aiaa-1998-1-921 frontiers of the "responsibly imaginable" in (civilian) aeronautics

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Copyright© 1997 American Inst itute

o f

A eronaut ics

and

Astrona utics Inc.

T H E

9 9 8 A IA A DR YDE N LE C T UR E

FRO NTIERS O F TH E RESPO NSIBLY IM AG INABLE IN ( CIVILIAN) AERO NA UTICS

Dennis M. Bushnel l*

National Aeronautics and Space Administration

Langley Research Center

Hampton, Virginia 23681-0001

ABSTRACT

Pape r discusses future alternatives to curr ently deployed

systems which could provide revolutiona ry

improvements

in

metrics applicable

to

civilian

aeronautics with, in many cases, significant military

"spinoff." Specific missions addressed include subsonic

transports, supersonic transports, personal aircraft, and

space access. These alternative systems

and

concepts

are in many

cases

updates to know n "end point" designs

enabled by recent and envisaged advancements in

electronics, communications, computing,

CFD /nonlinear aerodynamics and

"D esigner

Fluid

Mechanics"-- in conjunction with a design appro ach

employing extensive synergistic interactions between

propulsion, aerodynamics and structures.

INTRODUCTION

The

last

50 years o f aeronautics have been truly

revolutionary. In the developed world, train and ship

long h aul passenger traffic has been replaced by aviation

and

aviation

has

assumed

a

dom inant role

in

warfare.

The list of revolutionary technological developments

during this period includes large swept wing near-sonic

transports of the B47-707 genre, supersonic cruise and

fighter aircraft, turbojets and ram jets, high strength

aluminums

and

com posites,

and a

vast array

of

avionics. Much of this progress occurred under th e

dominant m etric of higher, faster and larger is better and

was a

cont inuation

of

aeronautical development trends

since the early 1900's.

Today and for the fo reseeable future, the metrics, and

hence the nature of desired technological improvements

are "different." These "new" (civilian) metr ics include

AFFORDABILrTY (initial,

life

cycle), pro ductivity

(aircraft,

air

space), safety

and the

environment (noise,

pollution). Driving

these

metrics

are

global economic

competition (exacerbated

by the

demise

of the

"cold

war"),

an

increasing demand

for air

travel

in the

shorter

term, increasingly stringent environmental regulations

and the em erging competition from the ongoing

telecomm unications r evolution(s) wherein business

travel

in

particular

may

become increasingly replaced

by

"virtual"

interpersonal interaction via (eventually) 3-D

technology

such as holographic projection, virtual

reality

imm ersion, etc. Estimates indicate a 40 percent

business travel reduction over the next 20 years due to

the "teletravel" revolution with consequent (no n-trivial)

reductions

(the order of 1000 units) in CTOL transport

production (e.g., references 1 to 5). Such a reduction in

business air travel would leave recreational travel the

dominant market

sector,

a sector wh ich is extremely

price sensitive, p lacing

a

further premium upon cost

reduction technologies.

Corresponding updated military metrics are

also

lead by

AFFORD ABILITY, follow ed by responsiveness,

flexibility, lethality, survivability, logistic support and

standardization/interoperability.

The

present paper

places

major emphasis upon

the

civilian sector,

with

obvious

military "spinoff possibilities" noted.

The curren t approaches to

satisfying

these metrics

almost universally involve increment al/evolutionary

technological improv ements to the existing para digms

coupled with revo lutionary reductions

in

design cycle

time and "manufacturability" improvements in the

context

of an

"integrated product team." What

are

conspicuous by their absence are any major attempts to

satisfy these metr ics

via the

complementary approach

o f

inventing, developing, and deploying

farther

term/

advanced

technologies,

in

pa rticular advanced

configurations or

systems, with revolutionary

performance improvements. While traditionally

performance imp rovem ents have been used to enhance

speed o r reduce

fuel

consumption, they can obv iously

also b e employed to address the present metrics of

cost/part co unt/w eight, pro ductivity, safety and the

environment, wh ere advanced perform ance can have

*Chief Scientist, Fellow AIAA

1

American Institute

of

A eronautics

and

A stronautics

Copyright

© 1998 by the American

Institute

of Aeronautics and

Astronautics,

Inc.

No Copyright is

asserted

in the United States

under

Title

17,

U.S.

Code.

U.S.

Government has a royalty-free license

to

exercise

all

rights under

the

copyright claimed herein

for

Governmental

purposes.

All other

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are reserved by the

copyright

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1997

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truly dramatic payoffs. Also, much of the

major

aeronautical improvement

actually fielded

over the past

40

years, particularly

in the long

haul arena,

ha s

been

the result

of

propulsion technology,

primarily

higher

bypass

and turbine

inlet temperature,

which

provided

much of the technology for a near trippling of

seat

miles

per

gallon. Studies

and

suggestions

for

advanced

configuration concepts (e.g., references 6-11) have, in

general, not been

investigated

in depth nor

implemented. In

fact,

vehicle long term strategic

planning/research in the civilian aeronautical arena has

been

exceedingly sparse for decades. This has not gone

unnoticed and

calls

for renewed

longer term research

efforts

have

come

from both Dan Goldin, the

NASA

Administrator, and

Cong ress "Focus

NASA's

Aeronautics on Revolutionary New

C oncepts" (reference

12). The military has been much m ore proactive in this

regard

(e.g.,

references 13,14 and the

previous

"forecast" studies-e.g.,

reference

15).

The purpose of the

present paper

is to discuss example

advanced

concept approaches, across

the

speed range

and

for

space access, which may provide

revolutionary

changes and oppo rtunities in civilian aeronautics in the

future.

The context of this

advanced

concepts

discussion

is

that there

are no "m agic bullets," i.e.,

concepts which require no R&D , have no problems,

require

no

research

and

provide

guaranteed

(huge)

benefits.

All of these

approaches

require considerable

work to move them through the two

filters

which exist

between an idea and a deployed system. The first of

these

filters

is a

technical

one which

asks

the question

does it work." The

second,

and imm ensely important,

filter

is technological and

addresses

the

issue

of w hether

the concept

makes

sense in the "real

world"

when all the

illities are considered. Also,

most

of these concepts

are not new (see reference 16),

simply worthy

of being

readdressed in the context of new metr ics,

missions/requirements, available technology

levels

and

implementation ideas. A

central theme

is the use of

serious synergies

between prop ulsion, aerodynamic

and

structural systems

(see

reference

17). Many

of the

concepts discussed

herein

devolved

from

a

series

of

unpublished wo rkshops and studies

convened

by the

present

author

at

NASA Langley

in the late 80's-early

90's specifically

to

(re)address advanced concepts

in

various

mission areas (CTO L,

HSCT,

hypersonic

airbreathing

propulsion

and

personal

aircraft).

A major "bottom

line

suggested by this

(re)examination of advanced aeronautical concepts is that

aeronautics is far

from

operating near the limits of what

can be

accomplished either

physically o r

economically—

i.e., the

conventional

wisdom (which is

unfortunately

approaching

a

"self-fulfilling

prophecy")

that

aeronautics is a

"plateau"/"mature"

industry is

emphatically no t correct. There is such a wealth o f

alternative

concepts,

each of them offering truly

revolutionary possibilities,

that

the adaptation of even

one or a

limited number could lead

to a

"Renaissance"

in aerona utics (see

also

reference 18).

