December 2000

NASA/TM-2000-210547

Research Opportunities in Advanced

Aerospace Concepts

Gregory S. Jones, Linda S. Bangert, Donald P. Garber, Lawrence D. Huebner, Robert E.

McKinley, Jr., Kenneth Sutton, Roy C. Swanson, Jr., and Leonard Weinstein

Langley Research Center, Hampton, Virginia
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December 2000

NASA/TM-2000-210547

Research Opportunities in Advanced

Aerospace Concepts

Gregory S. Jones, Linda S. Bangert, Donald P. Garber, Lawrence D. Huebner, Robert E.

McKinley, Jr., Kenneth Sutton, Roy C. Swanson, Jr., and Leonard Weinstein

Langley Research Center, Hampton, Virginia
Available from:

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7121 Standard Drive 5285 Port Royal Road

Hanover, MD 21076-1320 Springfield, VA 22161-2171

(301) 621-0390 (703) 605-6000
iii

EXECUTIVE SUMMARY ........................................................................................................................................................1

INTRODUCTION.......................................................................................................................................................................2

What We Cannot Do (or Do Not Do Very Well): ..............................................................................................................2

BACKGROUND .........................................................................................................................................................................5

TECHNOLOGICAL DRIVERS .......................................................................................................................................................6

SOCIETAL DRIVERS ...................................................................................................................................................................7

CONSTRAINTS ...........................................................................................................................................................................7

VISION.......................................................................................................................................................................................8

NASA LARC & AAAC ROLES ...............................................................................................................................................8

REGULATORY INFLUENCE ........................................................................................................................................................9

AERONAUTICS .......................................................................................................................................................................10

Aeronautics Paradigm ......................................................................................................................................................10

Technology Drivers...........................................................................................................................................................10

Societal Drivers.................................................................................................................................................................11

SUBSONIC MILITARY AVIATION ..................................................................................................................................11

Military Aviation Enabling Technologies ........................................................................................................................11

SUBSONIC COMMERCIAL AVIATION ..........................................................................................................................14

Civil Aviation Enabling Technologies..............................................................................................................................14

Environmental Noise.........................................................................................................................................................15

Environmental Emissions..................................................................................................................................................17

Short Take Off and Landing and Vertical Take Off and Landing Aircraft.....................................................................19

PERSONAL OR GENERAL AVIATION:............................................................................................................................19

Flying Car: ........................................................................................................................................................................20

Commercial Micro-Air Vehicles: .....................................................................................................................................21

HIGH SPEED CONCEPTS...................................................................................................................................................21

Supersonic .........................................................................................................................................................................21

Hypersonic Flight..............................................................................................................................................................24

SPACE EXPLORATION.........................................................................................................................................................28

SPACE TRANSPORTATION CONCEPTS ........................................................................................................................28

Background........................................................................................................................................................................28

Current Status....................................................................................................................................................................29

PLANETARY EXPLORATION CONCEPTS ....................................................................................................................30

Mars Missions ...................................................................................................................................................................31

Other Missions ..................................................................................................................................................................32

Gas Giant Dirigible/ Bathyscaphe ...................................................................................................................................32

NON-AEROSPACE APPLICATIONS..................................................................................................................................34

GROUND VEHICLE APPLICATIONS...........................................................................................................................................34

WATER VEHICLE APPLICATIONS.............................................................................................................................................35

NON-TRADITIONAL AIR VEHICLE APPLICATIONS...................................................................................................................35

BIOMIMETICS ..........................................................................................................................................................................35

ENVIRONMENTAL SYSTEMS ...................................................................................................................................................36

DISCUSSION ............................................................................................................................................................................37
iv

ADVANCED DESIGN AND CFD TECHNIQUES .........................................................................................................................40

CONCLUDING REMARKS AND RECOMMENDATIONS ............................................................................................43

RECURRING “ADVANCED AEROSPACE CONCEPT” THEMES INCLUDE:..................................................................................43

RECOMMENDATIONS ..............................................................................................................................................................44

What We (NASA) Should Aim to Achieve: .......................................................................................................................44

What We (AAAC) Should Do to Achieve our Aims in the Long Term: ...........................................................................44

What We (AAAC) Should Do to Achieve our Aims in the Near Term:............................................................................45

REFERENCES..........................................................................................................................................................................47

Web Sites: ..........................................................................................................................................................................48

APPENDIX ................................................................................................................................................................................49

EXAMPLES OF CURRENT ADVANCED CONCEPTS...................................................................................................................49

Blended-Wing-Body ..........................................................................................................................................................50

Strut Braced Wing Concept ..............................................................................................................................................51

Adaptive Vehicle Control..................................................................................................................................................52

Box Wing ...........................................................................................................................................................................53

Supersonic Biz Jet .............................................................................................................................................................54

Personal Air Vehicles........................................................................................................................................................55

Multi-body Inboard Wing .................................................................................................................................................56

Automated Package Delivery: A Virtual Memphis..........................................................................................................57

Intermodal Transports ......................................................................................................................................................58

Access to Space - Reusable Launch Vehicles...................................................................................................................59
1

EXECUTIVE SUMMARY

The Aerodynamics, Aerothermodynamics, and Acoustics Competency (AAAC)

formed four teams to develop white papers. The four teams were to address, respectively,

Advanced Aerospace Concepts, Flow and Noise Control, Advanced Design Tools, and

Hypersonic Airbreathing Propulsion Flowpath. The stated purpose of the teams was to:

∑ Provide a basis for an aerodynamics R&T vision for the next ten years that can

be marketed both internally and externally.

∑ Provide a basis for developing future branch and competency portfolios

consistent with the vision.

∑ Assist in maintaining, growing, and developing core competencies.

Each team was to address requirements for

∑ Physical and chemical modeling

∑ Computational tools

∑ Experimental facilities

∑ Instrumentation and measurements

It was further stated that performance enhancement, noise and emissions reduction,

multidisciplinary integration, and certification by analysis were important considerations.

The focus was determined to be the next ten years starting with the current state of the art.

This paper documents the efforts of the Advanced Aerospace Concepts team to

meet those requirements. The team decided to focus on advanced technologies, rather

than catalog every unusual aircraft that has ever been attempted. The team feels strongly

that individual vehicles, platforms, or airframes should be just a focus for demonstrating the

benefits of technologies that can then be widely applied to many platforms. However, the

team also recognizes the benefit of using the study of advanced vehicles as a tool to

uncover new directions for technology development.

In order to dispel the myth that "aerodynamics is a mature science" an extensive list

of "What we cannot do, or do not know" was enumerated. A zeit geist, a feeling for the

spirit of the times, was developed, based on existing competency research and on previous

and concurrent efforts to develop similar research goals. Technological drivers and the

constraints that might influence these technological developments in a future society were

examined.

The present status of aeronautics, space exploration, and non-aerospace

applications, both military and commercial, including enabling technologies are discussed.

A discussion of non-technological issues affecting advanced concepts research is

presented. An appendix is provided containing examples of advanced vehicle

configurations currently of interest.
2

INTRODUCTION

Civil aviation, particularly commercial, business, and personal airplane flight,

provides the finest example of why aerodynamics is incorrectly considered to be a fully

mature discipline. All of the airplanes look alike; only the scale differs. The reason for this

is quite simple. Like the 12-meter yachts of America's Cup sailboat racing from a few years

ago, nearly all of the vehicles flying today are finely tuned examples of a single concept.

Also, like the 12-meter yachts, these airplanes are exquisite masterpieces of technological

development trapped in an evolutionary dead end. The limitation of the sailboat-racing

world was a self-imposed one originating from the viewpoint that racing was an exercise of

personal skill rather than technological prowess. In the case of current aircraft, the

limitation appears to be a consequence of an inadequate design paradigm.

The essential question regarding today’s aircraft is the following: Have we actually

reached a point where only incremental changes are possible, due to a restricted

design space? Do we understand the physics well enough to model or change the

design space for complex flows that will lead to a revolution? The answer is no and

thus the mantra of revolutionary and advanced aerospace concepts remain elusive at best.

To determine how to invest the resources of the AAAC it is important to understand

where shortcomings are that influence enabling technology for revolutionary advances in

aerodynamics. In this light, a list has been started to answer the following question, with

the implication that these are areas where the AAAC can and should invest in

technology development: What is it that we CANNOT DO or DO NOT KNOW in

aerodynamics today? (While the list contains a wide spectrum of information, it is not

meant to be all-inclusive and should be reviewed and updated as these ideas are

addressed or as new ones arise.)

What We Cannot Do (or Do Not Do Very Well):

∑ Aircraft certification by analysis.

 Accurately predict aircraft performance through entire operating envelope.

(limitations include safety factors, acoustics signatures, emissions, etc.)

 Accurately predict safety factors related to maneuverability, particularly

near the ground. Also cannot predict the impact of time-dependent flow

control devices on aircraft performance.

 Accurately predict configuration effects on aeroacoustic emissions and

performance for revolutionary concepts (e.g. integrated aerodynamic

design with accurate noise predictions).

 Accurately predict impact of chemically reacting flows resulting in inability

to determine environmental emissions
3

∑ Accurately Predict/model/design general aircraft performance including

 Design capability for aircraft other than ones that are “cigar-shaped with

wings” is limited due to lack of empirical methods that the current design

techniques depend on.

 If the design of an aircraft were focused on a single important revolutionary

metric (e.g., zero drag, noiseless, emissionless, etc.), what would such a

vehicle look like?

 Short cycle time for designing process, allowing extensive exploration of

design space.

 Integration of CFD and CAA to accurately predict noise signatures

 Multidisciplinary research, leading to knowledge of multidisciplinary effects

and design [e.g., integration of engines, control surfaces, and other

technologies (structures, materials, electronics, mirco-, nano-technology,

etc.)]

 CFD and experimental measurements of three-dimensional, time-dependent

flow fields including the laminar-to-turbulent transition process for bounded

and free shear flows, fully turbulent flows, flow separation/reattachment

phenomena, shock/boundary-layer interactions, and mixing flows.

 Advanced 3-D codes generally rely on empirical turbulence modeling and are

not always reliable resulting in performance inaccuracies.

 Flight scaling for all speed regimes including very low and high Reynolds

number flows, vortex management, high lift, scramjet combustion, etc.

 Understanding/Modeling/Prediction/Integration of propulsion systems to

airframes.

 Sonic-boom control/alleviation.

 Virtual aerodynamics and flight control, e.g., virtual flaps, porosity effects,

circulation control, flow vectoring, etc.

 High lift systems

 Predict steady state or unsteady flow separation (leading to a inability to

reliability predict aircraft lift or drag)

 Predict Boundary layer Reattachment and/or re-laminarization

 Impact of passive/adaptive/ active flow control, structural control, and noise

control technologies for revolutionary vehicle designs

 Predict/model/measure thermal radiating flow fields for atmospheric ascent

and entry.

 Basic understanding/prediction/modeling of extraterrestrial atmospheres to

determine a vehicle’s ability to travel within them.
4

∑ The limitations listed above are often related to a lack of knowledge of

detailed physics associated with

 Scaling effects associated with size, speed, temperature, density, etc., (e.g.

Reynolds number, Mach number, Nusselt number, etc.)

 Flow Separation

∑ Induced flow separation (i.e., merging of shock-induced and trailing- edge

separated flows, vortex bursting, etc.)

 Unsteady flows

 Control concepts that address unsteady shocks, free-shear fluctuations,

circulation, and separation.

 Predict/model/design general “unsteady” aerodynamics for rotors,

propellers, engines, active flow devices

 Predict Buffet onset/post buffet characteristics.

 High-speed, high-dynamic-pressure separation phenomena of nonaxisymmetric bodies for multistage access to space, store separation,

maglev launchers, and autopackage delivery.

 Exploitation of inherently unsteady flow fields, requiring knowledge of

advanced flow physics, flexible structures and unsteady flight control.

 Aeroacoustic fields

 Acoustical analytic and computational tools, such as sonic fatigue loads

(especially for flexible structures), flow/structure interaction, a robust

noise/structural interaction model, physics-based jet/combustion/airframe

noise prediction technique, interior noise modeling and mid-range

frequencies, a robust long-range sound propagation model, and a

combined human/machine acoustic detector.

 Studying psychoacoustics to determine the annoyance metrics of aircraft

noise.

 Understanding/Measuring/Modeling/Prediction/Application of real gas flow

physics associated with all flow regimes, including continuum flow, noncontinuum flow, and slip flow

 Combusting Flows

 Understanding/Modeling/Prediction/Application of chemically reacting gas

flows.

∑ Some of the limitations listed above are also related to the available

experimental and computational tools such as

 Visualization/immersive simulation to be able to handle/assimilate large

quantities of data, both computational and experimental.
5

 Exploit multiprocessor supercomputer architecture development based on the

latest PC chips for maintaining/improving computing capacity necessary for

doing research better.

 Experimental techniques that capture complex 3-D time dependent off body

flows are too slow, expensive, difficult to use, integrate into available wind

tunnel facilities.

 Mach number limit for scramjet performance and operability.

 Hypersonic ground test facilities capabilities with the correct gas

chemistry, true temperature, and pressure is limited to Mach 6 to 8.

 Investing adequate time to perform literature searches to maintain knowledge

of current research within disciplines, travel for face to face meetings, etc.

The present workload of most researchers precludes sufficient time for these.

One’s own imagination and physics limit the fundamental ideas of “advanced” and

“revolutionary” aerospace concepts. Sources of this imagination can be jumpstarted by

staying abreast of the state of the art, interacting with those of similar mind, and even

reading science fiction. The ability to start with a multitude of ideas and filter them into a

few concepts that will revolutionize the transportation industry is too often plagued with

costs, timing, and a lack of committed support or dedication. The complexity of flight has a

colorful history of ups and downs. This paper will focus on concepts that will lead to

revolutionary vehicles. Many of these concepts will springboard from efforts that have

previously failed and are being re-visited with new technology enablers. Some of the

suggested concepts are radical applications of aeronautics that will satisfy economic and

societal drivers.

The paper is broken into sections that will cover aeronautics, space exploration, and

non-aerospace applications. Both military and commercial applications will be discussed

where appropriate. The required technology development section is intended to highlight

areas of research and development necessary to achieve timely and revolutionary

advanced concepts.

BACKGROUND

In the fall of 1999, the Aerodynamics, Aerothermodynamics, and Acoustics

Competency (AAAC) formed four teams to develop white papers. The four teams were to

address, respectively, Advanced Aerospace Concepts, Flow and Noise Control1

, Advanced

Design Tools2, and Hypersonic Airbreathing Propulsion Flowpath3. The stated purpose of

the teams was to:
6

∑ Provide a basis for an aerodynamics R&T vision for the next ten years that

can be marketed both internally and externally.

∑ Provide a basis for developing future branch and competency portfolios

consistent with the vision.

∑ Assist in maintaining, growing, and developing core competencies.

Each team was to address requirements for

∑ Physical and chemical modeling

∑ Computational tools

∑ Experimental facilities

∑ Instrumentation and measurements

It was further stated that performance enhancement, noise and emissions reduction,

multidisciplinary integration, and certification by analysis were important considerations.

The focus was determined to be the next ten years starting with the current state of the art.

This paper documents the efforts of the Advanced Aerospace Concepts team to meet

those requirements.

