aerospace

Review
The U.S. Air Force Next-Generation Air-Refueling System:
A Resurgence of the Blended Wing Body?
Guilherme Fernandes * and Victor Maldonado

Mechanical Engineering Department, Texas Tech University, Lubbock, TX 79409, USA; victor.maldonado@ttu.edu
* Correspondence: gdufflis@ttu.edu

Abstract: The interest in flying wings dates as far as the early years of the aviation age. Early
investigations of the feasibility of the concept demonstrated increased aerodynamic efficiency and
reduced fuel consumption. However, structural, engine integration, and stability and control issues
prevented further development. In the 1990s, a new concept, the blended wing body (BWB), was
created to alleviate some of the concerns of flying wings while maintaining increased efficiency
and adding further benefits, such as reduced pollutant and noise emissions. Despite the promise,
technical hurdles once again proved to be a deal breaker and, as of 2024, the only successful flying
wing is the B-2 Spirit, an extremely complex and expensive aircraft. Nowadays, with the world
quickly transitioning towards cleaner energy, the interest in the BWB has been renewed. The latest
technological advancements in the aerospace industry should make its development more plausible;
however, passenger comfort issues remain. Surprisingly, the BWB development may come from
an unexpected application, as a tanker aircraft. As the U.S. Air Force is seeking a replacement to
hundreds of aging tankers, a startup company was recently funded to develop the concept and
build a prototype. In this study, we explore the history of blended designs from its early days,
highlighting its opportunities and challenges—and why the design is an intriguing fit for application
as a tanker aircraft.

Keywords: USAF; NGAS; tanker; BWB

Citation: Fernandes, G.; Maldonado,

1. Introduction
V. The U.S. Air Force Next-Generation
Air-Refueling System: A Resurgence
1.1. Brief History of Flying Wing Designs
of the Blended Wing Body? Aerospace It is appropriate to state that, in the early days of aviation, configurations resembled
2024, 11, 494. https://doi.org/ neither the dominant tube and wing nor flying wing configurations, with most aircraft
10.3390/aerospace11060494 featuring a wooden structure and fabric wings. The term flying wing is adequate to describe
tailless aircraft that have no distinct fuselage, carrying its crew, payload, and fuel inside
Academic Editor: Dimitri Mavris
the wing structure. The first aircraft with a design feature resembling a flying wing was
Received: 1 May 2024 the Dunne D-8 (1911), a tailless biplane featuring swept wings and washout to prevent
Revised: 12 June 2024 tip stall and increase pitch stability [1]. In the 1920s, a series of tailless aircraft, known as
Accepted: 17 June 2024 Westland–Hill pterodactyl, demonstrated looping and rolling capabilities [2].
Published: 20 June 2024 The semi flying wing, developed by Jack Northrop in 1928, had no fuselage but still
featured vertical stabilizers [3]. Intrigued by the aerodynamic benefits of fewer non-lifting
surfaces, Northrop’s developments led to a pure flying wing in 1940, the N-1M “Jeep”. The
design, shown in Figure 1, showed decent handling ability [4], demonstrating the potential
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
of the flying wing configuration. However, it also suffered from engine overheating due
This article is an open access article
to its placement within the airfoil. Still, the promise prompted the USAF to pursue the
distributed under the terms and
development of a flying wing bomber, the Northrop N-9M (1942).
conditions of the Creative Commons Parallel to the developments by Northrop in the 1930s, the Horten brothers worked on
Attribution (CC BY) license (https:// a flying wing concept in Germany. The first aircraft to fulfill the definition of flying wing
creativecommons.org/licenses/by/ was the Horten I glider (1933). After a requirement for a heavy bomber by the Luftwaffe in
4.0/). 1943, the Horten brothers designed the Ho 229 (Figure 2).

Aerospace 2024, 11, 494. https://doi.org/10.3390/aerospace11060494 https://www.mdpi.com/journal/aerospace

Aerospace 2024,
Aerospace 2024, 11,
11, 494
x FOR PEER REVIEW 22of
of 23
23

Figure 1. Northrop N-1M “Jeep” flying wing (Courtesy of Smithsonian National Air and Space Mu-
seum. Available at https://airandspace.si.edu/collection-media/NASM-NASM2015-04014 (accessed
on 25 May 2024).

Parallel to the developments by Northrop in the 1930s, the Horten brothers worked
Figure 1. Northrop
on a flying wingN-1M “Jeep”
concept flying wing The
in Germany. (Courtesy of Smithsonian
first aircraft National
to fulfill Air and Space
the definition Mu-
of flying
Figure 1. Northrop N-1M “Jeep” flying wing (Courtesy of Smithsonian National Air and Space Mu-
seum. Available at https://airandspace.si.edu/collection-media/NASM-NASM2015-04014 (accessed
wing Available
seum. was the Horten I glider (1933). After a requirement for a heavy bomber by (accessed
at https://airandspace.si.edu/collection-media/NASM-NASM2015-04014 the Luft-
on 25 May 2024).
waffe
on in 1943,
25 May 2024).the Horten brothers designed the Ho 229 (Figure 2).
Parallel to the developments by Northrop in the 1930s, the Horten brothers worked
on a flying wing concept in Germany. The first aircraft to fulfill the definition of flying
wing was the Horten I glider (1933). After a requirement for a heavy bomber by the Luft-
waffe in 1943, the Horten brothers designed the Ho 229 (Figure 2).

Figure2.2.The
Figure TheHo
Ho229
229in
inflight
flight(public
(publicdomain).
domain).

The flying
The flying wingwing configuration
configuration was was favored
favored by by thethe Horten
Horten brothers
brothers duedue toto its
its high
high
aerodynamic efficiency.
aerodynamic efficiency. The The design
design was was also
also thethe first
first flying
flying wing
wing powered
powered by by turbojet
turbojet
engines. ItIt featured
engines. featured flaps,
flaps, elevons,
elevons, and and dragdrag rudders
rudders for for control
control and and stability.
stability. The
The design,
design,
Figure 2. The Ho 229 in flight (public domain).
described as
described as “decades
“decades ahead ahead of of its
its time”
time” [5],[5], first
first flew
flew ininthe
thefinal
finalmonths
monthsof ofthe
thesecond
second
World
World War II in 1945. However, after a fatal accident with its test pilot, the aircraftnever
War II in 1945. However, after a fatal accident with its test pilot, the aircraft never
The flying wing configuration was favored by the Horten brothers due to its high
entered
enteredserial
serialproduction
production[6]. [6].
aerodynamic efficiency. The design was also the first flying wing powered by turbojet
Following
Following the end of of the
thewar,
war,prototypes
prototypeswere were sent
sent to to
thethe
U.S. U.S.
for for evaluation
evaluation [7],
[7], and
engines. It featured flaps, elevons, and drag rudders for control and stability. The design,
and the German
the German designs
designs werewere extensively
extensively studied
studied by American
by American engineers,
engineers, which
which likely likely
con-
described as “decades ahead of its time” [5], first flew in the final months of the second
contributed
tributed to the to the further
further developments
developments by Northrop.
by Northrop. The YB-35,
The YB-35, built asbuilt as a scaled-up
a scaled-up version
World War II in 1945. However, after a fatal accident with its test pilot, the aircraft never
version of the N-1M, first flew in 1945. The YB-35 was powered
of the N-1M, first flew in 1945. The YB-35 was powered by four piston engines, each acti- by four piston engines,
entered
each serial production
activating [6].
vating a pair ofacontra-rotating
pair of contra-rotatingpropellers. propellers.
Some ofSome of the airframes
the airframes were converted
were converted to carry
to Following
carry early jetthe end
engines of the designated
and war, prototypes as were sent to the U.S. for evaluation [7], and
YB-49.
early jet engines and designated as YB-49.
the German
At designs were extensively from studied byhandling
American engineers, which likely con-
At the
the time,
time, thetheaircraft
aircraftsuffered
suffered frompoor poor handlingand andengine
engineproblems,
problems,and andthe the
tributed towas
program the further developments inby Northrop. The YB-35, built [8].
as a scaled-up version
program was ultimately cancelled in
ultimately cancelled favor
favor ofofthe
theB-36
B-36 program
program [8]. UpUp to that
to that point,
point, de-
of the N-1M, firstthis
development flew in 1945. The design YB-35 was waspowered by four piston engines, each acti-
velopment ofofthis revolutionary
revolutionary design was constrained
constrained to
to military
military applications—like
applications—like
vating
most a pair of contra-rotating propellers. Some of the airframes were converted to carry
most cutting-edge
cutting-edge inventions
inventions in in the
theaerospace
aerospaceindustry.
industry.The TheU.S.
U.S. AirForce
Air Force also
also shifted
shifted its
early jet engines and designated as YB-49.
its focus towards supersonic aircraft in the early 1950s. Nevertheless, the lessons learned
At impacted
greatly the time, the thedevelopment
aircraft suffered of the fromfirstpoor
flyinghandling and engine
wing to enter problems, decades
serial production and the
program was ultimately
later, the Northrop Grumman cancelled in favor of the B-36 program [8]. Up to that point, de-
B-2 spirit.
velopment of this revolutionary design was constrained to military applications—like
most cutting-edge inventions in the aerospace industry. The U.S. Air Force also shifted its
Aerospace 2024, 11, x FOR PEER REVIEW 3 of 23