DESIGNER FLUID MECHANICS

Most of the

configuration concepts discussed

herein rely

on a set of technologies developed extensively over the

last

30

years which

can be

collectively termed "D esigner

Fluid

Mechan ics."

These technologies include

laminar

flow control (reference 19), m ixing

enhancement

(e.g.,

reference 20), separated flow

control (references 18 and

22), vortex control (reference

23),

turbulence control

(e.g., r eference 24),

anti-no ise,

favorable

wave

interference (reference 25),

and

even "designer

fluids."

In most cases, these

have

been

taken to the

flight

test

stage and beyond and are

thus

ready, in

various

manifestations,

fo r inclusion in, and to

pro vide enabling

technology/ capability

for,

synergistic advanced aircraft

concepts.

Consider, fo r example, the currently

applicable aircraft

metrics-productivity, safety, environment and

affordability. These would

be

greatly

enhanced by

major simultaneous reductions in wake vortex hazard

(reference 23),

drag-due-to-lift

(references 26-34)

and

friction

drag (references 35,36).

The Designer

Fluid

Mechanics

literature just

cited offers an extensive array

of alternative

reduction

approaches

for each

metric/flow

phenomena. If the

lists

of the various

approaches

are

"merged" and "simultan eous solutions" sought, the

resultant configurational

imp lications strongly suggest

several

revolutionary

configurational alternatives, fo r

example, to the current 707-DC8 CTOL

transport

paradigm.

Th is serves to illustrate the

thought

processes followed

to develop the advanced

configurations

described

in the

following sections.

ALTERNATIVE

SUBSONIC

LONG

HAUL

TRA NS PORT CONFIGURATIONS

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Astronautics



Copyr ight© 1997 American Inst i tu te o f A e ronau t ics and Ast rona ut ics Inc.

Strut/Truss-Braced Wings with W ing-Tip Engine

Pfenninger has long

advocated

strut

bracing

to

improve

the

performance

o f conventional

transports

(e.g.,

references 37 and 38, see also

references

39-44). The

resulting (bending,

torsion)

structural benefits allow

reduced wing thickness and sweep, resulting in a

tremendously enhanced and easily m aintained (reduced

sensitivity to

rough ness/insect

remains/ice clouds,

reduced

cross

flow) extent of natur al-to-easily

forced

low

drag

laminar flow, as

well

as increased

span.

The

latter

allowed a

r eduction

in

wing

chord,

further

enhancing the extent of laminar flow, as well as

enhanced

takeoff and climb performance and

reduced

vortex hazard.

Plenninger's designs

fo r

such aircraft

yielded L/D values in the 40's,

over

twice current

levels. The

concept

was not, how ever, adopted

primarily

because

th e

extensive wing

span did not fit

the

FAA "80

m eter

box

fo r

airport gate com patibility

and disbelief that a

transonic

strut braced

wing

could be

designed with acceptable

shock drag (and obtain laminar

flow

on the strut/truss). Obviously

strut-bracing

is

routinely

em ployed on low(er)

speed

aircraft. The

latter

objection

is probably not

valid

in light of

today's

CFD capabilities. "W e build what w e can com pute"

(reference 45) and we hav e been too long constrained in

aircraft design to linear theory and consequent

"linear

thinking."

The span o f a

strut braced

configuration can probably

be

reduced

to the 80 meter requirement and the

overall

performance

retained

if an alternative

approach

is

employed

for major

drag-due-to-lift reduction-wing

t ip

engine

placement (also enabled by strut/truss bracing).

Whitcomb

(reference

46) and others (e.g.,

references

47

to 55) have

shown

that up to the order of 50 percent

DD L reductions are obtainable using t his approach,

which

pro bably requires circulation control

in the

empennage region

and

utilization

of

thrust vectoring

o n

all engines to

handle

the "engine

out"

problem. The

Pfenninger approach

o f

wing sweep/thickness

reductions and consequent natural-to-easily forced

wing

laminar flow

as enabled by strut

bracing

is

also

retained.

Synergistic

prop ulsion-aerodynamic

interaction

The use of tip engines fo r aerodynamic drag-due-to-lift

and

wake vortex reduction is

part

of an overall

paradigm shift in aircraft design where a configurational

concept

is sought in the context of an "open

thermodynamic system," i.e., synergistic use is made

of

the energy added by the

propulsion system.

Historically, aerodynamic theory is almost totally

predicated

upon analysis within

a

closed

system" (no

energy added

within the control

volume).

By necessity,

as

speed

is

increased,

increasing use has

been made

of

favorable pro pulsive interactions— wing pro pulsive pre-

compression/engine

nacelle favorable

interference

lift at

supersonic speeds

and,

at hypersonic conditions, th e

entire

undersurface of the body is an integral part of the

engine flow path for the airbreather case. However,

little

"first

principles"

work

is available

regarding

favorable

interactions

in the

subsonic case.

A

theoretical

construct for

aerodynamic optimization

in

an

open system

is not yet extant, but

there

are

several

discrete

exam ples, in addition to the

whig-tip

engine

case,

where

synerg istic airframe/ aerodynamic-

prop ulsion integration has been

studied

and in some

cases even applied.

These include

circulation contro l

wing flow which offers up to a factor of 4

increase

in

CL

compared to

conventional flaps

and

slats with

tremendous

reductions to

part count"

(reference

56),

wake/boundary layer ingestion into the propulsion

system [order of 15 percent increase in propulsion

efficiency (e.g.,

reference 57)] including synergistic

interaction with an LFC suction system (reference

58);

thrust vectoring for control (e.g., X-31, tail-less

fighters, etc.) and, hyperson ically, fo r lift enhancement

and thrust requirement reduction; utilization of a

leading edge

region

LFC

suction system

for high

lift

during

takeoff;

the

ejector

wing f or improved structural

and aero efficiency

(reference

59); wing tip injection for

wake vo rtex

and drag-due-to-lift

reduction

an d myriad

propulsive-augmented high lift schemes. An additional

major oppor tunity

f or

synergistic propulsion-

aerodynamic

interaction

is the

"Goldschmied"

wing (o r

body), a

takeoff

on the Griffith

wing (reference 60).

The

basic

concept is to po sition the propulsive inlet in

the recompression region on the wing/afterbody to

effectively place sinks inside the

body

and convert

much of the

balance

of the

afterbody into

a

stagnation

region instead of having a rear stagnation point.

Estimates

indicate up to a 50

percent

"cancellation"

of

the

body fr iction

drag

is possible via this "static

pressure thrust" approach (references 61-65). An

additional,

but to

this point

unevaluated, approach is

use of

inflatable

surfaces to provide "mission

adaptation."

A particularly attractive

application

of

this

would

be

tailoring

of the

afterbody

region o f

hypersonic

cruise

vehicles

during

acceleration through

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Co p y r i g h t © 1997 Amer ican Institute

o f

Aerona ut ics

and

Ast rona ut ics Inc .

the transonic speed

regime

to alleviate the tremendous

transonic "drag pinch" problem

on configurations

designed fo r efficient hypersonic cruise/

acceleration/airbreathing.

Double Fuselage

Conventionally, double fuselage/multi-body

aircraft

have

been employed to pr ovide span-load distribution

and accrue

the associated

structural w eight benefits

(reduced

wing

bending

mom ent) without going

all the

way to a

"blended

wing

body'Vspanloader

configuration

i.e.,

providing

such

benefits

v ia

"con ventional" (e.g.,

"comfortable") technology. Total

aircraft

drag is also

reduced, primarily due to

favorable effects

on drag-due-

to-lift

(e.g.,

references 9 and 66-70).

