structures technology for future aerospace systems

Structures technology for future aerospace systems

Ahmed K. Noora,*, Samuel L. Vennerib, Donald B. Paulc, Mark A. Hopkinsd

aCenter for Advanced Computational Technology, University of Virginia, NASA Langley Research Center, Hampton, VA 23681, USAbNASA Headquarters, Washington, DC 20546, USA

cAir Force Research Laboratory, Wright Patterson Air Force Base, OH 45433, USAdBoeing Phantom Works, St. Louis, MO 63166, USA

Received 5 August 1998; accepted 10 January 1999

Abstract

An overview of structures technology for future aerospace systems is given. Discussion focuses on developments

in component technologies that will improve the vehicle performance, advance the technology exploitation process,and reduce system life-cycle costs. The component technologies described are smart materials and structures,multifunctional materials and structures, a�ordable composite structures, extreme environment structures, ¯exibleload bearing structures, and computational methods and simulation-based design. The trends in each of the

component technologies are discussed and the applicability of these technologies to future aerospace vehicles isdescribed. Published by Elsevier Science Ltd.

Keywords: Structures technology; Aerospace systems; Smart materials and structures; Multifunctional structures; Composite struc-

tures; Extreme environments; Load-bearing systems; Computational methods

1. Introduction

Structures technology encompasses a wide range of

component technologies from materials development

to analysis, design, testing, production and mainten-

ance. Materials and structures have largely been re-

sponsible for major performance improvements in

many aerospace systems [1]. The maturation of compu-

tational structures technology and the development of

advanced composite materials witnessed during the

past 30 years have improved structural performance,

reduced operational risk, and shortened development

time. The design of future aerospace systems must

meet additional demanding challenges [2]. For aircraft,

these include a�ordability, safety and environmental

compatibility [3]. For military aircraft, there will be a

change in emphasis from best performance to low cost

at acceptable performance. For space systems, new

challenges are a result of a shift in strategy from long-

term, complex, and expensive missions to those that

are small, inexpensive and fast.

Materials and structures, in addition to enabling

technologies for future aeronautical and space systems,

continue to be the key elements in determining the re-

liability, performance, testability, and cost e�ectiveness

of these systems. For some of the future air vehicles,

the development and deployment of new structures

technologies can have more impact on reducing the

operating cost and the gross weight than any other

technology area (see Figs. 1 and 2). An overview of

government-sponsored programs on structures technol-

ogy is given in Ref. [4]. The treatment of future direc-

tions in structures technology in a single article must

Computers and Structures 74 (2000) 507±519

0045-7949/00/$ - see front matter Published by Elsevier Science Ltd.

PII: S0045-7949(99 )00067-X

www.elsevier.com/locate/compstruc

* Corresponding author. Tel.: +1-757-864-1978; fax: +1-

757-864-8089.

E-mail address: [email protected] (A.K. Noor).

Fig. 1. Projected percentage reduction in subsonic transport operating cost in 2020 resulting from deploying new technologies.

Long-haul/high capacity: (1) conventional, (2) blended-wing body, (3) long-haul capacity conventional, (4) medium range intracon-

tinental, (5) regional jet (courtesy of NASA Langley Research Center).

Fig. 2. Projected vehicle total gross weight reduction percent. Supersonic: (1) long-haul, (2) premium service, (3) business jet; long-

haul, high capacity subsonic, (4) conventional, (5) blended-wing body; global air cargo, (6) long haul, (7) short haul; STOL, (8)

medium range intercontinental, (9) short-haul high capacity; short-haul/vertical lift, (10) tiltrotor (courtesy of NASA Langley

Research Center).

A.K. Noor et al. / Computers and Structures 74 (2000) 507±519508

necessarily be selective and brief. The focus of the pre-sent article is on developments in component technol-ogies that will improve the vehicle performance,

advance the technology exploitation process, andreduce system life-cycle costs. The component technol-ogies are grouped into six categories, namely:

. Smart materials and structures

. Multifunctional materials and structures

. A�ordable composite structures

. Extreme environment structures

. Flexible load-bearing structures

. Computational methods and simulation-based de-sign

The development of each of the component technol-ogies is a multidisciplinary activity, which involvestasks in other disciplines. In this article, the trends in

each of the component technologies are discussed andthe applicability of these technologies to future vehiclesis described. Materials technologies for future aero-

space systems are discussed in Refs. [5,6].