Aerospace 2024, 11, 494 focus towards supersonic aircraft in the early 1950s. Nevertheless, the lessons learned 3 of 23
greatly impacted the development of the first flying wing to enter serial production dec-
ades later, the Northrop Grumman B-2 spirit.
Between 1950
Between 1950 and
andthe theearly
early1980s,
1980s,development
developmentof offlying
flyingwings wingswas wasdormant,
dormant,savesave
for
forone
onedevelopment.
development. After
After WW2,
WW2, Reimar
Reimar Horten
Horten immigrated
immigrated to toArgentina
Argentinaand andwas
wasthethe
program
programdirector
directorofofthe
theFMAFMAI.Ae I.Ae38,38,aa four-engine
four-engineexperimental
experimentaltailless taillesstransport
transportaircraft
aircraft
based
basedon onHorten’s
Horten’sprevious
previousdesigns.
designs. TheThe IA IA 38
38 suffered
suffered from fromissuesissuesshared
sharedby byprevious
previous
flying wing aircraft; it proved to be difficult to control and was
flying wing aircraft; it proved to be difficult to control and was underpowered, resultingunderpowered, resulting
in
inpoor
poorperformance,
performance,while whilethetheengines
enginesalso alsosuffered
sufferedfrom fromoverheating.
overheating.The Theprogram
programwas was
ultimately
ultimatelycancelled
cancelledin in1962
1962[9,10].
[9,10].
The
The advanced
advanced technology bomber bomber (ATB) (ATB)program
programstarted
startedinin1979 1979and andculminated
culminated in
in thedevelopment
the developmentofofthe thestealth
stealthbomber,
bomber,which which first
first flew
flew in in 1989.
1989. TheThe development
development of of
stealth
stealthcapabilities,
capabilities, along
along with
with thethe technical
technical challenges
challenges of of the
the flying
flying wing
wing configuration,
configuration,
resulted
resulted in development costs of USD 23 billion in 2021 money [11]. ForFor
in development costs of USD 23 billion in 2021 money [11]. comparison,
comparison, de-
development
velopment costs for stealth fighter aircraft were around USD 44 billion the F-22 [12]
costs for stealth fighter aircraft were around USD 44 billion the F-22 [12]and
and
the
theUSD
USD74 74billion
billion F-35
F-35 [13],
[13], both
both in in 2021
2021 money.
money. Unlike
Unlike in in the
the F-22
F-22 and
and F-35
F-35 programs,
programs,
development
developmentcosts costsper
peraircraft
aircraftwere
wereexacerbated
exacerbateddue due to
to the
the limited
limited number
number of of orders
orders (21).
(21).
Despite
Despite the high cost, the three-decade operational success and strategic naturethe
the high cost, the three-decade operational success and strategic nature of of
B-2 prompted
the B-2 prompted the U.S. Air Force
the U.S. Air Forceto procure
to procurethe development
the development of itsofflying wing
its flying successor,
wing succes-
the
sor,B-21 Raider.
the B-21 As of
Raider. AsFebruary
of February 2024,
2024, thethe
B-21B-21(Figure
(Figure 3) 3)is isbeing
beingflight
flighttested
testedand
anditsits
scheduled entry into service is in 2027
scheduled entry into service is in 2027 [14,15]. [14,15].

Figure3.3.B-21
Figure B-21Raider
Raiderstealth
stealthbomber.
bomber.Courtesy
Courtesyof
ofthe
theU.S.
U.S.Air
AirForce.
Force.

Othernotable
Other notablerecent
recentflying
flyingwing
wingdesigns
designsinclude
includeseveral
severalunmanned
unmannedaerial aerialvehicles
vehicles
suchas
such asthe
theNorthrop
NorthropGrumman
GrummanX-47B,X-47B,the theLockheed
LockheedMartin
MartinRQ-170
RQ-170sentinel,
sentinel,thetheRussian
Russian
SukhoiS-70
Sukhoi S-70Okhtonik,
Okhtonik,and andthe
theIndian
IndianDRDODRDOGhatak.
Ghatak.China
Chinaisisallegedly
allegedlyalso
alsodeveloping
developing
aastealth
stealth flying
flying wing
wing bomber,
bomber, known
known as Xi’an H-20. No Nopublic
publicdetails
detailsare
areavailable,
available,but
buta
aPLAAF
PLAAFrecruiting
recruitingvideo
videoshows
shows the
thesilhouette
silhouetteof of
a flying wing
a flying wingbomber
bomber similar to the
similar B-2
to the
[16].
B-2 [16].
Shortly before
Shortly before the
thefirst
firstflight
flightof
ofthe
theB-2,
B-2,when
whenNASANASAprompted
promptedthe the“renaissance
“renaissanceof of
long
longhaul
haultransport”
transport”in in1988
1988[17],
[17],industry
industryand and academia
academia scrambled
scrambledto to develop
develop aa new
new con-
con-
cept.
cept.The
Theresulting
resultingblended
blendedwing wingbody
body(BWB)
(BWB)configuration,
configuration,which
whichfeatures
featuresaadistinct,
distinct,lift
lift
generating
generatingcenterbody
centerbodyandandconventional
conventionalwings,
wings,resulted
resultedininhuge
hugeimprovements
improvementsregarding
regarding
fuel
fuel burn, takeoff weight,
burn, takeoff weight,pollutant
pollutantemissions,
emissions, andand installed
installed thrust
thrust when
when compared
compared to
to con-
conventional tube and wing aircraft [18]. The compact design provides
ventional tube and wing aircraft [18]. The compact design provides structural, structural, aerody-
namic, and payload synergy [19]. A flight test model for one of the earliest designs, the
X-48C, is shown in Figure 4.
Aerospace 2024, 11, x FOR PEER REVIEW 4 of 23

Aerospace 2024, 11, 494 4 of 23

aerodynamic, and payload synergy [19]. A flight test model for one of the earliest designs,
the X-48C, is shown in Figure 4.

Figure4.4.X-48C
Figure X-48Cflight
flighttest
testmodel
model(Courtesy
(Courtesyof
ofNASA).
NASA).

Projectsfeaturing
Projects featuringBWBBWBconfigurations
configurationswere weredeveloped
developed mostlymostly in inthe
theU.S,
U.S,especially
especially
regardingthe
regarding themain
mainproject
projectby byMcDonnel
McDonnelDouglas/Boeing,
Douglas/Boeing, the the “Silent
“Silent Aircraft
Aircraft Initiative”
Initiative”
and the
and the NASA
NASA“Quiet “Quietgreen
greentransport”
transport”project.
project. Development
Development also also happened
happened in in other
other
countriesto
countries toaalesser
lesserextent,
extent,most
mostnotably
notablythrough
throughthe theMOB,
MOB,VELA,
VELA,SAX-40,
SAX-40,NACRE,
NACRE,and and
AFCA projects (European Union/United Kingdom) and the
AFCA projects (European Union/United Kingdom) and the TsAGI institute (Russia). TsAGI institute (Russia).
TheBWB
The BWBdesign
designalso
alsofaced
faced a number
a number of of technical
technical challenges,
challenges, namely
namely structural
structural de-
design
and
signmanufacturing,
and manufacturing, as wellas as stability
well and control
as stability issues,
and control bothboth
issues, of which also also
of which existed for
existed
the
for early flying
the early wing
flying designs.
wing Additionally,
designs. Additionally,for for
a long
a long haulhaul
transport
transportaircraft, passenger
aircraft, passen-
comfort
ger comfortwas also
was an issue
also an [20].
issueOverall, despitedespite
[20]. Overall, the promise, the complexity
the promise, of the design
the complexity of the
and lackand
design of further
lack ofinterest
furtherby the aviation
interest by theindustry
aviationprevented the conceptthe
industry prevented from becoming
concept from
abecoming
commercial product.
a commercial product.
In
In recent
recent years,
years, as
as the
the world
world hashas transitioned
transitioned towards
towards cleaner
cleaner energy,
energy, regulatory
regulatory
agents
agentshave
havebeenbeenpushing
pushingfor for more
more energy
energy efficient
efficient alternatives
alternatives suchsuch asas electrified
electrified aircraft.
aircraft.
These
Thesetrends
trendsrenewed
renewed thethe
interest in alternative
interest designs
in alternative such as
designs the BWB.
such as theAmerican startup
BWB. American
company
startup company JetZero has been working on this design for a commercial aircraftyears,
JetZero has been working on this design for a commercial aircraft for a few for a
but
fewayears,
new opportunity prompted aprompted
but a new opportunity shift in thea design
shift inpriorities.
the design priorities.
1.2. The USAF NGAS Program
1.2. The USAF NGAS Program
The U.S. Air Force (USAF) is currently seeking replacements for its aging tanker fleet,
The U.S. Air Force (USAF) is currently seeking replacements for its aging tanker fleet,
consisting of more than 460 aircraft as of February 2024. In 2023, the USAF launched the
consisting of more than 460 aircraft as of February 2024. In 2023, the USAF launched the
Next-Generation Air-Refueling system program through a request for further information to
Next-Generation Air-Refueling system program through a request for further information
industry [21,22]. The program, formerly known as KC-Z, emphasizes aircraft survivability
to industry [21,22]. The program, formerly known as KC-Z, emphasizes aircraft surviva-
in contested environments, as well as the capacity to operate from unprepared airfields.
bility in contested environments, as well as the capacity to operate from unprepared air-
However, other mission requirements are not specified.
fields. However, other mission requirements are not specified.
Shortly after, American company JetZero revealed its BWB tanker concept, allegedly
Shortly after, American company JetZero revealed its BWB tanker concept, allegedly
capable of carrying twice as much fuel while requiring half the power of the KC-46 Pega-
capable of carrying twice as much fuel while requiring half the power of the KC-46 Pega-
sus [23], the most modern tanker currently employed by the USAF. JetZero was ultimately
sus [23], the most modern tanker currently employed by the USAF. JetZero was ultimately
awarded a USD 235 million contract to build the first prototype of the tanker and partnered
awarded
with a USD
Northrop 235 million
Grumman, contractwith
the company to build the experience
the most first prototype of the
on flying tanker
wings, and
to build
the concept [24]. An artist impression of the tanker BWB is shown in Figure 5.
 partnered with Northrop Grumman, the company with the most experience on flyin
Aerospace 2024, 11, 494 wings, to build the concept [24]. An artist impression of the tanker BWB is shown
5 of 23in Figur
5.