An advanced double fuselage approach could attempt to

delete

th e

conventional

outer

wing

panels

and

only

retain a,

largely unswept/long

chord, whig section

between

the fuselages. This requires prodigious drag-

due-to-lift reduction, a requirement which can be

addressed via design of the fuselages as wing-tip "end

plates and the

individual

fuselage emp ennage as

"winglets,"

i.e.,

the tails become

thrusting

surfaces in

the presence of the

wing

vort icitv wrapping around the

fuselage(s).

For this

case,

the "m idwing" can becom e the

site

of the

gear

(to

allow

use of con ventional runways), engines

"buried" at the rear of the wing to accrue the

benefits

o f

"boundary layer ingestion" and extensive (natural/

suction) lam inar

flow.

The

major payoff

would accrue

from

making

the

fuselages detachable/interchangeable

to prov ide a civilian

"sky-train"

with

enhanced

productivity. The midwing portion which does all the

"flying" could be in the air nearly "around the clock"

with freighter and/or

passenger modules, thereby

nearly

doubling

the pro ductivity/duty cycle and "return on

investment"

(reference

71).

Such an approach would

allow

a restructuring of the

airline capital

investment,

with the

airlines "o wning" their fuselages

and

leasing

them

from a

"rent-a-wing"

company. Obviously,

military

versions

could have

cargo,

troop, and

refueling

fuselages-providing a quantum jump in

m ilitary

flexibility

and productivity.

Biplane/Ring

Wing

Another alternative configuration is a "back-to-the-

future"

relook at a

long-haul biplane (reference

72 , see

also

references 9 ,73 and

74).

Recent

work (reference

72) indicates a wide spectrum of benefits for

such

an

approach, again in the context of

present-to-future

advances

in CFD, materials, controls, "Designer Fluid

Mechanics,"

etc. techno logies.

Some

of

these

estimated

benefits

a re

major-60

percent

reduction in

wing weight, order of 30 percent

increase

in L/D and

reduced vortex hazard. The related ring whig (e.g.,

references 75 and 76) is also worth

r evisiting.

JIJMBO AIRCRAFT

As

this

paper

is written, the requirement

for,

and

interest

in, "Jumbo A ircraft" (>600

PAX)

is in a state

of

uncertainty. Airbus is proceeding

with

an enlarged

"conventional"

aircraft

study/development, B oeing has

placed

its corresponding effort on the "back burner" and

the fate o f a developing

Douglas

aircraft blended wing

body

effort

is dependent upon the

outcome

of the

proposed Boeing-McDonnell Do uglas "m erger." There

is

little doubt that

a

larger size/updated version

of the

747

would have greatly

improved

performance

(e.g.,

reference 77). At some level the major players

(U.S.,

Europ eans, Russians) are studying the technology fo r a

jumbo aircraft in the 800+ PAX range which is,

"different"-some variant

of the

spanloader

or

"blended

wing body"

(BWB).

From reference 78, "machines

with

more

than

1,000

seats will

have to be

designed.

For such m achines, the

unstable flying wing,

quite

probably equipped

with

a canard,

will

be the imposed

solution."

(Bernard Ziegler,

Senior

Vice President,

Engineering, A irbus). Jumbo options

aside from

conventional and BWB include multi-body and wing-in-

ground (WIG) effect The multi-body is a viable

candidate

but the WIG

probably

is not. Study of the

extensive Russian

work in the WIG

arena indicates

several nontrivial problem areas for the WIG vis-a-vis

the long

haul

transport

mission-operation near the

surface in

high

density air engendering a

high drag

level,

structural and pro pulsive weight inefficiencies

associated

with w ater impact and takeof f thrust

requirements

and

safety

problems

associated

with

operation at the extremely low altitudes (fractions of

the wing chord) required to attain

appreciable

ground

effect benefits

on

cruise L/D.

The

success

of a

"deployed version,"

the B -2 bomber,

has renewed interest, worldwide, hi spanloader/blended

wing-body

aircraft The major performance benefits of

such

aircraft

are required to address the po tential "killer

issues fo r jumbo aircraft-noise

and

vortex hazard

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Amer ican

Inst i tute of Aeronaut ics and Ast ronaut ics Inc.

engendered by their great

weight.

It is not clear,

although jumb o

aircraft

will obviously carry

more

passengers individually,

whether

airport

passenger

throughput will go up or down with the introduction of

jumbos—more

PAX per

aircraft

bu t perhaps

less

aircraft/unit time in terms of

takeoffs/landings

due to

wake vortex/noise/passenger handling delays.

Obvious benefits of

spanloader

aircraft include large

increases in L/D

(due

primarily to the demise of

fuselage

wetted

area/skin

fr iction)

and reduction in

empty and gross takeoff weight. The design

approach

"puts

the lift

where

the

load

is" fo r a requisite

size

aircraft with a

phy sical

wing th ickness sufficient to

allow passenger seating within t he

wing.

The

technological

challenges

(as

well

as the

oppor tunities)

of an

800+

PA X

long haul

B W B transport are

tremendous but at

least

thus

far evidently

"wo rkable."

These challenges include

thick wing sections

(possibly

with

hybrid LFC and synergistic ("Goldschmied")

propulsion

integration),

non-circular pressure

vessels,

stability

and

control,

emergency

passenger egress, ride

quality, airport

com patibility

an d

very

high R eynolds

number

aerodynamics-both

high lift

and cruise. The

B WB design inherently contains a surfeit of internal

volume and is therefore highly conducive to enhanced

range, cargo operation and passenger comfort In regard

to the latter benefit,

sleeper

versions of the B W B

could

provide

very interesting comp etition

to the

HSCT

for trans-Pacific

routes

in

terms

of

enhanced

comfort/lower price

versus

shorter tra nsit time/higher

price.

The

USAF "New

W orld

Vistas"

study (reference

14)

specifically

called out the BWB approach as an

excellent candidate to

provide

enhanced

"global

reach"

airlift

capability—in

conjunction with pr ecision

(GPS-

guided) delivery pallets

as an

alternative

to the

vehicle

design

decrements and

vulnerability

of

landing

"in

theater." The large payload/volume and extraordinary

range of BW B transports also pro vides extraordinary

capability fo r high

capacity

paratroop drops,

cruise

missile or UCAV carriage/launch, "AW AC S" m issions

and

"aerial replenishmen t" References 9 and

82-87

provide an entre into the BWB literature, reference 88

describes

a

very

different Russian spanloader

approach.

THE

CHANNEL WING

The channel whig is an interesting example of an

advanced configuration w hich was somewhat ahead of

its

time.

The

concept

was o riginally developed,

essentially

empirically, in the

40's

an d

early 50's

as a

STOL

light

aircraft and several versions w ere flown,

this is not

just

a paper airplane. The essential

technology

consists of a sem icircular airfoil

"channel"

underneath, and surrounding on the

underside,

"standoff

wing

mo unted engine nacelles such that the

engine-induced airflow

produces

sizable lift on the wing

channel

at

zero-to-low forward speed, providing

dramatic

STOL

capability. The

developers

of the

configuration claimed, but there was never any

satisfactory proof that the approach w as capable of

near

VTOL performance.

Development

of this

concept

has been essentially dormant since the

late

50's except

for

Soviet

research-Antonov produced

an advanced

prototype-demonstrator in the

late 80's

termed the AN-

181 (reference

89).

