle structures which have the primary role to provide/maintain necessary external aerodynamic shapes while withstanding the flight loads and environments may be broadly classified under two headings :

In addition to primary structures, there are other structural elements, such as, fuel tanks of control systems, pressurisation gas tanks of liquid stages, appendages and module mountings, brackets, etc which are not usually subjected to aerodynamic/primary loadings, are called secondary structures. Designing of both the structural systems need careful attention. Challenge lies in designing and realising the structures with least mass employing low factor of safety and most systems demand innovative design concepts and accurate analysis. Each of the stages are interconnected through structures known as interstage structures. Interstage structures not only interconnect the two stages, but also house various avionics/propulsion packages and mechanisms that are required for the proper functioning of the various stages and the launch vehicle as a whole. The major elements of launch vehicle structures consist of motor cases, tankages and interstage structures. The design features of interstage structures for launch vehicles are mainly dealt in this paper. ehicle Structures — Launch V Vehicle ypes of Construction Prevailing T Types Launch vehicle structures are usually of different construction types employing variety of material choices . The major emphasis is to realise very reliable low mass structure meeting the strength and stiffness requirements of the launch vehicle. Any failure on the part of the structure will result in total failure of the mission and here lies the importance of the intuition of a structural designer. Designing, realising and qualifying the launch vehicle structures are very complex tasks. Decision as to the choice of material and method of construction is the key in realising cost–effective near optimum structure. Unlike ground–based structure, the factor of safety used in designing these structures are very low ( ~ 1.25) and thus demand accurate analysis of load bearing capacity of each element of structure and mutual interaction among various structural elements. Various structures commonly used in a launch vehicle are · Motor case/ tankages; · Interstage structures; · Base shrouds;

· Primary structures; and

· Heatshields/payload fairings to protect spacecraft; and

· Secondary structures.

· Thrust transfer joints, etc.

Primary structures are load–bearing structures and provide the necessary strength and stiffness and thus are responsible for the performance of the vehicle structure. Stage motors, tankages, interstage structures, interface joints and payload fairings belong to this category.

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These structures can be designed as monocoque, semi– monocoque or skin stringer, closely stiffened shell, isogrid/ waffle, truss/framed, etc. The main criteria are light weight, ease of fabrication/assembly and cost, meeting the strength and stiffness requirements. 31

A brief description of the present structural configurations and concepts used in the design of interstage structure for launch vehicles is given here · Monocoque, · Semi–monocoque, · Ring stiffened shells,

Truss

Monocoque

Closely stiffened

Isogrid

· Stringer, skin with frames, · Closely stiffened, · Integrally stiffened structures, · Isogrid, · Waffle, and · Sandwich. Monocoque structure implies an unstiffened shell with practically no or very few transversely placed ring frames called bulkhead–type stiffening rings. Fabrication is easy in the case of monocoque shell type structure. Launch vehicle structures are mostly subjected to compressive load. For this type of loads, it is seen that monocoque construction is suitable for light loaded structures of small diameters and pressurised structures. Monocoque constructions are usually adopted for motor cases (solid stages), propellant tanks (liquid stages) and interstage structures of small diameter, say up to 0.5 m. For interstage structures of larger diameters say up to 1 m, semi–monocoque/skin–stringer type construction consisting of a shell, stiffened with longitudinal stiffeners called stringers and transversely with rings (bulkheads) is adopted. For still larger diameters, closely stiffened shell construction is being presently resorted to. A closely stiffened structure consists of a thin shell stiffened with closely spaced stringers and a few number of bulkheads. In this type of structures, the thin shell can buckle or cripple at a load much lower than the buckling strength of stingers. However, the stringer along with the effective skin width can be designed to have adequate strength against failure. The bulkheads provide adequate stiffness so as to provide intermediate support to the skin and stringer. This helps in reducing column length and increases the overall bulking strength. However, the enormous effort and time required for riveting the components are strenuous. Integrally stiffening means, instead of structures produced from separate skin and stiffeners, structures are constructed by milling of solid blocks/plates. The structures are generally to be of waffle type when the machined out pockets are square or rectangular and is of isogrid construction when the pockets are equilateral triangles in shape. 32