Figure5.5.Artist
Figure Artist impression
impression of aof a blended
blended wingwing
body body
tankertanker
aircraftaircraft (Courtesy
(Courtesy of JetZero).
of JetZero).

The
Themission
mission requirements
requirements for the
forNGAS have not
the NGAS been
have specified
not or published,
been specified but it is
or published, but
reasonable to assume the aircraft will be sized to match the performance of tanker
is reasonable to assume the aircraft will be sized to match the performance of tanker air aircraft
currently in service with U.S. Air Force. For instance, the KC-46 Pegasus is designed to
craft currently in service with U.S. Air Force. For instance, the KC-46 Pegasus is designe
cruise at Mach 0.80 and 35,000 ft for 7350 nautical miles with a speculated fuel payload of
to cruise
120,000 lbsat Mach
[25], 0.80the
though and 35,000
range andftpayload
for 7350are
nautical
heavilymiles with a speculated
mission-dependent. fuel payloa
The new
aircraft configuration allows further design flexibility a priori, but the mission requirements The new
of 120,000 lbs [25], though the range and payload are heavily mission-dependent.
aircraft
are configuration
ultimately allows
set by the U.S. further design flexibility a priori, but the mission require
Air Force.
ments are ultimately set by the U.S. Air Force.
1.3. Structure of the Present Work
1.3. In this review
Structure of thepaper, a number
Present Work of aspects of the BWB design are explored in the
following manner. Research developments are presented in chronological order, separated
In this review paper, a number of aspects of the BWB design are explored in the fo
by major design areas, highlighting the early associated advantages or challenges. Each
section is manner.
lowing Research
complemented developments
by latest are presented
research developments in chronological
and aerospace order,
technologies thatseparate
by major
help address design
these,areas,
as wellhighlighting
as how thesethe early associated
potentials/issues advantages
uniquely or the
apply to challenges.
BWB Eac
section is complemented by latest research developments and aerospace
tanker design. The design of the BWB with regards to its numerous features is a highly technologies tha
help address
coupled process,these,
and anas wellwas
effort as made
how to these potentials/issues
separate these as much asuniquely
possible.apply to the BW
However,
some
tanker of the information
design. presented
The design in all
of the BWB sections
withwill point to other
regards design aspects.
its numerous features is a highl
coupled process, and an effort was made to separate these as much as possible. However
2. Potentials and Challenges of the Blended Wing Body Design
some of the information presented in all sections will point to other design aspects.
2.1. Aerodynamic Considerations
During a great portion of the first century of aviation, industry efforts focused on
2. Potentials and Challenges of the Blended Wing Body Design
efficiency improvements of the classical tube-and-wing configuration. However, with a
2.1. Aerodynamic
growing demand forConsiderations
environmentally friendly and aerodynamically efficient aircraft which
could During
carry a large number
a great portion of passengers overcentury
of the first a long range and having
of aviation, reach the
industry limitsfocused
efforts of o
conventional design, NASA prompted [17] studies that renewed the
efficiency improvements of the classical tube-and-wing configuration. However, with interest in alternative
aircraft configurations.
growing demand for environmentally friendly and aerodynamically efficient aircraf
The first major blended wing body design study, by Liebeck [26], focused on the design
which could carry
of a commercial a large
airliner number
to carry of passengers
800 passengers over aover
7000anm
long range
range at aand having
cruise Machreach th
limits of
number of conventional design, NASA
0.85 and was projected to enterprompted
service in 2020.[17] studies that
Due to the renewed
blended the of
nature interest i
alternative aircraft configurations.
the airframe and lower wetted area, the design featured increased aerodynamic efficiency,
Theinfirst
resulting a 20%major
higherblended wingratio,
lift-to-drag body 27% design
lowerstudy, by Liebeck
fuel burn, and total[26], focused
installed on the de
thrust.
However,
sign of a the aspect ratio
commercial was low,
airliner resulting
to carry 800inpassengers
faster induced
overdrag increase
a 7000 with lift
nm range at a cruis
coefficient,
Mach number generating a very
of 0.85 and lowwasoptimum
projected lifttocoefficient [27]. The
enter service cross-sectional
in 2020. Due to thearea was natur
blended
uniformly distributed among the span to minimize wave drag [28,29].
of the airframe and lower wetted area, the design featured increased aerodynamic effi
The lack of horizontal and vertical stabilizers also reduced corresponding friction and
ciency, resulting in a 20% higher lift-to-drag ratio, 27% lower fuel burn, and total installe
induced drag penalty, further increasing aerodynamic efficiency [30]. Through the use
thrust. However, the aspect ratio was low, resulting in faster induced drag increase wit
Aerospace 2024, 11, 494 6 of 23

of (then) advanced composite materials in the fuselage, the takeoff weight (TOW) was
reduced by 15% and the operating empty weight (OEW) by 12%, compared to similarly
sized conventional aircraft.
The development of the BWB concept by the same team resulted in a series of follow-up
studies [18,28,31], which culminated in the development of the Boeing BWB-450 commercial
aircraft concept. The model underwent successful, full-scale Reynolds number wind-tunnel
testing, with excellent agreement with simulations regarding aerodynamic characteristics.
In a collaboration between NASA and Stanford University, the BW-17 radio-controlled
model was built, which demonstrated good handling qualities despite the lack of a vertical
tail. This comprised the first wave of BWB studies, which happened in the United States
between the 1990s and early 2000s and consisted of NASA-Industry-Academia collaborations.
The BWB is a highly integrated aircraft configuration, with strong coupling between
aerodynamic design, structural design and manufacturing, stability and controls, perfor-
mance, and passenger comfort. The early tradeoffs pointed to the necessity of multidis-
ciplinary optimization, where the focal point of the design (i.e., optimal aerodynamic
performance) was achieved at a compromise of other design characteristics.
The superior aerodynamic qualities were indeed the driving aspects behind the de-
velopment of the BWB configuration over conventional aircraft. Roughly a decade after
the beginning of developments in the U.S., parallel research studies conducted in the
EU and UK elaborated on superior aerodynamic performance and weight savings of the
configuration through design optimization algorithms.
As early as 2000, a study [32] at Cranfield University proposed a baseline BWB similar
in configuration to the Boeing BWB-450. Subsequent efforts focused on the design optimiza-
tion, such as the MOB—multidisciplinary optimization of a blended wing body [33,34],
which integrated aerodynamic, structural, aeroelastic, and flight mechanics modelling
through low and high-fidelity analysis tools. The algorithm allows for the optimization of
range for a constant MTOW, which depends on the lift-to-drag ratio and structure weight.
The VELA projects [35], which focused on the development of a large passenger aircraft
with requirements similar to [26], reported a 10% lower TOW and 4–8% improvement in
aerodynamic efficiency [27]. A study by TsAGI, in Russia, presented an optimal blended
configuration with a lift-to-drag ratio of 25 at Mach 0.85 [30]. Lower total drag and
improved aerodynamic efficiency were also reported in further studies [36,37].
Design constraints included a higher thickness-to-chord ratio to accommodate for
passengers and cargo, as well as a cruise deck angle of less than 3 degrees for passen-
ger comfort—a higher angle would result in inadmissible reduction of the passenger
cabin [30]. The cruise deck angle requirement demands the use of positive aft-cambered
airfoils [38,39]. This generates a nose-down pitching moment, however, increases the trim
requirement [18,40]. To further illustrate the highly coupled nature of the BWB design, it is
worth mentioning that sweep and trim are commonly used to set control surface deflections
and minimize trim drag, also affecting deck angle requirements [41].
Usually, aircraft design strives for an elliptical lift distribution for optimal aerodynamic
efficiency [42]. However, the BWB is aimed to operate in the transonic regime, where wave
drag, rather than induced drag, is dominant. A research study [36] found that the elliptical
lift distribution creates a strong shock at the outer wing due to local lift, resulting in
increased wave drag and reducing aerodynamic performance.
In a further research study, the same authors proposed an averaged elliptical/triangular
distribution [43] (shown in Figure 6) to reduce shock strength while maintaining superior
aerodynamic performance. A purely triangular distribution is preferable if the design goal
is to minimize wing bending loads. A moderately loaded outer wing [18] is also useful to
optimize wetted area while reducing the wing bending moment and thus structural weight.
Aerospace 2024, 11, x FOR PEER REVIEW 7 of 23

design goal is to minimize wing bending loads. A moderately loaded outer wing [18] is
Aerospace 2024, 11, 494 also useful to optimize wetted area while reducing the wing bending moment and thus
7 of 23
structural weight.

Figure
Figure 6. 6. Different
Different design
design liftlift distributions
distributions forfor a BWB
a BWB design.
design. Reprinted
Reprinted from
from [43],
[43], with
with permission
permission
from Elsevier.
from Elsevier.