The original

40 's-to-50's

era research on the channel

wing

was, by necessity, highly empirical an d resource

constrained (see

references

90-97). The concept

provides a classic

example

of an

approach

worth

revisiting

with

updated

tools

and technology to

ascertain

th e

extent

to w hich its STOL performance

could

approach

VTOL. In

particular,

th e

incorporation

of circulation control in the classical

sense

(via

blowing imm ediately upstream an d above the trailing

edge),

when combined with

the engine-induced flow

over the wing,

should further

augment the

lifting

capability of the

configuration. Additional technology

updates/approaches of possible interest include (power-

on) CFD,

flow

separation

avoidance an d control, wing

laminar

flow fo r cruise

performance

and inboard strut-

bracing along with the all important

controls

issues.

The

po tential

fo r such an

updated channel

wing

approach to address the "V-22 niche"

should

be

determined as the

channel

wing

could

po ssibly

incorporate very

interesting

lifting capabilities with

a

fairly high

cruise speed at reduced w eight/cost compared

to the V-22

(with

it s

engine rotation requirements

an d

associated systems penalties).

HIGH

SPEED

CIVIL

TRANSPORT

The increasing importance of "Pacific Run" air travel

and the requisite

long transpacific

flight

times

for the

current subsonic transpo rts have renewed

interest

in a

Mach 2 class

"high speed

civil

transport."

Such a

device

is nom inally a "flying fuel

tank"

to a much

greater extent than the subsonic case, due to the

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addition of

appreciable

wave

drag.

This large

fuel

fraction

(order of one half to two thirds of gross takeoff

weight)

makes the designs

exceedingly sensitive

to drag

level and drag

reduction.

A mere 1 percent drag

reduction

can

yield

a IS Klb

reduction

in take off

weight.

To

first order

the

cruise drag

is

nearly equally

partitioned between skin friction, volume wave drag

and wave/vortex drag-due-to-lift. Large drag reductions

are pot entially available

via

"Designer

Fluid

Mechanics," e.g.,

laminar

flow control,

favorable

wave

interference

and flow separation control as

well

as

configuration tailoring to produce an elongated lifting

line

(references

25 and 99). The

following

advanced

configuration

concepts provide

enhanced performance

alternatives

to the

conventional

"double-delta"

planforms. Drag r eductions theoretically

attainable

include

up to 70+

percent

of the skin

friction

via

laminar

flow

control

and a very large

fraction

o f the

volume wave drag via favorable wave interference.

Parasol W ing

This

is an old

approach wherein

reflections of the

fuselage nose

shock pro vides

favor able interference lift

and subsequent afterbody region thrust

(references

4,

99-102).

Estimated

L/D

impro vements

are in the

range

of 25 to 30 percent Advanced

technologies

required to

accrue these

benefits

include flow separation

control

for

the

shock-boundary

layer interaction regions and

fluidics o r va riable physical geometry to work the "off-

design"

performance

issues. The

reduction

in volume

wave drag may allow

reduced

wing sweep and

consequent greatly enhanced LFC application.

Strut-Braced "Extreme Arrow"

Pfenninger has also

advocated

an

externally

strut-braced

HSCT with truly revolutionary cruise performance-an

L/D of order 20,

over tw ice

that of the

best

of the

current

appro aches

(references 103 and

104).

The strut

bracing allows use of an extreme arrow wing planform

with

m inimal w ave drag-due-to-lift,

increased aspect

ratio

and extensive laminar flow

("controlled").

Mid-

wing fuel canisters are used to provide

favorable

wave

interference an d

load

alleviation

with

extensive

"natural"

laminar flow on both the fuel canisters and

the fuselage.

Other A lternative HSCT

Approaches

Northrup has studied a "reverse delta"

configura tion

fo r

purposes of obtaining extensive regions of "natural"

laminar

flow

on the wing (reference 105, see also

reference 106). "Novel" general concepts

with

application across

th e

configurational

spectrum include

use of flow separation control at cruise to allow full

exploitation

of inviscid design precepts

(reference

25).

The current

design

approach

entails investigation

of

advanced (primarily inviscid

flow-driven)

aero concepts

in

th e

wind tunnel

to

determine flow separation

boundaries in

terms

of vehicle attitude and geometry.

The p otential

performance

of the

advanced concept

is

then

degraded accordingly.

Utilization of "designer

fluid m echanics" technology in a proactive manner fo r

cruise should

allow

recovery

of

this flow separation-

induced

performance

loss. Potential

benefits

include

enhanced wing leading edge

thrust,

increased upper

surface lift, increased fuselage

lift/camber

(reduced

wave

DDL)

and

enhanced

perform ance of favo rable wave

interference (via shock-B.L. separation

control)

as well

as

"bird-like"

flight

fo r enhanced ah-

system

productivity/noise reduction at lower speeds.

Another

"alternative" is to

consider

a "multi-stage"

aircraft (e.g.,

references

25 and 107). In-flight refueling

and some type

of "take-off

assistance"

are

obvious

ways

to

"multi-stage,"

but what is

specifically

suggested

herein is that

since

th e

HSCT

is a "flying

fuel

tank"

the aircraft that lands is very

different/lighter

weight

than

the vehicle at

takeoff. Therefore,

the

heavy fuel/noise control/high lift devices and gear

(gear

weight approaches fuselage weight for an HSCT)

required for the

"takeoff ' condition

could

perhaps

be

positioned toward the rear of the craft and

"detached'V'flown

back"

once airborne. The vehicle is

thus

no t

burdened throughout

it s

flight

by

apparatus

uniquely

required

f or

mission initiation only.

If economics or

environmental considerations

dictate a

lower

altitude/low er

speed

(« 50

Kft,

M

=1.5)

than an

obvious "approach of choice would be the

R.T.

Jones

yawed whig

(references

108-110) which

is capable,

from reference 108, of doubling

speed

and increasing

PAX

load-out 25 p ercent v is-a-vis the 747 for

essentially

the

same

fuel

burn.

Another exceedingly interesting but at this point

highly

speculative

approach to HSC T

(and

also CTOL)

optimization is through

wave drag reduction

via

"plasma Control"/ electro-gas-dynam ics. The

conventional shock-plasma Interaction literature is

clustered about two "end points" in terms of param eter

space-planetary (collision-less)

bow

shock form ation

in

6

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Amer ican

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the solar

wind and

inertial-confined fusion.

Of

particular interest in the former case is Venus, which

has no

m agnetosphere

and is therefor e subject to

electro-gas-dynam ics as opposed to

magneto-electro-gas

dynamics

phenomena

(e.g.,

reference

111).

Examination of the

phy sics

of shock-plasma

interactions (c.f.,

reference

112) indicates

that, in the

absence

of a magnetic

field ion -acoustic waves

are

expected

to be dom inant. Research in

Russia

(e.g.,

reference

113)

and the

U.S.

(reference

114) indicates

unexpectedly large experimental effects for a "new

arena of shock-plasma interaction— weakly ionized

plasmas

at

conventional

gas dynamic conditions

(continum

flow, supersonic/hypersonic

Mach

numbers). Typical requisite experimenta l conditions

are 10

12

electrons/cc, but

relatively

high electron

temperature

(vis-a-vis ion/neutral

temperature)

and

placement

of

electrons well

ahead of the

unmo dified

bow shock. Conventional wisdom (e.g., reference

115)

would indicate

that

such weak ionization should

have

little-to-no

effect

on the

neutral

gas

dynam ics.