Waffle Figure 1 T ypes of structural constructions Types

Waffle structure is a lattice of ribs forming an array of contiguous repetitive square or rectangular pattern of integral stiffening of skin of the structure as shown in the Figure 1. The first step in developing a procedure for minimum weight balanced design of integrally stiffened waffle plates is to put together a method of analysis that adequately predicts the behaviour of such plates. The rectangular waffle plate with integral orthogonal stiffener is assumed as a simply supported plate subjected to membrane and / or bending loading. This is to be designed so that gross buckling, local buckling and yielding do not occur in any of the given load condition. In developing the behaviour functions, it is assumed that the structural material used exhibits an ideal elastic–plastic behaviour. Isogrid structure is a lattice of ribs forming an array of contiguous repetitive equilateral triangular pattern of integral stiffening of skin of the structure. The triangular ribs behave entirely as an isotropic material giving rise to the name isogrid. The isogrid structure having the advantage to withstand both compressive and bending loads makes this type of construction attractive to the structural designers. Isogrids can effectively carry combined loads because of the arrangement of ribs. Therefore, either skin or lattice can be locally stiffened to handle local loads or discontinuities from cutouts. This choice offers more design flexibility than available with rectangular stiffening systems. They permit local skin buckling in large lightweight structures without triggering instabilities. The arrangement of ribs behaves iso– tropically, distributing the properties uniformly throughout

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the entire framework, thus rendering simplicity for analysis. The cost of fabrication and machining these panels for large structures are, however, expensive and machine intensive. Truss/framed construction (Figure 2) is employed for internal structures which are within the interstages or heatshield or open type of structures. These structures are mainly employed for inner branches of interstages and for providing isolating effects due to thermal gradient in two adjoining structures. Launch V ehicle Structures — Vehicle Present Material Choices Material choice plays a key role in the design and subsequent fabrication of the structure. Depending on the requirements, materials need to be selected either to satisfy strength criterion, as in the case of pressurised system, or stiffness criterion, as in the case of structures subjected to predominantly compressive loads. The designer selects materials with high specific strength (UTS /ρ) and high specific stiffness (E /ρ) along with other desirable characteristics. Materials commonly used in aerospace structure design are:

Exotic materials like Al, lithium, boron Al, etc are also used in various designs. Earlier, material barrier was a major obstacle to further development in aerospace industry. Currently to meet requirements of high–speed aircraft, design engineers are forced to utilise materials up to, and beyond, their practical limits. To meet the demands, new material mixtures, or composites were developed; these were strong but light and able to withstand severe temperature and corrosive conditions. Composites, in fact, seem to represent ‘dream’ materials with tailormade properties. Presently, fibre composite materials are finding more and more applications in structure design as one can get desired properties from these materials by careful lay up sequences and result in low mass structures. Sandwich construction is a special kind of laminate consisting of a thick core of weak, lightweight material sandwiched between two thin layers (called ‘face sheets’) of strong material (Figure 2). This is done to improve structural strength without a corresponding increase in weight, ie, to produce high strength–to–weight ratios. But advantages of sandwich structures are limited to those subjected to the flexural load.

· Alloy steels, namely, 15 CDV6, maraging steel, stainless steel, etc;

Launch V ehicle Structures Design and Vehicle Engineering : Current Practices

· Alloys of aluminium, namely, AA 2014, AA 7020, AA 2219, AA 2024, etc; and

The procedure followed in the design of structures is shown in Figure 3.

· Titanium alloys — Ti6Al4V.

Having decided upon the material choice, construction type and estimated loading sizing of the various structural elements, such as, thickness of shell, number and cross– section of longitudinal stiffeners and bulkheads, cross– section of end rings interface joints to carry the specified design loads are handled using structural mechanics approach. The design should ensure a positive margin of safety. This margin indicates excess strength over the required strength.