2. Smart materials and structures

Smart structures sense external stimuli, process the

sensed information, and respond with active control tothe stimuli in real or near-real time. A response canconsist of deforming or de¯ecting the structure or com-

municating the information to another control center.Smart materials deform or de¯ect the structure by

changing their physical properties when subjected to

electric, magnetic or thermal loads. An extension ofthis is the intelligent, self-healing vehicle whose built-in

redundancy and on-board self-inspection detects

damage and responds with autonomous adjustmentsand repair.

The active elements in smart structures can be

embedded in or attached to the structure. Typical sen-

sors include ®ber optics, piezoelectric ceramics andpolymers. Embedded sensors can be either discrete or

distributed to provide built-in structural quality assess-

ment capabilities, both during material processing andvehicle operation. Sensors can also be used for moni-

toring in-service or environmental loading, and for

shape sensing. Typical smart structure actuatorsinclude shape memory alloys (SMAs), piezoelectric and

electro-strictive ceramics, magneto-strictive materials,

and electro- and magneto-rheological ¯uids andelastomers.

The ®rst applications of smart materials and struc-

tural concepts will be on rotorcraft blades, aircraft

wings, air inlets, engine nozzles, large deployable pre-cision space systems and robust microspacecraft.

Expected bene®ts include enhanced handling qualities

(by changing control surface shape to manipulate liftor reduce drag, producing twist in aircraft wings or

helicopter rotor blades, or a�ecting ¯ow conditions

Fig. 3. Future tailless military aircraft will use smart materials (checkerboard area), twisting wing, expandable fuel cell, and compli-

ant trailing edge (courtesy of Air Force Research Laboratory, Wright Patterson Air Force Base, OH).

A.K. Noor et al. / Computers and Structures 74 (2000) 507±519 509

over the lifting surface); vibration suppression (includ-

ing ¯utter and bu�et control); alleviation of noise andvibration; and monitoring of vehicle health. For spacesystems, smart structures provide a robust design

approach for meeting precision requirements. They cansigni®cantly reduce cost and schedule by decreasingthe requirements on analysis, development testing,

hardware process testing, and quality control.The activities pertaining to three smart structures

concepts are described subsequently: shape adaptivestructures and aerodynamic load control; structuralhealth monitoring; and vibration and noise suppres-

sion. The Department of Defense and NASA arestudying the concept of shape adaptive structures and

aerodynamic load control. Design concepts include air-foil warping, camber shaping/control surface defor-mation, and variable sti�ness structures. The goal of

this research is to enhance ¯ight vehicle performance(while reducing weight and the need for discrete, exter-nal control surfaces). Some adaptive structures con-

cepts twist the airfoil, vary its camber, and deformleading and trailing edge control surfaces through

SMA actuation to enhance maneuvering and lift. Suchtechniques can increase aircraft survivability andreduce drag (Fig. 3).

Active aerodynamic load control can be achieved viaself-straining actuators (SMAs or piezoelectric devices)embedded within the structure. The actuators expand

or contract on command. This changes the shape ofthe active airfoil element, which in turn changes the

aerodynamic load on the lifting surface. The mostlikely candidates for smart material load control arevery ¯exible surfaces such as the High Altitude Long

Endurance (HALE) aircraft, or smaller surfaces suchas missile ®ns. Future piezoelectric materials will haveto withstand harsh environments and be expansive

enough to deform large aerodynamic surfaces.Anisotropic actuators will be needed to control bend-

ing and torsion response independently.One near-term use for smart structures is monitoring

of vehicle health [7±9]. An onboard-distributed ®ber

optic network, connecting sensors to processors, canbe used for this purpose [10]. One type of sensor being

developed will measure the `sounds' of crack growth-transducers emit acoustic signals throughout the struc-ture and measure changes in the structural response,

indicating crack initiation or growth at remote sites.Other sensors detect and measure separation (delami-nation) of composite material layers. Each processor

receives signals and analyzes an array of sensors todetermine if and where damage has occurred.

Two areas of special interest are reducing the oscil-lations of primary structure due to unsteady externalforces, and reducing the transmission of acoustic

energy through the structure. Active control systemsthat use piezoelectric actuators are being developed.