Rather than catalog everything that has ever been done, the team decided to

develop a zeit geist, a feeling for the spirit of the times, that was based on existing

competency research and on previous and concurrent efforts to develop similar research

goals4,5,6. To forecast a specific vehicle description far into the future requires the ability to

understand the technology development strategy AND the requirements placed on the

system by the society that will use that technology7,8,9,10,11. This paper will not focus on

specific vehicles but will examine technological drivers and the pressures or trends that

might influence these technological developments in a future society.

Technological drivers

Technological drivers are technical developments in all fields that act as enablers,

providing new means to do things that have either been impossible or prohibitively difficult

in the past. Primary technical drivers are those that are particularly applicable to flight and

have experienced or can be expected to experience significant advancements. In addition

to advances in specific individual areas there is the synergy that can be expected when

advances in different areas can be combined in novel ways. Where atmospheric flight and

space flight are concerned, high-density energy storage and vehicle weight are critical.

Advances in fuel cell efficiencies may well provide sufficient energy densities for

commercial flight allowing for electric powered engines. Composite materials have been in

development for many years and have not as yet delivered levels of reliable strength to

weight performance that had been hoped. There has been considerable progress, though,

and there is reason to expect that such progress may accelerate. Electron beam curing

may enable use of asymmetric laminates without the residual thermal stresses caused by
7

high temperature autoclave curing, thereby saving considerable weight and making

aeroelastic structures more feasible. So-called nanotubes may provide yet another order of

magnitude increase in strength and stiffness over conventional fibers12.

Advances in miniaturizing electronic components have already gone to nearly

miraculous lengths and further progress is expected. With miniaturization have come

speed and increased storage capacity to the point that it is likely a well designed and

programmed flight computer can be expected to outperform any human pilot. Airplane

design could then be expanded beyond the limitations imposed by human reflexes and

endurance. Sufficient reduction in the size of components leads to a synergy with

revolutionary consequences: sensors, processors, and actuators embedded in materials

can yield 'smart' materials that have the potential of exhibiting characteristics unlike

anything seen before. Materials, structures, and even entire systems could behave

organically while possessing strength, stiffness, and tolerance for environmental conditions

beyond any organic material.

Societal drivers

Societal drivers are social developments that are indicative of changes in the way

people view the world and the relative importance they place on the various costs and

benefits of daily life. The most significant societal driver appears to be safety. People don't

want to die, or even get a scraped elbow. No technical advance will be permitted that

reduces safety. Corresponding to safety is reliability, in that safety depends on reliability of

flight systems, but also in the sense of the reliability of schedules. Other drivers are desires

for physical comfort, quiet (no perceived noise), and low cost. Reducing environmental

degradation is far more controversial, but is still very clearly an important societal driver.

Due to the rapid advances in technology in nearly all aspects of modern life an emerging

societal driver is the desire for what might be termed ubiquitous magical technology: instant

gratification of purely personal desires to communicate and to know. While this is primarily

related to the electronics revolution, it will inevitably spill over into every phase of life.

Constraints

Constraints can be thought of as brakes rather than drivers. There are a number of

significant constraints to the development and implementation of new technologies. One is

the belief that infrastructure is limiting. It is assumed that any new vehicle must fit within

the existing set of airports, facilities, and traffic control systems. The world has an

enormous amount of capital invested in the existing infrastructure, but it should be

remembered that none of it existed just a few years ago. Airplanes don't take off and land

on rails because the pioneers of flight didn't try to fit within the existing infrastructure.

Another constraint is governmental. Governments are conservative and they make rules
8

that are based on what is, not on what might be. Rules-based constraints can be

overcome as long as they are recognized and dealt with as research progresses. There

are also constraints based on public perception. No manufacturer will build a truly different

airplane because the public reaction to such a thing is to imagine the headlines associated

with its first crash. This is a very serious problem and can only be handled by making

safety essentially fool proof. While there are technical constraints, real technical

constraints are based on the physics of the natural world and we are far from these limits in

most areas. The greatest technical constraints are limited imagination and lack of

knowledge. If we can just imagine something new, we can pursue the knowledge that will

enable its realization.

Vision

So, what might this future world look like? Despite advances in communications, air

travel will continue. Airplanes will be comfortable and easy to fly, have inexpensive tickets,

easy to maintain, use little fuel, be so quiet as to startle by their unheralded appearance,

affect the environment little more than a bird, allow passengers to communicate seamlessly

with the outside world, and they will not crash. They will be immensely large, very small,

and every size in between and they will not crash. Some will be fast, but they will not

crash. Pilots will not fly them, instead they will either command them or negotiate with

them, but crashing will not be one of the options. Oh, yes, they will also look very funny

compared to what you see today.

Space travel will be fairly common though the numbers will still be small compared

with air travel. People and materials will leave the earth’s atmosphere and return. Travel

to other planets will occur, and exploration of the planets will acquire a new urgency in the

search for new resources. For those planets with an atmosphere, a large part of the

exploration will be done with atmospheric based vehicles.

Until the need to fight each other can be eliminated, military use of air and space will

expand. The imperative to capture the high ground will not change, except that the ground

is no longer the limit and for that matter, the sky is no longer the limit! Military aerospace

vehicle performance will far exceed the ability of a human to go along for the ride. This will

reduce or eliminate the pilot from the on board cockpit.

NASA LaRC & AAAC Roles

Having envisioned a possible future, there remains the question of our role in

developing it. The NASA Strategic Plan is delineated in the 1998 NASA Policy Directive

(NPD)-1000.1. NASA is an investment in America's future. As explorers, pioneers, and

innovators, we boldly expand frontiers in air and space to inspire and serve America and to

benefit the quality of life on Earth. In the coming years, NASA will implement programs to
9

achieve a three-part mission encompassing Scientific Research, Space Exploration, and

Technology Development and Transfer. NASA's overall program, as outlined in the

agency's Strategic Plan, is composed of four Strategic Enterprises, Aerospace Technology,

Earth Science, Human Exploration and Development of Space, and Space Science.

The Langley contribution to the NASA Vision is to be the world leader in

pioneering science and innovative technology to enable U.S. aeronautical and space

preeminence. The Langley Mission statement is: In alliance with industry, other agencies

and academia, we develop airframe and synergistic space frame systems technologies to

enable preeminence of the U.S. civil and military aeronautics and space industries. In

alliance with the global research community, we pioneer the scientific understanding of the

Earth's atmosphere to preserve the environment.

The role of the AAAC is to develop useable research and technology results that

enable program offices to meet program objectives and that enable the Agency to develop

future aerodynamic, aerothermodynamic, acoustic, and hypersonic airbreathing propulsion

systems and wind tunnel testing technologies. AAAC objectives are to provide relevant

technologies and quality technical information, meet planning and execution commitments

to program offices, provide and sustain AAAC areas of expertise to meet Agency

requirements, and reduce cycle-time and/or cost of conducting experiments without

compromising safety.

Regulatory Influence

The role of regulations in the development of aerospace vehicles must not be ignored. The

FAA Strategic Plan sets targets for improving safety, security, and system efficiency. The

mission of the U.S. Environmental Protection Agency is to protect human health and to

safeguard the natural environment — air, water, and land — upon which life depends.

These government organizations directly affect planning by NASA LaRC, and the AAAC

because of their regulatory nature. Airplanes must be safer and quieter if FAA regulations

require it; airplanes must reduce emissions if EPA regulations require it. The lack of

motivation of the airframe and/or engine companies and their stockholders to implement

revolutionary technologies can often be attributed to the conservative nature of the FAA,

EPA, and other regulatory agencies. The influences of these regulatory agencies on the

business of designing, building, and flying aircraft and spacecraft must be considered as

the planning for the future technology is advanced. It is also important to interact with

these agencies so that regulations can be modified with advances in technologies that

make air travel safer and certification faster.
10

AERONAUTICS

In this section the present status and numerous possible advancements of military and

commercial aviation are discussed including enabling technologies. A poignant exposition

of the actual evolutionary state of aerodynamics, which dispels the widespread myth that

aerodynamics is a mature field (where mature suggests that there are no frontiers to

conquer) is given. The development and integration of individual components, (e.g. cruise,

high-lift, propulsion, structure, noise, controls, etc.) are far from being optimized or mature.

Both fixed-wing and rotary-wing aircraft are considered.

Aeronautics Paradigm

A major transformation has occurred in the landscape of the commercial aircraft

sales market in the last decade. Previously, the United States (U.S.) dominated the world

sales market for commercial and military aircraft, with large airframe companies such as

Boeing, McDonnell Douglas, and Lockheed competing for the lion's share of those markets.

The emergence of the Airbus consortium made the Europeans very competitive in

commercial airline products. To remain viable throughout the 1990’s the larger companies

consumed many of the smaller airframe companies. The consolidation of several large

companies also occurred. Even with consolidation, these companies are currently only

dominating the U.S. marketplace. The eroding U.S. market share therefore requires a new

direction based on innovative and creative technological advancements for transport

aircraft if these trends are to be reversed.

Although there is much resistance from the airframe manufacturers to revolutionary

changes in the conceptual design of commercial transport aircraft, there clearly needs to be

an effort to consider alternatives. In so doing, there is the possibility to achieve a

revolutionary change in design that is clearly superior to the present design.

Technology Drivers

In deciding how to exit the perceived dead end into which aviation has evolved, it is

important to look to societal drivers for direction. Technological drivers should only be used

to determine the size of the first step. The relative priority to place on conflicting societal

drivers then becomes a matter of some importance, but the marketplace is the proper

venue for those decisions. The role of the AAAC is to decide how it is best suited to

develop technologies; “based on societal drivers” that can be delivered to the marketplace

for resolution.
11

Societal Drivers

Several recurring themes that influence many of the societal drivers have been identified as

key areas that NASA will address: Safety, Noise, Efficiency, and Environment. It is not

unusual for these drivers to be in conflict with one another. It is also recognized that

military and commercial requirements place different demands on these key areas. The

following sections will highlight each of the areas and identify several advanced concepts

for military and commercial applications.

SUBSONIC MILITARY AVIATION

Many of the advances in commercial aviation have been brought about through

military applications (see appendix). The military’s need for performance and supremacy

continues to push the aerodynamic envelope and technologies that support it. Survivability

on the battlefield may soon drive aircraft to be pilotless, generating a new class of aircraft

with new requirements. The Defense Advanced Research Projects Agency (DARPA) is

virtually synonymous with advanced concepts military research. NASA's work should be

complimentary to DARPA, DOD, and other government programs.

While advances in military aviation may be pushing toward unmanned aircraft, the

enabling technologies that will necessary for these vehicles could also have applications to

civil aviation. NASA is in a unique position to bring appropriate military technologies to civil

applications.

Military Aviation Enabling Technologies

Flow Control

Managing the flow fields around complex aircraft components has been examined

for decades (aerodynamic control surfaces, vortex generators, fences, etc.). It is recent

advances in materials, miniaturization, and improved speed in electronics that have opened

the door to new and innovative ways of controlling macro and micro-scale flow

characteristics. A broad range of active and passive flow control techniques can be used to

manage flow that will affect performance, noise, safety, etc. Applications may include

Adaptive Performance Optimization (“virtual shaping of an aerodynamic surface to ensure

that the aircraft is always flying the optimal configuration for a particular flight regime);

control of all types of flows (laminar, turbulent, vortex, combusting, mixing, or separated);

anti-noise; favorable wave interference; and thrust vectoring. With these flow control

techniques, significant improvements in aircraft performance may be possible, from the

largest transports to the smallest uninhabited air vehicles. Aircraft structure could be freed

from flight control functions, dramatically reducing weight. Flight control could be possible
12

where there currently is none (munitions). Aircraft configurations could be enabled that

were previously prohibitive.

At optimum, flow control systems will be closed-loop. That is, sensing the flow,

comparing that flow to a model of the desired condition, calculating what actuation authority

is necessary, and implementing it. The requirements for and understanding of this enabling

technology is still in the embryonic stages. Critical to the success of implementation of

these technologies will be the understanding of unsteady flow physics these micro devices

exert on the global flow. This includes the scaling of the steady and unsteady spatial and

temporal flows that are being controlled; flow sensors; closed-loop controls; CFD modeling

for prediction and design of turbulent/unsteady flows; and the devices themselves (design,

bandwidth, durability, susceptibility to environmental effects (rain, bugs, etc.), control

power, etc.).

While these tools are being developed, much can be learned by intelligent

application and exploitation of these devices even before the details are fully understood.

Experimental work should not be put aside until the CFD codes are robust. For nearerterm payoff, flow control can be applied in a more localized fashion either in conjunction

with conventional controls or with shape-control. The reader is referred to a separate White

Paper dealing exclusively with flow and noise control, where the details of required

research are presented.

Shape Control

As early as the Wright Brothers, flight control was implemented through macro-scale

shape control, or wing warping. Put in more modern terms, heavy, complex flight control

systems (flaps, ailerons, spoilers, elevators, and rudders) can be eliminated through the

use of bendable skins with "smart" underlying structure. Sometimes called "continuous

moldline" or "morphing," this technology also has the benefit of eliminating flap edges,

which are a source of noise and increased radar signature. Although the technology is

largely unproven, it is believed that some combination of shape and flow control will be

necessary for applications where shape control alone would cause flow separation (e.g.,

high-lift systems) which would reduce the effectiveness of the system. If shape-control

systems can react fast enough, the total structural weight of the aircraft can be reduced

through active aeroelastic control. Currently the structures and materials disciplines

include control effector research in this area, but are often unaware of the details of the

aerodynamic applications, and the attendant requirements, so much more multidisciplinary

work is needed. And when a structures practitioner asks an aerodynamicist what a

continuous flap shape should be, there is often no answer because so many years have

been devoted to optimizing flap gaps and overhangs. Some basic "what if" work needs to
13

be accomplished to understand what shapes are desirable from a lift/drag/control

standpoint.

On a smaller scale, micro-vortex generators and passive porosity can be considered

shape control if they are selectively deployable. The explosion in the availability of micromachining techniques and in micro-electrical mechanical systems (MEMS) devices are

bringing such applications closer to fruition. Although it will be discussed more in the

"Uninhabited/Micro Vehicles" section, understanding low Reynolds number flows and

having the computational tools to predict the behavior and design of such systems is critical

for the widespread acceptance of micro flow/shape control devices.

Cloaking

High performance and reduction of observables, whether visual (contrails), acoustic,

or at frequencies beyond the visual and audible range, is usually of paramount importance

to military missions because they directly affect the survivability of the aircraft and the pilot.

While these considerations may at first seem to have only military applications, they

actually offer many possible civil aviation benefits. If flap edge noise can be eliminated

through continuous moldlines or if jet noise can be directed away from the "threat" area for

military vehicles, then noise can be similarly reduced/directed away from near-airport

communities. Similarly, minimum length convoluted diffusers are being designed into the

Blended Wing-Body (BWB) aircraft and other innovative engine applications.

Preventing contrails for military and civil applications is important. Current thought

suggests that contrails are a bigger contribution to atmospheric pollution than NOx or CO2.

Research into an environmentally friendly means of disbursing or eliminating contrails has

direct applications to both the military and civil aviation fleets.