Outer facing material Waffle core BI–DIRECTIONAL Facing material Stringer Facing material Corrugated core

Truss core UNIDIRECTIONAL

Honeycomb core ORTHOTROPIC Figure 2 T ypical sandwich construction Typical

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Though sizing of various structural elements and thus the structural design is carried out meticulously, this has to be finally translated into engineering drawings for realising the structures. This step is known as detail engineering and standard design practices like rounding of sharp corners, mitigation of stress concentration, local stiffening at cut out/joining regions, incorporation of fabrication, assembly/inspection features, tolerances, etc are introduced into the design and necessary alteration/ modifications to the sizes already arrived at are carried without sacrificing design margins. This is the most important step and a good design will only emerge when a good engineering is incorporated into the design. Designed structures need to be rigorously analysed to establish the adequacy of structure individually or as a whole. This is usually required to take care of the complex 33

Start

Define configuration/constraints, load, service condition, functional requirements, design input Material selection and type of construction, design load factor determination Sizing/resizing of the structure and preliminary/ modified design realisation Rigorous analysis of failure modes and margin evaluation for structure member

No

Is the design OK

Specifications — Configuration — Interfaces — Environmental conditions — Loads, temperature

STRUCTURES

Materials — Selection criteria — Jointing — Fabrication — Availability

Design and Analysis — Design concepts — Structural analysis

Yes Release of design drawings Review of design and correction, if any Release of fabrication drawings Realisation of the hardware Qualification level structural

Is the structure safe under test

No

Yes Clearence for further fabrication of flight hardware

End Figure 3 Design flowchart

loading and boundary conditions as well as interaction between various structural elements and the effect of adjoining structure which can not be adequately addressed while carrying out the detail sizing task. The most common and versatile technique is finite element method (FEM), which permits analysis with complex loading, difficult boundary conditions and irregular boundaries. Although many design aspects are involved in the effective utilisation of materials, particularly under severe environmental conditions, three basic factors, namely, design, structure and materials are summarised in Figure 4. The optimum design of a system requires consideration of all three factors simultaneously. This interplay between design, materials and structures are presently computerised and are effectively utilised to optimise a structure during its design phase based on 34

Figure 4 Design optimisation

stress, static displacements, buckling strength and natural frequency. Previously this design practice was done in a serial order and was really time consuming. Though, advanced analytical techniques and exhaustive analyses are carried out during the design phase, for structures subjected to complex loading and complicated boundary conditions, it is difficult to simulate in analysis the exact behaviour of joints and other complexities leading to high stress regions and gradients. In addition, it is difficult to quantify and account for assembly/machining stresses locked up in the fabricated structures, which in turn may degrade the designed performance of the structure. Also, the structure which are mainly subjected to compressive loading and prone to buckling at very low levels of stresses, are very difficult to be handled through analysis and predicted buckling capacity are often found erroneous as these can not account for boundary conditions, imperfections, assembly and machining stresses satisfactorily. Qualification of structures through simulated loading and interfaces, establishes/demonstrates its ability to withstand design loads satisfactorily. It also testifies the adequacy of the structure design, assumptions in design and performance of structures in spite of various cut outs and discontinuities resulting in high stress/strain gradient and their reinforcement schemes. Another important feature which will be brought out by these tests is the adequate margin in the structure for the locked in machining, assembly stresses, which are locked during the fabrication and are not accurately quantifiable. Qualification test also establishes the design adequacy of structures, which are prone to failure by buckling and is difficult to predict analytically. However, the high costs and time involved in the qualification level test like fabrication and assembly of the prototype hardware, test