A Alowlowwing
wing loading
loading configuration
configurationcontributes
contributes to reduced
to reduced take-off and landing
take-off speeds,
and landing
resulting
speeds, in decreased
resulting required
in decreased requiredfield field
lengths [20] [20]
lengths andand improving
improving the the
aircraft’s ability
aircraft’s abil-to
ityoperate from
to operate unprepared
from unprepared airfields.
airfields.If the engines
If the enginesareare
mounted
mounted onontoptopof of
the fuselage,
the fuselage, the
theinlets areare
inlets also protected
also protected from
from foreign
foreign object debris
object debris(FOD)
(FOD) byby thethe
fuselage.
fuselage. These
Theseaspects
as-
match
pects the requirements
match the requirements put put
forthforthby the
by USAF,
the USAF,as discussed
as discussed in Section 1.2. 1.2.
in Section
Additionally, low wing loading enables a higher rate of climb. This, inin
Additionally, low wing loading enables a higher rate of climb. This, turn,
turn, reduces
reduces
the necessary airspeed to generate supplementary lift for ascending
the necessary airspeed to generate supplementary lift for ascending to higher altitudes. to higher altitudes. Fur-
Furthermore, low wing loading augments the sustained turn capabilities of a BWB aircraftby
thermore, low wing loading augments the sustained turn capabilities of a BWB aircraft
byenabling
enablingitittotoproduce
producegreater
greater liftlift
forfor
a given
a givenengine
engine thrust
thrust when
when compared
compared to conventional
to conven-
tube-and-wing aircraft configurations.
tional tube-and-wing aircraft configurations.
The
The outboardwing
outboard wingisishighly
highlyloaded,
loaded,and andoutboard
outboard slatsslats are
are necessary
necessary for for stall
stall protec-
pro-
tion [18]. Outer wing flow remains attached, as well as in the
tection [18]. Outer wing flow remains attached, as well as in the centerbody region, due centerbody region, due toto
significant lateral flow relieving compressibility effects. Ideally, the stall should begin inin
significant lateral flow relieving compressibility effects. Ideally, the stall should begin
thethe centerbody
centerbody oror
inin
thethe
kinkkink region,
region, keeping
keeping the
the ailerons
ailerons effective
effective andand avoiding
avoiding pitch-up
pitch-up
moment during stall [18,28].
moment during stall [18,28].
In a related investigation [44], the aerodynamic impact of sweep angle variations
In a related investigation [44], the aerodynamic impact of sweep angle variations on
on a BWB aircraft featuring constant twist and airfoil sections was exploded. The study
a BWB aircraft featuring constant twist and airfoil sections was exploded. The study in-
involved adjusting the leading-edge sweep angle of the outer wing from 40◦ (forward
volved adjusting the leading-edge sweep angle of the outer wing from 40° (forward
sweep) to 55◦ (backward sweep). The results indicated that forward sweep mitigates tip
sweep) to 55° (backward sweep). The results indicated that forward sweep mitigates tip
stall tendencies but concurrently elevates wave drag, resulting in a diminished lift-to-drag
stall tendencies but concurrently elevates wave drag, resulting in a diminished lift-to-drag
ratio. Conversely, aft sweep generates a nose-down pitching moment, thereby enhancing
ratio. Conversely, aft sweep generates a nose-down pitching moment, thereby enhancing
longitudinal stability, albeit at the expense of increased wave drag.
longitudinal stability, albeit at the expense of increased◦ wave ◦drag.
Varying the sweep angle within the range of 20 to 40 notably enhances the lift-to-
Varying the sweep angle within the range of 20° to 40° notably enhances the lift-to-
drag ratio by up to 80% at the optimized sweep angle of 38.6◦ , primarily attributable to
drag ratio by up to 80% at the optimized sweep angle of 38.6°, primarily attributable to a
a substantial reduction in wave drag. However, further increments in aft-sweep angle
substantial reduction in
led to diminishing wave drag.
returns, reducing However, further increments
aerodynamic efficiency and in aft-sweep
requiringangle led
structural
toreinforcement
diminishing returns, reducing aerodynamic efficiency
to withstand elevating heightened bending moments and stress.and requiring structural rein-
forcement to withstand
Moreover, elevating
a significant heightened
aft-sweep anglebending
results in moments and stress.of aircraft weight
the displacement
and center of gravity, resulting in adverse longitudinal moments and subsequent aug-
mented trim drag. The pitching performance, as well as other control and stability charac-
teristics, are explored in detail in Section 2.3.
Aerospace 2024, 11, 494 8 of 23

Aerodynamic Considerations in Light of the NGAS Program

Improved aerodynamic efficiency is a desirable trait in modern aviation. Only in
rare circumstances it is not the main driving factor behind aircraft design (i.e., for military
applications when speed, maneuverability or stealth are prioritized). The next generation
tanker has mission requirements that closely match the performance of current commercial
aircraft regarding everything but its payload.
With that in mind, just as in commercial aircraft, the superior aerodynamics of the
BWB serves a tanker aircraft well. A recent research study [25] compared the blended wing
body tanker to a conventional tube and wing aircraft currently in service (Boeing KC-46
Pegasus). The blended wing body has a better lift-to-drag ratio and significantly lower
takeoff weight (28%), fulfilling the same mission while requiring the thrust installed in
narrow body aircraft such as the A320 Neo.
The efficiency of the BWB compared to conventional aircraft raises questions as to why
the BWB has not become a commercial product thus far. This is likely related to the mission
requirements of the projects developed in the 1990s; all major projects in the USA, EU,
Russia, or UK required the aircraft to be sized as a long haul, very high-capacity transport.
Passenger comfort and emergency egress were also key issues. These will be addressed in
Section 2.5.3.
The current landscape of aviation shows a preference for twin-engine, shorter capacity
aircraft over multi engine designs such as the Boeing 747 or Airbus A380. This trend
was allowed by the development of reliable turbofan engines featuring higher thrust-to-
weight ratios and lower specific fuel consumption, decreasing operational and maintenance
costs. In the early stages of BWB development, it was speculated that conventional air-
craft would be gradually retired as the BWB entered commercial operations. Despite the
aerodynamic advantages, the sizing of the aircraft at the time aimed at the succession of
the 747/A3XX [30]. The industry ultimately moved in the opposite direction, disproving
that claim.

2.2. Structure and Manufacturing

The BWB configuration offers efficient payload distribution and the possibility of
over-the-wing engine placement. The centerbody generates lift due to its low aspect
ratio, thereby mitigating wing loading. These characteristics serve to minimize wing
bending moment and shear forces, resulting in favorable inertia relief and consequently
reduced structural weight [45,46]. Additionally, the integration of the fuselage and outer
wings reduces the overall wetted surface area, leading to a higher wetted aspect ratio and,
consequently, a structurally more efficient wing design [47].
The structural design for the earliest versions of the BWB consisted of upper and
lower surface panels, rounded leading edge functioning as spar, rear main spar, and outer
ribs [18,28], which proved to weigh more than a conventional fuselage. The structure of
the outer wing section was similar to that of the wing in a conventional tube and wing
aircraft. The inner section, which contains the passenger cabin, must be designed with
pressurization and wing bending loads as requirements.
Unlike in a conventional aircraft, where the two loads are separately supported by
the fuselage and wing, respectively, the inner section is responsible for both loads simulta-
neously. Taking pressure loads in a non-cylindrical vessel presents an enormous design
challenge due to non-linear stresses which could result in severe deformations under
extreme maneuvers or gusts [48–50].
The initial structural concept [31] utilized a thin, arched pressure vessel located above
and below each cabin. In this configuration, the pressure vessel skin takes the load in
tension independently of the wing skin. Alternatively, the second concept employed a thick
sandwich structure for both the upper and lower wing surfaces, with both cabin pressure
loads and wing bending loads supported by the sandwich structure.
A potential safety concern arises with the separate arched pressure vessel concept. In
the event of a rupture in the thin arched skin, the cabin pressure would need to be sustained
 and below each cabin. In this configuration, the pressure vessel skin takes the load in ten-
sion independently of the wing skin. Alternatively, the second concept employed a thick
sandwich structure for both the upper and lower wing surfaces, with both cabin pressure
loads and wing bending loads supported by the sandwich structure.
Aerospace 2024, 11, 494 A potential safety concern arises with the separate arched pressure vessel concept. 9 of In
23
the event of a rupture in the thin arched skin, the cabin pressure would need to be sus-
tained by the wing skin, necessitating that the wing skin be sized to accommodate the
pressure
by load.
the wing Consequently,
skin, necessitating once
thatthe
thewing
wing skin
skinis be
dimensioned to meet this requirement,
sized to accommodate the pressure
the inner pressure vessel theoretically becomes redundant.
load. Consequently, once the wing skin is dimensioned to meet this requirement, the inner
The vessel
pressure coupling of both pressure
theoretically becomesand wing bending loads is also concerning as far as
redundant.
fatigue
Theiscoupling
concerned. of Since the pressurized
both pressure and wing cabin faces its
bending design
loads loadconcerning
is also on every flight [28],
as far as
the BWB
fatigue is centerbody must the
concerned. Since be designed
pressurized with thatfaces
cabin requirement
its design in load
mind, onfurther adding
every flight to
[28],
the BWB
the weight of the fuselage.
centerbody must beAlso, deformation
designed with that of the aerodynamic
requirement in mind,surface
further could
addingsignifi-
to the
cantly affect
weight of thethe aerodynamic
fuselage. advantages of
Also, deformation of the
the blended
aerodynamicdesignsurface
[48]. Preliminary calcula-
could significantly
tions the
affect [30]aerodynamic
have shownadvantages
that ratherof thick upper and
the blended lower
design panels
[48]. would calculations
Preliminary be necessary to
[30]
have
carry shown that rather
both pressure and thick
bendingupper and lower panels would be necessary to carry both
loads.
pressure
Earlyand
in bending
the design loads.
of the first generation BWB aircraft, special emphasis was placed
on theEarly in theofdesign
problem containingof thecabin
first generation
pressure in BWB blended aircraft,
designsspecial
throughemphasis was placed
a simplified, two-
on the problem
dimensional of containing
beam-column cabin [49].
analysis pressure
In a in blended designs
comparison betweenthroughellipticala and
simplified,
multi-
two-dimensional
bubble designs, the beam-column
latter were analysis
found to[49].take In a comparison
internal between
cabin pressure elliptical
loads and
efficiently
multi-bubble
through balanced designs, the latterstress
membrane were found to take
in inner internalsegment
cylindrical cabin pressure
shells loads efficiently
and inter-cabin
through balanced the
walls. Generally, membrane
stressesstress
are onein order
inner cylindrical
of magnitude segment
highershells
thanand inter-cabin walls.
in conventional tube
Generally, the stresses
and wing fuselages are internal
since one order of magnitude
pressure primarily higher than
results in in conventional
bending tube and
stress instead of
wing fuselages since
skin membrane stressinternal
[48]. pressure primarily results in bending stress instead of skin
membrane stress [48].
Multi-bubble designs bridge the gap by featuring cylindrical fuselage sections, as
shown in Figure 7.designs
Multi-bubble The cabin bridge
floorstheand gap by featuring
partitions cylindrical
help support the fuselage
torsion and sections,
bending as
shown in
loads [28]. Figure 7. The cabin floors and partitions help support the torsion and bending
loads [28].