Actuators will be attached to the skin and substruc-tures of the vehicle. Alleviation of these dynamic loadswill increase structure life and reduce maintenance

time and costs [11].Some elements of smart materials and structures

technology are already being demonstrated; however,

several technical challenges must be overcome beforethe technology can be incorporated into future oper-ational vehicles. The challenges include de®ning thefatigue life characteristics of smart actuators attached

to realistic aircraft structures in an operational en-vironment, and developing the maintenance and repairprocedures for embedded actuators. Other technical

issues that must be addressed to realize the full poten-tial of the technology involve structure fabricationmethods, reliable actuator material, lightweight struc-

tural materials capable of physical and virtual shapechanges, and recon®gurable adaptive control system.The basic theory for controlling smart structures in

static and dynamic environments is presented in Ref.[12]. Reviews of recent developments in smart ma-terials and structures and their applications are givenin Refs. [13±24]. The various couplings between mech-

anical, thermal, electric, and magnetic ®elds for smartmaterials are depicted in Fig. 4. Computationalmethods for smart materials and structures are

described in Ref. [25].

3. Multifunctional materials and structures

Multifunctional structures (MFS), in addition tosupporting loads, use sensors to detect and evaluateloads or failure, and to interact with the surrounding

electromagnetic environment. MFS represents a newmanufacturing and integration technology by whichcommunications and electronics equipment are inte-

grated into conformal load-bearing structures.Advances in large-scale integrated electronics packa-ging, lightweight composite structures and high-con-

Fig. 4. Couplings between mechanical, thermal, electric and

magnetic ®elds in smart materials and structures.


ductivity materials enable the technology. In MFS,

electronic assemblies (multichip modules), miniature

sensors and actuators are embedded into load carrying

structures, along with associated cabling for power and

data transmission. This level of integration e�ectively

eliminates traditional boards and boxes, large connec-

tors, bulky cables, and thermal base plates, yielding

major weight, volume and cost savings.

Current MFS research for aircraft addresses the

antenna/airframe proliferation, integration, and main-

tenance. Present ¯ight vehicle designs have almost 100

antennas occupying roughly 60 apertures (Fig. 5). To

accommodate electromagnetic windows, these antennas

require local reinforcement of the airframe structure,

increasing structural weight and cost. Externally

mounted antennas degrade the aerodynamic perform-

ance and require extensive maintenance. Integrating

antennas and other electronics into load-bearing skin

structures will be a common concept in future air ve-

hicles. It will yield lower cost, lighter weight airframes,

increased antenna performance, and lower manufactur-

ing and maintenance costs. These multifunctional

structures may also enable greater data transfer rates

and increased aircraft surface area for additional sen-

sors and transmitters.

MFS technology o�ers signi®cant savings in the

mass and volume of spacecraft by eliminating electrical

chassis and cabling, and placing most sensors and bat-

teries on the bus structure, which also provides struc-

tural and thermal control (Fig. 6). The technology

o�ers several bene®ts to future spacecraft: its inherent

modularity supports low-cost mass production and

assembly, signi®cantly reducing life-cycle costs; it elim-

inates cables and connectors, minimizing `touch-labor'

needed during ®nal spacecraft integration, whose

robustness and reliability it increases; it enables

Fig. 5. RF multifunction structural apertures.

Fig. 6. Multifunctional structural panel with integral electronic, structural and thermal control.


reworkable `plug-n-play' spacecraft; and it provides

integrated thermal control, ¯ex interconnect architec-ture for power distribution, and data storage and elec-

tronics into load-bearing structures.Structural integration of complex aero-¯uidic and

thermo-mechanical subsystems into future aerospacevehicle design is being studied. The system-level pay-

o�s of integrating these technologies will be realizedonly through simultaneous development across all dis-

ciplines involved in designing 21st century aircraft.Structural integration must begin early in the prelimi-

nary design phase of the system development for thesepayo�s to materialize. Advanced subsystem devices,

novel structural concepts, design/analysis methods, andmanufacturing approaches will be assimilated to quan-

tify reductions in weight and assembly costs.Technologies of interest for structural integration

are ¯exible skin panels, integral cooling for thermalmanagement, ¯uidic jets for aerodynamic control, inte-

gral electric and hydraulic lines, and actuators whoseattachment hardware may be built directly into the air-

frame substructure. Composite materials will be used

extensively in these integral designs because of theirunique thermal and mechanical tailorability [26±29].

