Environmental performance of emerging supersonic transport aircraft
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WORKING PAPER 2018-12
Environmental performance of emerging
supersonic transport aircraft
Authors: Anastasia Kharina, Tim MacDonald, Dan Rutherford*
Date: July 17, 2018
Keywords: supersonic, aircraft fuel efficiency, NOX, noise, ICAO
SUMMARY and route. In the best-case scenario, international agreements that call for
the modeled SST burned 3 times as reductions in CO2 , the International
Three U.S.-based startup companies
much fuel per business-class passen- Civil Aviation Organization (ICAO)
are working to develop new super-
ger relative to recently certificated projects that CO 2 emissions from
sonic transport (SST) aircraf t for
subsonic aircraft; in the worst case, international aviation will triple from
planned entry into service in the mid-
it burned 9 times as much fuel com- 2018 to 2050, given current trends
2020s. There are currently no inter-
pared to an economy-class passenger (ICAO, 2013, 2016).
national environmental standards for
on a subsonic flight.
such aircraft, which last flew in 2003. To mitigate this rise in CO2 emissions,
Policymakers are considering whether These findings suggest two pathways ICAO established two aspirational
to develop new specific SST standards for further development of commercial goals for international flights: fleet-
or to apply existing standards for sub- SSTs. First, manufacturers could maxi- wide fuel efficiency improvements of
sonic aircraft to the new designs. mize the likelihood of meeting existing 2% annually through 2050, and zero
environmental standards by develop- net growth of aviation CO2 emissions
This paper provides a preliminary
ing new aircraft based upon advanced, after 2020 (ICAO, 2010). In March
assessment of the environmental per-
clean sheet engines. Second, policy- 2017, ICAO formally adopted new
formance of new commercial SSTs.
makers could establish new environ- global aircraft CO2 emission standards
Results suggest that these aircraft are
mental standards specifically for SSTs for member states to implement start-
unlikely to comply with existing stan-
based upon the performance of poorer ing in 2020. ICAO’s Carbon Offsetting
dards for subsonic aircraft. The most
performing derivative engines. Such and Reduction Scheme for Interna-
likely configuration of a representative
standards would allow for increased tional Aviation (CORSIA) is expected
SST was estimated to exceed limits for
air pollution, noise, and CO2 relative to to come into effect around the same
nitrogen oxides and carbon dioxide
new commercial aircraft. time (ICCT, 2017).
(CO2) by 40% and 70%, respectively.
A noise assessment concludes that
Aircraf t d eve lopm e nt is c a pital -
emerging SSTs are likely to fail current INTRODUCTION intensive and risky; the vast majority
(2018) and perhaps historical (2006)
landing and takeoff noise standards. Aircraft produce about 3% of global of projects are undertaken by large
carbon dioxide (CO2) emissions and airframe manufacturers, often with
On average, the modeled SST was 11% of all CO 2 emissions from the substantial government suppor t.
estimated to burn 5 to 7 times as much transportation sector (EIA , 2018). A more recent phenomenon is that
fuel per passenger as subsonic air- The aviation sector is one of the of startups, often backed by major
craft on representative routes. Results fastest-growing sources of green- companies such as Boeing and Lock-
varied by seating class, configuration, house gas emissions globally. Despite heed Martin, developing new aircraft
Acknowledgments: We thank Malcolm Ralph, Darren Rhodes, Tim Johnson, Ben Rubin, Vera Pardee, Brad Schallert, and Amy Smorodin for their advice and
thorough review. This study was funded through the generous support of the ClimateWorks Foundation and the Joshua & Anita Bekenstein Charitable
Fund.
* Kharina and Rutherford are with the International Council on Clean Transportation. McDonald is a Ph.D. candidate in the Department of Aeronautics and
Astronautics at Stanford University.
© INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION, 2018 WWW.THEICCT.ORGENVIRONMENTAL PERFORMANCE OF EMERGING SUPERSONIC TRANSPORT AIRCRAFT
designs. Examples include compa- Table 1. SST startup companies.
nies developing electric-powered
Company Aerion Spike Boom
aircraft, such as Zunum Aero and
Aircraft type Business jet Business jet Airliner
Joby Aviation, and three companies
in the United States developing new S-512 Quiet
Aircraft name AS2 —
Supersonic Jet
supersonic transport (SST) aircraft:
Target entry into service 2025 2023 2023
Boom Supersonic, Spike Aerospace,
and Aerion Corporation. Boom is Target speed Mach 1.4 Mach 1.6 Mach 2.2
developing a commercial airliner, Target maximum range 7,780 km 11,500 km 8,300 km
while Spike and Aerion are focusing Low-boom technology? No
“Quiet supersonic
No
on supersonic business jets. flight technology”1
Virgin Group
The potential return of supersonic Corporate customers Flexjet — Japan Airlines
CTrip
flights could have large environmen-
tal and noise pollution consequences. 1
Spike Aerospace (2017b).
In 2015, aviation was responsible for
about 800 million metric tons of CO2 performance of their designs. The Spike Aerospace (2017a), based in
emissions, or about as much as the work is meant to inform policymak- Boston, is collaborating with Sie-
German economy. New supersonic ers’ thinking about future standards mens, MAYA Simulation, Greenpoint
aircraft could lead to further emission for new supersonic designs until such Technologies, BRPH, Aernnova, and
increases if they are less fuel-efficient time that higher-fidelity data is made Quartus Engineering Inc. Dubbed
than new subsonic aircraft. available. S-512, Spike’s supersonic business jet
is targeted to fly at Mach 1.6. Unlike
The previous generation of civil super-
Aerion and Boom, Spike’s aircraft
sonic aircraft, the Aérospatiale/BAC BACKGROUND design would be powered by two
Concorde and Tupolev Tu-144, took
engines, not three. It claims to use a
their first flights five decades ago. AIRCRAFT “Quiet Supersonic Flight Technology”
Currently, there are no environmental
There have been two commercial in designing its project airplane.
standards applicable to new super-
supersonic vehicles in the past: the
sonic designs. ICAO’s Committee on Aerion Corporation, Spike’s competi-
Aérospatiale/BAC Concorde and the
Aviation Environmental Protection tor in delivering the first supersonic
Tupolev Tu-144. Seventeen Tu-144s
(CAEP) is now developing noise and were manufactured, including 14 pro- business jet, aims to perform the first
emission certification standards for duction aircraft that flew commer- flight of its AS2 aircraft in 2023 and to
supersonic aircraft (FAA, 2018a). cially 102 times before being decom- bring it into service in 2025. It is the
missioned in 1978 (NASA , 2014). only company to identify its engine
This paper presents a preliminary m a n u f a c tu re r : G e n e r a l El e c tr i c .
Concorde, while equally limited in
analysis of a new commercial SST’s Aerion collaborated with Airbus in air-
production, had a more substantial
performance in terms of fuel burn, craft design and signed an agreement
service life. It flew its first scheduled
CO2 and nitrogen oxide (NOX ) emis- for co-development with Lockheed
supersonic passenger service in 1976
sions, and landing and takeoff (LTO) Martin, the developer of NASA’s low-
and the last in 2003. Both aircraft
noise. Other environmental factors, boom flight demonstrator experimen-
were powered by turbojet engines
including sonic boom, particulate tal plane (or X-plane). This business
with afterburners, which led to high
matter, and stratospheric water vapor jet is targeted to fly at Mach 1.4 using
fuel burn and takeoff noise.
have not been addressed. The analy- three engines.
sis uses publicly available data, expert Three companies in the United States
engineering judgment, and an open- are currently developing new SSTs: B o o m S u p e r so n ic is d eve l o pin g
source aircraft conceptual aircraft Spike Aerospace, Aerion Corpora- a 55-seat commercial jet capable
design tool (SUAVE). The analysis tion, and Boom Supersonic. Spike and of operating at Mach 2 . 2 with a
addresses a key data gap since manu- Aerion are both focusing on business design range of 4,500 nautical miles
facturers are currently releasing little jet models, whereas Boom is develop- (8,300 km). Boom is not developing
information about the environmental ing a supersonic airliner. a specific technology or design to
2 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION WORKING PAPER 2018-12ENVIRONMENTAL PERFORMANCE OF EMERGING SUPERSONIC TRANSPORT AIRCRAFT
suppress sonic boom; instead, it is The ICAO noise standard does not not be applied to new supersonic
relying on the use of a newer engine include a regulatory limit for super- designs (ICAO, 2014b).
