Economic Viability Assessment of NASA’s Blended Wing Body
N3-X Aircraft
C. Goldberg
∗
, D. Nalianda
†
P. Pilidis
‡
and R. Singh
§
Cranfield University, Bedfordshire, MK43 0AL, United Kingdom
Abstract
Numerous novel aircraft concepts are under development that aim to achieve dramatic increases in
efficiency and reductions in emissions in comparison to current aircraft. Research into these concepts
typically focuses on performance aspects to establish whether the aircraft will be capable of meeting
developmental goals. However, the final goal of such concepts is to progress to viable commercial products.
Economic viability assessments are therefore an integral part of the development process to ensure a
sustainable industry. The key question to address is whether a high efficiency aircraft concept can
translate into an attractive product from an economic perspective.
This research performed an economic viability assessment of NASA’s N3-X aircraft, a blended wing
body aircraft with a distributed boundary layer ingesting propulsion system. The sensitivity of the aircraft’s
direct operating cost to changes in acquisition price and maintenance cost was predicted to establish
maximum cost margins for the aircraft. In a May 2017 fuel price scenario, the N3-X could be no more
than 25% more expensive than the baseline aircraft to remain economically viable. Introducing a carbon
tax or fuel price jump widens the margin for increased costs. Aircraft cost estimates for the aircraft
predict an acquisition cost from 11–37% more expensive than the baseline. In combination with the
direct operating cost sensitivity analysis, the N3-X is predicted to need to capture 30% of the aircraft
market up to 2035.
Nomenclature
Acronyms
BLI = Boundary layer ingestion
BWB = Blended wing body
CER = Cost-estimating relationship
DOC = Direct operating cost
IRR = Internal rate of return
MPCM = Manufacturing process cost model
PCM = Parametric cost model
SEE = Standard error of estimate
SFC = Specific Fuel Consumption
TERA = Techno-economic and Environmental Risk Assessment
TRL = Technology readiness level
Symbols
1 Introduction
The development of new, more efficient technology is an integral part of the growth of the aviation industry.
Whilst a modern aircraft is superficially similar to its predecessors, the tools and technologies used in their
design have led to dramatic improvements in efficiency in comparison to their predecessors. As with any
∗
Research Assistant, Propulsion Engineering Centre, Cranfield University.
†
Lecturer, Propulsion Engineering Centre, Cranfield University.
‡
Head of Centre, Propulsion Engineering Centre, Cranfield University.
§
Professor Emeritus Gas Turbine Engineering, Cranfield University.
1
Proceedings of 53rd AIAA/SAE/ASEE Joint Propulsion Conference 2017, AIAA Propulsion and Energy Forum,
10 - 12 July 2017, Atlanta, GA, USA, Paper number AIAA 2017-4604
DOI:10.2514/6.2017-4604
©2017 AIAA. This is the Author Accepted Manuscript.
Please refer to any applicable publisher terms of use.
commercial industry, a core driver behind the development of new technologies is to offer a product that
increases profitability. In aviation, this is typically gained through reductions in fuel consumption, a cost that
can account for more than a quarter of an aircraft’s direct operating cost [1]. However, the global community
in the 21
st
century has become increasingly aware of its impact on the environment. This awareness has
added an additional goal to the development of new technology: the mitigation and reduction of industry’s
impact on the environment. The key aspects of this goal are the reduction of aviation’s energy consumption
(i.e. fuel use) and the reduction of emissions, especially carbon dioxide, oxides of nitrogen, and noise. The
goals of reducing energy consumption lies in line with the typical course of development in aviation to lower
fuel consumption and hence reduce fuel cost. However, aviation bodies have targeted dramatic reductions
in energy consumption and emissions that cannot be met without the development of revolutionary new
technologies [2, 3].
As commercial entities, airlines and manufacturers are naturally profit-oriented. It is therefore inevitable
that the decisions made will be influenced by economics, be it operating strategies or the purchase of new
assets. In a high fuel price environment, the financial penalties of operating older, less-efficient aircraft are
high. The purchase of new aircraft and interest in new technology is therefore relatively lower. In an opposite
scenario, the cost of operating older aircraft is less significant as fuel contributes less to the overall operating
cost. Older aircraft may therefore be brought out of storage rather than investing in new technology [1]. It
is therefore vital that new technologies can be shown to have the potential to provide profitable solutions to
manufacturers, owners, and operators and ensure that they are commercial viable.
