structures technology for future aerospace systems

1. Structures technology for future aerospace systems
Ahmed K. Noora,
*, Samuel L. Vennerib
, Donald B. Paulc
, Mark A. Hopkinsd
a
Center for Advanced Computational Technology, University of Virginia, NASA Langley Research Center, Hampton, VA 23681, USA
b
NASA Headquarters, Washington, DC 20546, USA
c
Air Force Research Laboratory, Wright Patterson Air Force Base, OH 45433, USA
d
Boeing 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, ¯exible
load 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 is
described. 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: a.k.noor@larc.nasa.gov (A.K. Noor).

2. 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

3. 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, and
reduce 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 involves
tasks in other disciplines. In this article, the trends in
each of the component technologies are discussed and
the applicability of these technologies to future vehicles
is 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 to
the stimuli in real or near-real time. A response can
consist 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 of
this is the intelligent, self-healing vehicle whose built-in
redundancy and on-board self-inspection detects
damage and responds with autonomous adjustments
and repair.
The active elements in smart structures can be
embedded in or attached to the structure. Typical sen-
sors include ®ber optics, piezoelectric ceramics and
polymers. Embedded sensors can be either discrete or
distributed to provide built-in structural quality assess-
ment capabilities, both during material processing and
vehicle operation. Sensors can also be used for moni-
toring in-service or environmental loading, and for
shape sensing. Typical smart structure actuators
include shape memory alloys (SMAs), piezoelectric and
electro-strictive ceramics, magneto-strictive materials,
and electro- and magneto-rheological ¯uids and
elastomers.
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 lift
or 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

4. over the lifting surface); vibration suppression (includ-
ing ¯utter and bu€et control); alleviation of noise and
vibration; and monitoring of vehicle health. For space
systems, smart structures provide a robust design
approach for meeting precision requirements. They can
signi®cantly reduce cost and schedule by decreasing
the requirements on analysis, development testing,
hardware process testing, and quality control.
The activities pertaining to three smart structures
concepts are described subsequently: shape adaptive
structures and aerodynamic load control; structural
health monitoring; and vibration and noise suppres-
sion. The Department of Defense and NASA are
studying 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 deform
leading and trailing edge control surfaces through
SMA actuation to enhance maneuvering and lift. Such
techniques can increase aircraft survivability and
reduce drag (Fig. 3).
Active aerodynamic load control can be achieved via
self-straining actuators (SMAs or piezoelectric devices)
embedded within the structure. The actuators expand
or contract on command. This changes the shape of
the active airfoil element, which in turn changes the
aerodynamic load on the lifting surface. The most
likely candidates for smart material load control are
very ¯exible surfaces such as the High Altitude Long
Endurance (HALE) aircraft, or smaller surfaces such
as missile ®ns. Future piezoelectric materials will have
to 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, can
be 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 to
determine if and where damage has occurred.
Two areas of special interest are reducing the oscil-
lations of primary structure due to unsteady external
forces, and reducing the transmission of acoustic
energy through the structure. Active control systems
that use piezoelectric actuators are being developed.
Actuators will be attached to the skin and substruc-
tures of the vehicle. Alleviation of these dynamic loads
will 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 before
the technology can be incorporated into future oper-
ational vehicles. The challenges include de®ning the
fatigue life characteristics of smart actuators attached
to realistic aircraft structures in an operational en-
vironment, and developing the maintenance and repair
procedures for embedded actuators. Other technical
issues that must be addressed to realize the full poten-
tial of the technology involve structure fabrication
methods, reliable actuator material, lightweight struc-
tural materials capable of physical and virtual shape
changes, 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 given
in Refs. [13±24]. The various couplings between mech-
anical, thermal, electric, and magnetic ®elds for smart
materials are depicted in Fig. 4. Computational
methods for smart materials and structures are
described in Ref. [25].
3. Multifunctional materials and structures
Multifunctional structures (MFS), in addition to
supporting loads, use sensors to detect and evaluate
loads or failure, and to interact with the surrounding
electromagnetic environment. MFS represents a new
manufacturing and integration technology by which
communications 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.
A.K. Noor et al. / Computers and Structures 74 (2000) 507±519510

5. 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.
A.K. Noor et al. / Computers and Structures 74 (2000) 507±519 511