These properties may eventually allow for incorpor-ation of highly conductive elements, embedding of

electrical sensors and shape memory materials, and in-tegration in the structural design of micro actuators

and subsystems.Payo�s to be realized by future aerospace vehicles

through the use of multifunctional structures includereduced operational costs and weight, and improve-

ments in mission e�ectiveness. For future air vehicles,advanced structural concepts will be combined with

integral ¯ight systems to provide signi®cant reductionin part count and increased volumetric e�ciency. The

savings realized in operational and sustainment costsfor systems with increased robustness and durability

may o�set the probable increase in cost per pound ofthe airframe, attributed to inclusion of electronic or

mechanical subsystems. The maximum payo�s to bemeasured in e�ciencies at the system level can only be

Fig. 7. Advanced grid sti�ened structure.


attained by considering the airframe to be a multifunc-tional structure early in the development of the con-

ceptual design and simultaneously optimizing thedesign across all disciplines. Previously unimagined de-sign e�ciencies and synergies will be achieved.

Ultimately though, the transition of multifunctionalaircraft structural concepts to next generation aircraftwill require resources focused on development of tech-nologies to ensure their a�ordability, durability, and

supportability in the operational environment. Thesequantities will be essential to continued developmentof future systems including uninhabited aircraft and

hypersonic vehicles.

4. A�ordable composite structures

For many future aircraft, the use of composite pri-

mary structures, along with other structures technologyimprovements, could have more impact on a�ordabil-ity than any other technology area. The cost of manu-facturing composite structures has proven to be the

largest obstacle to their widespread use. This is becauseof design and manufacturing approaches that use com-posite materials in the conventional `metals fashion' of

assembling large numbers of mechanically fastenedparts.A�ordable composite structures can be achieved by

proper material selection, changing load paths, usingrobust low-cost manufacturing and joining/assemblytechniques, and developing approaches for subsystemintegration [30±32]. A coordinated design approach

involving larger, integrated components to maximizeproducibility, quality, and design e�ciency is neededto fully exploit the weight and cost bene®ts of compo-

sites. This will require composites to be considered asearly as possible in the design process so that loadpaths are de®ned that o�er manufacturability and do

not penalize the composite structure's e�ciency.The low-cost composite manufacturing processes

include tow placement, resin transfer molding, resin

®lm infusion, pultrusion and nonautoclave processing.A promising structural concept for low-cost automated

manufacturing is Advanced Grid Sti�ened (AGS)structures, which evolved from early isogrid sti�eningconcepts, and features a lattice of rigid, interconnected

ribs (Fig. 7).Because the properties of composite materials are

directionally dependent, they enable a structure'sstrength and sti�ness to be tailored in directions that

allow the most e�cient management of airframe loads.Advancements in manufacturing using ®ber placement,adhesive bonding, textile structures and low-cost tool-

ing will let designers fully exploit the bene®ts of com-posites.Naturally occurring composite structural members

have evolved with extremely complex load paths forsystem performance. This is prevalent in the skeletalformation in birds' wings, the damage tolerant struc-

ture of beetle shells, and the directionality of ®bers intree limb-to-trunk attachments. The ability to tailormaterial properties will allow structural designs toachieve similar complexity. These technologies will be

transferred into the automotive, railroad, marine, andinfrastructure sectors to produce low-cost vehicle load-bearing frames and bridge structures.

Several government and industry programs havefocused on a�ordable composite structures.Noteworthy among these are the NASA Advanced

Composites Technology (ACT) Program; theComposites A�ordability Initiative (CAI) in the USwhich is a joint service (Air Force, Navy/industry)

e�ort; and the A�ordable Manufacture of CompositeAircraft Primary Structures (AMCAPS) Program inthe UK.