Aeropropulsive control

Although much more "near-term" than the previously discussed technologies, it is

recognized that aeropropulsive control (also known as "thrust vectoring") has already been

flight-demonstrated on the XF-22, X-31 and SU-37. Thrust vectoring is of particular interest

to the applications of uninhabited or micro vehicles. This is to save the weight and

complexity of a separate flight control system. Although adequate flight control, even in

engine-out conditions, is essential for man-rated aircraft, the same may not always be true

for uninhabited air vehicles where risk, loss, or recovery issues are different. For example,

an unmanned aircraft could utilize a parachute system for engine-out conditions, with

recovery at a later time, or self-destruct to prevent capture of the technology.

Conventional flight controls in high-performance fighters are already capable of

generating more "g" forces than a pilot can endure. Uninhabited vehicles with more

complex control systems could be much more maneuverable than manned fighters over the
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whole range of flight conditions. Although much research was accomplished13 in the field of

thrust vectoring in the 1970s - 1990s, lighter, less mechanically complex thrust vectoring

systems will be required for small uninhabited vehicles, and would have a payoff for larger

vehicles as well.

Military Micro vehicles

Aerodynamics at very low Reynolds Number is currently not well understood, and

can be very different from aerodynamics at the typical "millions" Reynolds number scale.

Understanding of the flow physics at these scales, computational tools to predict their

behavior, and finally, design tools for such vehicles must be developed. Multidisciplinary

optimization of these vehicles is critical when every gram of weight counts. Further

integration of propulsion and control systems and acoustic considerations must be

addressed from the outset. MEMS may also play an important role in the control of micro

vehicles. At these physical sizes, unsteady (periodic) aerodynamics (“flapping-wing”

technology) may also be a design option.

Military Rotorcraft

A military capability for vertical takeoff and landing (VTOL) at unprepared sites will

be needed as long as ground troop insertion, supply, and evacuation into a battlezone is

required and there are enemy ground troops to eliminate. Rotorcraft fills a unique niche in

that they achieve VTOL without significant harm to the landing surface, unlike jet VTOL.

Advanced configurations which have more heavy-lift capability (quad-tilt rotor, for example)

and are more survivable ("stealthy") will be needed for the foreseeable future.

SUBSONIC COMMERCIAL AVIATION

Historically, commercial aviation has benefited from the high-risk research

conducted on military aircraft. However, the advances in military applications can only go

so far, as civil aviation faces many more regulatory restrictions. These restrictions force

differences in military and civil aviation in the areas of environmental noise, environmental

emissions, and safety.

Civil Aviation Enabling Technologies

There are many similarities in the enabling technologies for both military and civil

aviation. The definition of what is revolutionary and what is not will be different for military

and commercial aviation. However, flow control, shape control, and aeropropulsive control

are enabling technologies that are common to both. Safety and the tolerance of the

community often govern the differences in the thrust for developing the required enabling
15

technologies for military and/or civilian flight where aircraft fly. This is emphasized in the

environmental noise and environmental emissions issues described below.

Environmental Noise

One area in which societal desires most clearly argue for a revolution in aircraft

configuration is that of noise. New and future regulations will reduce the noise impact on

the community surrounding airports. Past programs have made substantial progress in

reducing aircraft noise within the framework of the current airframe designs. There is room

for some further progress within the same framework, but the increasing popularity of flight

may well render such progress moot. Thus, noise reduction must be considered a primary

technology area that requires attention.

 Aircraft noise can be divided into two categories: interior and exterior. Interior noise

affects the people in the airplane and exterior noise affects everything else. Efforts to

control interior noise can presumably be applied to any aircraft configuration. While some

airplane configurations may be easier to manage than others, it is difficult to see how

reduction of interior noise requires fundamental new configurations. Reduction of exterior

noise, on the other hand, favors certain concepts and configurations over others. Exterior

airplane noise is an issue involving source noise level, frequency content, proximity, and

directivity. Efforts to date have focused on source noise level. Substantial further progress

will require shifting attention to the other issues.

Sound is reduced by distance, and higher frequencies are reduced more than lower

frequencies because of differential atmospheric absorption. Keeping airplanes away from

people is then one useful way of reducing noise. This is already done with many aircraft

taking-off at steep angle of attacks then throttling back the engines once the aircraft has

reached a safe altitude. These procedures are necessary to meet existing noise

restrictions around the world. More could be done in this area, but with fewer expectations

on current airplanes. Assisted take-off like those used on aircraft carriers could be done

with silent runway based systems using electric motors. The airplane engines would not be

required to run at a high power setting until the airplane was farther away. This would have

the further benefit of reducing wear on the engines. Approach to landing is now done along

a 3° glide-slope that keeps the airplane close to the ground over great distances. A much

steeper angle would keep the airplane farther away from people. Deeply throttling back the

engines during landing would reduce the noise even more. Safety drives the existing glide

slope requirement as well as the need to keep the engines running at a relatively high level.

The Space Shuttle currently makes fairly steep unpowered landings; so it is evident that

such a thing is possible. The safety benefits of this type of approach could be tremendous.

An airplane designed to make unpowered landings, especially slow landings, would not be
16

in any more jeopardy of crashing due to engine failure than crashing a conventional

airplane at a 3° glide-slope.

Since the atmosphere better absorbs higher frequencies than lower ones, sound

sources on airplanes should be designed to generate less low frequency noise even at the

expense of additional high frequency noise. Human response to sound is a complex field

of study, but certain simple concepts can be used. One is that pure tones are bad. They

stand out, they are noticeable, and they are identifiable and distinguishable from other

sounds. A mosquito makes very little noise, but its very identifiable whine gets your

attention. The broadband noise of an airplane, on the other hand, is little different

qualitatively from the broadband noise of other sources.

The directivity of airplane noise is an area that has seen little attention and yet

shows some promise. The only exterior airplane noise that matters to people is the noise

that reaches them. If an airplane makes more noise but that noise is directed away from

people, they won't be bothered by it. Tailoring noise sources to have directivity patterns

where noise is directed upward will further reduce the annoyance caused by airplanes.

And yet, addressing noise sources (such as engines or flaps) as individual items that are in

some sense independent of the airplane is part of the paradigm that needs to be

abandoned. Tailoring engine noise directivity is one solution to reducing noise on the

ground. A better solution is to place the engine above the airplane where the airplane

structure shields the ground (and the people there) from the noise. The very idea of

designing an airplane with low noise as an important design parameter established at the

beginning of the design cycle is a paradigm shift of the first magnitude.

The Blended Wing Body airplane is an example of this paradigm shift, but it doesn't

go far enough. Using large engines with very high bypass ratio fans to reduce jet noise is

one way to reduce source noise levels. But moving the engines forward allows shielding of

aft noise as well as forward noise. Noise can be further directed upward by “scarfing” the

inlet and exhaust nozzles. If this is not an ideal cruise configuration, the engines can be

designed to be moved during flight from take-off positions to cruise positions (and then to

landing positions, if necessary) where each engine position and orientation is accompanied

by a unique nozzle configuration.

Another way to reduce noise is to use many small engines instead of a few big ones.

Small engines could be designed to have greater high-frequency noise content that would

be absorbed by the atmosphere. Very small engines could be distributed and/or embedded

in the wings with inlet and exhaust both being positioned atop the wing. Designing an

airplane with many small engines has favorable safety implications. Loss of an engine is

less important for an aircraft with thirty engines than it is for an aircraft with one or two.

Even unpowered flight generates noise as air flows over the structure. If propulsion

noise is reduced or shielded, flow noise becomes a more obvious issue. At the low speeds
17

of take-off and landing, a large part of the problem is the noise from turbulent flow

interaction. The very high-lift devices that make possible the slow approach to landing are

responsible for this noise. Generating additional lift to make possible a slower and much

steeper approach to landing would seem to be a self-defeating proposition. Understanding

the sources of airframe noise, though, makes it possible to reduce that noise for

conventional wing and landing gear design. Fairings could be used on landing gear and

relatively simple changes to slat and flap design could reduce their noise. More advanced

circulation control over wings, fuselage, or their blended structure would generate high lift

with less noise on more modern designs. This control could be from Coanda surface

blowing, compressor bleed blowing, micro-machined low-power synthetic jets, or fluidic

actuators. Active smart structures, vortex generators, and variable wingspan along with

riblets and fringed trailing edges would begin to make an airplane approach bird-like

efficiencies.

With noise generation dependent on the fine details of flow fields, the only way to

significantly reduce noise while maintaining safe, efficient vehicles is to more fully

understand the flow over moving structures. Understanding and manipulating turbulent and

unsteady flow are vital. There is a need for an advanced unsteady turbomachinery

aerodynamics CFD codes and a high order duct acoustics propagation codes. On a larger

scale there is a need to develop a propulsion/airframe aeroacoustics (PAA) code that can

use CFD results that describe the 3-D flow field over an entire propulsion/airframe

structure. These codes would be used to determine the effect of the local flow field on

propulsion noise generation as well as the effects of shielding, diffraction, and reflection on

the propagation of that noise. With a better understanding of the flow field into engines, the

proper blend of passive treatment, adaptive liners, and active noise control can be

ascertained. The use of these codes in trade studies is at least as important as their use

for physical understanding. Designing for environmental effects is far more effective than

dealing with inadequacies later.

Environmental Emissions

Another area where societal drivers argue for a revolution in aircraft configuration or

operation is that of chemical emissions. As with noise, chemical emissions are an area of

concern not only for those who fly, but even (or especially) for those that do not. Concern

for emissions has traditionally focused on carbon dioxide, oxides of nitrogen, and

particulate. Only more recently has the issue of water vapor emissions been raised. It has

been suggested that global warming has been caused as much by anthropogenic cirrus

clouds (derived from aircraft contrails) as by excess carbon dioxide. Thin, high-altitude

cirrus clouds have been shown to have a net warming effect on the earth as opposed to the

net cooling effect of thick clouds. Simply changing to hydrogen fuel would not reduce water
18

vapor emissions though it might reduce production of ice crystal nucleation sites. Changing

to a closed-cycle propulsion system, adding precipitation-enhancing dopants to the fuel, or

flying where cirrus clouds don't form are potential solutions. This is an area where clearly

more research is truly needed.

Emission reduction can be achieved in at least three ways:

∑ new technology incrementally applied to conventional designs to gradually

reduce chemical emissions, evolutionarily,

∑ apply existing technology in new ways to allow the removal of systems from an

aircraft, i.e., lose weight and/or change operational characteristics, revolutionary

or evolutionarily,

∑ devise radically new technologies and concepts that completely eliminate

chemical emissions, revolutionarily.

Evolutionary emission reduction methods include all of the usual incremental

technological “innovations”. Examples include direct fuel efficiency improvements (via

advanced engine materials and components or flow path improvements), weight reduction

(primarily via new materials systems or structural advances; secondarily via vehicle resizing

due to fuel efficiency gains or drag reductions), and lift-to-drag ratio improvements (e.g.,

increased aspect ratio, airfoil tweaking, or drag clean-up).

Revolutionary emission reduction can rely upon extensive changes to the airframe,

onboard systems, and aircraft operations to minimize weight and engine-on time. Methods

could include innovations such as maglev catapults to reduce or eliminate takeoff

emissions and/or eliminate landing gear, high-bandwidth communications to enable

ground-based digital flight control systems and minimize onboard avionics, thrust vectoring

to eliminate tails, designer aerodynamics (e.g., circulation control, passive porosity,

synthetic jets, etc.) to enable virtual control surfaces, and synthetic vision to eliminate flight

crew and passenger windows.

Both the evolutionary and many revolutionary approaches to emission reduction

have a major drawback: emissions are not completely eliminated. Based on recent

analysis, even if a true zero-emissions aircraft were introduced into the fleet within the next

10-15 years, the CO2 overall emissions from the civil fleet will not return to 1990 levels (per

the Kyoto Agreement) until about 2050. If a new type of aircraft that reduced CO2

emissions by 80% were introduced around 2020, emissions would finally level off around

2050 at 1990 levels plus 67%. True zero-emission aircraft (at least in terms of one suitable

for revenue service) are far beyond the current state-of-the-art. Propulsion is the major

hurdle. Fuel cells of various types could hold promise, but aircraft that are powered by fuel

cells gain weight as they operate (vs. losing weight in the current paradigm). The fuel cell

by-products (e.g., water) could be jettisoned in-flight to lose the weight, as long as the byproducts do not persist in the atmosphere. Liquid hydrogen (LH2) has similar issues. If LH2
19

is used as fuel (like Jet-A today), the only resulting emission is water vapor. Beaming

power from the ground or from space to the aircraft does not currently look feasible due to

atmospheric effects on the beam.

Short Take Off and Landing and Vertical Take Off and Landing Aircraft

Helicopters satisfy a number of transportation, commercial, and emergency and

needs due to their vertical take-off/landing and hovering capability. In general, helicopters

travel at a forward speed of around 200-mph, have flight duration of two to three hours, and

have a range of about 600 miles. They have higher fuel consumption than fixed-wing

aircraft for travel over a given distance. Some improvements have been made in forward

speed by introducing jet or propeller systems in compound helicopters. Advancements in

materials have made helicopters lighter, stronger, and safer. A major problem with

helicopters continues to be the vibration and noise levels they produce. Tilt-rotor and tiltwing aircraft are hybrids in that they have the vertical take-off/landing and hovering abilities

of helicopters but have the high-speed horizontal flight abilities of airplanes. They will travel

roughly twice as fast and twice as far as conventional helicopters. As with helicopters,

vibration and noise are major issues.

Vertical take-off and landing (VTOL) and short take-off and landing (STOL) aircraft pose a

somewhat more difficult aerodynamics challenge than conventional aircraft. Advanced

transport airplanes that land slowly and steeply won't significantly reduce the need for those

that hover and land vertically. Using structure to shield noise has significant penalties

when thrust is directed downward, so other solutions must be found. Shrouding rotors, at

least small ones such as those found on tilt-rotors, may provide a solution. Another

approach is to provide adaptive blades that change length or active blades made of smart

materials that change twist distribution to alleviate noise while maintaining performance.

Regardless of which approaches are used to enhance efficiency or reduce noise and

vibration, an understanding of unsteady flow is vital. These aircraft experience periodic and

random unsteady flow over airfoils under steady state, unsteady flow during transition

between flight regimes, and complex unsteady flow/structure interactions during take-off,

hover, and landing.

PERSONAL or GENERAL AVIATION:

Commercial and business aviation benefits the most from substantially new vehicle

configurations, but general aviation (or personal aviation) also has room for improvement.

If the AGATE and SATS programs yield only a fraction of the increase in flying that is

projected, noise will become a significant issue. Most of the small airfields from which such

vehicles would fly are located in areas where people are used to lower noise levels. Cost

considerations generally lead to intermittent-combustion powered, propeller driven vehicles
20

that work best in a tractor configuration, but small turbine engines have tremendous

advantages if cost can be reduced. Many-bladed, lightly loaded, highly-swept, very thin

propellers can be used to reduce noise, but only if coupled with a narrow, symmetric

cowling that reduces unsteady loading on the propeller. Concurrent design of airframe and

propeller with low noise as a primary consideration is necessary. High technology must

continue to flow into even the less expensive aircraft.