How ever, the

ob servations

indicate

virtual

replacement

of the usual

speed

of sound by a higher speed wave

system the

order

of the ion-acoustic velocity and

consequent reduction in effective Mach number and

body

drag.

The reducio-ad-absurdium implications are

truely

revolutiona ry-an increase in

drag

rise

Mach

number from 0(.85) to 0(2+),

HSCT's

which

a re

similar

in configuration to CTOL transports and CTOL

transports

with greatly increased wing thickness/reduced

sweep, etc.

The

keys

to

such

leaps

are the

verification

of the

available observations,

understanding of the

operative physics and obvious

developmental

aspects

associated with

high temperature

electron

generation

and

placement

in and ahead of the vehicle flow field.

That an increase in sound speed can reduce wave

drag

has long

been

known

(e.g.,

references 116 and

117),

but

heating per se is

simply

not an

energetically

efficient

approach to obtaining such a sound

speed

increase/drag

reduction.

What is "new" is the

possibility o f employing new

physics (e.g.,

coulombic-induced com munication-ion-acoustic

waves)

and the

exceedingly

low

power levels

which the

existing

data

base

suggests are required. Some type of

"amplification

system" may be operative to achieve

such dramatic influences

upon

gross flow

field

features

from

such a small energy input/low

ionization

fraction.

A

current line of investigation in that

regard involves

dynamic processes including ion-acoustic

instabilities

(e.g., reference 118).

PERSONAL AIRCRAFT-THE

VTOL

CONVERTICAR

The

developed

nations entered the

1900's

with a

transportation system (for people) centered upon the

horse, the

railroad

and the steamship, with associated

travel times

the

order

of

hours-to-days/weeks,

depending upon distance. In the closing years of the

same

century,

th e

automo bile

has

long

supplanted the

horse and the fixed whig aircraft has nearly

driven

the

railroads

and steamship

compan ies from

the long haul

passenger business. Travel

times have shrunk

to

minutes-to-hours. In the process of supplanting

older

transportation systems,

these

newer approaches

have

had a

profound influence upon

th e structure of

modem

societies.

In the

U.S.,

cities

have

expanded out of

18th

century seaports and 19th century

railheads, where

much of the developed region w as by necessity

within

walking distance of the transportation

terminals,

into

tremendous suburbs with

attendant reductions

in

crowding/increased

opportunity for

individual

home

ownership

etc.,

etc.

The existing

transportation

system fulfills

a

variety

o f purpo ses, including travel

to and

from work

and

stores,

and for

various

business,

service and pleasure related activities.

This

portion

of the

present

report

centers

upon

future

possibilities/options for a

specific

portion of the

transportation spectrum, short-to-moderate range,

nominally

from

10's to

100's

of

miles.

The current

dominant transportation

mode fo r

this mission

is the

automobile, which,

possibly

more

than a ny

other

single technical achievement, has

enabled

the current

life style enjoyed by the developed nations. In this

process,

the auto has created

m assive safety problems

(order

of

40,000

deaths/year due to highway accidents)

and has been responsible for the expenditure of truly

prodigious sums

on

roads,

bridges, along with

pollution-induced health

and

m aterial degradation

remediation.

The current status of the auto

infrastructure is that w e

continue

to

clear

and

pave

more

of the watershed, contributing to air po llution,

flooding, desiccation,

th e

formation

of

heat islands

and

wildlife

habitat degradation. Also, the average trip

time

is

increasing

due to

suburbian expansion

and

increased

congestion, causing non-trivial

changes in

family life

as

travelers attempt

to

utilize non-traditional

time slots, o r suffer long/nonproductive comm utes. In

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C op y r i gh t © 1997 Amer ican Ins ti tu te of Aerona ut ics and Ast rona ut ics Inc.

the

U.S.

th e interstate

highway

system is (finally)

finished

and is already

clearly

overburdened and in

need

of very expensive repairs

and expansion, particularly in

the

urban areas.

Society cannot, easily or otherwise, continue to bear

the

costs imposed

by almost

sole reliance upon

the

automobile

fo r short-to-intermediate

passenger

transport, alternatives are

necessary

for the future— both

for

the

developed societies

and

those that

desire

to/are

developing.

Probably the

most

comm only

advocated

alternatives involve

some

form

of mass

transit, which

have,

along with tremendous capital costs, several

other drawbacks such

as

passenger wait time,

weather

exposure

and

lack

of

p rivacy, security, pride

of

ownership

and

personal stowage.

Additional drawbacks

are the

fact

that they

are not

portal-to-portal

and

there

is

no guarantee of

having

a

seat,

as well as an inherent

assumption

regarding

increased

population

density/concentration.

Undoubtedly, th e

future

mix of

short-to-intermediate transport systems

will include

both

mass tran sit and autom obiles of

some variety,

probably operated on

"intelligent"

highways to

improve safety and throughput/trip time (reference

119). The "smart highway " alternative is, however,

expensive and

very

limited in terms of growth

potential

(=0[factorof2]).

There is,

however, both

a n eed and an opportunity to

include in the transporta tion m ix a personal air

vehicle

which would

provide, percentage-wise, the same

increase

in

speed

(compared to the

auto

in traff ic), as

the auto

provided

over the horse. Personal air

transportation usable by "everyone" is both

revolutionary and the next logical step in the

development

of human infrastructure and corporal

communication. The

increased

speed of such a

capability, along with the

greatly reduced capital

requirements in

terms

of highways/bridges, etc.,

should

allow significant

increases

in the quality of

life

as well

as reduced sta te and national

public works

budgets.

Specific benefits include distribution

of the

population

over a

much

larger area allowing a more

peaceful/less

damaging

co-existence

of m an and nature, along

with

improved transportation

safety.

The vision is of

multilevel

highways in the

sky, contro lled

and

monitored by

inexpensive

electronics as

opposed

to

narrow, single level, exceedingly expensive "ribbons of

concrete" (e.g.,

reference 120).

Such air

systems/vehicles could also obviously be used fo r

longer haul, as are automobiles today. The various

wait

times

associated with comm ercial

air

travel, along

with the inefficiencies in

terms

of transit time of the

hub

and spoke

system

mitigate in favor o f reduced

overall

trip time for

slower,

but

more

direct,

travel via

personal aircraft (compared to the "faster" commercial

jet).

Various options

exist for personal aircraft

systems.

The discussion

herein will

address one such

option,

an automatic VTO L-converticar, and attempt to

defend

that

particular

recomm endation.

Certain

requirements/desirements

are comm on to any

personal

transportation

vehicle/system. These

include

short

transit time/high

speed,

direct portal-to-portal,

privacy and security, constant availability, personal

stowage and a

suitability

fo r use by the "n on-pilot."

The latter

necessitates from

the outset

that

an obvious

(and

probably

attainable) goal should

be an automatic

personal air

transport

system, autom atic

with

respect to

navigation

(e.g.,

references

121-123),

air

traffic

control

and

op eration. The technology to accomp lish this is

either

currently

employed by/for the

long

haul air

transport application, or in the

research/application

pipeline, thanks to the

m icrochip "electronics

revolution" and includes

GPS,

personal and other

communication

satellites

(44

broadband/mobile

communication

satellite systems "on the

books'Vin

progress)

and the

military investments

in

RPV 's,

AA V's, UA V's, UTA's, UCA V's, etc. Such

automatic

operation could

provide

vastly

improved

safety,

as the p reponderance (70 percent to 80 percent)

of

a ir

transport accidents have historically been

due to

"human

error."