5. Extreme environment structures

Hot structures is an enabling technology for air-frames and engines operating in the high-speed ¯ightregime required for future transpaci®c and transatmo-

Fig. 8. Temperature distribution in the X-33 reusable launch vehicle.


spheric vehicles (TPV and TAV), as well as for space

transportation systems (Fig. 8). Airframes will neednew lightweight structural concepts that can accept

high temperatures (400±1500 8F) and high acoustic

content (noise levels up to 170 dB). This creates an

entirely new environment within which large areas of

the vehicle will now be exposed, simultaneously, toextreme thermal and acoustic load levels. State-of-the-

art concepts for such vehicles can easily exceed twice

the weight of structures designed for nonextreme en-

vironments. Design life requirements of future systemsalso far exceed those of current vehicles. The greatest

potential for achieving the required weights and life

lies in the development of novel structural concepts

that use high-temperature polymer and ceramic matrix

composite (CMC) materials. Key materials mayinclude the Blackglas2 family of CMCs including bis-

maleimides, polyamides, and porous structures.

Fig. 9. In¯atable structures: (a) In¯atable solar sail; (b) Solar Orbital Transfer Vehicle (SOTV) showing two 7 � 10 m in¯atable

solar concentrators (courtesy of the Air Force Research Laboratory, Edwards AFB, CA); (c) a 5 m diameter in¯atable collector

(courtesy of NASA Langley Research Center and Air Force Research Laboratory, Wright Patterson Air Force Base, OH); (d) in-

¯atable laboratory being attached to Mars lander to increase the internal pressurized volume for the crew.


Bringing materials technology to maturity for TPVsand TAVs will require a full understanding of struc-

tural failure mechanisms. In addition, life predictionmethods for high-temperature polymeric or CMCstructural joints must be developed. They will be essen-

tial for creating durable and damage tolerant designsthat can meet the long life requirements.For future space transportation systems, advanced

materials and structural concepts are needed for pri-mary structures, leading edges/nose caps, cryotanksand thermal protection systems (TPS). For the primary

structure, candidate materials are high-temperaturepolymeric matrix and advanced metal matrix compo-sites. Reliable bonded and bolted joint concepts areneeded for these materials. Refractory composites,

active cooling and reusable ablators are considered forthe leading edges/nose caps. Composite and metallicsandwich constructions are candidates for the cryo-

tanks. The tanks must be integrated with the vehicleand a global health monitoring system during design.Candidate materials for TPS include ultra high-tem-

perature ceramic composites and long-life, low-costcarbon±carbon and CMCs. Use of refractory compo-site hot structures in the primary structure could elim-

inate the requirement for TPS.

6. Flexible load-bearing structures

Future ¯exible load-bearing aerospace structures

include in¯atable deployable aperture structures forantennas and radars, ¯ight and access doors withouthinge lines, in¯atable solar sails and re¯ectors; multi-

layer positional walls for satellites; ¯exible wall multi-layer structures for lunar and Mars habitats; and novel¯exible load-bearing concepts for aircraft structures([33±37] and Fig. 9).

In¯atable deployable structures o�er low-launchvolume and mass. Following the successful ¯ight ofthe in¯atable antenna experiment in May 1996, NASA

and DOD undertook a space in¯atable technologyprogram. It addresses concepts and component tech-nologies for long-life space missions of the 25-m class

apertures supported by large rigidizable structures.Studies focus on thin-®lm membrane materials, fabri-cation techniques, rigidization methods, in¯ation sys-tems and the interfaces between their structure and the

rest of the spacecraft. Rigidization methods usedinclude gel impregnation, cold rigidization, UV curing,yielding of aluminum laminate, and foam injection.

One in¯ation concept, an onboard gas generation sys-tem, uses chemical reaction between liquids or liquidsand solids. The gas generation system consists of a

reaction chamber, low-pressure liquid tank, and low-pressure valving.In¯atable structures are candidates for the Mars

transit vehicle and for the habitats on the lunar/Martian surfaces. The habitat will encounter large sur-

face temperature gradients, radiation from solar ¯aresand galactic cosmic rays, and micromechanicalimpacts. The primary force controlling the structural

design is the di�erence between internal and externalpressures (external pressure is nearly zero on the moonand less than 1% of the internal pressure on Mars),

and the structure behaves as a pressure vessel.In¯atable structures have low mass and a small ratioof stowable size to deployed volume for economical

transportation from earth. These structures can be pre-fabricated and tested on earth, then deployed on sitethrough controlled internal pressurization.Among the novel ¯exible load-bearing concepts con-

sidered for aircraft structures are the expandable fuelcell (EFC) and the compliant trailing edge. They arelocated on the external surface of the vehicle. EFCs

are conformal to the vehicle's outer moldline whenthey are empty, and are in¯ated when ®lled with fuel.They can signi®cantly increase the aircraft range (Fig.