6. 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 aerospace
vehicle design is being studied. The system-level pay-
o€s of integrating these technologies will be realized
only 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 these
payo€s to materialize. Advanced subsystem devices,
novel structural concepts, design/analysis methods, and
manufacturing 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 thermal
management, ¯uidic jets for aerodynamic control, inte-
gral electric and hydraulic lines, and actuators whose
attachment hardware may be built directly into the air-
frame substructure. Composite materials will be used
extensively in these integral designs because of their
unique 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 include
reduced 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 reduction
in part count and increased volumetric eciency. The
savings realized in operational and sustainment costs
for systems with increased robustness and durability
may o€set the probable increase in cost per pound of
the airframe, attributed to inclusion of electronic or
mechanical subsystems. The maximum payo€s to be
measured in eciencies at the system level can only be
Fig. 7. Advanced grid sti€ened structure.
A.K. Noor et al. / Computers and Structures 74 (2000) 507±519512

7. attained by considering the airframe to be a multifunc-
tional structure early in the development of the con-
ceptual design and simultaneously optimizing the
design across all disciplines. Previously unimagined de-
sign eciencies and synergies will be achieved.
Ultimately though, the transition of multifunctional
aircraft structural concepts to next generation aircraft
will require resources focused on development of tech-
nologies to ensure their a€ordability, durability, and
supportability in the operational environment. These
quantities will be essential to continued development
of 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 technology
improvements, 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 because
of design and manufacturing approaches that use com-
posite materials in the conventional `metals fashion' of
assembling large numbers of mechanically fastened
parts.
A€ordable composite structures can be achieved by
proper material selection, changing load paths, using
robust low-cost manufacturing and joining/assembly
techniques, and developing approaches for subsystem
integration [30±32]. A coordinated design approach
involving larger, integrated components to maximize
producibility, quality, and design eciency is needed
to fully exploit the weight and cost bene®ts of compo-
sites. This will require composites to be considered as
early as possible in the design process so that load
paths 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€ening
concepts, and features a lattice of rigid, interconnected
ribs (Fig. 7).
Because the properties of composite materials are
directionally dependent, they enable a structure's
strength 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 for
system performance. This is prevalent in the skeletal
formation in birds' wings, the damage tolerant struc-
ture of beetle shells, and the directionality of ®bers in
tree limb-to-trunk attachments. The ability to tailor
material properties will allow structural designs to
achieve similar complexity. These technologies will be
transferred into the automotive, railroad, marine, and
infrastructure sectors to produce low-cost vehicle load-
bearing frames and bridge structures.
Several government and industry programs have
focused on a€ordable composite structures.
Noteworthy among these are the NASA Advanced
Composites Technology (ACT) Program; the
Composites A€ordability Initiative (CAI) in the US
which is a joint service (Air Force, Navy/industry)
e€ort; and the A€ordable Manufacture of Composite
Aircraft Primary Structures (AMCAPS) Program in
the UK.
5. Extreme environment structures
Hot structures is an enabling technology for air-
frames and engines operating in the high-speed ¯ight
regime required for future transpaci®c and transatmo-
Fig. 8. Temperature distribution in the X-33 reusable launch vehicle.
A.K. Noor et al. / Computers and Structures 74 (2000) 507±519 513

8. spheric vehicles (TPV and TAV), as well as for space
transportation systems (Fig. 8). Airframes will need
new 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, to
extreme 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 systems
also 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 may
include 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 Â 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.
A.K. Noor et al. / Computers and Structures 74 (2000) 507±519514

9. Bringing materials technology to maturity for TPVs
and TAVs will require a full understanding of struc-
tural failure mechanisms. In addition, life prediction
methods for high-temperature polymeric or CMC
structural joints must be developed. They will be essen-
tial for creating durable and damage tolerant designs
that 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, cryotanks
and thermal protection systems (TPS). For the primary
structure, candidate materials are high-temperature
polymeric matrix and advanced metal matrix compo-
sites. Reliable bonded and bolted joint concepts are
needed for these materials. Refractory composites,
active cooling and reusable ablators are considered for
the leading edges/nose caps. Composite and metallic
sandwich constructions are candidates for the cryo-
tanks. The tanks must be integrated with the vehicle
and a global health monitoring system during design.
Candidate materials for TPS include ultra high-tem-
perature ceramic composites and long-life, low-cost
carbon±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 for
antennas and radars, ¯ight and access doors without
hinge 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-launch
volume and mass. Following the successful ¯ight of
the in¯atable antenna experiment in May 1996, NASA
and DOD undertook a space in¯atable technology
program. 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 used
include 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 liquids
and 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 ¯ares
and galactic cosmic rays, and micromechanical
impacts. The primary force controlling the structural
design is the di€erence between internal and external
pressures (external pressure is nearly zero on the moon
and 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 ratio
of stowable size to deployed volume for economical
transportation from earth. These structures can be pre-
fabricated and tested on earth, then deployed on site
through controlled internal pressurization.
Among the novel ¯exible load-bearing concepts con-
sidered for aircraft structures are the expandable fuel
cell (EFC) and the compliant trailing edge. They are
located on the external surface of the vehicle. EFCs
are conformal to the vehicle's outer moldline when
they 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 structural
sti€ness. The integrated ¯exible structures provide
more 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 operational
environment. 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 will
depend on the ¯exible airframe structures technology
to 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 of
computational technology including: computational
development of new materials and processes; accurate
prediction of damage initiation and propagation, and
of 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 large
spectrum of time and length scales. Distinct models are
A.K. Noor et al. / Computers and Structures 74 (2000) 507±519 515