and better aerodynamics than Con- sonic aircraft (ICAO, 2014a). In 2004,
corde’s to manage sonic boom. It is CAEP formed a supersonic task group The lack of supporting standards
developing a one-third–scale super- under its Working Group 1, which for both noise and engine emissions
sonic airplane that will demonstrate focuses on noise pollution. This group complicates the development of new
Boom’s technology prior to finalizing has been monitoring the development supersonic aircraft. Without interna-
its airliner design. Boom claims that its of supersonic technologies in order tional standards providing regulatory
aircraft “won’t pollute any more than to develop eventual en-route (sonic certainty that their aircraft can be sold
the subsonic business-class travel it boom) and LTO noise standards. and operated worldwide, major manu-
replaces” (Dourado, 2018). Table 1 facturers will be reluctant to invest in
summarizes key elements of these Although the United States is an new designs. But developing an emis-
active participant in CAEP negotia- sion standard within ICAO typically
three companies and their products.
tions, it is moving forward indepen- requires primary flight and engine test
Several government agencies are also dently to regulate supersonic aircraft data for a wide variety of aircraft types.
involved in supersonic development. noise. Citing concerns about sonic Negotiations among ICAO member
NASA has been a major player in SST boom, the Federal Aviation Admin- states can be slow. For example, the
technology development for decades, istration (FAA, 1973) banned civilian 2016 ICAO CO2 standard (ICCT, 2016)
including building several supersonic aircraft from flying faster than Mach took seven years, or more than two
X-planes. The agency’s next super- 1 over U.S. soil. This contributed to CAEP cycles of three years each, to
sonic X-plane, the Low Boom Flight effectively banning Concorde opera- develop. Typically, an additional four
Demonstrator, will be built by Lock- tions over the continental United years of lead time is provided before
heed Martin and delivered in 2021 States and limited its movement to new standards take effect.
(NASA, 2018). On the other side of the transoceanic routes. Language cur-
Atlantic, the European Union (EU) is rently incorporated into the 2018 FAA Two general approaches are under
funding the Regulation and Norm for reauthorization bill would undo that consideration: either to develop spe-
Low Sonic Boom Levels, or RUMBLE. restriction (U.S. Government Publish- cific, SST-only standards for new air-
RUMBLE1 aims to develop and assess ing Office, 2018). craft, or to require that new designs
sonic boom prediction tools, study comply with existing LTO NOX , noise,
Separately, the FAA (2018a) is initiat- and CO2 standards for new subsonic
the human response to sonic boom,
ing a process to develop U.S.-specific airplanes. Separately, both the eco-
and validate the findings using wind-
standards for civil supersonic aircraft nomic viability and public acceptabil-
tunnel experiments and flight tests.
noise, including a proposed rule for ity of new supersonics will depend in
This is part of the effort by the EU
LTO noise certification of supersonic
to assess the “social acceptability” part on their fuel efficiency. Four met-
aircraft. This rulemaking process may
of new designs to support European rics—NOX emissions, noise, cruise CO2
or may not be in line with ICAO’s
regulatory development. emissions, and mission fuel burn—are
standard setting. If the United States
explored in this paper.
sets its own standards for SSTs, other
POLICY countries may adopt operational
restrictions on those aircraft. Thus, METHODOLOGY
The development of new supersonic it is unlikely that the U.S. govern-
aircraft designs will be influenced ment will adopt a national standard SCOPE AND OVERALL
by international aviation standards. instead of coordinating internationally APPROACH
Those are decided by ICAO, the spe- through ICAO.