Current efforts, including AGATE, in General Aviation technology have focused

primarily vehicle technology. Langley has initiated the SATS program to examine an

integrated new civil air transportation network. The SATS program is looking at using GPS

and computer networks to direct flight traffic. It is also evaluating a synthetic vision system

to improve small aircraft visibility in inclement weather, and it is examining the coordination

of ground transportation. However it is not looking at the aerodynamic aspects of the

technology.

Research in low Reynolds flight and also in unsteady aerodynamics in low speed

flight would be of particular value to personal aviation aircraft. Combining a better

understanding of the aerodynamics with improved understanding of active stability and

control capabilities could be used to improve the safety and performance of lightweight

aircraft flying at low speeds and increase the smoothness of take-off and landing. This in

turn could make for a vehicle much easier to fly and thus attract a larger pool of pilots to

operate the aircraft.

Ad-hoc experimental aviation designs, made by both professionals and enthusiastic

amateurs have led the development of improved personal aircraft. There is certainly a

need for NASA aerodynamic researchers to also participate in this field. As the number of

personal aircraft increases, the problems of safety, comfort, noise, fuel efficiency, and

speed will become as important just as in commercial aviation.

Flying Car:

The rationale of using a personal flying car is to bypass the limitations imposed by

the road system (avoid heavy traffic, and travel where no good roads are available), and

also travel about as fast as aircraft, but point to point rather than multimode travel (car to

plane to car). The current state of technology is in a very early phase. Several concepts,

which include the Aerocar CRX, the Roadrunner, and the Lincair, have been conceptually

designed but are at an early stage of testing and refinement.

Several advances in technology or infrastructure are needed to improve this design

concept. First the use of different materials to reduce weight and increase durability need

to be employed. Materials that would minimize defect propagation and even allow for

passive repairs would be ideal for this technology.
21

The flying car would also benefit from the increase in information technology to

make air travel safer. Current UAV systems such as GlobalHawk, Darkstar, and Predator

have demonstrated automated guidance and control of aircraft. Derivatives of such

systems could simplify the task of flying allowing greater accessibility to the air. The flying

car would definitely benefit from studies on unsteady aerodynamics. Small cars can

typically be blown 1-2 ft on a highway traveling at moderate speeds due to cross wind

gusts. Studies to characterize unsteady cross-flow aerodynamics could aid in the

development of an active flow-correction-control system. These studies would also aid in

the take-off and landing portions of the air-car flight as well.

Commercial Micro-Air Vehicles:

Micro-Air Vehicles with cameras and on-board instruments can conduct police

surveillance, search for lost persons, assess damage from storm or earthquake, sample the

atmosphere, and do numerous other earth-based uses. In addition, they may be useful as

wide-ranging explorers on other planets. These types of vehicles need to be small, have

reasonable speed and endurance, and be reliable.

The current leading Micro-Air Vehicle is the “Black Widow”, made by Aerovironment.

This vehicle can fly at 43 mph for 22 minutes. Many of the present vehicle designs suffer

from frequent crashes due to their small size/mass and low speed. A paper by Dudley

showed that there is a distinct breaking point in the aerodynamics employed by flying

machines both biological and mechanized which occurs around the six-inch wing span.

The aerodynamics of flight for vehicles under a six-inch wing span is still not clearly

understood. An understanding of the aerodynamic forces at play in this low Reynolds

number regime should be examined to extend the size to as small a vehicle as practical.

Studies of low Reynolds number flight, flapping wing flight, hovering, and unsteady

aerodynamics are needed to begin to properly understand this class of vehicles. Currently DARPA

is searching for a fully controllable Micro-Air Vehicle for military surveillance and hazardous cloud

detection. Most research areas are focused on making a workable vehicle with size and range

specified. Very little fundamental aerodynamic work is being performed on this class of vehicles.

HIGH SPEED CONCEPTS

Supersonic

Commercial supersonic flight clearly falls within a separate regime from commercial

subsonic flight. While there have been many successful military supersonic aircraft for

decades, there have been no economically successful commercial supersonic aircraft. The

reason, simply put, is that the economics are different. That is the economics between

military and commercial aviation is different, and the economics between subsonic and

supersonic flight are different. Even so, with all of the success of supersonic military
22

aircraft, none of them have been transports. Commercial success in supersonic flight will

stem from designing vehicles that will generate sufficient income (by charging a premium

over subsonic flight, opening new markets, or by a currently unimagined paradigm shift) to

justify the additional expense of their development and operation. The constraints that

must be met are those of the long-range subsonic fleet, and then some. The additional

constraints are primarily environmental: sonic boom and emissions at a higher altitude.

Complicating the issue is that just meeting the constraints faced by the long-range subsonic

fleet is more difficult for a supersonic fleet because take-off and landing are low speed,

(hopefully) quiet events for which supersonic cruise configurations seem ill suited.

Enabling Technologies

Successful development and demonstration of the key technologies for quiet,

efficient supersonic cruise flight will enable valuable and unique vehicles, with a range of

military and civil applications important to the U.S. national security and economic

competitiveness.

These enabling technologies are well known and exist at varying stages of maturity.

Successful supersonic cruise vehicles will require: supersonic flight over land, with

minimization or elimination of the sonic boom; low-noise, efficient supersonic inlets,

engines and nozzles; high levels of aerodynamic cruise efficiency; high lift systems with

safe low speed flight, and advanced lightweight airframe structure. Progress on these

technologies was made in the recent U.S. High Speed Research Program. However, these

technologies were not fully matured, and more importantly, most of them were not

demonstrated in flight.

Methods for shaping supersonic airframes to minimize the sonic boom have been

developed and tested at small scale. The key remaining challenge is the validation of these

shapes at large scale in the real atmosphere. Moreover, simply shaping the airframe to

soften booms may not be sufficient. Other advanced concepts may be needed to achieve a

sufficiently low boom level. These technologies must be integrated in a manner that does

not detract from overall aerodynamic efficiency. Technology that allows pilots to determine

the location and loudness of the sonic boom on the ground may also be required. A nearterm test to determine the boom “floor” is highly desirable. If a supersonic lifting shape with

a minimum of boom-producing features (e.g., without inlets) creates a sonic boom greater

than the acceptable overland level (somewhere near 0.5 psf at the ground), further pursuit

of boom-softening technology may be irrelevant.

Attractive technologies for very efficient supersonic cruise aerodynamics include

drag reductions obtained through laminar flow14. Concepts for achieving this laminar flow

naturally via novel wing design and through active flow control have been explored but

require significant additional development and flight demonstration.
23

Technologies for the design of supersonic propulsion systems that are quiet during

takeoff and landing operations still require considerable work. The challenge is to deliver

suppression of engine noise without severe increases in weight and loss of engine thrust.

Advanced structures and materials technology concepts have been explored for

application to the high temperatures experienced in supersonic flight. Analysis shows they

provide substantial weight-reduction benefits. Further maturation and understanding of

manufacturing techniques is required. Weight has a powerful effect on sonic boom, but

boom levels low enough for overland supersonic flight cannot be achieved (on an

economically relevant transport) via weight reduction alone.

One may note that some combinations of technologies may be mutually exclusive on

supersonic aircraft (e.g., low-boom and supersonic natural laminar flow), at least, as the

phenomena are currently understood. This should emphasize even more strongly the need

for integrated multidisciplinary research on the technical challenges.

Shift in Direction

The solution to this apparent high-speed dilemma is to abandon concepts for

supersonic airplanes that are merely 'pointy' versions of conventional subsonic airplanes.

Some of the ideas that are proposed for developing a new subsonic fleet will work for a

supersonic fleet, although the implementation will be different. For example, the designer

can: use vectored thrust and flow control in place of conventional control surfaces; make

vehicles that are reconfigurable for take-off, cruise, and landing; include environmental

considerations early in the design process with noise; use propulsion concepts that are not

merely scaled up military fighter or bomber engines, but may include different engines for

different speed regimes: high bypass turbofan for take-off, turbojet for cruise, nothing for

landing; use an integrated structure that is not necessarily a fuselage with wings pasted on

and an empennage pinned to the tail of the donkey; and use synthetic vision instead of

windows so you can put the aircraft commander (pilot) where weight and balance dictate.

More new ideas will be required. Methods to eliminate (or manage) sonic booms head this

list. The primary recommendation is to stop assuming that an idea is bad because it was

discarded in the past, or is not ideal for subsonic flight. Trade studies should be performed

as close to the start of the design process as possible with the highest level of fidelity

available.

Needed key technologies could be successfully developed using a carefully planned

technology-development program involving conceptual design and analysis, ground tests

(wind tunnel, jet noise facilities, CFD, etc.), and flight tests in two stages. First, the key

design methods and advanced concepts should be validated to determine the achievable

performance levels for boom and noise reduction. Such results are essential to assess the

feasibility of future supersonic applications. The second stage of work would explore the
24

technology integration and validation for specific design applications of interest for both

military and civil applications, at sufficient fidelity and scale to allow confident extrapolation

of results to full-scale applications.

Hypersonic Flight

The arena of hypersonic flight within the atmosphere is one that has been studied for

decades, but has had applications limited to rocket-based flight to orbit (both ascent and

entry), as well as the NASA/Air Force X-15 rocket-based aircraft flight test program.

Exploitation of this speed regime within the atmosphere for military and commercial

benefits is still in the early stages. The primary benefit exploited from this speed regime is

flight time. A military vehicle traveling at five to ten times the speed of sound could provide

worldwide coverage in about four hours. Increasing speeds to higher than Mach 15 would

provide insignificant reductions in block time for either military or civil aircraft—the Earth is

simply not large enough to warrant it; therefore, this speed is probably the limit for

hypersonic cruise applications. However, speeds to Mach 25 are required for insertion to

low Earth orbit.

Military Applications

There are several hypersonic applications that are of interest to the military

community. The first of these are missiles and weapons that are highly agile and effective,

i.e., smart bullets. Advances in materials and controls technology could make hypersonic

rocket-based penetrators an important asset in future conflicts. Furthermore, as the enemy

continues to dig deeper into the Earth to avoid aerial attack, penetrator weapons that are

highly accurate may play an important role. Another high-speed, small-scale munition

application is the DARPA supersonic miniature air-launched interceptor. This would

provide cruise missile defense by using a low-cost infrared sensor on a small missile to

seek their large rear-aspect infrared signatures and overtake the inbound missiles from

behind. As smart aircraft systems, MEMS, and small-scale propulsion systems (~2-inch

diameter) continue to show promise in the lower speed regimes, exploitation at the higher

speeds is a natural progression to provide new levels of agility. This would be coupled with

sensor/feedback miniaturization from the electrical/computer-engineering world to provide

autonomously flying vehicles.

The second military application would be hypersonic weapon systems that would

exploit the oxygen in the atmosphere for the combustion process (instead of carrying it in

tanks or relying on solid propellant). They could be used for global-reach missions while

maintaining limited observability and exploiting high speed. These systems would require

hypersonic airbreathing engines, a concept that has been researched for almost 50 years,

but has seen NO flight demonstration. NASA’s Hyper-X program will finally bring the
25

scramjet engine concept out of the laboratory and test it in the atmosphere. One known

truth about hypersonic airbreathing propulsion is the need to formally integrate the

propulsion system with the airframe in the design process. Its importance is noted in the

fact that hypersonic airbreathing propulsion flowpaths is the subject of a separate white

paper in this series. The reader is strongly encouraged to peruse that document for

additional information in this area. While the basic concept of a scramjet is not new, the

technologies needed to make it a viable propulsion system option are not very mature. As

important as the component is to a revolutionary breakthrough, the systems studies needed

to increase their flight readiness level is an important avenue of research. Other issues of

importance to hypersonic airbreathers include the need for advanced materials, simplifying

systems, improving maintainability, and improved reusability by understanding the extent of

thermal and acoustic fatigue.

The uses of hypersonic airbreathing vehicles can also be extended to human-tended

flight, both for military applications (global reach reconnaissance, delivery, strike) and

commercial applications (Trans-Pacific passenger and cargo flights in three hours).

Access to Space

Aforementioned, hypersonic flight is required for access to space. Current launch

systems are either not reusable or only partially reusable. The future holds many promising

technologies for improving the safety, reliability, and affordability for space access. Among

these ideas are single-stage-to-orbit vehicles, that are either rocket-based or

rocket/airbreather based, and multi-staged vehicles that use airbreathing propulsion for the

lower speeds and rocket propulsion for higher speeds and orbital insertion.

One approach to hypersonic airbreathing space-access vehicles is with an overunder propulsion configuration with turbo-ramjets for the low speed system and scramjets

for the mid- and high-speed system. (Currently, the upper limit on scramjet operating

speeds is not known and is a worthy research topic on its own.) Obviously, there is an

altitude limit for operating scramjets (due to lack of sufficient oxygen) which requires that a

rocket-based system is needed for the final acceleration to attain low Earth orbit.

Scramjets are being flight-tested in the Hyper-X program, and complete vehicle

configurations are being studied in the Spaceliner 100 Program. These airbreathing

propulsion systems require airframe integration in the design of the vehicle. The vehicle

forebody serves as a compression surface for the inlet flow into the airbreathing engine,

and the afterbody serves as the nozzle expansion region for increased vehicle thrust.

Hence, an accurate understanding of the fluid flow over the complete vehicle is required,

including boundary-layer transition, separation zones, and combusting flowfields.

Furthermore, airbreathing-engine concepts are actively being considered as stages in a
26

multi-staged vehicle; thus, aerodynamics of vehicle separation is a critical enabling

technology.

Another propulsion system options for hypersonic airbreathing space-access vehicle

are the so-called rocket-based combined-cycle engines that utilize miniature rockets

integrated into a dual-mode scramjet flowpath. These miniature rockets can be used to

provide thrust at take-off and for orbital insertion. This method allows the rockets to be

used when scramjet performance degrades, but is hampered by the need to carry

additional oxidizer, reducing its payload fraction. One possible solution to the thermoacoustic fatigue issues for high-speed scramjets is an engine that utilizes fuel injected

upstream of the throat and ignites near the throat due to shock-induced pressure and

temperature rise. This concept is known as either a pre-mixed, shock-induced combustion

engine or an oblique detonation wave engine. Preliminary studies have shown a difficulty

in two areas: (1) getting the fuel to sufficiently mix with the captured air upstream of the

inlet, and (2) avoiding premature combustion upstream of the throat. However, if these

problems can be overcome, the result would be an engine with reduced weight and heat

load, resulting in increase fuel efficiency and thrust.

There are also some technologies related to airframe-propulsion interactions and

integration that can be exploited in the hypersonic regime. The first of these is favorable

shock wave interference, in which shock waves produced by elements of the geometry

judiciously interact with other parts of the geometry to minimize or eliminate the vehicle’s

wave drag. This concept is a point design, but adaptable structures could make this a

multipoint-optimized design. Second, the design capabilities for waverider vehicles have

been extended to more complex generating flowfields to improve the captured engine inlet

flow. Conical derived waveriders possess a pressure gradient away from the surface,

yielding a non-uniform flowfield for the compressed flow entering the inlet. Using an

osculating-cone inverse design technique, flowfields are nearly uniform in the area

upstream of a representative engine position and good vehicle volumetric efficiency can be

achieved. Finally, a concept for utilizing deflection of the gross thrust of a scramjet engine

may be able to be used to both decrease the wing size and weight at a given altitude or to

increase the cruise altitude of a prescribed configuration. All of these concepts have

potential merit, but analytical, computational, or experimental assessment has been very

limited.