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rigs and fixtures and adaptors calls for alternatives to minimise the number of structural tests required for a launch vehicle. AEROSP ACE STRUCTURE — THE FUTURE AEROSPACE DEMANDS Presently the often–quoted requirements in space missions are fly faster, soon, often, higher, safer and cheaper. Time is running out with the need for improvements in aerospace technology, highly ambitious space missions and programmes minimising space cost, such that, it has become imperative for aerospace design practices to quickly dwell on and aspire for improvements and needs required for the future. In addition to it, major technology drivers for aerospace include reducing launch volume; enabling the construction of space and surface infrastructure through modular elements; extending the performance and lifetime of systems operating in extreme environments; and providing systems with integrated diagnostic and adaptive capabilities. Aerospace designers are now forced to come out of the cocoon that they had woven around themselves: the cocoon of design, materials and structures. Aerospace structures have to be designed with multi– disciplinary optimisation techniques as well. A few major challenges are · Thermo–mechanical behaviour of structures at re– entry for reusable launch vehicle; · Thermal buckling phenomenon under thermal and mechanical load; · Fatigue strength with respect to thermal cyclic load; · Prediction of aging of structure travelling in adverse flight condition (like RLV moving out of atmosphere and re–entry in exactly a reverse regime); · Health monitoring and clearance for reusability; · Damage tolerant design for light weight structures; · Synchronisation of rocket and aero–plane design; · Joint/rivet under thermal load condition; and · Thermal isolation. These call for newer material development that can address to afore–mentioned issues. It is imperative now to reverberate with the activities for newer material development that will prove the backbone for scientific growth through full utilisation of adaptive composite, nano–material concept, piezo–electric fibre and actuator to make structure self–diagnostic and smart. The list is endless. This paper restricts to highlight a few aspects, namely : · Inflatable and rigidisable structures,

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· Self–healing / health monitoring structures employing, › Shape memory alloys, and › Elastic memory composite materials.

· Interface joints— stress–free in thermal environments, · Virtual qualification testing of structures, and · Multi–disciplinary design optimisation of structures. Precision Inflatable Structures As long as conventional structures find place in space programme, scientific community paralelly keep striving to optimise the structure to meet the stringent requirement. But now the time has come to explore structures beyond the conventional prevalence that has reached the plateau. Inflatable structure is one among such unconventional structures. Inflatable structures are inflated to required size and shape mostly through inert gas injection. It has significant edge over conventional structure, such as, · less expensive; · huge (50%) mass saving; · large (75%) package efficiency; · inherently strong because of less number of joints; · low production cost and high reliability of deployment; · inherently self–correcting and less complex engineering; ·easily adapted to symmetric shapes and curved surfaces; and · favourable dynamics and thermal response. This promises smaller vehicle for a given payload or a heavier payload with available vehicle. The technology involved is capable of addressing various space requirements owing to the merits it possesses. Challenges are many but a few need mention here. It is difficult to detail (a) material characterisation, (b) joints design and the interfacing with the conventional structure, (c) identification of location for the injection point and associated injection pump, (d) impact threat of debris, (e) prediction with the consideration of zero gravity for the qualification of inflatable structures, and (f) prediction of its behaviour in space for longer period; along with the development of general methodology/ tool/ formulation for comprehensive analysis and its realisation of inflatable structure. Large number of applications, as shown in Figure 5, can be foreseen in inflatable structures for space mission. Use of inflatable boom as stiffeners, inflatable ship for transportation of launch vehicle systems/subsystems, 35

solid. In most shape memory alloys, a temperature change of only about 10oC is sufficient to initiate this phase change. The two solid phases, which occur in shape memory alloys, are martensite and austenite. The shape memory effect is observed when the temperature of a piece of shape memory alloy is cooled to below the temperature Mf . At this stage, the alloy is completely composed of martensite, which can be easily deformed. After deforming the material, the original shape can be recovered simply by heating the wire above the temperature of Af . The heat transferred to the wire is the power driving the molecular rearrangement of the alloy, similar to heat melting ice into water, but the alloy remains solid. The deformed martensite is now transformed to the cubic austenite phase, which is configured in the original shape of the wire. Shape memory effect is currently being utilised in aerospace actuators and satellite release bolts.