cialized UN agency that sets recom- According to the FAA (2018b), jet
mended standards and practices for On the emissions side, ICAO has air- fuel consumption for commercial
civil aviation worldwide. Currently, craft engine emission certification flights accounts for more than 90%
international standards to support the standards for engines capable of of total U.S. jet fuel consumption
certification of new supersonic air- supersonic flight based on the Con- for both domestic and international
craft and engines are not in place. corde (ICAO, 2017a). However, in operations. Globally, general avia-
2007, CAEP agreed that these strin- tion is understood to be about 2%
1 https://rumble-project.eu/i/. gency limits are outdated and should of total aviation fuel use (GAMA ,
WORKING PAPER 2018-12 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION 3ENVIRONMENTAL PERFORMANCE OF EMERGING SUPERSONIC TRANSPORT AIRCRAFT
2009). Because business jets are a costs on its website. Relative to pres- recognize the availability of more
small contributor to overall emissions, ent-day subsonic service, supersonic advanced design and manufacturing
we decided to focus on commercial travel on Boom airliners is claimed to methods, such as using the area rule
supersonics in this analysis. save more than half the time it takes to to design a fuselage with reduced
fly trans-Atlantic, and a little less than wave drag. As a result, the overall lift/
The first step in the analysis is to iden- that for trans-Pacific, because the lat- drag ratio of the vehicle was improved
tify a representative commercial SST ter would require a refueling stop due by an average of 10% from the value
design. Boom Supersonic, the only to range limitations. Boom estimates initially calculated under the low-fidel-
company currently developing such one-way ticket prices for three routes: ity methods.
an aircraft, provided that basis. Boom $3,200 between Tokyo and San Fran-
has received support from a variety cisco, $3,500 between Los Angeles The engine model used in this work is
of investors and customers. In March and Sydney, and $2, 500 between a low-fidelity model built into SUAVE.
2017, Boom (2017) announced that it New York and London. The company High-level engine cycle parameters—
had received $41 million in funding also claims a fuel efficiency similar to namely bypass ratio, overall pressure
including from Y Combinator, Sam that of existing premium-class twin- ratio, and turbine inlet temperature—
Altman, Seraph Group, Eight Partners, aisle aircraft. are used as inputs into the model.
and others. Japan Airlines provided Efficiencies of the engine components
$10 million in addition to 20 aircraft The following sections describe the are based on technology level esti-
preorders, while Virgin Atlantic holds tools and assumptions used to esti- mates from Mattingly (2006). From
an option on 10 aircraft. All in all, mate NOX and CO2 emissions, noise, these parameters, the thermody-
Boom has received 76 aircraft com- and mission fuel burn of a reference namic equations are solved across the
mitments across five airline customers commercial SST based on Boom’s engine components to find the tem-
(Etherington, 2017). design. These values are compared perature and pressure at each com-
against current ICAO subsonic emis- bustion stage, along with the final exit
Boom is aiming for a 2023 entry into sion standards and the fuel burn of jet velocity.
service for its aircraft. Before build- equivalent commercial aircraft on rep-
ing its airliner, Boom is building a resentative missions. Piano 5 aircraf t performance and
one-third–scale demonstrator air- design software (Lissys Ltd., 2017)
craft dubbed XB-1 or “Baby Boom.” was used to compare estimated super-
TOOL SUMMARY sonic aircraft fuel burn with compara-
Subsonic flight tests are planned near
Boom’s hangar at Centennial, Colo- To evaluate new supersonic aircraft, ble subsonic transport. Two subsonic
rado, followed by supersonic flights we chose SUAVE (Lukaczyk et al., aircraft were chosen as baseline: Air-
at Edwards Air Force Base in South- 2015), an open-source conceptual bus A321LR (long range) and Boeing
ern California for technology valida- aircraft design tool with development B787-9. The narrow-body A321LR
tion purposes. currently led by Stanford University. It was chosen because of its similarity
was specifically designed to be able in overall weight and range capacity
At the start of this project, Boom was to evaluate the performance of uncon- to the Boom aircraft, which makes it
contacted for information about the ventional aircraft, including super- suitable for trans-Atlantic flights. The
design and performance characteris- sonic configurations. B787-9 was chosen to represent con-
tics of its aircraft. General confirmation ventional transoceanic travels on a
of publicly available data was provided, The aerodynamics model derived from twin-aisle aircraft without a refueling
but no detailed information regarding SUAVE was used for both subsonic stop. Both aircraft are state-of-the-art
engine specification was offered. and supersonic conditions (Lukaczyk subsonic aircraft at the time of writing
et al., 2015). These low-fidelity meth- and will be representative of newer in-
The following public information was ods have been validated against Con- service aircraft when new commercial
available to support this assessment. corde performance numbers. Details SSTs enter into service.