Figure 7. FEA
Figure 7. FEA analysis
analysis of
of the
the multi-bubble
multi-bubble concept
concept fuselage
fuselage (MBF)
(MBF) loaded
loaded with
with cabin
cabin pressure
pressure and
and
simulated
simulated aerodynamic
aerodynamic loadings
loadings [48].
[48].

The outer-ribbed shell prevents buckling due to external resultant compressive loads.
The initial results from these approximate finite element analyses indicate progressively
lower maximum stresses and deflections compared to the earlier study, but weight is higher
than the conventional B777/A380. A slight modification of the multi-bubble concept was
investigated by substituting inter-cabin walls for columns, and integrating outer panels to
decouple the loads and provide buckling stability [50].
Due to manufacturing concerns with multi-bubble fuselage designs, a Y-braced box
fuselage (Figure 8) alternative was developed with special resin-film-injected (RFI) stitched
carbon composite with a foam core. This configuration is a hybrid between the multi-bubble
 lower maximum stresses and deflections compared to the earlier study, but weight is
higher than the conventional B777/A380. A slight modification of the multi-bubble con-
cept was investigated by substituting inter-cabin walls for columns, and integrating outer
panels to decouple the loads and provide buckling stability [50].
Aerospace 2024, 11, 494 10 of 23
Due to manufacturing concerns with multi-bubble fuselage designs, a Y-braced box
fuselage (Figure 8) alternative was developed with special resin-film-injected (RFI)
stitched carbon composite with a foam core. This configuration is a hybrid between the
concept and the
multi-bubble separate
concept andpressure shellpressure
the separate conceptshell
[31],concept
efficiently
[31],taking structural
efficiently takingloads
struc-
while alleviating manufacturing concerns.
tural loads while alleviating manufacturing concerns.

Figure8.8.Y-braced
Figure Y-bracedbox
boxfuselage
fuselagefor
forBWB
BWBvehicles
vehicles[48].
[48].

Morerecently,
More recently,attempts
attemptshave
havebeen
beenmademadetotoinclude
includephysics-based
physics-basedmassmasspredictions
predictions
onshape
on shapeoptimization
optimizationalgorithms
algorithmsforforthe
theBWB
BWB[47].
[47].Most
Mostearly
earlyresearch
researchprojects
projectsassumed
assumed
extensiveuse
extensive useofofcomposite
compositematerials
materialsininas asearly
earlyasas1996
1996[28,51],
[28,51],although
althoughthis
thisassumption
assumption
wasmade
was madeexclusively
exclusivelyforforweight
weightestimations.
estimations.The TheTsAGI
TsAGIproject
projectwas
wasmore
moreconservative
conservative
andassumed
and assumeda afuselage
fuselagecomposition
composition consisting
consisting of of mostly
mostly aluminum
aluminum with
with minimum
minimum useuse
of
of composites [30]. Later developments [52] indicated the need for substantial
composites [30]. Later developments [52] indicated the need for substantial improvements improve-
ments beyond
beyond aluminum aluminum and composite
and composite structures. structures. The alternative
The alternative presented presented was a
was a unique
unique manufacturing
manufacturing process toprocess
exploit to
theexploit the orthotropic
orthotropic nature and
nature and unique uniqueadvantages
processing processing
ofadvantages
dry carbonoffibers.
dry carbon fibers.
As
Asanother
anotherdesign
designalternative,
alternative,ananoval
ovalfuselage
fuselagedesign
design[53]
[53]consisting
consistingofoffour
fourarcs
arcs
connected
connectedby byaaprismatic
prismaticbox
boxcreates
createsa alarge,
large,uninterrupted
uninterrupted internal
internalspace that
space allows
that forfor
allows a
flexible cabin configuration while more efficiently taking tension and compression
a flexible cabin configuration while more efficiently taking tension and compression loads. loads.

Structural
StructuralConsiderations
ConsiderationsininLight
LightofofthetheNGAS
NGASProgram
Program
The
The previous section describing the structureand
previous section describing the structure andmanufacture
manufactureconcerns
concernsfor
forthe
theBWB
BWB
isisrelatively short for a few reasons. Unlike in tube-and-wing aircraft, where the materials
relatively short for a few reasons. Unlike in tube-and-wing aircraft, where the materials
and
andmanufacturing
manufacturingtechniques
techniques change
changebutbut
both thethe
both wings andand
wings fuselage are designed
fuselage in thein
are designed
classical manner, the highly coupled design and odd fuselage shape of the BWB
the classical manner, the highly coupled design and odd fuselage shape of the BWB makes makes the
structural design unique for each aircraft.
the structural design unique for each aircraft.
The
TheBWB
BWB tanker aircraft isisalso
tanker aircraft alsounique,
unique,even
even among
among blended
blended designs,
designs, duedue
to itstopay-
its
payload; since no passengers are carried, the fuselage (other than the crew cabin) does
load; since no passengers are carried, the fuselage (other than the crew cabin) does not
not need to be sized with the internal pressure loading in mind. This creates immense
need to be sized with the internal pressure loading in mind. This creates immense oppor-
opportunities for weight savings and simplifies the design process. For the outer wing, we
tunities for weight savings and simplifies the design process. For the outer wing, we en-
envision a conventional design, featuring ribs and spars. The centerbody must be designed
vision a conventional design, featuring ribs and spars. The centerbody must be designed
to support wing bending loads as well as the fuel tanks. Due to the distributed weight, we
suggest the use of longitudinal beams to reinforce either a truss or monocoque construction.
There is ample opportunity to introduce modern materials in the NGAS structure. For
instance, aluminum-lithium alloys could substitute traditional aeronautical aluminum due
to lower density, improved mechanical properties, and corrosion resistance [54,55]. The
lack of necessity to account for cabin pressure facilitates the use of composite materials to
build the fuselage panels, resulting in further weight savings.
 tion.
There is ample opportunity to introduce modern materials in the NGAS structure.
For instance, aluminum-lithium alloys could substitute traditional aeronautical alumi-
num due to lower density, improved mechanical properties, and corrosion resistance
Aerospace 2024, 11, 494 [54,55]. The lack of necessity to account for cabin pressure facilitates the use of composite 11 of 23
materials to build the fuselage panels, resulting in further weight savings.
Each individual fuel tank needs to be pressurized for proper operation at very high
Each
altitude individual
[56]. The crewfuel tank
cabin needs
may to be pressurized
be separately designedforfor
proper operationpressure
a comfortable at very high
and
altitude [56].
mounted Theoverall
on the crew cabin may be separately designed for a comfortable pressure and
structure.
mounted on the overall structure.
2.3. Stability and Control
2.3. Stability and Control
The absence of horizontal and vertical stabilizers makes the BWB configuration un-
stableThe
andabsence of horizontal
inherently difficult toand vertical
control [15],stabilizers makes
also making the BWB configuration
it particularly unsta-
sensitive to lateral
ble and
wind inherently
gusts [38,57], difficult
as well asto other
control [15], also
adverse making
weather it particularly
conditions [58,59].sensitive
Winglettorudders
lateral
wind gusts [38,57], as well as other adverse weather conditions [58,59]. Winglet rudders
have been proposed as a partial substitute, but the idea was abandoned due to its com-
have been proposed as a partial substitute, but the idea was abandoned due to its complex-
plexity [24]. The inclusion of regular winglets also causes a decrease in flutter speed,
ity [24]. The inclusion of regular winglets also causes a decrease in flutter speed, which is
which is also potentially dangerous for aircraft structure [30].
also potentially dangerous for aircraft structure [30].
These problems are shared by flying wings, as discussed in Section 1.1. In a conven-
These problems are shared by flying wings, as discussed in Section 1.1. In a conven-
tional aircraft, movement in any of the principal axes is achieved by movement of specific
tional aircraft, movement in any of the principal axes is achieved by movement of specific
control surfaces and the axes are decoupled. For example, moving the rudder to generate
control surfaces and the axes are decoupled. For example, moving the rudder to generate
a yawing moment does not cause a significant pitching moment. On the other hand, in a
a yawing moment does not cause a significant pitching moment. On the other hand, in a
BWB or flying wing, movement of any control surface generates movement in more than
BWB or flying wing, movement of any control surface generates movement in more than
one axis, characterizing strong coupling and redundant motion [60–62]. One exemplary
one axis, characterizing strong coupling and redundant motion [60–62]. One exemplary
control surface arrangement is shown in Figure 9. There are twelve elevons which gener-
control surface arrangement is shown in Figure 9. There are twelve elevons which generate
ate pitch,
pitch, yaw,yaw,
andand
roll roll motion
motion simultaneously
simultaneously whenwhen activated.
activated.

Figure 9. Control surface arrangement for early BWB design [17].