The development of revolutionary or novel technologies entails a higher degree of risk than the evolutionary
development of conventional technology. Novel concepts require the investment of significant time and money
to ensure that the technology can move from a low technology readiness level (TRL) preliminary design through
to certification and commercial use. The development of new aviation technology can span a timeline that
means a new aircraft takes more than a decade to move from concept to a commercial product. Reflecting
this, aviation’s developmental goals also span long timelines, with NASA’s Subsonic fixed wing project settings
goals for 2035+ and Flightpath 2050 setting a goal for 2050. Long timelines contribute further uncertainty
to the development of new technology, as the environment in which the new technology must operate (in
terms of global policy, taxation, economic environment and fuel price) is an unknown. Given the risk and
uncertainty of novel technology, a large body of research covering all aspects of each novel technology is
necessary. Research at the preliminary stage typically focuses on predicting the performance characteristics
of the technology. Given the commercial orientation of the industry, it is vital to ensure that new aircraft
concepts remain profitable whilst also meeting established developmental goals.
This research presents the techno-economic analysis of a novel aircraft concept, NASA’s N3-X, designed
for a 2035+ to achieve 60% fuel saving versus a 2005 entry-into-service aircraft (the Boeing 777-200LR). The
aim of the research was to assess the economic benefits of the N3-X in comparison to the baseline aircraft
and to present a framework for predicting the economic viability of novel aircraft concepts.
2 Case Study Definition
The present research focuses on a case-study of the NASA N3-X conceptual aircraft, developed by Felder
et al [4, 5]. The N3-X aircraft is designed to reduce fuel/energy consumption by at least 60% relative to
a conventional 2005 entry-into-service aircraft. Following the established performance characteristics of the
selected baseline aircraft, mission level goals have been set for the N3-X aircraft in terms of payload and
range requirements. The N3-X is therefore ideally required to achieve a mission range of at least 7500 nmi
at Mach 0.84 with a full payload (payload mass equal to the aircraft maximum of 53,570 kg). In order to
achieve the required efficiency improvements, the aircraft makes use of a number of novel technologies in both
the airframe and the propulsion system. The aircraft propulsive power is provided by a distributed propulsion
system consisting of an array of propulsor fans which ingests the boundary layer of the airframe, along with
free stream air. Electrical power for the propulsor fans is produced by a pair of turbojet/turbogenerator type
engines through a superconducting system, cooled by liquid hydrogen. The N3-X airframe is a blended wing
body planform with main engines assumed to be embedded within the airframe (Figure 1). The embedded
engine and propulsor array location provide noise shielding to achieve the required noise targets [6]. Fuel burn
and emissions targets are achieved through utilisation of the boundary layer ingesting distributed propulsion
system and a blended wing body airframe [5].
2
65.5 m (215 ft)
41 m (135 ft)
Distributed Propulsor Array
Ingested Boundary Layer
Blended Wing Body
Embedded Engine
Figure 1: N3-X aircraft diagram
3 Method
There are numerous goals when optimising an aircraft design, such as achieving a low fuel consumption,
operating empty weight, or cost. However, an optimised aircraft design does not guarantee a profitable
solution, as competitors and alternative options may offer a more attractive solution. It is therefore useful to
apply tools that can compare performance and economics against these alternatives.
Aircraft costs resulting from performance are relatively easily identified as an extension of performance
assessments. However, further costs are contributed as a part two key uncertain costs: the aircraft’s ac-
quisition price and value, and its maintenance cost. This complicates the assessment of novel technology
from an economic perspective. Cost predictions may be created for aircraft in a number of ways. They can
typically be split into two categories; Parametric Cost Models (PCM) and Manufacturing Process Cost Models
(MPCM) [7]. These two categories of model attempt the overcome the difficulties of estimating the cost of
an engineering project from different perspectives and at different levels of fidelity. Parametric cost models
make use of historical data to establish a statistical relationship between variables. In the case of cost estima-
tion, this will be between the cost of the aircraft and the design parameter or parameters found to correlate
well with cost. The relationship between the dependent variable cost and its independent variables can be
determined using a regression analysis to create a cost estimating relationship (CER). MPCM models support
a bottom-up design process, which assess each component and the processes required for its manufacture,
building up to an estimation of the total aircraft cost. However, both techniques are difficult to apply to novel
or preliminary designs. There is insufficient design detail at the preliminary stage to enable a cost estimate
based on manufacturing process, whilst novel aircraft are not typically covered by the historical data used
to create parametric cost estimating relationships. This is therefore an aspect that must be addressed when
attempting to predict the economic performance of a novel concept. Research by Nalianda et al. developed
a sensitivity analysis method which predicts the influence that the two key costs have over an aircraft’s direct
operating cost [8]. The sensitivity analysis may be used to compare the direct operating cost of a novel aircraft
to that of a baseline aircraft to assess whether a novel concept is profitable and provides a good return on
investment. Using this form of analysis, the design of a new conceptual aircraft may be assessed from a
manufacturers or operators perspective for any fuel cost and/or emissions taxation scenarios. Subsequently,
the maximum viable acquisition price and maintenance cost for the aircraft may be predicted. For a given
set of novel technologies that are included in the design, a decision may then also be made as to whether the
established maximum cost boundaries are achievable. This method circumvents the difficulty of predicting
the acquisition or maintenance cost of a novel concept.