A three-view drawing on Boom’s are available in the initial SUAVE
website was used to develop the geo- paper, with minor refinements made The mission profile used in modeling
metric representation necessary for over time. Because the reference SST is based roughly on the profile flown
aerodynamic calculations (see below). aircraft is largely the same shape as by Concorde. We used the full mission
Boom also provides estimated routes, the Concorde, we expect that these profile to include the fuel burn used in
travel times, ticket prices, and fuel methods will hold. However, we also the climb portion of the mission. In the
4 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION WORKING PAPER 2018-12ENVIRONMENTAL PERFORMANCE OF EMERGING SUPERSONIC TRANSPORT AIRCRAFT
Table 2. Airframe parameters used for modeling.
Parameter Value Source
www.flightglobal.com/news/articles/dubai-boom-to-make-a-big-noise-
Maximum takeoff mass (kg) 77,000
at-show-about-shorte-442767
Design range (km) 8,300 https://boomsupersonic.com/airliner
Maximum passengers 55 https://boomsupersonic.com/airliner
Design speed (Mach number) 2.2 https://boomsupersonic.com/airliner
Length (ft) 170 https://boomsupersonic.com/airliner
Wingspan (ft) 60 https://boomsupersonic.com/airliner
Reference geometric factora (m2) 80 Estimated
Balanced field length (ft) 10,000 https://boomsupersonic.com/airliner
https://techcrunch.com/2017/01/12/boom-shows-off-its-xb-1-supersonic-
Cruise altitude (ft)b 60,000
demonstration-passenger-airliner
Medium-bypass-ratio https://blog.boomsupersonic.com/why-we-dont-need-an-afterburner-
Engine
turbofan, no afterburner a4e05943b101
a
Reference geometric factor, which approximates an aircraft’s pressurized floor area, is used to calculate the CO2 standard metric value. The metric
value is used to demonstrate compliance with ICAO’s CO2 standard (see below).
b
We reduced the cruise altitude slightly in our analysis to meet a lower average altitude more consistent with a cruise-climb to 60,000 ft.
SUAVE framework, missions are con- We used a three-view drawing avail- fuel burn but with a higher develop-
structed as a series of segments that able on Boom’s website to develop ment cost. Currently, no manufactur-
are further split into control points. the geometric representation neces- ers are producing commercial tur-
At each control point, conditions sary for aerodynamic calculations. bofan engines that could operate at
(altitude, speed, climb rate, etc.) are The measurements were made with Mach 2.2. An alternative approach,
specified, and the equations of motion Digimizer, a digital measurement tool described in Fehrm (2016), would be
are solved to determine aircraft per- (MedCalc Software, 2018). We used to develop a commercial SST using an
formance. This is done iteratively, seg- the tool to estimate all airframe geo- engine derived from an in-production
ment by segment, to determine the metric parameters except for wing military turbojet aircraft.
full mission performance. See Lukac- and vertical tail thickness. An Open-
zyk et al. (2015) for further details of VSP (Fredericks, 2010) model based We assume that the new supersonic
on these estimates is used to deter- aircraft will use a variable-geometry
the mathematics involved.
mine wetted areas. For the wing and nozzle, as Concorde did, that will be
tail thickness, we assumed that Boom capable of keeping the flow nearly
MODEL CONSTRUCTION would be able to create structures perfectly expanded. The result is an
Parameters for the SST model were slightly thinner than Concorde, reduc- idealized engine that provides accu-
determined primarily using publicly ing the wing’s maximum thickness rate values when operated near its
from 3% to 2.25%. The vertical tail is design point, which means that climb
available information on Boom’s air-
assumed to be slightly thicker than
craft. In general, Boom’s statements and cruise values are expected to
the wing at 3.5% of the chord length.
of its designed capability were taken be representative of a well-designed
See Table A1 in the Appendix for the
as truth; we did not modify estimates engine with the parameters specified
full list of geometric parameters.