10. Fig. 10. Computationally driven material development.
Fig. 11. Life cycle simulations in a distributed virtual environment.
A.K. Noor et al. / Computers and Structures 74 (2000) 507±519516

11. 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 ability
to model and assess ¯aws and damage in materials
under realistic service conditions, especially in regions
of high-stress gradients, such as joints and interfaces.
The maturation of this research will enable the design
of 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's
entire life-cycle demands a high-level of con®dence and
functionality. 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 accurate
prediction 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 with
damage and failure, development of a framework for
modeling material and structural damage, hierarchical
multiscale computational strategies, novel test
methods, measurement techniques and scaling laws,
and validation and veri®cation methodologies.
Intelligent simulation-based design (ISBD) refers to
simulation of the entire life cycle of the aerospace sys-
tem, from concept development to detailed design, pro-
totyping, quali®cation testing, operations, maintenance
and disposal (Fig. 11). This is a seamless process per-
formed in a distributed synthetic environment linking
geographically dispersed design and manufacturing
teams, facilities and resources. The conceptual, prelimi-
nary and detailed design phases will merge into a
single continuous design process with progressively
re®ned models (reducing the level of abstraction and
adding details to better match the evolving aerospace
system). A number of government and industry pro-
grams are currently devoted to various aspects of
ISBD. Computational tools in ISBD include high-®de-
lity, rapid modeling facilities and physics-based deter-
ministic, nondeterministic and qualitative simulation
tools for structures, aerodynamics, controls, thermal
management, power, propulsion and optics. They also
include tools for mission design, cost estimating, pro-
duct assurance, safety analysis, risk management, vir-
tual manufacturing, quali®cation testing and life-cycle
optimization. Realization of ISBD requires the inte-
gration and deployment of new technologies, including
high-capacity computing, communications and net-
working; synthetic/immersive environments; CAD/
CAM/CAE systems; product data management sys-
tems; computational intelligence and its soft computing
tools; 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 and
engineering teams. The teams are provided with tools
and facilities to signi®cantly improve their ability to
explore, generate, track, store and analyze di€erent
mission scenarios and alternative product development
processes. The technologies used in ISE are high-per-
formance computing, high-capacity communications
and networking, modeling and simulation, knowledge-
based engineering, computational intelligence, human-
centered computing, and product information manage-
ment. The virtual environment incorporates advanced
computational, 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 will
provide the means to optimize the combined perform-
ance of geographically dispersed multidisciplinary
teams. ISE should radically advance the process by
which complex science missions are synthesized and
high-tech engineering systems are designed, manufac-
tured and operated. The ®ve components critical to
ISE 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.
A.K. Noor et al. / Computers and Structures 74 (2000) 507±519 517

12. 8. Future directions
Not every area in the disciplinary ensemble of struc-
tures technology is expected to yield breakthroughs in
the near future. However, steady progress in all the
component technologies will improve the structural
eciency of aerospace systems. Demands for eciency
and multifunctionality will drive structures technology
to 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. Use
of FGMs can alleviate the high gradients of internal
stresses and strains resulting from the di€erent local
deformation ®elds included when dissimilar materials
are 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 are
built into the molecular structure. Among their poten-
tial applications are sprayable and adhesive batteries
or 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 and
electrical 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 design
and development of natural organisms is an integrated
process 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 will
address advanced load path management, innovative
materials processing, low-cost fabrication and other
technology 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 high
performance 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 a
change to missions and vehicles that are enabled by
innovative structures technology. The 21st century will
see aerospace vehicles made from computationally
designed materials at the atomic/molecular level to
accomplish 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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