3.1 Performance Modelling
Boundary Layer Ingestion
The boundary layer ingesting propulsion system of the N3-X was simulated from an integrated / net propulsive
force perspective. Inlet mass flow characteristics were approximated as the mass flow averaged properties for
the entire inlet stream for all flow conditions [9]. The propulsor array’s performance was otherwise calculated
using conventional one-dimensional gas dynamics methods. The propulsors were assumed to have a variable
area nozzle [4]. Fan mass flow, efficiency, and pressure ratio at alternative power settings for propulsors in
the array were determined from a scaled fan map.
3
Turbomachinery
Engine modelling and simulation was performed using an in-house gas turbine performance assessment tool
[10]. Using the tool, a model of the engine can be created from a selection of modules to simulate the engine
thermodynamic performance and predict gas properties of the individual gas turbine components. This in
turn allows for a detailed simulation of the overall engine performance.
The propulsive power for the baseline aircraft is provided by a pair of twin spool turbofan engines. The
performance of the power plants was simulated using public domain data available for the GE90-115B turbofan
engine. The performance data with which the engine was simulated and verified included sea level static thrust
(514kN) mass flow (1641kg/s) and bypass ratio at cruise (between 7.1 and 8.9) [11]. The design point of
the engine was fixed at a cruise altitude of 35,000 feet and Mach 0.85.
The main engines of the N3-X are required to provide power for the aircraft propulsor array through a
superconducting electrical system. The engines are therefore primarily power producing, although a small
amount of core nozzle thrust is produced to counteract the engine drag. Power requirements are defined by
the propulsor array, with each of the main engines providing half of the required power, including an assumed
transmission loss of 0.2% [5]. The engines were sized for the aerodynamic design point, with cruise defined
at 40,000ft and Mach 0.84 [12]. A number of key engine parameters for the simulated baseline and N3-X
engines are listed in Table 2. Further engine design and component efficiency parameters for the N3-X main
engines as assumed for this study may be found in the referenced sources [4, 5, 12].
Table 2: Engine simulation parameters for the baseline aircraft and N3-X main engines
Parameter Baseline N3-X
Design Point
Operating Pressure Ratio 42 64
Turbine Entry Temperature (K) 1503 1811
Bypass Ratio 7.8 N/A
Mass Flow Rate (kg/s) 658 20.4
Net Thrust (kN) 88.2 3.9
SFC (g/kNs) 15.4 N/A
Power Output (MW) N/A 14.6
Sea Level Static
Turbine Entry Temperature (K) 1755 1867
Mass Flow Rate (kg/s) 1648 52.3
Net Thrust (kN) 514 13.5
Power Output (MW) N/A 34.1
Aircraft Performance Simulation
Conventional aircraft mission simulation tools are designed to support standard aircraft configurations and
propulsion systems. However, there are a limited number of tools available to enable the simulation and
integration of the novel propulsion system architecture of the N3-X. Therefore, a custom aircraft performance
model was created for this study, to combine conventional aircraft simulation methods with a module to
simulate a novel and highly integrated propulsion system [9]. The mission performance model applied a point
mass approximation of the aircraft. Block fuel burn was estimated by splitting the aircraft mission into taxi,
take-off, climb, cruise, descent, and landing segments. No improvements to air traffic management were
assumed for 2035. The aircraft are therefore assumed to cruise at a fixed altitude.