for parameters such as maximum in Table 3. Although doubts have been
takeoff mass or engine bypass ratio Boom’s exact engine configuration raised about the capability of con-
based upon our own expert judge- has yet to be announced. The com- structing an efficient engine with the
ment. This may lead to overestimating pany aims to develop an aircraft with properties specified (Fehrm, 2016),
the real-world performance of the ref- a medium bypass–ratio turbofan using we provide this analysis assuming that
erence SST aircraft. The high-level air- an existing core and no afterburners. such an engine can be designed. We
frame parameters used in the model A clean-sheet turbofan engine would therefore expect that the calculated
are shown in Table 2. provide lower noise, emissions, and emissions will be optimistic.
WORKING PAPER 2018-12 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION 5ENVIRONMENTAL PERFORMANCE OF EMERGING SUPERSONIC TRANSPORT AIRCRAFT
Three engine models were devel- Table 3. Engine parameters used for modeling.
oped to span the range of possible
performance: most likely (derivative Configuration
Parameter
turbofan), best case (clean-sheet Best Most likely Worst
turbofan), and worst case (derivative Clean-sheet Derivative Derivative
Engine type
turbojet). For the most likely case, turbofan turbofan turbojet
we examined an engine expected to Bypass ratio 3.0 3.0 —
be usable on the Aerion AS2 based Overall pressure ratio 15 15 13.8
on the CFM56 (Fehrm, 2018). In its Turbine inlet temperature (K) 1850 1650 1800
expected Mach 1.4 flight condition
Landing and takeoff thrust
with refanning, the engine would 40,000 50,000 30,000
available (lbf)
have a lower-pressure compressor Top of climb thrust available (lbf) 7,600 8,200 9,200
(LPC) pressure ratio of 2, a high-
pressure compressor (HPC) pres- aircraft by Fehrm (2018). In this case, assumed aerodynamic improvement
sure ratio of 10, and a turbine inlet the compressor outlet temperature is over the Concorde. Improvements
temperature (T4) of 1650 K. To adapt limited to below 700°C. This engine of 20%, 10%, and 0% (no improve-
this for Mach 2.2 flight, we assume provides a worst-case scenario esti- ment) in the lift-to-drag ratio were
that the pressure ratio is limited by mate on engine performance, although assumed for the best-case, most
the temperature at the compressor it is unlikely to be used for commercial likely, and worst-case configurations,
outlet as a result of material tem- aircraft because of excessive noise. respectively.
perature limits in the compressor An aircraft based on this existing
(Fehrm, 2016). This provides us with military engine could be brought to
a HPC compression ratio of about FUEL EFFICIENCY
market more easily and more cheaply
7.5. We also assume a bypass ratio DETERMINATION
than one using a turbofan.
of 3, consistent with Boom’s stated The fuel efficiency of the reference
engine plan. This may be optimistic We checked that these engines are SST was compared to existing sub-
given the resulting high ram drag on capable of producing the required sonic standards and aircraft using
Mach 2.2 operations. ta ke of f a n d cruise th rust . B oom two metrics: the ICAO CO 2 metric
claims a takeoff thrust in the range value (CO2MV) (ICCT, 2016) and mis-
To represent the best-case scenario, of 15,000 to 20,000 lbf per engine sion fuel burn.
we cre ate d a n a dva n ce d e n gin e ( Trimble, 2017 ). We estimate the
aimed at meeting NASA’s N+2 goals need for about 7,000 to 8,000 lbf The ICAO CO2MV is based on the
for supersonic aircraf t (Welge et per engine in cruise and somewhat maximum specific air range (SAR),
al., 2010). In this case, the assump- more than that for climb. Our analy- which is a measure of cruise fuel
tion of a clean-sheet design allows sis indicates that this performance efficiency. The MV is expressed as
us to vary T4 to find the maximum is possible for all three engines with 1/(SAR × RGF 0.24), where RGF is the
efficiency. We assume a compressor resizing. 2 All engine models use the reference geometric factor deter-
outlet temperature limit of 720°C for same component efficiencies. Table mined by multiplying the pressur-
this design (Welge et al., 2010). This 3 summarizes the engine parameters ized fuselage length by the fuselage
provides a somewhat more efficient used in the modeling. width. This approximates the amount
engine. As a clean-sheet design, this of usable space in the aircraft. We
engine would be more advanced, Uncertainty in the aerodynamic per-
estimate the pressurized length of
formance of the representative SST
and therefore more complex and the reference SST to be about 35 m
was captured by varying the level of
costly, than the derivative turbofan and the width to be about 2.3 m.