In addition, it

makes

personal air

vehicle

transportation

available to the general public,

as

opposed

to the few who

have

the opportunity,

wealth,

and physical character istics to become pilots,

as well as reducing the

unit

cost by an order of

magnitude or

more

due to the concomitant

vast

increases in production ra te/market.

Conventional wisdom holds tha t, to be successful, an

alternative

transportation

system must be not

only

faster,

but

also

relatively

inexpensive. The

costs

involved in any system include a cquisition, operation,

maintenance, and depreciation . To be com petitive with

the

automo bile

a

personal

VT OL -converticar should

have

an acquisition

cost

in the vicinity of a quality

automobile. Although in terms of the

current

helicopter industry, this

is a

ridiculous target,

th e

advantages

of a

production

run of millions

instead

of

American Institute of Aeronautics and Astronautics



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1997

Amer ican

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hundreds, along

with

a

recent offering

of a

single seat

helo

for $30K (references 124 and 125) and a

two-place

"gyroplane" for

$20K,

all at

small

production runs

makes the outlook to

achieve

such a

go al possible

if

no t

probable

(see

reference

126).

Operational costs

include fuel, insurance, parking fees,

etc., and

need

no t

be greater than the

auto.

Maintenance is considerably

greater fo r present

helos

than for autos, and

therefore

this issue

would have to be

addressed

in any personal

helo

technology

development

program.

All-weather operation is also a requirement , the same

all-weather capability one now has in an automobile,

which is by no m eans

absolute.

Extremely heavy rain,

extreme winds, ice and snow

will

all either

slow

or

stop the auto, and similar restrictions will probably

hold

for the personal

helo.

Obviously

the evolving

"detect

and

avoid"

technology could be

utilized

by the

personal

helo

(either on or off board) to increase

safety

vis-a-vis

extreme

weather. In terms of speed and range,

the helo must provide a significant speed advantage or

it is simply n ot viable. As compared t o a fixed wing

personal aircraft, the helo speed

advantage

is

much

less

vis-a-vis th e auto, but at a

nominal

factor o f 4

(for

th e

traffic case) still sufficient

We are

currently spending

significant sums to

gain

a factor of 2+ in the

high

speed civil transport program (vis-a-vis subsonic

transports).

Another

key issue is rider acceptance in

terms of acoustics, vibration, r ide

quality,

and

reliability/safety.

All of these technical

areas

will

require

further

work, although the

helo

community ha s

made significant

strides in

these already

and

considerable further

gains/technological

advances are in

the pipeline. A final major set of issues involve

community acceptance

in

terms

of

acoustics

and

downdrafts during near surface operations. Again, more

work is needed, but th ese could be

addressed

by

operational as

well

as

technological

approaches.

Previous approaches to the

"personal

helicopter" have

mainly considered existing

machines

as opposed to the

advanced

technology/ farther

term

vision discussed

herein

(e.g., references 127-129). There have been,

however,

calls fo r such an approach

(references 130,

131).

Over the years, particularly

since

the 1930's, there have

been suggestions,

and in

some cases

strident

calls,

fo r

the developm ent and marketing o f personal

aircraft

Although

"general aviation" has

made

considerable

advances,

the "aircraft for the

masses" never

really

caught on for a variety of

reasons,

mainly

involving

COST,

requisite techno logy

readiness

and an absolute

requirement that th e "operator" be a pilot e.g.,

non-

automatic

operation.

History

is replete

with

examples

of

concepts

which are

good ideas

and which

keep

resurfacing until

the

technology base

is

ready.

An

obvious example is the gas turbine engine. Since the

last personal aircraft campaign in the

late

40's-50's,

major

strides

have

occurred in

several

enabling

technologies.

These

include

light weight miniature,

inexpensive

and trem endously

capable electronics/

computing,

lightweight

composite

m aterials

with

"nearly infinite" fatigue life, com putational fluid

mechanics, smart-to-brilliant

ma terials/skins, flow

control

of

several

types and

active controls/load

alleviation.

Such

advances significantly change the

personal aircraft discussion,

par ticularly

for the helo.

"The helicopter looks,

35 to 40 years

after

its

invention,

to be

poised

in the po sition t he

fixed

wing

aircraft

were in the

late

40's and

early

50's,

again

40

years

after t he

first

flights

w ere being

made"

(reference

132).

In

particular,

the

personal helicopter would

profit from much

of the sizable investment made in

military

m achine

research,

albeit the civilian

application is in

many

ways less severe in terms of

"rough

usage" etc. This

is

again directly analo gous

to

the fixed wing

situation where the 707

class

of

transport aircraft

profited

immensely from/was enabled

by , the military investments in swept wing/jet

propelled bombers/tankers/transports.

Key helo -specific

technologies

either

available or in the

pipeline include com posite blades

with

10,000

hour

fatigue life, the hingeless-bearingless roto r with low

drag

hub,

automatic health monitoring to allow

significant reductions

in

m aintenance costs, anti-

vibration and anti-noise fo r enhanced rider comfort,

automatic piloting and navigation/nap-of-the-Earth

operation, and composite structure and smart

skins

fo r

flow and load control (see, for example, references 133-

140).

There

are several "system s level" issues and critical

choices

regarding

the

personal aircraft which served

as

key discriminator s in the selection of the

par ticular

personal

aircraft discussed

herein, a

helo-converticar.

The

first

such

issue

is

whether

th e

personal aircraft

(either "fixed" o r rotary wing) should be a

separate

air

vehicle,

or a

"conv erticar," i.e.,

a

combination

autom obile and air

vehicle capable

of

economically

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performing both missions. Economics

and

utility

strongly

favor

the

"converticar" option.

There are

numerous

elements

commo n to both the air and ground

vehicles, such as passenger

com partm ents,

engines,

etc.

and therefore, if it is technically feasible to

reduce

the weight of an auto to

what

is

reasonable

for an air

vehicle, then

a

single

device should be considerably

more economical (initial cost

as

well

as maintenance-

wise) than buying and maintaining tw o

separate

vehicles, part icularly when one considers the present

cost

of autos. Simplex

estimates

of the

flight-specific

component w eights indicate

a

value

of

less than 1000

pounds,

indicating

that,

with shared

utilization

of

common systems such as the engine, the

"all-up"

weight of the

converticar could

be in the

(reasonable)

range

of

3000+

pounds. From an operational

viewpoint,

usage

as well as m aintenance-wise, a single

vehicle

should be

much

more

convenient,

obviating

the

need for a

rent-a-car

in the

vacinity

of

one's

destination. Once the converticar option is

selected,

some decision/recomm endation

has to be made

regarding the prov ision for the

"air-unique"

components,

pa rticularly

th e

lift-producing surfaces

which require, fo r

reasonable levels

of drag-due-to-lift,

non-trivial

span/aspect ratio.

Option s include towed

"trailored" wings (utilized in early versions of the

converticar),

fixed wings of inherently low aspect

ratio

for

readability (reference 141), airport

"rent-a-wing"

concessions

where the

wings

are attached prior to , and

removed

at the conclusion of,

flight,

and

telescoping/folding

wings. The

present

author favors

the telescoping/folding option

as

offering

the

best

comprom ise between

convenience and

performance.