Figure 9. Control surface arrangement for early BWB design [17].
Since all control surfaces are part of the wing, activation increases drag and reduces lift,
Since all control surfaces are part of the wing, activation increases drag and reduces
characterizing strong coupling between stability, control, and aerodynamic characteristics.
lift, characterizing strong coupling between stability, control, and aerodynamic character-
For example, activation of the elevons to increase the angle of attack causes substantial loss
istics.
of lift,For example,
causing activation
the aircraft of the elevons
to plunge to increase
before reaching thethe angleangle
desired of attack causes[63].
of attack substan-
tial loss of lift, causing the aircraft to plunge before reaching the desired angle of attack
Early in the history of BWB design, it was assumed that the airplane would be statically
[63].
unstable in order to assume high cruise efficiency [28]. In the initial designs, control surfaces
consisted of elevons and drag rudders, all located in the trailing edge, an arrangement
identical to that of Northrop’s flying wings of the 1940s [30]. Inboard control surfaces
consist of plain hinged flaps and generate mostly pitching moment, and rolling moment
to a lesser degree; outboard control surfaces consist of elevons and drag rudders, which
generate significant rolling and yawing moments [60,64].
Due to short moment arms, the BWB has low pitch and yaw authority [65,66]. The
design is also subject to high yaw rates and auto-rotation tumble [32]. Providing sufficient
yaw control is especially difficult in the one engine inoperative regime, and research has
 consist of plain hinged flaps and generate mostly pitching moment, and rolling moment
to a lesser degree; outboard control surfaces consist of elevons and drag rudders, which
generate significant rolling and yawing moments [60,64].
Due to short moment arms, the BWB has low pitch and yaw authority [65,66]. The
Aerospace 2024, 11, 494 design is also subject to high yaw rates and auto-rotation tumble [32]. Providing sufficient 12 of 23
yaw control is especially difficult in the one engine inoperative regime, and research has
shown that winglet rudders are insufficient [61]. The use of drag rudders or crocodile flaps
is preferable
shown for yawrudders
that winglet controlare[31].
insufficient [61]. The use of drag rudders or crocodile flaps
Early in for
is preferable theyaw
development
control [31].of the BWB, it was known that a complex flight control
Early
system, in the development
consisting of the BWB,
of multiple, rapidly moving it was known
control that a[18,28],
surfaces complex wouldflight
becontrol
neces-
system,
sary. Theconsisting of multiple,
trailing edge rapidly
elevon chord movingare
fractions control surfacesby
determined [18,28], would
wing spar be necessary.
location, char-
The trailing
acterizing edge elevon chordcoupling.
a structural–control fractions are determined
Trailing by wingshould
edge surfaces spar location,
be as largecharacter-
as de-
izingconsiderations
sign a structural–control coupling. Trailing edge surfaces should be as large as design
allow [60].
considerations
Control laws allow
need[60].
to efficiently allocate control surfaces to minimize actuator rate,
hingeControl
moment, laws
andneed to efficiently
horsepower allocate [28,31],
requirements control and
surfaces
the BWBto minimize
relies on actuator rate,
a full author-
hinge
ity moment,
digital flightand horsepower
control system for requirements
stabilization, [28,31], andand
control, thetrim.
BWBThe relies on apilot
BWB full authority
does not
digital flight
directly controlcontrol
any system
surfacesfor stabilization,
during flight [60].control, and trim.
The main Thegoals
control BWB are pilotroll
does
modenot
directly control any surfaces during flight [60]. The main control
damping, coordinated turn, and roll response shaping. An open loop analysis shows that goals are roll mode
damping, coordinated
traditional SISO designturn,facesand roll response
fundamental shaping.[67].
limitations An open loop analysis shows that
traditional SISO design faces fundamental limitations [67].
There are no dedicated trim devices. Pitch stability is usually achieved through a
There are
combination of no dedicated
wing sweep, trim devices.
the use Pitch stability
of reflexed airfoils inis the
usually achieved
centerbody through
section, anda
combination of wing sweep, the use of reflexed airfoils in the centerbody
cambered airfoils in the outer wing section [38]. These design choices, when combined section, and
cambered airfoils in the outer wing section [38]. These design choices,
with proper wing twist, also generate an optimal lift distribution. A design featuring aft- when combined
with proper
mounted wingmay
engines twist, also generate
be chosen an optimal
to generate lift nose-down
additional distribution. A design
pitching featuring
moment. The
aft-mounted engines may be chosen to generate additional nose-down
use of positive, aft-cambered airfoils, associated with the cruise deck angle requirement, pitching moment.
The use ofapositive,
generates nose-downaft-cambered airfoils, associated
pitching moment, increasingwiththe the
trimcruise deck angle
requirement requirement,
[18,40].
generates a nose-down pitching moment, increasing the trim requirement
Trim stability can be partially achieved through proper wing twists, although the [18,40].
Trim stability can be partially achieved through proper wing twists, although the
magnitude of washout necessary to stabilize the aircraft may be quite high, ranging from
magnitude of washout necessary to stabilize the aircraft may be quite high, ranging from 8
8 to 10 degrees [68]. This greatly affects the designed lift distribution, supporting the fact
to 10 degrees [68]. This greatly affects the designed lift distribution, supporting the fact
that an elliptical lift distribution is not necessarily optimal for a BWB.
that an elliptical lift distribution is not necessarily optimal for a BWB.
There is strong coupling between fuselage design, aerodynamic performance, and
There is strong coupling between fuselage design, aerodynamic performance, and
handling characteristics. A study comparing different fuselage shapes with respect to mo-
handling characteristics. A study comparing different fuselage shapes with respect to
ment characteristics [30] found that blended designs with large front chord extension have
moment characteristics [30] found that blended designs with large front chord extension
unsatisfactory moment characteristics, as shown by design FW-103 in Figure 10. Interest-
have unsatisfactory moment characteristics, as shown by design FW-103 in Figure 10.
ingly, JetZero’s BWB design resembles design FW-102, with reduced centerbody span,
Interestingly, JetZero’s BWB design resembles design FW-102, with reduced centerbody
which features satisfactory moment characteristics over the operational envelope.
span, which features satisfactory moment characteristics over the operational envelope.

Figure 10. Moment behavior of select blended wing body configurations. Reprinted from [30], with
permission from Elsevier.

Performance investigations for the static stability margin by the TsAGI group [30]
C
indicated a degree of static stability close to zero (CmL ≤ 0) during takeoff and landing,
C
while the instability in cruise flight should be limited by CmL ≤ 0.3. In tube and wing
configurations, a specific static margin can be achieved simply by selecting a wing position
relative to the fuselage [28], which is not an option for the BWB design. The most obvious
alternative is to change the placement of the engines. The static margin varies greatly with
geometry design. Past designs feature static margin values ranging from −15% degrees [28]
 Performance investigations for the static stability margin by the TsAGI group [30]
indicated a degree of static stability close to zero (𝐶 0) during takeoff and landing,
while the instability in cruise flight should be limited by 𝐶 0.3. In tube and wing con-
figurations, a specific static margin can be achieved simply by selecting a wing position
Aerospace 2024, 11, 494 relative to the fuselage [28], which is not an option for the BWB design. The most obvious
13 of 23
alternative is to change the placement of the engines. The static margin varies greatly with
geometry design. Past designs feature static margin values ranging from -15% degrees [28]
to slightly positive [27]. In general, for alternative aircraft designs, a positive static margin
to slightly positive [27]. In general, for alternative aircraft designs, a positive static margin
from55to
from to15%
15%isisdesirable
desirable[69].
[69].
The fuselage configuration
The fuselage configuration does does notnot impact
impact rollroll
andandyawyaw stability,
stability, which
which are prefer-
are preferably
ably achieved through continuous motion of the outer wing surfaces,
achieved through continuous motion of the outer wing surfaces, the elevons, and drag the elevons, and
drag rudders. Attempts to improve lateral stability include the use
rudders. Attempts to improve lateral stability include the use of belly flaps, which alsoof belly flaps, which
also enhance
enhance rotation
rotation at takeoff
at takeoff and landing
and landing [63]. [63]. Differential
Differential control
control surfaces,
surfaces, located
located in the in
the lower
lower centerbody,
centerbody, werewere tested
tested withwith positive
positive results
results [38].Morphing,
[38]. Morphing,seamless
seamlesstrailing
trailing
edge devices were also investigated to substitute traditional drag rudders
edge devices were also investigated to substitute traditional drag rudders [61], providing [61], providing
reduceddrag
reduced dragwhile
whileproviding
providing both
both crocodile
crocodile flapflap
andand aileron
aileron modes
modes at theatsame
the same
time.time.
The
use of split drag rudders is not new in flying wings, being a design feature of bothboth
The use of split drag rudders is not new in flying wings, being a design feature of the B-2the
B-2 Spirit
Spirit bomber
bomber (Figure
(Figure 11) of
11) and andtheofupcoming
the upcoming B-21 B-21 Raider.
Raider.

Figure11.
Figure 11.Split
Splitdrag
dragrudders
rudderson
onB-2
B-2Spirit.
Spirit.Courtesy
Courtesyof
ofthe
theU.S.
U.S.Air
AirForce.
Force.