that near-term SST manufacturers
2 Note that LTO thrust exceeds what is Maximum SAR measures the distance
are likely to deploy. needed because engines must be sized for
top-of-climb thrust. Additional uncertainty
(km) traveled per mass (kg) of fuel
Our final engine is a turbojet based is also present at takeoff because the under optimal flight conditions. To
on an in-production military engine design mass flow near zero speed is not calculate it, the aircraft is simulated
available. It is expected that engines for
(EJ200) with constraints suggested new SSTs will be derated to bring them in at a variety of altitudes and cruise
in the series of articles on supersonic line with LTO requirements. speeds to find the condition that
6 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION WORKING PAPER 2018-12ENVIRONMENTAL PERFORMANCE OF EMERGING SUPERSONIC TRANSPORT AIRCRAFT
gives the maximum value. In addition
to finding the maximum cruise SAR,
we also find the maximum SAR at
ANC
Mach 2.2 and at subsonic conditions.
Those values are important because 5500 km
3200 km 5600 km
SSTs will be required to fly over many 8200 km
LHR
countries subsonically and will likely NRT SFO JFK
LAX
not fly at maximum supersonic SAR if
the associated speed is too low. Supersonic route
Subsonic route
We next analyzed mission fuel burn. Shared route
This is defined as gate-to-gate fuel 6600 km
12,100 km
burn minus fuel used to taxi the air-
craf t. To determine this , we use
climb and descent segments similar
PPT
to what Concorde performed, along
with a cruise segment at constant SYD 6100 km
altitude. Although we expect that the
new supersonic aircraft will perform
a cruise-climb to reduce fuel burn Figure 1. Routes investigated. (Source: GCmap.com)
slightly, this would reduce mission fuel
burn on the order of only 1 to 2%. Mod- south out of ANC to clear Alaska’s 2017b). 4 Freight was assumed to be
eling an optimal trajectory is therefore southern edge. negligible for narrow-body aircraft
not necessary to reach the level of and the SST, whereas for wide-body
accuracy targeted in this study. The analysis distinguishes between aircraft it was assumed to be 16%
premium (both supersonic and sub- of total payload for flights operat-
Three different origin-to-destination sonic) and economy (subsonic only) ing between North America and the
missions were chosen for the analysis, service. We assumed a load factor of Pacific or Southeast Asia/Oceania,
corresponding to routes highlighted 80% for economy and 60% for pre- and 34% of total payload for flights
by Boom: San Francisco–Tokyo (SFO- operating between North America
mium passengers based on Bofinger
NRT), Los Angeles–Sydney (L A X- and East Asia (ICAO, 2018).
and Strand (2013). 3 To apportion fuel
SYD), and New York–London (JFK-
use between premium and economy
LHR). Because of the SST’s shorter To determine the aircraf t takeof f
seats on the subsonic aircraft, we
design range, we assume that a refuel- weight for each SST mission, we first
ing stop would be needed in Anchor- assigned a weighting factor of 2 to 1 used the maximum takeoff weight in
age (ANC) and Tahiti (PPT) for the first to account for the greater floor area a generic full-range mission of 4,500
two routes, respectively. These routes of long-haul premium seats (ICAO, nautical miles. The landing weight was
and distances are shown in Figure 1. determined, corresponding to the sum
3 Transparent load factor data for both premium of the payload, reserve fuel, and aircraft
Mission distances were assumed to be subsonic and supersonic seating is limited.