In terms of reducing the cost of access to space, a significant amount of payload

either is or could be made to withstand significantly high g-loads, e.g. fuel and building

materials. This would permit the use of a new type of launcher that utilizes sequentially

triggered blast waves within an evacuated tube to accelerate a payload-carrying projectile

to near orbital velocity in a relatively short distance. Issues that need to be addressed for
27

such a vehicle include nose-tip survivability at these speeds (near sea level), controllability

of the projectile, and structural design to withstand the high g-loads.

In the farther term, another approach to boost payload to orbit would be with

electromagnetic radiation, such as microwaves or lasers. This would result in

environmentally friendly propulsion systems (e.g., magnetohydrodynamic drives) with little

or no exhaust. Currently, proof-of-concept tests have been for low speeds and altitudes.

Other approaches to advances in hypersonic propulsion include high energy density fuels

and fusion power. The high energy-density fuels are being studied by the Air Force, while

an investment of modest size and duration is required to determine the potential feasibility

of fusion propulsion and power.

Before leaving the topic of hypersonic vehicles, it is important to note that, aside

from pulse facilities, there are no ground test facilities that completely simulate hypersonic

flight (both in terms of conditions and gas constituents). Detailed ground testing is needed

to validate concepts and preliminary designs prior to flight test development. Efforts should

be made to fully understand the requirements of a full-simulation hypersonic ground test

facility and work towards eliminating that void.

The emerging field of molecular nanotechnology provides some potential

technological applications for hypersonic flight and space access, although they are still

highly speculative. Molecular nanotechnology is defined as the thorough three-dimensional

structural control of materials, processes and devices at the atomic scale [Globus, et. al.,

1998]. For launch vehicles, the construction of material systems by arranging atoms on the

surface of diamonds (diamondoid materials) could provide a strength/mass ratio that is over

50 times higher than for titanium, thus reducing the vehicle weight so that it could actually

be lighter than the payload. In the nearer term, improved structural materials are being

addressed using carbon nanotubes. Furthermore, carbon nanotubes are being studied for

use as vehicle thermal protection. The current methods of carbon nanotube production

typically create a tangled mat of nanotubes with a very low mass/volume ratio. These

tubes should be able to withstand the high temperatures like graphite composites, but

being in a tangled mat may prevent or limit ablation.
28

SPACE EXPLORATION

SPACE TRANSPORTATION CONCEPTS

The major thrust of the NASA's Space Transportation Program is the development of

a Reusable Launch Vehicle (RLV) to replace the aging Space Shuttle. The critical

requirements of this new vehicle, designated 2nd Generation RLV, are that the payload

costs will be a factor of 10 less expensive and it will be a factor of 100 safer than the

current Shuttle with an operational date of 2010. The current program is actually planned

out to 2040, leading to a 4th Generation RLV that is 1,000 times less expensive and 20,000

times safer than the current Shuttle. The timeframe of the present paper is 10 years; thus

the concentration in this section will be primarily on the 2nd Generation concepts. Note that

the current Shuttle is denoted as the 1st Generation RLV, even though it is only partially

reusable.

Background

The current state of the art in reusable launch vehicles and the U.S.’s premier

launch vehicle is the Space Shuttle (albeit only partially reusable), which was first launched

in 1981. Since that times several studies have been performed to define both reusable and

expendable new launch vehicles. NASA began a serious initiative in 1994 originating from

the Access to Space study in which NASA was given the responsibility for developing

reusable launch vehicles and the Air Force responsibility for expendable launch vehicles.

NASA decided the overall concept with the greater payoff but with more risk was a fully

reusable single-stage-to-orbit (SSTO) vehicle. Also, any new operational vehicle should be

a commercial venture with NASA assisting in technology development. From this

beginning two X-vehicles were started in partnership with industry to help develop the

technology such that a decision could be made in 2000 to proceed with a commercially

viable SSTO RLV to replace the Shuttle. The vehicle designated X-33 began with three

competing concepts in Phase I. The concepts were a vertical-take-off vertical-landing

(VTVL) vehicle proposed by McDonnell Douglas and two vertical-take-off horizontal-landing

(VTHL) vehicles: a winged vehicle proposed by Rockwell and a lifting body proposed by

Lockheed. Lockheed's lifting body concept was selected and became the X-33 vehicle.

This vehicle will fly a sub-orbital trajectory reaching a Mach number of approximately 13.

As originally proposed by Lockheed, the X-33 would be a scale prototype of their actual

RLV named the VentureStar.

At the same time, a smaller scale flight vehicle denoted as X-34 was begun in

partnership with Orbital Sciences, with the major objectives of studying rapid turnaround

and airline type operations, which selected a winged vehicle with a planform similar to the
29

shuttle. The vehicle would be launched from an L-1011 before firing it's rocket engine to

reach Mach numbers from 4 to 8. More recently the X-37 vehicle, awarded in competition

to Boeing, was added to the technology base. This vehicle is a lifting body, which will be

launched by the Shuttle or an expendable launch vehicle, will reach low Earth orbit and

reenter similar to the Shuttle. The new technologies for the X-37 encompass reentry

technologies and orbital operations.

There are two additional X-vehicles which were not directly funded by the Space

Transportation Program but do impact the technology and possible concept selection for

future generation vehicles. One is the X-38 vehicle, which is developing technology for a

reusable Crew Return Vehicle (CRT) for future use on the International Space Station. The

X-38 is a lifting body and uses a guided parafoil for landing after the high-speed direct

reentry. The other vehicle is the X-43 (from the Hyper-X program) which will provide flight

data for a hydrogen-fueled scramjet at Mach 7 and 10. The vehicle possesses an airframe

integrated propulsion system similar to the former National AeroSpace Program (NASP)

vehicle. The technologies and vehicle concepts from the described X-vehicle programs will

influence the new Space Transportation Initiative.

Current Status

There are several significant changes in the current space launch initiative from the

original Access to Space study. The first requirement is that the full reusability of the

vehicle must have a factor of 10 reduction in payload launch cost. Second, while safety

has always been of first priority within the agency, this is the first time that actual factors of

increased safety (100x) have been defined. Third, a much wider range of concepts can be

proposed and considered as viable candidates. The vehicle must be fully reusable but it

does not have to be a SSTO vehicle. Therefore, new two-stage-to-orbit (TSTO) concepts

and also Shuttle Derived concepts are possible. As part of the Integrated Space

Transportation Plan there will be at least two concepts competitively selected as large Xvehicles in 2001 fiscal year, and smaller technology flight demonstrations throughout the

program. The new date for the selection of the concept for the 2nd Generation RLV is 2005

and will be put into operation in 2010.

The safety requirements primarily impact the method of providing crew-carrying

capability. Several concepts are being proposed to meet the crew safety requirements and

generally involve separation of vehicles. For instance, the crew compartment is integral to

the main vehicle but self-contained, or the crew compartment is a separate vehicle

attached to the outside of the main vehicle. In either case, a separation of vehicles occurs

in the event of an emergency threatening crew safety. The aerodynamics of such a

separation, which potentially could occur anywhere from a low supersonic to hypersonic

flight condition, must be accurately defined to insure a safe separation. Furthermore, the
30

flight abort conditions for the increased safety requirements will require improved

aerodynamic performance over a much larger flight envelope than developed in the Shuttle

program or previously considered.

As mentioned, the X-38 vehicle is a technology demonstrator for an emergency

Crew Return Vehicle stationed at the International Space Station. However, within the

current Integrated Space Transportation Plan there is a decision point in 2003 as whether

to developed a Crew Return Vehicle (CRT), a Crew Transfer Vehicle (CTV), or a Crew and

Cargo Transfer Vehicle (CCTV). Concepts for the three different missions may include the

X-37, the X-38, or the HL-20 (a previously studied lifting body concept). The vehicles could

be launched by an expendable launch vehicle or the RLV and perform small cargo and

crew transfer as well as an emergency vehicle at the Space Station. These concepts may

be the one selected as the externally attached vehicle discussed in the previous paragraph.

Technologies are being studied that could impact the selection of an overall vehicle

concept. Launch technologies include catapult launching (similar to a plane launched from

an aircraft carrier) or maglev (magnetic levitation launch with magnetic fields) as groundbased launch aids in first stages of multi-staged vehicles.

The development of advanced high-temperature materials is being conducted for the

thermal protection system (TPS). Current RLV concepts are generally blunt configurations

in order to reduce the aerodynamic heating during the entry phase of the mission which

impacts the type and weight of the TPS. If such materials can be developed with sufficient

temperature resistance then it is possible that the leading edges of the concepts could be

made thinner leading to a higher lift-to-drag vehicle with increased cross range. This would

lead to an entirely different class of vehicle requiring definition of high performance

aerodynamics and an even more critical examination of transition to turbulent flow.

Any RLV concept is a highly integrated package dependent on many space

technologies. Aerodynamics and aerodynamic heating are both enabling technologies

critical to the design of any RLV. The aerodynamic performance establishes whether the

vehicle will fly, and the aerodynamic heating sets the TPS requirements establishing

whether a vehicle will survive. The Langley Research Center has been a major partner for

atmospheric flight mechanics in all of the X-planes. Langley is the designated lead center

for "airframes" and "atmospheric flight mechanics" within NASA's space transportation

program.

PLANETARY EXPLORATION CONCEPTS

This category of vehicles includes those vehicles that travel through the

atmospheres of planets or moons of the planets or Earth reentry after scientific visits to

other celestial bodies, i.e., planetary missions requiring aerodynamic performance and
31

heating analysis for design of the spacecraft. As will be discussed, in recent years

aerodynamics is being used for orbital spacecraft as well as direct entry probes.

Until recently, the state-of-the-art was that an orbiting spacecraft was placed into

orbit by retrofiring jets after the transit travel to the planet. This required weight for the jet

gas thus increased the launch weight and weight carried in transit. Entry probes [Apollo,

Pioneer Venus probes, the Galileo (Jupiter) probe, Viking probes (Mars), and Pathfinder

(Mars)] were blunt configurations with high drag required to decelerate the high entry

velocities to about Mach 2 where a parachute was then deployed for further deceleration

before landing. The probes were released from an orbiter spacecraft (or carrier bus) prior

to the spacecraft going into orbit.

NASA has renewed the planetary exploration program under the "Faster, Better,

Cheaper" mantra which replaces a few larger missions with many inexpensive missions.

With this renewal there are recent, current, or planned missions which rely on

aerodynamics rather than retrofire for success of the mission. These missions use aeroassist technology such as aerobraking where the spacecraft or carrier bus dips slightly into

the planet's atmosphere hundreds or thousands of times over months for the required

velocity decrement to go into orbit, or aerocapture in which the velocity decrement is taken

with a single deeper pass through the upper atmosphere in a short period of time for orbit

insertion. Both aerobraking and aerocapture require aerodynamic performance, stability

analysis, and heating analysis in the transitional flow regime between free-molecule and

continuum flight mechanics. If an entry probe were part of the mission, then it would enter

from orbit at a lower velocity than from a high-speed transit velocity. The trade off in a

mission is whether the weights associated with aeroassist technology, such as thermal

protection material from increased heating, is lower than the fuel weight of retrofiring.

Mars Missions

Mars is the largest exploration program with NASA committing to fly a mission during

every launch window opportunity, which occurs every two years. The long-term view

includes a crewed mission. There are other missions of which those of importance to

aerodynamics and heating are sample return entries to Earth such as Stardust (interstellar

and cometary dust) and Genesis (Sun solar wind). In 1997 the Mars Global Surveyor was

the first to use aerobraking for an operational spacecraft. This may become the standard

for all future Mars orbiters including the satellites for the Mars telecommunication network.

Aerocapture has not been used to date; but, is being considered for the Mars orbiter portion

of the Mars Sample Return Mission and for the human exploration of Mars including the

Mars entry and the Earth return. Aerodynamic configurations with lift-to-drag in the range

of 0.3 to 0.7 are being studied which include elliptically-blunted raked cones, blunted largeangle cones at angle of attack, ellipsled (elliptically blunted cylinder), and straight or bent
32

biconic or triconic shapes. Extensive studies of aerodynamic performance, stability, and

heating characteristics are required to design aerobraking vehicles for human exploration to

Mars. The Langley Research Center is the lead center for aeroassist technology

With a maturing Mars program it is becoming necessary to consider two new

technologies which could impact the required aerodynamic performance. These are

precision landing and hazard avoidance. In the past we were satisfied just to land in the

general area. Now we want to avoid hazards that may destroy the lander or severely

impact the mission and we want to land at a precise point on the planet. Being able to land

at a precise location is an enabling technology for manned mission where the vehicle must

land near the operational base but not hit it. The entry vehicle will require improved

aerodynamic performance and guidance systems for these technologies to be successful.

Other Missions

For detailed study of other worlds that have atmospheres, one option is to maintain

vehicles aloft in these atmospheres to conduct scientific research. A vehicle that is able to

move while aloft has a much greater ability to uncover knowledge about these worlds. The

requirements for such vehicles are heavily dependent on knowledge of the extraterrestrial

atmospheric composition. These concepts could take the form of “flying” vehicles, such as

the proposed Mars Plane, or “floating’ vehicles, like sensor-laden balloons or parachutes.

Vehicles such as this could be used to study the gas giant planets, Mars, or Venus. One

key to research in this area is to accurately use the knowledge of the atmospheric

composition to perform adequate analysis, simulation, and experimental testing. At

present, computational modeling has not compared well with experimental results, due to

different shear-layer and separated-flow predictions. The possibility also exists to utilize

larger flyers with highly flexible wings. The biomechanics of using inflatable or tensionedfabric cords for this application still need to be addressed. Finally, whatever the final

configuration for the flyer/floater, it must be designed for efficient storage during space

transit, survivability during atmospheric entry, and deployability when on station.

Gas Giant Dirigible/ Bathyscaphe

The gas giants of our solar system (Jupiter, Saturn, Uranus, and Neptune) and to a

lesser extent Titan have very thick atmospheres and extremely high pressures on their

surfaces. The best way to initially study these planetary atmospheres would be with balloon

or dirigible carried instrument packages carried fairly high in the atmosphere at a safe level

of pressure. High Reynolds number unsteady flows need to be better understood for these

programs.

In addition to the high floaters, it may be possible to send instruments to much

greater depths in the atmosphere, and for some cases, even to the surfaces. For these
33

cases, Bathyscaphe-types of carriers are needed. In addition, the interior of Jupiter’s moon

Europa may be mostly liquid water. If a Bathyscaphe with instruments was placed below

the ice surface, it could explore this huge volume of water. The hydrodynamics of such

vehicles might be greatly different than in Earth’s ocean, so studies might be needed to

examine such flows as might be encountered.
34

NON-AEROSPACE APPLICATIONS

NASA’s could leverage their expertise in aeronautics and space to advancing nonaerospace applications such as ground and water transportation systems. Although not a

transportation system, many “Environmental Systems” could also benefit from NASA’s

discovery and technology development programs. Application of NASA technologies to

non-aerospace applications has traditionally happened through spin-offs of focused and

basic research programs.