Thestructure
The structureof ofthe
themorphing
morphingtrailing
trailingedge
edgedevice
deviceconsists
consistsof ofcomposite
compositeouter outerskin
skin
layersand
layers andstringers
stringersand andinner
innermiddle
middleskins.
skins. On
On the
the one
onehand,
hand,thetheouter
outerskins
skinsneed
needto tobe
be
deformable
deformablein inorder
orderto toachieve
achieverequired
requiredflap flapdeflections,
deflections,butbutononthe
theother
otherhand
handthey
theyneed
need
to
tohave
havesufficient
sufficientstiffness
stiffnessin inorder
ordertotoprevent
preventlateral
lateral deformations
deformations under
under cruise
cruise air
air loads,
loads,
increasing
increasingdesign
designcomplexity.
complexity.
In
In aa BWB, thethe stability
stabilityandandcontrol
controldesign
designgroupgroup must
must decide
decide if the
if the airplane
airplane can can
sac-
sacrifice control
rifice control surfaces
surfaces originally
originally intended
intended forfor
rollroll
to to
bebe pitch
pitch effectors
effectors [60];
[60]; thethe simulta-
simultane-
neous activation
ous activation of of control
control surfaces
surfaces is aisproblem.
a problem. Activating
Activating surfaces
surfaces for for pitch
pitch leaves
leaves no
no ele-
elevons available for roll or yaw control. Morphing
vons available for roll or yaw control. Morphing trailing edge trailing edge devices may help alleviate
help alleviate
this
this issue.
issue.
An
An important
important requirement
requirement for for the
the BWB
BWB operation
operation isis control
control in in the
the one
one engine
engine outout
regime,
regime, which
whichexacerbates
exacerbates the thealready
alreadyexisting
existingcontrol
controlchallenges.
challenges. The The use
use of
ofsplit
splitdrag
drag
rudders
rudders as outboard elevons
as outboard elevonsprovides
providesextraextrayawyaw control
control in the
in the low-speed
low-speed engine-out
engine-out con-
condition [30], in addition to providing speed brakes for
dition [30], in addition to providing speed brakes for low-speed flight. low-speed flight.
Thrust vectoring as a means to enhance control was not incorporated for the complex-
ity [40], despite promising results in pitch motion [70], as well as because critical sizing
conditions for control surfaces occur during idle engine conditions and thrust vectoring
would be insignificant in that operational regime [60]. Additionally, FAA certification
requirements also impose that aircraft control should not be impaired after any single
failure of the stability system [60].
The BWB requires advanced control allocation algorithms which typically assume
linear control surface effectiveness [62]. Wind tunnel experiments have shown that the angle
of attack and surface deflection have the strongest effect on control moment nonlinearities.
The center-section planform, leading edge sweep, and relative size of the front and rear
chord extensions greatly influence the behavior of moment characteristics at high angles of
attack [30].
Aerospace 2024, 11, 494 14 of 23

Losses at maximum deflection angles and control surface interaction effects are sig-
nificant, therefore, the authors recommended the inclusion of control allocation selection
and performance evaluation in early design stages to avoid costly redesigns. The inclusion
of control considerations in preliminary design also helps alleviate hydraulic power con-
sumption. The BWB features significant hinge moments due to large control surface areas,
combined with high deflection rates in order to safely control the longitudinal instability,
which may result in actuator mass penalty [57].

Stability and Control Considerations in Light of the NGAS Program

Stability issues are inherent to the BWB configuration, and a design effort must be
made to compensate for the obvious absence of vertical and horizontal stabilizers. Overall,
there is no one-size-fits-all control allocation algorithm, and the system must be fine-tuned
to each blended wing body configuration. The goal is to design a system that provides
sufficient control authority while minimizing the size of the elevons and, therefore, the
hydraulic power.
Previous research placed important emphasis on the range for the center of gravity of
the aircraft [30], which directly affects the trim. A very unique feature of the BWB tanker
helps alleviate static stability and trim problems: its payload, comprised of fuel. During
ferry flight, the aircraft’s own fuel consumption can be drawn strategically from certain
fuel tanks to keep the aircraft trimmed and stable.
The same idea applies to the refueling operations. In operating regimes where there is
not enough fuel in the tanks to achieve this effect (such as the aircraft’s return to base after
the fuel payload has been transferred), the aircraft weight is significantly lower, resulting in
increased authority of the existing control surfaces. Most advanced military aircraft feature
multiple fuel tanks and advanced fuel transfer systems [56].
One specific development greatly facilitates the development of the control system for
the NGAS BWB: the partnership between JetZero and Northrop Grumman, which designed
and manufactured the only flying wing currently in service, the B-2 Spirit bomber. It must
be noted that Northrop Grumman successfully designed a control system for a flying wing
with 1990’s technology. The design of its successor, the B-21 raider, is currently ongoing,
which will most likely prompt a major update in the control system. The use of a similar
technology for the NGAS will likely reduce development costs.

2.4. Noise
The original blended wing body features reduced a lower acoustic signature due
to the centerbody shielding the engine noise [18,71]. An experimental study featuring a
scaled model reported reductions in the noise that radiated downward into the forward
sector between 20 and 25 dB [72]. For the chosen configuration, featuring engines mounted
with nacelles in the aft section of the fuselage, it was observed that noise associated
with the exhaust radiates into the sector directly below the model downstream, reducing
shielding efficiency.
Despite the early promise, studies specifically regarding the noise generated by the
BWB configuration were scarce, even in the early 2000s. The earliest studies [73,74] investi-
gated a noise-driven design featuring integrated Propulsion-Airframe-Aeroacoustic (PAA)
technologies, which resulted in noise benefits at the compromise of performance.
Most specific studies focused on acoustic emissions started after 2006 [75], as part
of the “Silent Aircraft Initiative” (SAI) by Cambridge, MIT, and NASA. As mentioned
in Section 2.1, the BWB is a highly integrated design, and prioritizing one design aspect
compromises the others. Overall, an optimized aircraft for reduced noise emissions tends
to have a blended nature [76].
When a BWB airframe was designed with noise as the primary target [4,40], the aircraft
noise at an airport perimeter was found to be 62 dBA, near the background noise level for
a highly populated area. The design features engines partly buried within the fuselage,
resulting in imperceptible takeoff and landing, and presenting a significant environmental
Aerospace 2024, 11, 494 15 of 23

benefit. The SAI was driven by the aggressive requirements set by the NASA N+2 program,
aimed at developments regarding environmentally responsible aviation [77]. Despite
the focus on noise, the culminating design of the SAI, the SAX-40 still featured a 25%
improvement in fuel burn compared to conventional aircraft.
During approach and landing, the airframe generates the most noise, which increases
as the approach speed increases [40,75]. Lower approach speeds require lower stall speeds,
which in turn generate more drag. The tradeoff between cruise aerodynamic performance,
stability, departure characteristics, and noise [39] is exacerbated by the fact that the BWB
does not feature trailing edge flaps (since there is no trimming surface to compensate the
nose-down pitching moment, resulting in less approach noise [18]), requiring a higher
approach angle of attack and resulting in higher induced drag. The SAI included noise
assessment methods coupled with extensive use of multidisciplinary optimization tools
such as Wingmod [78].
The absence of flaps also negatively affects the departure characteristics [31]. The BWB
also has lower wing loading, therefore, the maximum lift coefficient happens at high angle
of attack. The higher induced drag is generated by use of the elevons, as well as drooped
leading edge slats [30] (which also generate further noise).
The landing gear is responsible for the majority of noise generated during landing,
caused by unsteady flow structures [79]. Usually, the landing gears are strategically located
in lower velocity regions. The blended shape is such that local velocity under the fuselage is
almost the same as free stream; in conventional aircraft, this is only about 80% (generating
circulatory flow). Therefore, the BWB landing gear will generate more noise. The slats also
pose a significant contribution, and technological developments in the area are proposed to
minimize their acoustic signature [77].
During takeoff, the turbulent mixing of the high-speed jet is responsible for the
majority of the noise. The BWB features a multi-engine, low specific thrust configuration
which allows the fan to operate at part-speed during takeoff. This increases the benefit of a
variable area nozzle regarding fan and jet source noise reduction [80]. The SAI proposed a
faired undercarriage and smooth lifting surfaces, optimally shaping the centerbody and
increasing passive circulation and reducing noise levels. However, the faired undercarriage
increases weight [81].
A number of novel noise technologies with potential application for the BWB were
investigated, with modest benefits. For instance, the distributed propulsion system in the
centerbody enables a substantial amount of acoustic treatment in the exhaust duct, such as
the use of extensive acoustic liners [82,83]. Sakaliyiski et al. [84] investigated the potential
of perforated drag plates, and Shah et al. [85] proposed the treatment of the trailing edges
by the deployment of brushes to reduce airframe self-noise, although the noise reductions
were limited to 4 dB [86]. Nozzle chevrons and pylon treatments were also proposed [87].
As part of the Quiet Green Transport (QGT) initiative by NASA, BWB aircraft were
shown to be adequately quiet even when featuring open-rotor propulsion [88]. The study
reported the design to be quieter than conventional aircraft, though at the time it was
speculated that this alternative would face stiff competition by designs featuring future,
quieter turbofan engines. Noise studies regarding a BWB powered by hydrogen fuel
cells [89,90] indicated that distributing the propulsion system into several engines instead
of a few large ones tends to increase the frequency of the engine noise, leading to greater
atmospheric attenuation.
Overall, the SAI studies successfully showed that, even when noise is the design
priority for a BWB, the aerodynamic improvements over conventional aircraft are still
significant. This is achieved through the integration of noise assessment tools [80,91] on
the fuselage design methodology. However, some of the technologies proposed by the SAI
were then perceived as high-risk [81], including the BWB concept itself, the thrust vectoring
system [39], the landing gear design, and the cost of the program.
Additionally, some of the noise-reducing features of the BWB require changes to the
operational rules in the terminal area, such as displaced threshold, and use of a variable-
Aerospace 2024, 11, 494 16 of 23

area nozzle on takeoff [75]. The SAI did not publish effective perceived noise level (EPNL)
calculation details, which definitely would present a hurdle in the regulatory process.
It is also appropriate to discuss the complexity of performing a noise assessment
for the BWB. While multiple noise prediction algorithms exist, such as the fast-scattering
codes developed at N.C. State [83,92], these make the consideration of potential flow,
neglecting the well-known effects of turbulence on noise generation [93]. Other noise
assessment techniques include numerical simulations, experimental data and empirical
correlations [77].
Special noise assessments for BWB’s lack the possibility of experimental validation.
Shielding, for example, can only be assessed through wind-tunnel experiments on scale
models. However, even when experimental setups were used [72,92], no attempt was made
to simulate the noise emission characteristics of the engines. Overall, at the time of the
SAI, noise shielding prediction for full configuration aircraft and realistic conditions were
not available.
More recently, a perception-based noise study [94] investigated the flyover of a BWB
variant. It has been shown that the blended designs are substantially less annoying than
current tube-and-wing long-range aircraft of similar range and mission for take-offs as well
as for landings. For the best BWB variant, noise annoyance was reduced by 4.3 units for
departures and by 3.5 units for approaches on an 11-point scale. The main reason for these
findings seems to be the acoustic shielding by the body of the extended fuselage, which
was found to be an important factor in reducing sound levels in the order of 10–20 dB.