Figure 5 T ypical applications of inflatable structures Typical

sheathing for transporting temporarily assembled space structures to launch pad, floatation recovery system after splash down, inflatable landing system for dropping space structure on moon, inflatable catapult (an aid for smooth separation), space habitation/space tourism, inflatable auditorium etc. For the use of robotics, shape memory alloyed structure can be imparted good aesthetics by attaching proper inflatable covering which will prevent any damage to the system by avoiding any sharp corner of robot through a smooth and symmetric inflatable attachment. A few experiments have already been conducted by the developed countries like putting inflatable re–entry and descent technology (IRDT) demonstrator into orbit and succeeded in gathering relevant researching data. But still much to be done and research should be continued. Self–healing/Health Monitoring Structures Employing S MA ’s and E MC SMA MA’s EMC Shape memory alloys (SMA) are metals, which exhibit two very unique properties, pseudo-elasticity and the shape memory effect. The most effective and widely used alloys include NiTi (nickel–titanium), CuZnAl, and CuAlNi. The two unique properties described are made possible through a solid state phase change, ie, a molecular rearrangement, which occurs in the shape memory alloy. Typically, when one thinks of a phase change of a solid to liquid or liquid to gas change, is the first idea which comes to mind. A solid state phase change, is similar in that a molecular rearrangement occurs, but the molecules remain closely packed so that the substance remains a 36

However, there are still some difficulties with shape memory alloys, which must be overcome before they are to find wide applications in space structures. These alloys are still relatively expensive to manufacture and machine compared to other materials, such as, steel and aluminum. Fatigue is also a problem for shape memory alloys. Components of machinery made of a SMA can not be cycled through the same loading (twisting, bending, stretching etc) many times. Most SMAs survive 2000 cycles, while a steel piece, for example, may withstand the same conditions for hundreds of thousands of cycles. Elastic memory composite (EMC) materials are now developed which enables the practical use of the shape memory properties in fibre–reinforced composites and other speciality materials. EMC materials are similar to traditional fibre–reinforced composites except for the use of an elastic memory thermoset resin–matrix. The elastic memory matrix is a fully cured polymer, which can be combined with a wide variety of fibre and particulate reinforcements and fillers. The unique properties of the matrix enable EMC materials to achieve high packaging strains without damage. Raising the temperature of the EMC material and then applying a mechanical force to induce strains, the shape memory characteristics enable the high packaging strains to be ‘frozen’ into the EMC by cooling. Deployment (ie, shape recovery) is effected by elevating the temperature. The temperature at which these operations occur is adjustable. At lower temperatures, the performance of EMC materials follows classical composite laminate theory. At higher temperatures, EMCs exhibit dramatically reduced stiffnesses due to significant matrix softening of the resin. Adequately addressing the mechanics of the ‘soft–resin’ will enable the EMC materials to provide repeatable stowage and deployment performance without any damage and or changes in performance. Products fabricated from these materials can be deformed and reformed repeatedly.

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Products utilising EMC materials can be fabricated with conventional composite fabrication processes and tooling. The properties of EMC materials are summarised as · can be formulated with low cost components; · use standard existing polymer and composite manufacturing processes; · regain original shape with applied heat, no other external force is required; · possess widely adjustable deformation and reformation temperatures; · suitable for repeated deformation and reformation cycles; · reform accurately to original shape; · maintain high strain capability when heated; · enable large volume reduction for packing; and · issues, such as, shelf–life, chemical reaction, toxicity, explosion hazard, or environmental impact are not of concern. The applications for these revolutionary new materials are broad ranging, including mission–enabling components for spacecraft, performance enhancing and cost saving industrial and medical applications, deployable equipment for emergency and disaster relief, and improvements in the performance of sports equipment. EMC materials are ideally suited for deployable components and structures because they possess high strain–to–failure ratios, high specific modulus and low density. By contrast, most traditional materials used for deployable structures have only two of these three attributes. Stress–free Interface Joint for Aerospace Applications — a Design Concept using Smart Key Fastening T echnique Technique The design of joints in aerospace structures subjected to differential temperature is typically carried out with the interfacing joints tailored to compensate/withstand for thermal effects. This mode of cure for a problem often entails use of special exotic design methods and costly materials. The aim of this concept developed is to prevent the effects of thermal stress caused due to temperature gradients in flanged joints, so that, conventional joints can be adopted. Here, a new design concept of ‘smart key’ is introduced. The smart key is the active fastener used in place of a bolt in the flanged joint of a launch vehicle. The smart key is I–shaped (Figure 6) and is inserted radially from the outer diameter (OD) of the two interfacing flanges. On the