The next critical cho ice is between conventionaV 'fixed

wing" operation and a VTOL device. An essential

difference is that the

fixed wing machine/operation

requires an

airport.

There are

many

thousands of GA

airports in the U.S. and one would have to

begin

and

end the air

portion

of the

trip

at one of

these.

In the

opinion of the

present author, this

is

simply

too

restrictive and contravenes several of the fundamental

purposes of the personal air vehicle

such

as

independence

of/reduced

requirement for large civil

works,

portal-to-portal transportation,

and access to

remote sites

(remote

from roads,

etc.).

The VTOL

option

would

allow

development/usage of

currently undeveloped nations/regions at a fraction of

the

cost

of the roads/bridges, etc. usually required fo r

such development,

and at much

less

disruption to the

environment (reference 142). The estimated off-shore

mark et for such a device is the order of

$.5T/year.

Conversion from ground to

flight

and back again for a

helo-converticar

requires only a

relatively

hard surface

with a

diameter

the order of 25 f t, something which

could be placed at

intervals

along side th e

existing

highway system to provide convenient ground-to-air

"merging" away from existing b uiltup housing areas

to

minim ize acoustic/downdraft etc.,

influences

upon the

population.

Further advantages of the helo include the

provision for both lift an d prop ulsion in a

single

device

during

air

operation

and ATC

"margin"

(in the

event

of

an A TC conflict the vehicles

involved

could

"hover"

or

locally land while the

problem

is addressed/resolved).

Another

major option

involves the extent to which the

operation

in the air

mode

should be automatic as

opposed to

p ilot/human derived. While

sport

mo dels

could be

somewhat

human-controlled (within the

confines of the A TC/safety

regulations)

the optimal

solution

is clear. The

portion

of the

population

physiologically capable

of becoming

pilots

is not

large

and

there is considerable cost and time involved in

doing so, m ost accidents are due to

pilot

error

(reference

129), and the AT C system requires, for the large

numbers ultimately envisaged, autom atic operation.

Therefore,

a

user-orientated personal

air

capab ility

should,

ultimately, be

automatic

in operation as

well

as navigation and ATC, as

already

suggested

herein.

A personal

transportation machine

capable of

both

ground and

(VTOL)

air

operation

could be an

automobile with

an 1C

engine (reference

144),

probably

initially

a

two-seater

and at

least somewhat pilot-

controlled,

which

is light enough to also fly and

which

has

built into

its

roof

a n

erectable

low

drag, large taper

(reference 145) rotatable hub with a diameter consistent

with the

vehicle

width

containing

the order of four o r

mo re telescoping/

folding

rotor

blades.

In addition, a

rear deck vertical fin is

required

w ithin wh ich is a,

perhaps electrically driven, tail

rotor.

Alternative

approaches

include circulation contro l on the

"afterbody" o r a tandem/counter-rotating rotor system.

As stated several times in this

discussion ,

th e central

issue is COST (see the quote from Henry Ford in

reference

146) and

usability.

As a

result

of

technological advances in

several

areas, many of them

momentous, and the tremendous requirement/market fo r

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Co p y r i g h t © 1997 American Inst i tu te o f Ae rona u t ics and A s t ronau t ics Inc.

such an

affordable/user-friendly capability,

the

issue

of

personal air transportation should be revisited. The

probable

course

of developm ent for personal air

transportation is par allel to that of the automo bile in

the early

1900's.

The

initial

m achines were expensive

("rich m an's toys") with many impediments to their

operation such as poor roads, noise

sensitivity

and

laws

which were

in

many cases "anti-automobile." Once

industrialists (e.g., Henry

Ford)

addressed the

problem

via "design to cost/PRICE,"

simplicity

(any color as

long

as

it's black)

and m ass pro duction, the

price

dropped

drastically and the resulting widespread

sales/utilization of the

pro duct revolutionized,

in

many

ways, our

entire society (see also references

147 and

148).

The key to such a mass market/affordability fo r

the

VTOL

converticar is

automatic

operation via a

largely existing infra structure and technology base.

Militarized versions

would

provide the armed

forces

with a

(ro botic/automatic) flying jeep

capable of

meeting the

b attlefield

mobility requirements for the

"Army after next"

(beyond

"Force

21 ).

Such

an

"automatic" VTO L-capable machine would

also

provide, in a "future world" of increasing prevalent tele-

commuting and "electronic cottages" affo rdable/robotic

delivery of requisite food supplies for the carbon-based

inhabitants as

well

as go ods ordered on the

"net'Vshopping channels.

SPACE ACCESS

There are tw o disparate space

access missions with

fundamentally

different

but

overlapping metrics--

inexpensive

transportation of "pounds to orbit" and

"space warfare." The

former m ission

is

vital

to

economical/ viable

space

utilization (environment al

monitoring, navigation, com munications, natural

resource observation,

space

m anufacturing,

astro/solar/planetary physics, planetary exploration

etc.)

and is

currently

being worked by NASA via the

X-33 and X-34 programs. The space warfare m ission

was the subject of the NAS P program of the

late 80's

and early

90's

and the continuing/increasing

requirement for a

military

T ransatmo spheric Vehicle

(TAVy'space

plane"

is clearly

stated

in

several

recent/ongoing

A ir

Force planning

studies

(e.g.,

Spacecast 2020, Air Force 2025, New World V istas).

The

rationale for a military

space plane includes

battle

space

awareness, global precision strike and space

control.

The present writer

suggests

that the

different

metrics/requirements fo r these tw o m issions may

perhaps best be satisfied by

disparate approaches~an

advanced rocket for inexpensive space access and an

advanced

airbreather

which could uniquely provide

the

required flexibility

for the military

space plane

missions. The increased dry w eight/cost

associated

with

airbreathing engines makes th e

hypersonic

airbreathing

option problem atical

for the

pounds

to

orbit"

mission but its capability f or self ferry, enhanced

cross

range,

orbital

plane change, recall,

enhanced

launch window, in-atmospheric abort/assured payload

return,

dispersed launch

sites,

orbit/deorbit/reorbit,

etc.

uniquely

satisfy various space w arfighting metrics (see

references

149-152).

Considering first the

"pounds

to orbit" m ission for

both

civilian an d

military

applications, over th e years a

large number of studies have "mixed and m atched" the

various

multitudinous

technical/technological

space

access options available, fo cusing o n (significantly)

reduced cost—with n o obvious clear winning

combination." The following suggested advanced rocket

involves

emerging

technologies

not yet included in

these previous

systems

studies with the intent of

addressing

the prime

m etric—affo rdabili

ty. The

suggested

technological elements of a space access

advanced

rocket

include

propulsion,

fuel and aero

components.

The

suggested

primary propulsion

cycle

is a

pulse-detonation

wave rocket

engine,

a technology

which is successfully emerging from

several

years of

Phase

2 SBIR research projects. Potential benefits

include lightw eight,

order

of 15

percent

Isp

increase

(constant

volume as opposed to constant

pressure

combustion) and up to a factor of 40 r eduction in

pressure level requirement for the turbine feed pumps

with attendant major reliability

and

cost

benefits

compared to current (i.e., SSME ) practice/requirements

(e.g., references

153-157).

Significant additional thrust

up to the

Mach

6

regime

is ava ilable via a lightweight

exhaust

region

shroud/ejector

designed

using NA SP-

level and

beyond

mixing augm entation

technology.