Today many of the roadblocks to “general transportation systems” that are imposed

upon NASA are largely political. Many agencies have isolated themselves through budget

and manpower requirements internal to that agency, including NASA. The state of the

nation’s highways and roadways gives new meaning to gridlock. The aging highway

systems are in dire need of a paradigm shift. The opportunity to develop alternatives to

roads may relieve the staggering gridlock in and around our large cities by moving people

via mass transportation systems and personal air highway systems. The debate regarding

mass transportation systems will continue and be difficult to sell because of the freedom

that a personal vehicle provides. A paradigm shift in this area will be driven by costs and

future limitations set by environment and safety regulations. The technology to develop a

safe, quiet, and environmentally sound air highway system will happen. What do we need

to do to make it happen sooner than later?

Non-aerospace technologies can be categorized into four groups, Ground, Water,

Non-traditional Air, and miscellaneous applications. It can be noted that some applications

can be multi-functional or have applications in two or more of these categories.

Ground vehicles include automotive, trains, motorcycles, trucks, and buses. Water

vehicles include ships, submarines, torpedoes/missiles, hydrofoils, sailboats, and even jetskis. Non-traditional air applications include bird flight (biomemitics), smart bullets,

aerosols, internal engine compartment cooling, nuclear cooling systems, advanced mixing

systems, and flight in different atmospheres. Examples of several miscellaneous

applications could be weather control systems (tornadoes & hurricanes), pollution detection

systems (rivers, smoke stacks, biohazard tracking), energy systems (wind turbines, water

turbines), safety systems (wind brakes, parachutes), biomedical systems (heart valve and

blood flow systems or other non-Newtonian fluids), and sports (runners, skiing, sky diving,

etc.).

Ground vehicle applications

NASA’s involvement in ground vehicles has been limited due to the automotive

industry focus on style rather than efficiency. Only until recently has the industry been

required to meet government regulations on fuel efficiency, thereby giving some emphasis
35

to aerodynamics. While Henry Ford proposed roadable airplanes as early as 1940, the

status of the nation’s highway systems may be the catalyst needed to make them a reality.

The ability for the nation’s automotive industry to utilize NASA’s technologies has been, in

general, untapped. Passive porosity, unsteady aerodynamics, aeroacoustics, and vortex

control are several areas that potentially have application to the future of ground vehicles.

Water vehicle applications

Developing specific technologies related to platforms such as ships, submarines,

and underwater weapons have typically been left to DOD and the shipbuilding industry.

Today’s water vehicles, particularly water munitions, are evolving into multifunctional

vehicles. Examples include vehicles that swim undetected for awhile then fly to a specific

destination. To develop such systems, NASA could collaborate with the DOD to make an

impact. The commercial water applications such as hydrofoils, sailing ships/boat, personal

watercraft, etc. could see development in the areas of increased speed, efficiency, load

capacity, maneuverability, reduced noise, and safety. Unsteady aerodynamics and

hydrodynamics are key areas for such development.

Non-traditional Air vehicle applications

Non-traditional air vehicle applications are areas that NASA is best suited to provide

expertise because it closely resembles research that NASA has already committed to. The

advent of smart materials that are lighter and stronger gives us the opportunity to rethink

speed, size/scale, and shape. New materials, microelectronics, and communication

systems make previous impossibilities a new reality. While steady state aerodynamics

have been developed to the edge of maturity, our unsteady aerodynamic and acoustic

interaction knowledge base across the speed range is beginning to develop with many

unanswered questions. Designer fluid mechanics is being proposed to replace

conventional control systems. However, accurately scaling the unsteady 3-D

microstructure and predetermining the appropriate interactions are not possible today. To

develop micro-aircraft or machines that fly in other atmospheres will require a greater

understanding of scaling to low Reynolds number at very high speeds.

Biomimetics

Biomimetics is simply the engineering of a system or process that is inspired by and

imitates nature. The philosophic view is that there is no need to reinvent the wheel for

every innovation. Also one can note that: Nature runs on solar energy. Nature recycles

everything. Nature fits form to function. Nature curbs excesses from within. Nature taps

power to the limits. By looking at existing systems in the natural world, a researcher may

benefit from the 60-140 million years of evolutionary enhancement that has gone into a
36

system. Biological systems have the greatest untapped potential to revolutionize how we

design, build and use future space systems. They are quite simply the most robust and

efficient systems in the known universe.

NASA Langley would be ideally suited for such a role. While five of the ten NASA

research centers are primarily focused on space research and enhancement, NASA

Langley has been known more for its structure and material research, along with its

aeronautical and fundamental research activities. To incorporate biomimetics into its

current research arsenal would be an easy task for NASA Langley. The basic

infrastructure of computing, chemicals, power, and building facilities are already in place.

With its research focus, biomimetic programs could begin immediately in the areas of

biologically inspired materials, structures, and aerodynamics such as self-assembling,

amorphous structures, and flapping wing flight technology.

Continuing and broadening the effort could establish NASA Langley as a biomimetic

paradigm pioneer. Using nature as a model, Langley could shift its own infrastructure into

an environmentally and ecologically balanced entity. The result could be a facility capable

of supporting itself on its own power and causing little to no harm on the environment

through biologically inspired power production and pollution safe practices. By instituting a

biomimetic research paradigm, NASA could enter into partnerships with the DOE, Dept. of

Agriculture, DOT, and the EPA. NASA would not only show that it was interested in space

but the Earth as well.

Environmental Systems

Applications of aerodynamic technologies to “Environmental Systems” can range

from wind turbine aeroacoustics, pollution detection, unsteady wind loading on buildings,

biological warfare (application, prevention, and detection), and weather systems (e.g.

tornadoes alleviation). New partnerships would be necessary with DOT, DOE, NOAA,

DOD, etc. Advances in materials and active flow control systems will open doors to

enhancing mixing that could be applied to internal combustion engines, as well as the

cooling systems used to manage the automotive and/or aircraft cooling systems.

The fluid mechanics of many biomedical systems (blood, digestion, etc.) can also be

addressed by CFD and measurement technologies developed by NASA. Miniaturization of

probes applied to active flow systems has spin off applications in many biomedical fields.

While many opportunities exist for non-aerospace applications of the aerodynamic

technologies that NASA can develop, it will require manpower and resources that the

agency/center/AAAC may not have. It is important to have the critical mass of people

and/or resources that can focus on the aforementioned opportunities, or the probability for

success is at low risk.
37

DISCUSSION

Perhaps one of the reasons that aerodynamics is incorrectly considered a mature

science by some is that aerodynamic solutions to problems are often misappropriated in

systems studies. Drag reduction due to effective propulsion-airframe integration is often

book-kept as a specific fuel consumption reduction and, therefore, credited to the

propulsion discipline. Drag reduction enabled by increased aspect ratio is credited to

structures, because aspect ratio alone is a known drag benefit, but is enabled by structural

advances. Our current state of the art has been brought about with a "departmental"

approach to aircraft, and integration was often done as an afterthought with any resulting

problems fixed on an ad hoc basis.

It should be clear at this point that the field of aerodynamics, while well advanced in

many respects is not fully mature in that there are still many unanswered questions to

important problems where solutions would be of great benefit. In this paper we have

identified areas for research that show promise for providing those solutions. What we

have tried to avoid is any great emphasis on specific vehicles. Individual vehicles,

platforms, or airframes should be just a focus for demonstrating the benefits of technologies

that can be widely applied.

Although understanding physical phenomena and modeling/prediction of

unsteady/complex flowfields is essential, much can be learned by intelligent application of

many of the technologies described herein. Controlled flight was achieved decades before

3-D Navier-Stokes modeling was available, and we should not fall into the "if the only tool

you have is a hammer, then every problem is a nail" syndrome with supercomputing.

Applied aerodynamic and acoustic solutions should be pursued concurrently with the

development of prediction and modeling tools. This exercise is highly speculative in many

respects, but it is based on clear trends in technical developments in other areas as well as

on changes in societal values. We have identified areas for research, but we have not

offered suggestions on how to do that research.

Looking beyond issues of purely technical nature we can see a number of other

issues that affect the ability of researchers to solve the problems outlined in the previous

sections. A major recurring theme is the increasing interdependence of different disciplines

in the field of aeronautical engineering that is already known for the breadth of its scope.

Multidisciplinary design has existed for some time; what is needed now is the ability to

perform multidisciplinary research. The need for integrated research to provide answers to

complex problems is apparent. What is less apparent is how to facilitate in-depth research

that is multidisciplinary in nature within the current management framework. In the same

spirit as identifying areas for research without suggesting solutions, we wish to identify

areas that require management attention without suggesting solutions. Three broad areas
38

need to be addressed: resource allocation, organizational structure, and cooperative efforts

with organizations outside of NASA.

The most important resource at Langley is the staff. As research becomes more

integrated, attention to the allocation of skill mix between specialists, generalists, and

managers may become more important. There is no substitute for the highly educated,

experienced, and focused researcher who becomes an expert in a field. Langley has a fine

history in identifying and developing this type of researcher. What requires attention is the

extent to which generalists can be identified and trained, what skills they will need, and

precisely what role they will play. Performing the research to enable development of a

flexible structure with smart components that uses unsteady flow control for maneuvering

may be as impossible with only specialists as it would be with only generalists. We need to

develop a systematic way of balancing these competing needs in personnel development

and providing the incentives for various organizations to provide the necessary training.

The equipment and facilities necessary for the staff to do the work must also

change. The importance of the information technology revolution in determining the

direction of the future research should not be underestimated, nor should its role in

enabling and performing that research. As computers have increased in speed, the time

between successive generations of computer chips has decreased. The ability to keep

pace with this change and provide current tools for the researchers then becomes a

management problem. The ability to simultaneously test aerodynamics, electronics,

materials, and structures is vital to the development of some of the concepts suggested

here. Acquisition of new facilities or modification of old ones to meet these new

requirements, as well as management of those facilities, spans distinct management

entities in the current organization. We need to develop a systematic way to encourage

cooperation between organizational entities at Langley in developing facility requirements,

sharing funding and responsibilities, and distributing credit for the work.

Depending too heavily on program-based research tends to limit advances to

evolutionary steps. Planning and control can be too systematic and too heavily based on

easily achievable. Researchers developing revolutionary concepts must take risks and be

ready and able to radically change direction or focus quickly, and have authority to make

those changes commensurate with their responsibility to achieve results. This is not to say

that a researcher should not be answerable for their work. We need to develop a

systematic compromise between existing and new programs, and high-risk fundamental

research that will provide the technology breakthroughs that could revolutionize the

aerospace industry in the 21st century.

There are a number of outside organizations with which Langley cooperates in

research and some which not only influence the research that we do, but the extent to

which that research is accepted. We have no suggestions regarding cooperative research
39

with universities. Cooperative research with industry is another matter. Recent experience

with large programs focused on the immediate needs of industry with little regard for the

long-term needs of the country. The need to provide industry with short-term solutions is

real and should not be discouraged. Instead it should be placed in proper balance and not

allowed to interfere with legitimate medium term and long term research that provides us

the ability to meet industry's needs for the future.

In suggesting pursuit of research that may fundamentally change the appearance

and performance of aircraft, we must somehow cooperate with the FAA. The AGATE

consortium has already provided a perfect example of how this can be done: a cooperative

research effort involving NASA, industry, and the FAA from the onset developed a roundrobin method for certifying composite materials for flight that will save industry a significant

amount of time and money. By involving all interested parties from the beginning, each

party could become aware of the needs of the others and satisfy them with a minimum of

false starts and wasted effort. We need to develop a systematic way of directly involving

the FAA in NASA research at an early stage so that both organizations understand the

implications of the work for future regulatory action.

Under no circumstances should the investigation of advanced concepts be limited to

a white paper team. Instead it should be an on-going effort by each and every researcher

within AAAC. This was formerly accomplished by interaction with industry and academia

on a daily basis in the course of doing business. A short-term horizon, brought about by

the need to show immediate relevancy to industry, is partly to blame for eroding this

process, but we should take care not to let the pendulum swing too far in the other

direction, so that some interaction with industry is viewed as "good."

Furthermore, the AAAC should develop a systems approach to advanced aerospace

concepts. Setting the stage for developing and following through with advanced concepts

requires a multidisciplinary approach. Engaging in existing LaRC efforts (e.g. ASCA/SAB,

AVSTPO, and the Chief Scientist) and internal AAAC efforts will be critical in the filtration of

many ideas and advanced concepts into ones that we will modify or pursue. This would

require both generalists and specialists. With this mix, an open exchange and assessment

of ideas would result. Warning: care should be taken as to not increase the burden on

researchers without demonstrated commitment of the management to implement

recommendations.

Lastly, it is imperative that stakeholders understand the significance of where the

state of aerodynamics, aerothermodynamics, and acoustics has been, where it is, and

where it is going. In this light, a recommendation is made to provide the necessary

resources to create an interactive compact disk with this type of information. If this

information (via text, pictures, photos, video clips, and interactive demonstrations) could be
40

sufficiently captured and freely released, the misperception that aerodynamics is a “mature

science” could be properly handled.

Advanced Design and CFD techniques

Some of the ideas presented in this paper might be considered revolutionary, and

others are a revisit to ideas initially examined in the 1940's and 1950’s with today’s

technology. Nevertheless, they are a glimpse into the realm of possibilities, and they

probably represent the tip of the proverbial iceberg regarding possible alternative designs

and technological improvements to current aircraft. In the effort to delineate the best

course of action for research we need a powerful screening process. Thus, considerable

advancement is required in design tools, computational tools, measurement devices, and

wind tunnel facilities.

Aerodynamic calculations have demonstrated that Computational Fluid Dynamics

(CFD) can accurately predict the characteristics of high Reynolds number attached wing

flows15. With state-of-the-art CFD methods16,17 essentially, interference drag-free engine

installations at flight Reynolds numbers can be designed. However, these computational

methods cannot reliably predict flows with steady or unsteady separation. In addition, with

RANS modeling adequate resolution of high Reynolds number viscous flow over an aircraft

configuration requires millions of grid points and hours of computer run time, even with the

use of parallel processing. Such computational times are clearly not acceptable for design

techniques that can be applied on a routine basis (e.g., a two-hour analysis calculation

must be reduced to a few minutes)18-22. While advancements in computers and parallel

processing strategies will certainly lead to reductions in computational times, such

advancements will not be sufficient to provide the necessary computational capability for

complex flow problems such as buffet behavior, vortex bursting, and dynamic stall.

Powerful, efficient, and robust numerical algorithms will also be needed.

The next generation of design tools will need to have a broad spectrum of flow

modeling capability to allow sufficient flexibility in design. For example, flow analysis

should be possible with not only linearized inviscid and nonlinear potential flow methods but

also the Reynolds-averaged Navier-Stokes (RANS) equations. The RANS equations must

be solved with reliable turbulence modeling (i.e., sets of transport equations for turbulence

that have clearly defined ranges of application). Eventually, at least some portion of a flow

field should be solvable with direct numerical simulation. This level of analysis would avoid

the requirement for turbulence and transition modeling, which may not be sufficiently

reliable in certain complex flow situations. Appropriate modeling will be needed for both

chemically reacting and real gas flows. In addition to these options for flow modeling and

analysis, the design tools for aircraft and various other transportation vehicles will need to
41

have a multidisciplinary capability. Thus, the tools will need to include the effects of

aeroacoustics, emissions, propulsion/airframe integration (PAI), as well as the effects of

fatigue and life cycle. For a detailed discussion of design tools the reader is referred to the

associated White Paper entitled “Advanced Design and Diagnostic Tools”. In order to

construct such powerful design tools substantial improvements in physical modeling and

computational capability are necessary.