Weights and dimensions for the N3-X were obtained from referenced sources[4, 5, 12] in combination with
a 3-D model of the aircraft available in the public domain [13]. Weights and dimensions for the baseline
aircraft were obtained from publicly available sources[14]. Performance obtained from the simulation model
of the baseline aircraft was verified against actual payload-range performance information available for the
baseline aircraft[14]. The simulated baseline aircraft indicated a maximum payload range of 7,474 nautical
miles (<1% error), a maximum fuel range of 9,242 nautical miles (<1% error), and a maximum ferry range
of 10,218 nautical miles (2.7% error).
4
3.2 Economic Modelling
The economic performance of a product or project may be represented by identifying the operating cost and
revenue produced. Operating cost may be split into two components, direct and indirect [1]. Direct costs can
be easily associated with a project, such as materials, labour, or maintenance. Indirect costs are typically more
difficult to attribute to a single project, and will include administrative staff salaries and similar miscellaneous
costs. Although indirect operating costs must be included in a company-wide assessment, it includes costs
which may be distributed over a range of projects. Therefore, direct operating cost is more useful as a point
of comparison. The present study is considered from the perspective of an airline operator and assumes the
outright purchase and operation of a new aircraft. The direct operating cost is therefore the summation of
the following components [1]:
• Fuel
• Emissions taxation
• Maintenance (engine and airframe)
• Insurance
• Depreciation
• Interest repayment
• Crew salary
Both fuel cost and emissions taxation are related to aircraft performance parameters as determined by an
aircraft mission simulation. The emissions taxation component enables the simulation of alternative scenarios
and policies, such as carbon taxation. The remaining components are financial rather than performance
related and can contribute around three quarters of the overall aircraft operating cost [1]. Assuming the
aircraft is purchased through financing - rather than leased - interest on the cost must be repaid over the
aircraft life. Depreciation is not a direct outflow of money, however, it represents the decrease in value of
the aircraft over its life. The useful life and residual value of an aircraft at the end of its life depends on
operator policy [15]. For the present work, residual aircraft value was assumed to be 10% of the initial value
over a 20-year useful life. Insurance cost depends on the risk of operation, and was assumed to be 0.5% of
the aircraft value per year for the present research [1].
Investment cost analyses are often performed to assist in project and investment decisions. Amongst some
of the methods available the application of the concept of Net Present Value calculation (NPV) is quite
prevalent. NPV is an economic valuation concept that accounts for the fact that incomes or expenditures in
the future have less impact than their value at present. Further, a project’s profit and loss are weighted by
making use of a discount factor. Often the discount factor applied is the interest rate, or the weighted average
cost of capital (WACC), a value which accounts for the weighting of costs a company may attribute to debt
and equity. The discount factor represents the return on investment that would be required to exceed the
return achieved by investing the money elsewhere. When comparing a selection of project investments, the
one offering the highest NPV is the one most likely to be selected. Alternatively, the Internal Rate of Return
(IRR) may be calculated. This value is the rate for which the project NPV breaks even, i.e. expenditures
exactly cancel out revenue. A project should ideally exceed the minimum required return rate to be considered
a suitable investment. This minimum is represented by the WACC, which is 7-8% for the airline industry [16].
In the present work, the NPV analysis was made in terms of the difference in direct operating cost between
the baseline aircraft and N3-X, as revenues were assumed to be equal. The project IRR was subsequently
calculated based on this DOC difference, with a NPV formulation as follows [8]:
∆X =
life
X
n=1
∆DOC
(1 + IRR)
n
(1)
In this formulation, ∆X is equal to the difference in aircraft purchase cost), ∆DOC is the difference in direct
operating cost, and IRR is the real rate of return for which the project NPV is equal to zero. The term n
represents the years in the aircraft’s economic life, up to the assumed maximum of 20 years. Direct operating
cost was calculated as cost per flight. Fuel cost and emissions cost were calculated on a cost per flight basis
from the performance analysis. Insurance, depreciation and interest repayments were calculated as costs per
year and were subsequently distributed over the number of missions flown per year. Maintenance cost and
crew salary were per flight hour terms scaled to the length of the flight. A maintenance severity curve was
included to scale maintenance cost per flight hour to the flight length and flights per year. Flight cycles per
year were scaled based on flight length and reference data on the B777-200LR [17].
The effect of inflation can be included in the calculation by either modifying the cash flow or the rate
of return. In the first case, the cash flow is inflated using the current inflation rate and discounted using a
5