Estimates indicate

the

addition

of the ejector

could

significantly increase payload fractions

(e.g.,

references

158-161).

The suggested advanced

fuel

is

solid

hydrogen (with imbedded Boron lattice)

under

development

in the Phillips

Laboratory

HEDM

Program (reference

162).

In addition to the expected

densification benefits,

th e Air

Force

estimates up to a

25

percent

fuel

Isp

increase. Finally,

to

obviate aero-

control-dictated ballast and "packaging"

requirements

recourse

could

be made to "designer aerodynam ics"

11

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C opy r i gh t ©

1997

Amer ican Ins t i tu te of Ae ronau t ics and As t ronaut ics

Inc.

wherein dynamic pneumatic vortex generation is

utilized

to provide stable/contro lled flight in lieu of/in

addition to conventional

fins

and

"static

margin"

(e.g.,

reference 163), i.e., application of the NASA/USAF

HARV forebody vortex

control

technology for

fighter

aircraft and the

X-29

(unstable

vehicle)

control

approaches)

to the

(unstable)

rocket control problem.

Optimization of the

base

ejector approach suggests a

"different"

configuration-an

annular/hollow cylindrical

geometry

with

extremely (structural/cost)

efficient

torroidal fuel/oxidizer tanks

and both

"internal"

and

external

air

entrainment into

the

ejector propulsion

stream.

Considering next the m ilitary -specific space-access

requirements,

the

suggested airbreathing

cycles/technologies for the TA V/space warfare mission

have as th eir

core

(up to March=10) the

"conventional"

NAS P approach of

(with increasing

Mach number) low

speed system/RA MJET/diffusive-

burning

scramjet A key

finding from

th e

NASP effort

was

the criticality of the

high

Mach number (M>10)

propulsive

performance.

In

particular,

the internal

losses/penalties

(friction,

heat transfer, shock, mixing,

weight,

disassociation)

of the diffusive

burning

scramjet in the high

Mach

number regime are not

consistent with a viable m achine.

Therefore,

two new

cycles

are suggested for this

M>10

speed

regime.

For 10<M< =15 Langley has begun

working

a

PMSIC

(premixed shock induced

combustion)

approach w hich features forebody

fuel

injection/mixing, greatly reduced com bustor

length/losses/weight and an Isp

increase

the

order

of 25

percent (e.g., references

164-168).

For the "pullup"

into orbit portion

of the

flight path,

a

lox-augmented

scramjet, essentially a

high

Mach number RBCC is

suggested which

utilizes

flow-path em bedded linear

rocket motors

to

pressurize

the combustor/allow

airbreathing to higher altitudes (reference

149)

and

provide shear fo r

enhanced

m ixing and species for

nozzle catalysis. The key to a viable PMSIC engine is

the avo idance of "pre-ignition" upstream of the

combustor in the forebody

premixing

region along

with simultaneous

forebody injection/mixing

optimization

fo r

penetration,

fuel

addition thrust,

injection-induced com pression and forebody skin

friction/heat tr ansfer reduction.

In addition to

these basic cycles, other

technolog ies

could

be employed to

enhance

airbreathing prop ulsive

performance at

high

Mach n umber-thrust vectoring for

trim and

drag

reduction, m ultiple shock inlet, nozzle

flow catalysis to reduce the thrust

loss associated with

species

"freezing,"

fuel

thrust via (highly)

heated

fuel

injection

and forebody boundary layer transition delay

fo r

flow path friction/heat transfer loss

reduction.

With the

exception

of

limited application

of

fuel

heating and tran sition

delay

these

techno logies/cycles

were not employed in the various

NASP design

cycles.

Their utilization should

enable development of a

viable/affordable

"warfighting" space plane. An

additional requirement fo r such a development is a

T&E

engine test

facility

in the Mach 10 to 16

range

with th e

requisite

large size/long run time needed for

hypervelocity engine "certification" in terms of the

interacting

system

elements-combustion,

aero-

integration,

aero-thermal, aero-thermal

elasticity,

materials/ structures, coatings, seals, controls,

operability,

etc. Promising

fa cility research f o r this

requirement is underway at Princeton

University

and

MSB, Inc.

in

Butte, Montana.

The current

best bet

approach

is a combination o f

cold very high

pressure

air

storage/supersonic flow establishment

with

subsequent (serial) energy addition to a m oving stream

via e-beams

(or

MW/lasers)

and

subsequent

MHD

acceleration

(reference 69). The R&D

engine

test

requirements

can be met

viable

a

detonation driver

upgrade to the NAS A H ypulse facility (reference

170).

The

design

driver for

both

of

these

facilities

is the high

total

pressure required to simulate the forebody

(post

bow shock) flow for the PMSIC

class

of

engines

(e.g.,

simulation

o f

"combustor

entrance"

conditions

is not

sufficient).

Our problem in space access is emp hatically n ot a

dearth o f

prom ising technological possibilities,

there

are

many

more

than

m entioned herein. We are

emphatically not up

against physical/physics barriers

in terms of major

performance

improvement

approaches fo r

system w eight

reduction/affordability/flexibility, etc.

CONCLUSIONS

Advanced configuration and "fro ntier" aeronautics is

admittedly long term and high risk (by

definition)

but

constitutes, in the opinion of the author, the only

approach available which

can

seriously address

the

12

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Institute

of Aeronautics and

A stronautics



Co p y r i g h t ©

1997

Am erican Insti tu te o f Aeronaut ics and Ast ronaut ics

Inc.

productivity, cost,

safety and environmental

m etrics

in

a

truly

meaningful

m anner.

1.

Contained

within the discussion herein are

several relatively novel general approaches/concepts.

These

include, for

examp le, transport

aircraft

designed

and optimized in an open thermodynamic system

(utilization of

extensive pro pulsive/ aerodynamic/

structural synergisms) and utilization of

flow

control at

cruise

and otherw ise to accrue full

iiiviscid

performance

benefits.

2. A suggestion is made fo r developm ent of

automatic personal

aircraft

operation (via

GPS,

corns

satellite

constellations and RPV/AAV/UCAV, etc.

technology)

enabling a

large vehicle production

run and

an affo rdable revolution in

personal

mobility (VTOL

"converticar") and

many

aspects of our culture/econom y

($.5T/year

to $lT/year

market).

3. A multitude of

updated/modified

alternative

configurations

a re suggested fo r both subsonic and

supersonic long haul which have

revolutionary

potential. The

wealth

o f available possibilities/

approaches greatly increases

the

probab ility

that one or

more

of these

could "work"

in the

"real wor ld."

The

resource

level

currently expended in the area of

advanced

concepts/configurations

is, and has long

been,

"subcritical"

in

terms

of p roviding adequate evaluations

even of the potential of the ideas

currently

extant.

4. Suggestions are m ade for advanced (in

some

cases revolutionary)

appro aches to space

access-

for both

the

"pounds-to-orbit"

and

space w arfighting

missions.

5. In the opinion of the present

author,

we

should

return t o the invention/development and

utilization o f advanced

concepts

as a method of

working

affordability (along with

the

current design

cycle and manufacturability/"process"

approaches).

CKNOWLEDGEMENT Th e

author

is, and

will

forever be greatly indebted to the many in and out of

aeronautics who

have

had the courage,

foresight,

knowledge and creativity to seriously consider the

farther

term

possibilities

in aviation. Their work is

absolutely crucial to the health and

competitiveness of our industry and national defense

and, increasingly, to the enrichment of the human

experience.

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