Recent work in numerical algorithms strongly suggest that a factor of 100 speedup in

the time required to solve the governing equations of fluid dynamics for stationary flows is

possible. This has immense implications with respect to the use of Navier-Stokes solvers

in a three-dimensional design process that can be applied routinely. Furthermore, such a

fast steady flow solver can be extremely useful in improving the efficiency of unsteady flow

solvers for a variety of problems, especially those in aeroelasticity. For example, the dual

time-stepping algorithms for computing unsteady flows depend on a steady-state scheme

in order to obtain the solution at each time step. These algorithms have the advantage of

allowing large time steps and thus improved efficiency for many practical problems in

unsteady aerodynamics. There would be a major impact on the general performance of

these algorithms with a 100 times faster steady-state solver.

Until now the principal focus in the development of algorithms for fluids has been on

stationary flow solvers. However, unsteady flows are beginning to receive considerably

more attention. One reason for this is that most real flows have some degree of

unsteadiness. In an effort to optimize the design of aircraft, this unsteadiness cannot in

general be ignored in computational analysis. Another reason for the growing emphasis on

unsteady flows is the role that they play in many active flow-control devices (see White

Paper entitled ” Flow and Noise Control”). In order to analyze and design flow controls,

reliable and efficient unsteady flow solvers are needed. Reliable and accurate unsteady

flow solvers can allow the development of flow controls that offer the possibility of achieving

revolutionary advancements in the aerodynamic performance of flight vehicles.

Another important issue for unsteady flow analysis is flutter. Flutter involves the

interaction between unsteady aerodynamic loading and mechanical vibrations. In order to

ensure a suitable margin of safety for the structural components of an aircraft, it is

necessary to accurately predict the unsteady forces and moments. At the present time,

due to computational expense, unsteady loading for flutter analysis is primarily performed

with potential flow methods. Such methods allow transonic aeroelastic analysis for an

entire aircraft configuration in a computational time more than two orders of magnitude

smaller than that required by RANS methods. However, if there are strong vortical and/or

viscous effects, these methods are not suitable. Currently, new techniques are being

developed that involve flow modeling with the Euler and Navier-Stokes equations. The

prohibitive computing time for unsteady flow analysis is the major limiting factor for practical
42

use. Recent applications of dual time stepping methods, with multigrid methods for time

step solving, offer considerable encouragement in addressing this difficulty. Flutter

boundaries have been calculated with numerical algorithms coupling the unsteady Euler

equations and structural modal equations. The computational times have been reduced

substantially. Still, much greater improvements are needed before these schemes can be

used on a routine basis. The new generation of numerical schemes discussed earlier has

the potential to advance this capability for flutter analysis considerably.
43

CONCLUDING REMARKS AND RECOMMENDATIONS

The perception that aerodynamics is a mature science is flawed. It is true that our

capability to design aircraft in spite of our shortcomings has become routine, particularly for

cigar shaped vehicles with thin wings. However, upon careful examination, it is clear that

many of these shortcomings occur in flight regimes that are plagued with sociological and

technical issues that require advances in our capabilities. This is particularly true in

environmental issues related to noise and emissions. Moving the current aerospace

paradigms to revolutionary air vehicles will require both generalists and specialists.

Expanding the AAAC horizons and developing enabling technologies is the only way that a

“revolutionary” aircraft will emerge. While it is important not to lose sight of vehicle

concepts, it is more important for AAAC to focus on the enabling technologies and the

integration of those technologies that will lead to revolutionary aircraft.

The AAAC should position itself to leverage off of the expertise and advances of

other competencies and program offices to utilize state of the art enablers for strength,

weight, and flexibility. Some of the current enabling technologies that are outside AAAC

might include adaptive/self-healing materials and electronics, molecular nanotechnology,

smart structures, and seamless, failsafe, broadband universal communication.

To participate in the development of the “Noise Friendly”, “Green Machines”,

“Personal Air Cars”, “Crash-less aircraft” or other truly revolutionary and advanced vehicle

concepts, it will be necessary for the AAAC to integrate with “aerospace systems” groups

such as Systems Analysis Branch (SAB). Examples of current “Advanced Vehicle

Concepts” that SAB are working are shown in the appendix. Evaluating the technology

roadblocks to these vehicles will supplement the requirements for technology development.

However, as a warning, to establish priorities based on current vehicles alone will be short

sighted and potentially limit the development of technology enablers that will pay off in the

long term.

Prior to making advanced concepts recommendations it is useful to recognize

several recurring themes that are basic to the majority of advanced aerospace concepts.

Recurring “Advanced Aerospace Concept” themes include:

∑ Revolutionary concepts will not be cigar shapes that land and take off as airplanes do

today.

∑ The status of current prediction and design tools is inadequate for developing a

revolutionary aircraft. Expanding CFD and CAA to include complex aircraft features

that move and are inherently unsteady will be revolutionary (transition, separation, 3-

D vortical flows, noise propagation, etc.). Making these tools “user friendly” will give
44

the designer the benefit of non-linear analysis and more realistic systems integration

capabilities.

∑ Research in both temporal and spatial scaling for micro-, macro-, and mega-vehicles

will be building blocks for advanced concepts through the understanding and

application of complex physics.

∑ Need to develop unsteady Aerodynamics and Aeroacoustics enablers for flow control

(both active and passive). Micro-, macro-, and mega-flow control techniques are

expected to be a major influence on advanced concepts.

∑ Utilize flow control, shape control, and aeropropulsive control that is quiet and safe.

∑ Most high-risk research is being done today is for advanced military aircraft.

Historically most revolutionary advanced aerospace applications can be traced to

military aircraft. NASA’s role should be to supplement, augment, support, or lead

such high risk military research activities, then capitalize on appropriate applications

to be used for civil aviation. We need to develop or enhance our ties to DOD and

DARPA activities.

∑ New propulsion technologies are critical to aircraft across the speed range. The

integration into the airframe systems remains a top enabling issue, particularly for

supersonic and hypersonic vehicles.

∑ Multi-disciplinary research requires unencumbered exchange and communications

between researchers, regardless of organization.

Recommendations

What We (NASA) Should Aim to Achieve:

∑ Aerospace vehicles that do not crash, ever, for any reason.

∑ Aerospace vehicles that do not change the characteristics of the atmosphere with

chemical emissions.

∑ Aerospace vehicles that are so quiet they are not noticed by people on the ground

(35dB below current technology).

∑ Aerospace transportation systems that integrate seamlessly with ground

transportation systems.

What We (AAAC) Should Do to Achieve our Aims in the Long Term:

∑ Develop good working relationships with other Langley, NASA, governmental,

industrial, and university entities that clearly delineate these aims.

∑ Develop a coordinated plan with other Langley, NASA, governmental, industrial,

and university entities that specifies roles and responsibilities.
45

∑ Develop the ability and the work environment to perform multidisciplinary research,

integrating propulsion, structures, acoustics, etc.

∑ Develop a complete understanding of flow separation and separation control23.

Focused areas of research should include revolutionary high lift systems that will

enable quiet and safe take off and landings.

 Active and Passive Flow Control

 Circulation Control Wings

∑ Develop a complete understanding of transition and turbulence.

 Active and Passive Flow Control

∑ Develop a complete understanding of every aspect of unsteady flow, from periodic

unsteadiness to unpredictable unsteadiness23 due to unexpected encounters with

(chaotic) atmospheric turbulence.

∑ Develop a complete understanding of combusting flow.

∑ Develop a complete system of physics based tools for prediction of noise source

generation, noise/structure interaction, and long-range noise propagation.

∑ Develop a complete understanding of interior noise, active noise control, and human

perception of noise.

∑ Develop experimental facilities to test integrated concepts24.

What We (AAAC) Should Do to Achieve our Aims in the Near Term:

∑ Focus efforts on improving conceptual design tools to prevent conservatism from

eliminating promising vehicle concepts and to prevent ignorance from promoting bad

vehicle concepts.

∑ Make development and evaluation of advanced concepts a major, ongoing process

within the competency. Form a working group to Interface with “Systems Analysis

Branch”. Leverage off vehicle concepts to ensure near term expectations are being

met but do not sacrifice basic research that will enable long range programs. Infuse

SAB with advanced tools and training to improve evaluation of advanced concepts.

 Bring in advanced aircraft design course for AAAC to establish a baseline for how

airplanes are currently designed and built (should be offered on a regular basis).

∑ Promote advanced research tools including new facilities

 Develop concepts, hardware, and software to enable immersive visualization of

theoretical and experimental results.

 1 meter polysonic (0 < Mach < 5, high Reynolds number) acoustic wind tunnel

 Hypersonic flight replication tunnel (5 < Mach < 18) with meaningful test duration.

∑ Provide resources (money) controlled at the branch level to facilitate pursuit of new

concepts through enabling decision making by the experts.
46

∑ Develop advanced CFD algorithms with improved accuracy and reduced

computational effort for multiprocessor supercomputers as well as desktop

computers.

∑ Create bridges to cross-pollinate the AAAC and create a critical mass that ebbs and

flows as necessary.

 Develop a mentoring program with the appropriate incentives to cross train

researchers and support personnel to capitalize on experience.

 Initial emphasis should be on fundamental fluid mechanics and acoustics

 Initial emphasis should be on time-dependent structural dynamics and fluid

mechanics

∑ DO NOT PICK A FEW GRAND CONCEPTS THAT ARE EASY TO SELL WITH

VIEWGRAPHS BUT ARE DOOMED TO FAILURE! NASP was a failure because it

was never built, but the research it promoted was not a failure. HSCT was a failure

because it was never built, but HSR, the research it promoted, was not a failure.

Instead, return to the past practice of solving individual problems that arise from

pursuit of advanced concepts. The vehicle will be built when research (including

concept demonstrators) has solved the problems, not when the date of the last preprogrammed milestone is reached.

∑ Enabling technology development should include but not be limited to

 Steady-state and time-dependent Viscous Flow technologies (including CFD and

experimental Reynolds number dependent physics and applied research)

 Active and Passive Flow Control technologies (aerodynamic and acoustic)

 ESTOL Transport technologies (take off and landing speeds less than 50 knots)

 Personal Air Vehicle technologies

 Noiseless Vehicle technologies (including ESTOL, personal, and supersonic

(boomless) vehicles)

 High Performance Vehicle technologies (cooperating and leveraging off of military

applications)

 Micro Air Vehicles (leveraging off of biomimetics)

 High altitude and long endurance Vehicle technologies

 may couple to Micro Air Vehicles, Flow Control, and advanced structures

 Extraterrestrial exploration

 Real gas research

 High Speed Vehicle technologies (supersonic and hypersonic technologies

emphasizing propulsion integration and combusting flow physics)

 Access to space

 Agile missiles
47

REFERENCES

1. Flow and Noise Control, White Paper (unpublished), December 2000

2. Advanced Design and Diagnostic Tools, White Paper (unpublished), December 2000.

3. Hypersonic Airbreathing Propulsion Flowpath, White Paper (unpublished),

 December 2000.

4. New World Vista, Air and Space Power for the 21st Century, USAF Scientific Board,

1999

5. L. D. Huebner, et al. Transportation Beyond 2000: Technologies Needed for

Engineering Design, NASA CP 10184, Parts 1 and 2, February 1996.

6. McKinley, R. E., “Advanced Concepts Overview”, presented at Patuxent River Naval Air

Station, August 1999.

7. Bushnell, D. M., “Frontiers of the “Responsibly Imaginable” in (Civilian) Aeronautics”,

AIAA Paper 98-0001, The 1998 AIAA Dryden Lecture, January 1998.

8. Bushnell, D.M. “Application frontiers of “designer fluid mechanics”--visions versus reality

or an attempt to a.vswer the perennial question “why isn’t it used?”, AIAA paper 97-

2110, June 1997

9. Beach, H.L., Bushnell, D.M., “Aeronautical Facility Requirement into the 2,000’s”, AIAA

Paper 90-1375, June 1990

10.A.K. Noor, S.L. Venneri, “Future Flight”, Mechanical Engineering, September 1994

11.Rosen, C., Burger, R., Sigalla, A., “Aeronautical Technology 2000 – A Projection of

Advanced Concepts, AIAA Paper 84-2501, October 1984

12.Globus, A., and et al.: NASA Applications of Molecular Nanotechnology. The Journal of

the British Interplanetary Society, volume 51, pp. 145-152, 1998.

13.S. F. Yaros, et al., Synergistic Airframe-Propulsion Interactions and Integrations, NASA

TM-1998-207644, March 1998.

14.Powel, A. G., et al., “Feasibility and Benefits of Laminar Flow Control on Supersonic

Cruise Airplanes”, NASA-CR-181817, July 1989

15.Lynch, F., “Subsonic Transport Study Results”, presented at NASA Langley Research

Center, 1999.

16.Research Opportunities in Configuration CFD, White Paper, 1998 (unpublished).

17.Mavriplis, D. J., “Large-scale Parallel Viscous Flow Computations Using an

Unstructured Multigrid Algorithm”, NASA/CR-1999-209724, November, 1999.

18.Jameson, A., “Time Dependent Calculations Using Multigrid, with Applications to

Unsteady Flows Past Airfoils and Wings”, AIAA Paper 91-1596, June 1991.

19.Alonso, J. J. and Jameson, J., “Fully-Implicit Time-Marching Aeroelastic Solutions”,

AIAA Paper 94-0056, January 1994.

20.Edwards, J. W. and Thomas, J. L., “Computational Methods for Unsteady Transonic

Flows’, AIAA Paper 86-0107, January 1987.
48

21.Caradonna, F. X., “The Application of CFD to Rotary Wing Flow Problems”, AGARD

Report No. 781, Aerodynamics of Rotorcraft, November 1990.

22.Berkman, M. E., Sankar, L. N., Berezin, C. R., and Torok, M. S., “A Navier-Stokes/Full

Potential/Free Wake Method for Rotor Flows”, AIAA Paper 97-0401, January 1997.

23.J.D. McLean, et.el, Study of the Application of Separation Control by Unsteady

Excitation to Civil Transport Aircraft, NASA CR-1999-209338, June 1999

24.Aerodynamics of Wind Tunnel Circuits and their Components, AGARD-CP-585, June

1997

Web Sites:

∑ Lockheed Martin Aeronautics Company, See Image Gallery

http://www.lmtas.com/index.html

∑ Boeing Commercial Airplanes Group Overview,

http://www.boeing.com/commercial/overview.html

∑ Air Force Research Lab, http://www.afrl.af.mil/va.html

∑ Wright Labs, http://www.wrs.afrl.af.mil/

∑ AFSOR, http://afosr.sciencewise.com/

∑ Related GOVT links via AF: http://www.mn.afrl.af.mil/public/othrsite.html

∑ Army Research Office, http://www.aro.army.mil/index.htm

∑ NANOSCIENCE Programs, http://www.aro.army.mil/phys/pcog99a.htm

∑ Office of Naval Research, http://www.onr.navy.mil/

∑ Defense Link, http://www.defenselink.mil/

∑ DARPA home site, http://web-ext2.darpa.mil/body/main.html

∑ DARPA Tech99, Presentations, http://www.darpa.mil/darpatech99/

∑ DARPA, Micro-adaptive flow control, http://www.darpa.mil/tto/programs/mafc.html

∑ DARPA micro propulsion, http://design.caltech.edu/micropropulsion/

∑ DARPA Tactical Technology Office, http://www.darpa.mil/tto/

∑ Bell Helicopter, http://www.bellhelicopter.textron.com/index.html

∑ Sikorsky Helicopter Co., http://www.sikorsky.com/

∑ Airbus, http://www.airbus.com/family_line.html

∑ Aircraft photos, http://www.airliners.net

∑ Personal Aircraft, http://www.moller.com/index.html

∑ LaRC 2000 Biomimetics, http://kvass.larc.nasa.gov/cgi-bin/biomimetics/home.pl
49

APPENDIX

Examples of Current Advanced Concepts

In recent years a variety of alternative or advanced concepts have been considered

for the subsonic and supersonic transports. A few examples are given here, and a number

of additional ones can be found in the work of LaRC’s Chief Scientist Dennis Bushnell7,8,9.