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Figure 6 Smart key concept

application of any temperature gradient to the top interfacing structure, the smart key allows free radial movement and prevents any stress build up on the account of temperature. The top and bottom flanges of the smart key lock into recesses provided in the OD of the flanges and efficiently function to prevent the interfacing structure from separating out on the application of external load. Thus, the joint with smart keys along with the slotted flanges function as good as any interfacing joint carrying external load without any influence of stress due to thermal effects. The new design concept arose from the modifications made to the typical bolted flanged joint. In the conceptual design of the joint active fastener is meant by a fastener replacing the conventional bolt with one that allows radial freedom for the top flange. The fastener is held onto the flanges externally. Mutil–disciplinary Design Optimisation of Structures Multi–disciplinary design optimisation (MDO) of structures is a methodology that has evolved out of the need for combining the role of designers in the areas of propulsion, control and guidance, thermal, trajectory, aerodynamics (Figure 7) along with the current practices of design optimisation. This helps in widening the input parameters during the conceptual design phase itself rather than working in a narrow environment. Also life cycle disciplines like manufacturability, supportability and cost Structures

Control and guidance

Aerodynamics MDO Thermal

Propulsion

Trajectory Figure 7 Multi–disciplinary design optimisation

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analysis are included in the MDO concept. MDO has now emerged as a new engineering discipline and plays a key role in the development of advanced aerospace vehicles. The major obstacle in implementing MDO technology is the complexity that it has to cater to. Studies on multi– objective optimisation with objective parameters depending on a whole lot of aerodynamic design parameters other than materials, structure and design are called for. In MDO, the decision variables are many, objectives and constraints are expensive to evaluate, and there are multi conflicting objectives. Therefore, robust solutions are required for the flight hardware and decisions on a common platform have to be taken from a variety and large number of feasible solutions. For reusable launch vehicles and for future space missions, MDO is quite unavoidable and has to be developed. CONCLUSIONS As contemplated, important areas are elaborated for the point of view of present practices and future aspirations. It significantly calls for a new dimension in science and technology. Since motto is ‘cheaper, better and faster’, so challenges increase manifold. Future ambitious missions crave for newer dimensions like comprehensive utilisation of novel and efficient structures like inflatable structures, intelligent structures/material etc and at the same time overcoming the associated difficulties, so that, payload gain should be maximised. This would augur an era of low cost access to space.

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Apart from it, for the design and analysis, concepts like damage mechanics etc gains much importance to bolster various failure criteria and hence predicting correct margin of safety. This salient point supports the better usage of lighter, stronger and stiffer material that endorses the material related research towards self–reliance. Testing again poses lots of challenges. Since, testing costs are exorbitantly high, new concepts are coming up like ‘testing without testing’ or ‘testing through FE simulation’. One way is to bring down the effect of load on structure. This can be achieved if external load causes minimum excitation to the concerned structures through concepts like active cancellation, stiffening of path through the utilisation of various smart structures. This, in turn, would alter the dynamic character of the path and bring down the excitation, reaching the structure from the inclement load by aerodynamics, thrust oscillation and propulsion. Thus, prediction of the behaviour of structure will be much easier as structure experiences least excitation and fundamental theory would be sufficient. All afore–mentioned issues refer to multi–disciplinary activities that exhort scientific community from all the disciplines to possess a total understanding of the future challenges and work as a team . REFERENCE 1. Proceedings of the Colloquium on Multi–disciplinary Design

Optimization, VSSC, January 28, 2004.

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