Noise Considerations in Light of the NGAS Program

Acoustic emissions are a critical aspect involved in any aircraft designed for low
detectability. Given the special emphasis placed by the NGAS on aircraft survivability, it is
possible that an attempt will be made to minimize the airframe noise. However, for a tanker
aircraft designed to fly at high altitude and Mach number, other stealth elements are likely
to be prioritized, such as the radar cross-section and infrared signature. The noise benefits,
however, present a significant development regarding the commercial use of the BWB.

2.5. Miscellaneous
2.5.1. Propulsion
At the time of conception, the BWB was seen as an ideal platform to integrate advanced
propulsion concepts. Alternative propulsion solutions such as propfan engines [30] and
hydrogen fuel cell distributed propulsion [90] were evaluated, with limited potential upside.
For propfan engines, the cruise Mach number would be reduced, and the complexity of
hydrogen fuel cell design, in addition to added weight, held the concept back. More
recently, hybrid-electric propulsion was also investigated for BWB designs [95].
The most extensively studied advanced propulsion concept is the use of boundary
layer ingestion (BLI) to reduce ram drag and increase propulsive efficiency at
cruise [15,31,49,96,97]. The benefits are offset by lower inlet pressure recovery and in-
creased flow distortion before the compressor stage, in addition to increased fan noise, and
it was speculated that some sort of flow control had to be used so that distortion levels
were acceptable [75].
As discussed in Section 2.1, minimal aft camber enhances the external pre-compression
of upstream flow in the BLI engine configuration [40,98]. This provides uniform flow at the
engine inlet, reducing the challenges associated with the use of BLI propulsion. To satisfy
these requirements, a multivariate optimization is required to ensure conflicting constraints
are satisfied.
Some research has also been performed regarding embedded propulsion systems [75,91],
resulting in lower installation weight and lower nose-down pitching moment, as well as
improving flow separation and maximum lift coefficient. However, this type of installation
is also prone to inlet flow distortions [99].
 at the engine inlet, reducing the challenges associated with the use of BLI propulsion. To
satisfy these requirements, a multivariate optimization is required to ensure conflicting
constraints are satisfied.
Some research has also been performed regarding embedded propulsion systems
Aerospace 2024, 11, 494 [75,91], resulting in lower installation weight and lower nose-down pitching moment, as
17 of 23
well as improving flow separation and maximum lift coefficient. However, this type of
installation is also prone to inlet flow distortions [99].
Despite the number of different propulsion alternatives, JetZero ultimately decided
Despite the number of different propulsion alternatives, JetZero ultimately decided for
afor a conventional
conventional design,
design, placing
placing conventional,
conventional, high bypass
high bypass turbofan
turbofan engines
engines on pylons
on pylons at the
at the rear section of the fuselage (Figure 12). Ultimately, it appears that the benefit of
rear section of the fuselage (Figure 12). Ultimately, it appears that the benefit of alternative
alternativealternatives
propulsive propulsive is alternatives is marginal
marginal compared compared
to the to the design
design hurdles hurdles
and cost and cost
of developing
of developing
new technologies.new technologies.

Figure12.
Figure 12. Artist
Artist conception
conceptionof
ofthe
theNGAS
NGASduring
duringmission.
mission. (Courtesy
(Courtesy of
ofJetZero).
JetZero).

The
The only
only way
way the
the authors
authors envision
envision thetheNGAS
NGASaircraft
aircraftto
tofeature
featureadvanced
advancedconcepts
concepts
like
like embedded propulsion, is if the U.S. Air Force places immense emphasis on stealth,
embedded propulsion, is if the U.S. Air Force places immense emphasis on stealth,
like
like in
in the
the B-21
B-21 Raider.
Raider. However,
However,unlike
unlikethethe B-21
B-21 (which
(which has
has to
to penetrate
penetrate enemy
enemy territory
territory
undetected
undetected by by air
air defenses),
defenses), the
the NGAS
NGAS willwill likely
likely operate
operate out
out of
of highly
highly contested
contested zones,
zones,
where
whereallied
alliedfighters
fighterscancanprovide
provideescort.
escort.
Obviously,
Obviously,the theNGAS
NGASshould
should still feature
still stealth
feature elements
stealth elementsandand
minimize noise/infrared
minimize noise/infra-
emissions and radar scattering as much as possible, but embedded
red emissions and radar scattering as much as possible, but embedded propulsion propulsion is likely not
is likely
worth the additional complexity and
not worth the additional complexity and cost. cost.

2.5.2. Stealth
2.5.2. Stealth
Flying wing/BWB designs have outstanding radar scattering characteristics due to
Flying wing/BWB designs have outstanding radar scattering characteristics due to
the blended nature of the airframe [100], as well as the capacity to easily integrate radar
the blended nature of the airframe [100], as well as the capacity to easily integrate radar
absorbing materials [101] on the curved fuselage. Overall, there is enormous potential to
absorbing materials [101] on the curved fuselage. Overall, there is enormous potential to
reduce the aircraft’s radar cross-section (RCS), such as on the B-21 Raider [102].
reduce the aircraft’s radar cross-section (RCS), such as on the B-21 Raider [102].
Some other stealth elements have been previously discussed, such as noise (Section 2.4)
and infrared signature. There is a limit to reductions in infrared signature due to engine
location. This may make the aircraft vulnerable to the infrared search-and-track (IRST)
systems of enemy fighter aircraft [103,104]. It is speculated that modern IRST systems can
detect targets at around a 100 km distance [105], placing the enemy fighter well within
radar range and beyond-visual-range (BVR) capability for escort fighters.
As explained in the previous section, the increased infrared signature is likely an
acceptable compromise considering the design costs associated with burying the engines
inside the fuselage.

2.5.3. Aircraft Safety, Passenger Comfort, and Emergency Egress

Compliance with emergency egress rules is a major challenge for BWB configurations
due to its shape and fuselage configuration [31]. For example, regulations state that the
emergency exits must be above the waterline in case of a water landing [30]. Previous
research has shown that the internal configuration of aisle widths, consisting of position,
alignment, and format is more important than the aircraft shape [64], and successful
emergency egress can be achieved through proper design.
Another issue that cannot be easily alleviated through design is passenger comfort.
In a blended configuration, most passengers do not have direct vision over a window.
Aerospace 2024, 11, 494 18 of 23

Additionally, passengers away from the centerline are subject to higher g-loads over turns,
deteriorating ride quality [28].
The authors opted to include these challenges to illustrate further design hurdles
which contributed toward the BWB not entering commercial service. For a tanker aircraft
such as the NGAS, the only “passengers” are the pilots and the refueling boom operator,
rendering passenger-related issues a non-factor.

3. Conclusions
In this review paper, we discussed the history of blended wing body designs—from the
early days of aviation to the successful B-2 Spirit bomber—highlighting its advantages and
challenges and exploring the reasons why the BWB never entered into commercial service.
We firmly believe that the BWB, originally conceptualized as a long-haul transport air-
craft, will likely become the U.S. Air Force Next-Generation Air-Refueling system (NGAS).
The superior aerodynamic performance of the blended design allows it to fly further while
burning less fuel, greatly enhancing refueling operations. The reduced radar signature,
noise, and infrared emissions are also particularly interesting for military application in
highly contested airspace.
The technical advancements in the past three decades and unique features of tanker
aircraft have put JetZero in a great position to address the challenges that prevented
the development of the BWB for passenger applications back in the 1990s. For a tanker
aircraft, passenger-related issues are a non-factor; the development of new manufacturing
techniques and materials greatly reduces the complexity of manufacturing complex shapes
for the aircraft structure. Dealing with cabin pressure loads, which created major structural
design hurdles in the past, is not necessary.
The last major issue to be addressed is stability and controls of the blended design.
New technologies such as artificial intelligence present great potential to reduce design
complexity. Also, this problem has been successfully tackled by Northrop Grumman in
the development of the B-2 Spirit flying wing. Not so curiously, JetZero partnered with
the company to manufacture the BWB tanker prototype. The existing know-how, added
to (likely) major control updates for the new B-21 Raider bomber, puts JetZero in prime
position to deal with these challenges.
It must be reinforced that the BWB is a highly coupled design, with numerous tradeoffs
between aerodynamic performance, stability and controls, structural design, and stealth
characteristics, and a complex optimization process is necessary. Ultimately, time will show
if the BWB tanker will enter serial production for the NGAS project. Its upcoming first
flight, scheduled for 2027, will likely be the deciding factor.

Funding: This research received no external funding.

Data Availability Statement: Not Applicable.
Conflicts of Interest: The authors declare no conflict of interest.

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