Very Large Subsonic Transport

 In order to increase the capacity of the current airport system, as well as increase

the capacity of freighter transports, the design of a very large subsonic transport (VLST) is

being considered in both the U.S. and Europe. Aircraft capable of carrying 600 to 800

passengers over global distances are being designed. Technical problems in designing a

VLST include satisfaction of aircraft noise requirements, the fabrication of large composite

structures, the control of large flexible structures, engine emission requirements, alleviation

of a strong wake, and utilization of current airports. In an effort to solve these problems the

following technologies will be extremely important: Active control systems, flow control

systems, and simple, effective high lift systems.

Long Endurance Aircraft

Many missions have been identified for uninhabited aircraft that can fly for very long

duration, (weeks, months, years, etc.) at high altitudes. These missions include

environmental monitoring,

communications, reconnaissance,

and weapons platforms. An

example of this technology is the

Pathfinder. The “Pathfinder-Plus” is

a solar-powered, propeller-driven

aircraft with a duration of 2000-3000

hours at altitudes greater than

70,000 feet. These high altitude

vehicles operate in a low Reynolds

number regime where real gas

effects are not well understood.
50

Blended-Wing-Body

A revisit to the blended wing body (BWB) idea introduced in the 1940’s has produced

several transport designs. Generally, these designs consider integration of the engines

with the BWB.

• Benefits

– Fuel efficiency >20% increase

– Weight ~10% decrease

– Direct Operating Cost (DOC) ~12%

decrease

• Challenges

– Non-cylindrical pressure vessel

– Stability and control at envelope

edges

– Vortex hazard alleviation

– Loading distribution

– Propulsion/Airframe Integration

(PAI)

– Noise propagation

• Status

– NASA/Boeing project to

build 14.2% RPV for S&C

work underway. Includes 1%

spin tests, 2% rotary tests,

and 3% static tests

AT

Advanced Vehicle Systems Technology Program
51

Strut Braced Wing Concept

One design concept receiving attention involves a strut-braced wing. The wing

thickness and sweep can be reduced, which facilitates natural to easily enforced laminar

flow with the bracing. In addition, lift to drag ratio (L/D) of 40 (twice as large as current

transport values) can be achieved.

• Benefits

– Significant emissions, weight, and

cost reductions (6-12%) due to

lighter wing, less drag, & reduced

fuel burn

• Challenges

– Strut design and integration (aero

and structures)

– Propulsion/Airframe Integration

– Laminar flow optimization

– Candidate for tip engine integration

• Status

– Currently on hold, but will be

addressed in the future

Wing Tip Engines

Another concept being examined is the use of wing tip engines. It allows a reduction

in aerodynamic drag due to lift and vortex wake. The strut braced wing concept may be a

technology enabler for such a concept.

AT

Advanced Vehicle Systems Technology Program
52

Adaptive Vehicle Control

• Benefits (quantitative analysis pending)

– Increased survivability

– Lower weight, higher performance

control surfaces and systems

• Challenges

– Design and integration of

innovative aero and structural

gadgets that allow full envelope

trim and maneuvering for tailless

concepts while avoiding surface

breaks

• Status

– AVSTPO projects working aero

and structures gadgets (RACRSS

& Morphing)

– Phase 1 REVCON project to build

RPV pending

• Examples Include:

– Oscillatory excitation

– Synthetic jets for pulsed

vortex generation

– Passive porosity

– Inflatable Gurney flaps

AT

Advanced Vehicle Systems Technology Program
53

Box Wing

• Benefits

– Modest (~3-6%/available seat mile)

benefits in fuel efficiency and DOC

– ~20% smaller spotting factor than

equivalent conventional transport

– May enable STOL operations and

reduced noise footprint

• Aero Challenges

– Simple direct lift system (STOL)

– Increasing biplane gap to raise

efficiency

– Drag at intersections

– Propulsion/Airframe Integration

• Status

– NASA work on hold for now

– Lockheed Martin studying for KC-135

replacement

AT

Advanced Vehicle Systems Technology Program
54

Supersonic Biz Jet

• Benefits

– If sonic boom is low enough to

allow overland supersonic cruise,

the economics are favorable for

development of this class of

vehicle

• Challenges

– Sonic boom minimization (shaping,

weight reduction, lift tailoring, etc.)

– Propulsion/Airframe Integration

• Status

– Recently completed near-field

boom model tests (favorable

results)

– DARPA proposing a program for

similar vehicles

AT

Advanced Vehicle Systems Technology Program
55

Personal Air Vehicles

• Benefits

– Major reduction in doorstep-todestination travel times for the

average individual

– Competitive costs with shared

ownership

• Challenges

– Air Safety - Smart systems (aero,

control, GNC, etc.)

– V/STOL for PAV’s

– Propulsion/Airframe Integration

– Noise propagation

• Status

– Intense studies underway within

Advanced Concepts Group of ISAT

AT

Advanced Vehicle Systems Technology Program
56

Multi-body Inboard Wing

• Benefits

– Potential for large

reductions in induced

drag

– Possibly reduced

vortex strength

– Large capacity with

simple systems

• Challenges

– Induced drag reduction

characterization

– Landing gear integration

– Propulsion/Airframe Integration

• Status

– No current work, but under

consideration within Advanced

Concepts Group of ISAT
57

Automated Package Delivery: A Virtual Memphis

Modular aircraft (freighters) circulate on fixed racetracks between major purchasing

“hubs” (above weather and hub traffic). Delivery vehicles (hitch-hikers) detach from

the freighters to deliver packages to

neighborhood collection centers, while

outbound deliveries hitch a ride from the

collection center to the freighter & on to their

final destinations. Vehicles cross between

racetracks to reach other

national/international destinations. Robotic

collection centers notify customers of

deliveries. Freighters are composed of

“Lego”-like diamond-shaped delivery vehicles, plus a lead vehicle with management

functions, main power, etc. Extensive leverage of SATS, Advanced Structures &

Materials, Propulsion, Controls, Aerodynamic, and Smart/Morphing activities.

Enabling Technologies:

• Wideband communication with the National Airspace System

• “Designer” aerodynamics (make bricks fly)

• Controls (vehicle management, virtual control surfaces, fluidic flow vectoring, ...)

• Advanced materials and

structures (electromagnetic

glue, recyclable airframe, ...)

• Compact, very cheap

propulsion systems

• Systems integration (maglev

launcher, package retrieval,

security, ground delivery, ...)

Freighter

Inlet

Nozzle

Hitchhiker

RearView

Top View

AT

Advanced Vehicle Systems Technology Program
58

Intermodal Transports

• Benefits

– 2-3X greater profitability for

operators due to increased

utilization (despite increased per

trip costs)

• Challenges

– Low noise operations to

accommodate intermodal

configurations (24 hour and 7

day/week operations)

– Structural integration of cargo

pods

• Status

No current work, but under consideration within Advanced

Concepts Group of Inter-Center Systems Analysis Team (ISAT)

AT

Advanced Vehicle Systems Technology Program
59

Access to Space - Reusable Launch Vehicles

X-33

The X-33 is the flagship technology demonstrator in

NASA's Space Transportation Enterprise. The X-33 is a

half-scale prototype of a reusable launch vehicle (RLV)

called the "VentureStar." It will dramatically lower the cost

of putting a pound of payload into space from $10,000 to

$1,000.

X-34

NASA’s X-33 and X-34 rocket planes are suborbital

technology demonstrators, operating at speeds up to 15

and 8 times the speed of sound, respectively.

X-37

The X-37 will be the first of NASA’s fleet of reusable

launch vehicle experimental demonstrators to operate in

both the orbital and re-entry phases of flight. The robotic

space plane will play a key role in NASA’s effort to

dramatically cut the cost of putting payloads into space.

Capable of being ferried into orbit by the Space Shuttle or

an expendable launch vehicle, the X-37 will operate at

speeds up to 25 times the speed of sound and test

technologies in the harsh environments of space and

atmospheric re-entry.

X-38

NASA's X-38 technology demonstrator for the Crew

Return Vehicle may become the first new human

spacecraft to travel to and from orbit in the past two

decades, a spacecraft developed at a fraction of the cost

of past human space vehicles.
60

STOL and VTOL

Rotorcraft, or perhaps VTOL aircraft, poses a somewhat more difficult challenge.

STOL Airplanes that land slowly and steeply won't significantly reduce the need for those

that hover and land vertically. Using structure to shield noise has significant penalties

when thrust is directed downward, so other solutions must be found. Shrouding rotors, at

least small ones such as those found on tiltrotors, may provide a solution. Another

approach is to provide adaptive blades that

change length or active blades made of

smart materials that change twist distribution

to alleviate noise while maintaining

performance.

Rotorcraft

Helicopters are rotary-wing aircraft that satisfy a number of transportation,

commercial, emergency and military needs in today’s society due to their vertical takeoff/landing and hovering capability. In general, helicopters travel at a forward speed around

200 miles/hr, have flight duration of two to three hours, and have a range of about 600

miles. They have higher fuel consumption to travel a given distance compared to that

required by a fixed wing aircraft. A major problem with helicopters continues to be the

noise levels they produce. Some improvement has been made in the forward speed

(reaching 345 miles/hr) by introducing jet or propeller systems (compound helicopters).

Advancements in materials have made helicopters lighter, stronger, and safer. Innovative

technology developments in the areas of flight and flow controls could dramatically improve

safety, noise, and aerodynamic performance of helicopters.

Tilt Rotors

A tilt rotor aircraft is an airplane with the vertical take-off/landing (and hovering)

capability of a helicopter. These vehicles have rotors that behave like a turboprop when

tilted 90 degrees. Tilt rotor aircraft are

expected to provide transportation for short

distances (roughly 300 miles). At the

present time a tilt rotor aircraft is being built

by Bell Boeing (designated the Bell Boeing

609). This vehicle cruises at the speed of

316 miles/hr with a L/D of 8, and it has a

range of 870 miles. It will travel twice as fast

and twice as far as a conventional

helicopter. However, maintenance costs/flight hour may be about four times more than for

a conventional turboprop. Reduction of these costs as well as reduction of noise and
61

emissions will obviously be important issues for the success of the tilt rotor aircraft,

especially if the claim by Bell Boeing of revolutionizing air travel is to be realized.

Rotary Wing CFD

At the present time Navier-Stokes computations for helicopter rotor analyses are still far

outside the realm of routine calculations. Furthermore, while much progress has been

made in developing and improving the efficiency of Euler solvers for inviscid flows, such

solvers continue to remain too expensive for routine calculations of helicopter

aerodynamics, which frequently involves unsteady flows. Potential flow techniques

currently provide the primary methods for computing the aerodynamics of helicopters.

There are five principal areas of aerodynamics to consider for helicopter rotor

analyses: 1) Rotor wake prediction, 2) compressible aerodynamics, 3) component and flow

interactions, 4) retreating blade dynamic stall, 5) fuselage flows. The first three areas are

not viscous dominated, allowing inviscid flow analysis for aerodynamic predictions, which is

sometimes coupled with boundary-layer analysis. The rotor wake is the most important

issue in the case of hover. Currently, there is no reliable prediction capability for the

detailed effect of rotor blade geometry on the wake. For high-speed forward flight, detailed

knowledge of the wake is necessary to predict vibratory loading. The structure of

advancing rotor wakes is not understood and unsteady loading cannot be reliably

predicted. Also, at high-speed forward flight compressibility effects can be quite important.

Rotor blade tip flows can be supersonic with shocks, resulting in high drag. Such flows are

generally unsteady and strongly three-dimensional. Reliable techniques are not available

that include boundary-layer effects and provide coupling with potential methods for global

rotor analysis.

Viscous effects can play a strong role in the aerodynamic areas of retreating blade

dynamic stall and fuselage flows. The retreating blade of a rotor can experience dynamic

stall due to high lift coefficients caused by blade flapping and cyclic input (produced by

propulsion). Vortex interactions for retreating blades further amplify this problem. There is

no consistent predictive capability for even two-dimensional, let alone three-dimensional,

unsteady stall behavior. In the case of a helicopter fuselage, significant flow separation can

occur, making it difficult to compute the parasite drag.
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

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Washington, DC 20503.

1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE

December 2000

3. REPORT TYPE AND DATES COVERED

Technical Memorandum

4. TITLE AND SUBTITLE

Researach Opportunities in Advanced Aerospace Concepts

5. FUNDING NUMBERS

WU 522-17-41-30

6. AUTHOR(S)

Gregory S. Jones, Linda S. Bangert, Donald P. Garber, Lawrence D.

Huebner, Robert E. McKinley, Jr., Kenneth Sutton, Roy C. Swanson, Jr.,

and Leonard Weinstein

 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

8. PERFORMING ORGANIZATION

REPORT NUMBER

 L-18030

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA/TM-2000-210547

11. SUPPLEMENTARY NOTES

White Paper

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified-Unlimited

Subject Category 01 Distribution: Nonstandard

Availability: NASA CASI (301) 621-0390

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

This report is a review of a team effort that focuses on advanced aerospace concepts of the 21st Century. The

paper emphasis advanced technologies, rather than cataloging every unusual aircraft that has ever been

attempted. To dispel the myth that "aerodynamics is a mature science" an extensive list of "What we cannot do,

or do not know" was enumerated. A zeit geist, a feeling for the spirit of the times, was developed, based on

existing research goals. Technological drivers and the constraints that might influence these technological

developments in a future society were also examined. The present status of aeronautics, space exploration, and

non-aerospace applications, both military and commercial, including enabling technologies are discussed. A

discussion of non-technological issues affecting advanced concepts research is presented. The benefit of using

the study of advanced vehicles as a tool to uncover new directions for technology development is often

necessary. An appendix is provided containing examples of advanced vehicle configurations currently of

interest. 

14. SUBJECT TERMS

Aerospace Technology Drivers, Advanced Vehicle Concepts, Advanced Aerospace

15. NUMBER OF PAGES

68

Concepts, Civil Aviation, Military Aviation, General Aviation 16. PRICE CODE

A04

17. SEC U RITY CL ASSIF IC AT ION 

O F REPO R T

Unclassified

18. SEC U RITY CL ASSIF IC AT ION 

O F TH IS PA GE

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Unclassified

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