3)The compliant trailing edge integrates structures and

control technologies into a continuous trailing-edge

surface that fully complies with requirements for aero-dynamic performance, ¯ight control, and structuralsti�ness. The integrated ¯exible structures providemore e�cient control surfaces (reduced maneuver

drag) than do conventional designs.Practical use of these novel concepts will require

development of technologies to ensure their a�ordabil-

ity, durability, and supportability in the operationalenvironment. Many future military air vehicles includ-ing long-range transport aircraft, special operations

aircraft with vertical takeo�/landing (VTOL) require-ments, and long endurance reconnaissance aircraft willdepend on the ¯exible airframe structures technologyto meet operational performance goals.

7. Computational methods and simulation-based design

High-®delity ®nite element models are routinely used

to predict the loads and responses of aerospace ve-hicles. Advances are still needed in several areas ofcomputational technology including: computationaldevelopment of new materials and processes; accurate

prediction of damage initiation and propagation, andof the safe life of the vehicle; intelligent simulation-based design. The latter refers to the seamless process

of simulating the entire life cycle of the aerospace sys-tem before physical prototyping.Computationally based material development is a

new paradigm in material synthesis. It is based on mul-tiscale material and process modeling spanning a largespectrum of time and length scales. Distinct models are


Fig. 10. Computationally driven material development.

Fig. 11. Life cycle simulations in a distributed virtual environment.


used, starting with atomistic models, including ®rst

principles quantum mechanical methods, and progres-sing in size from microscopic to mesoscopic and conti-nuum models. The next stages include the realm of

modeling processes and, ultimately, system modeling(Fig. 10). Research activities will focus on the abilityto model and assess ¯aws and damage in materials

under realistic service conditions, especially in regionsof high-stress gradients, such as joints and interfaces.

The maturation of this research will enable the designof materials at the atomic/molecular level to be multi-functional via the speci®cation of particular properties.

Realizing this technology's potential will require devel-opment of broad-based computer simulation, as re-

liable as experiments with cross-discipline interactions.It will also require the practicing community's accep-tance of the results from such techniques.

Modeling and simulation of an aerospace system'sentire life-cycle demands a high-level of con®dence andfunctionality. Among the weak links in the process is

the reliable prediction of structural failure modes, ulti-mate strength, residual strength, and fatigue life.

Simulation tools for damage initiation and propa-gation are needed, as are methodologies for accurateprediction of safe operating cycles for airframes and

propulsion systems and of the useful life of space sys-tems. Accomplishing this task will require an under-

standing of the physical phenomena associated withdamage and failure, development of a framework formodeling material and structural damage, hierarchical

multiscale computational strategies, novel testmethods, measurement techniques and scaling laws,and validation and veri®cation methodologies.

Intelligent simulation-based design (ISBD) refers tosimulation of the entire life cycle of the aerospace sys-

tem, from concept development to detailed design, pro-totyping, quali®cation testing, operations, maintenanceand disposal (Fig. 11). This is a seamless process per-

formed in a distributed synthetic environment linkinggeographically dispersed design and manufacturing

teams, facilities and resources. The conceptual, prelimi-nary and detailed design phases will merge into asingle continuous design process with progressively

re®ned models (reducing the level of abstraction andadding details to better match the evolving aerospacesystem). A number of government and industry pro-

grams are currently devoted to various aspects ofISBD. Computational tools in ISBD include high-®de-

lity, rapid modeling facilities and physics-based deter-ministic, nondeterministic and qualitative simulationtools for structures, aerodynamics, controls, thermal

management, power, propulsion and optics. They alsoinclude tools for mission design, cost estimating, pro-duct assurance, safety analysis, risk management, vir-

tual manufacturing, quali®cation testing and life-cycleoptimization. Realization of ISBD requires the inte-

gration and deployment of new technologies, includinghigh-capacity computing, communications and net-

working; synthetic/immersive environments; CAD/CAM/CAE systems; product data management sys-

tems; computational intelligence and its soft computingtools; knowledge-based engineering and virtual manu-

facturing.An extension of the ISBD concept is the intelligent

synthesis environment (ISE) being developed by NASA,UVA and JPL. ISE e�ectively combines leading-edge

technologies to build and assemble a widely distribu-ted, integrated collaborative virtual environment link-

ing diverse, geographically dispersed science andengineering teams. The teams are provided with tools

and facilities to signi®cantly improve their ability toexplore, generate, track, store and analyze di�erent

mission scenarios and alternative product developmentprocesses. The technologies used in ISE are high-per-

formance computing, high-capacity communicationsand networking, modeling and simulation, knowledge-

based engineering, computational intelligence, human-centered computing, and product information manage-

ment. The virtual environment incorporates advancedcomputational, communication, networking facilities

and tools, and information system based cognitive and

perceptual aids for creative design and decision mak-ing. The environment is adaptable and intelligent with

respect to end users and hardware platforms. It willprovide the means to optimize the combined perform-

ance of geographically dispersed multidisciplinaryteams. ISE should radically advance the process by

which complex science missions are synthesized andhigh-tech engineering systems are designed, manufac-

tured and operated. The ®ve components critical toISE are human-centered computing, infrastructure for

distributed collaboration, rapid synthesis and simu-lation tools, life-cycle integration and validation, and

cultural change in the creative process (Fig. 12). The®ve components are described in Refs. [38 and 39].

Fig. 12. Major components of ISE.


8. Future directions

Not every area in the disciplinary ensemble of struc-

tures technology is expected to yield breakthroughs inthe near future. However, steady progress in all thecomponent technologies will improve the structural

e�ciency of aerospace systems. Demands for e�ciencyand multifunctionality will drive structures technologyto develop new capabilities in which material selection

and structural forms are highly complex and inte-grated.Several material technologies o�er extraordinary far-

term opportunities such as functionally graded, func-tion-integrated, nanophase and biomimetic materials.

For functionally graded materials (FGMs), novel pro-cessing techniques are used to produce engineered, gra-dual transitions in microstructure, composition, and

properties to satisfy spatially varying functional per-formance requirements within a single component. Useof FGMs can alleviate the high gradients of internal

stresses and strains resulting from the di�erent localdeformation ®elds included when dissimilar materialsare joined to form a component. This technology

could allow researchers to develop complicated struc-tures without using conventional methods of joining.Function-integrated materials are extensions of smart

materials. The sensing functions that use photons,mechanical forces and magnetic or electric ®elds arebuilt into the molecular structure. Among their poten-

tial applications are sprayable and adhesive batteriesor solar cells for aircraft wings, to convert solar energy

to electrical power and store it, and sprayable struc-tural composites that have a switchable antenna func-tion to receive and process information or to provide

low observability on demand.Nanophase materials are produced by consolidating

ultra®ne particles of the same atoms as their common

forms. Mechanical, optical, chemical, magnetic andelectrical properties of nanophase materials can be tai-lored to meet speci®c needs through controlling the

size of their constituent grains.Biomimetic technology aims at producing new ma-

terials by mimicking the synthesis, processing and

properties of materials found in biological systems.Among the unique and useful characteristics of such

systems are multifunctionality, hierarchical organiz-ation, self-repair, adaptability and durability.Moreover, biological structural systems do not dis-

tinguish between materials and structures. The designand development of natural organisms is an integratedprocess in which component functions are multiple and

result in a cost-e�ective, durable structure.Future structural research issues involve integrating

existing and new materials into functional systems with

high-quality and low-cost features. Future e�orts willaddress advanced load path management, innovative

materials processing, low-cost fabrication and othertechnology challenges to enable more a�ordable,

lighter, higher, stronger and sti�er, safer and more dur-able vehicles for di�erent ¯ight regimes, and for plane-tary atmospheric entry and ¯ights throughout the solar

system. These activities will dispel the notion that highperformance can be achieved only at high cost. Today,structures technology is derived from mission and ve-

hicle requirements. A cultural shift is needed to e�ect achange to missions and vehicles that are enabled byinnovative structures technology. The 21st century will

see aerospace vehicles made from computationallydesigned materials at the atomic/molecular level toaccomplish a variety of missions. Programmable multi-functional materials and structures will be able to

adjust their shape and their mechanical, electromag-netic, optical and acoustic properties on demand.

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