On subsonic operations, Delta reported an empty weight. This landing weight was
great-circle distances if there is not a estimated first-class load factor of 57% in then reduced to account for the 60%
substantial land mass under the route. 2015 (Anderson, 2015). For supersonics, the load factor assuming 100 kg per pas-
Because the great-circle distance for Concorde’s load factors were highest between
London and New York and between Paris and senger. This provided the expected
the ANC-NRT route includes about New York for British Airways and Air France, landing weight for each origin/desti-
1,000 km traveling over Alaska, we respectively. BA’s load factors between JFK and nation-specific mission. The mission
estimated a subsonic flight over Alaska LHR were reported to be between 50 and 60%
in 2002 (Kingsley-Jones, 2002) and as high
at a speed matching optimal subsonic as 73% in the first six months of operations in 4 Bofinger and Strand (2013) calculated a
SAR. This created a slightly slower (15 1978 (Witkin, 1978). Air France achieved load business-class to economy-class emissions
min) but more fuel-efficient flight path factors above 60% on its Paris–New York and multiplier of 1.86 to 2.71 for flights where the
Paris–Rio de Janeiro routes (ibid.). Other routes, passenger weight share is 12.5% of the total
relative to a purely supersonic, longer- including Paris–Caracas and London–Bahrain, aircraft weight, close to the simple average of
distance route that requires flying experienced load factors well below 60%. subsonic flights in this analysis.
WORKING PAPER 2018-12 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION 7ENVIRONMENTAL PERFORMANCE OF EMERGING SUPERSONIC TRANSPORT AIRCRAFT
was then solved iteratively to deter- so we instead used exit jet velocities emissions of the reference SST air-
mine what takeoff weight (and there- to investigate likely noise character- craft relative to existing subsonic
fore fuel burn) would be needed to istics. The jet velocity is found by tak- standards. Values for the most likely,
land at the specified weight. ing a mass-weighted average of the best-case, and worst-case configura-
core and fan velocities from SUAVE’s tions are summarized, along with their
In modeling the mission fuel burn of engine model. The accuracy of these regulatory values and the year each
subsonic aircraft, default Piano 5 val- jet velocities depends on the engine’s standard takes effect. Regulatory
ues for operational parameters such as capability to operate at an ideal noz- limits for NOX are set as a function of
engine thrust, drag, fuel flow, available zle area ratio, which assumes that the overall pressure ratio, whereas stan-
flight levels, and speed were used. engine has a variable nozzle that can dards for CO2 are set as a function of
reach the necessary outlet area. Boe- aircraft maximum takeoff mass. NOX
Cruise speeds were set to allow 99%
ing has determined that a jet velocity emission estimates were not avail-
maximum specific air range. Flight
of 1,100 ft/s will be sufficient to meet able for the worst-case configuration
times were estimated by including
20 min of taxi time (both in and out) Chapter 3 minus 10 to 20 EPNdB 6 because emissions data is not avail-
for all flights, plus 30 min for refuel- (Welge et al., 2010). Research sug- able for military engines.
ing for routes longer than the range of gests that the lateral noise limit is
the key determinant of passing noise As Table 4 indicates, the reference
the SST (Figure 1).
standards stricter than Chapter 3, so SST is unlikely to meet existing com-
we focused specifically on that value. mercial aircraft standards. It exceeded
EMISSIONS AND NOISE allowable LTO NOX limits by 38% in the
NOX emissions were estimated for the most likely configuration, and CO2MV
most likely and best-case scenarios
RESULTS limits by 52% to 115%, with a most likely
using emissions data for in-produc- Table 4 summarizes the results for exceedance of 67%. The best-case,
tion engines in the ICAO engine emis- LTO NO X and cruise CO 2 (CO2MV) advanced clean-sheet engine was
sions databank (ICAO, 2018).
Table 4. Modeled NOX and CO2 performance of SST aircraft by configuration.
An exponential curve of NOX emission Configuration
indices versus overall pressure ratio Pollutant Standarda Year Parameter Most
(OPR) for current CFM56 (CFM56-5 Best Worst
likely
and -7) engine data was used to
Overall pressure ratio 15 15 13.8
adjust the emissions values. 5 These
SST (g/kN) 18 40 —b
engines use rich-quench-lean (RQL) NOX CAEP/8 2014
Standard (g/kN) 29 29 —b
combustor technology. For the most
likely (derivative turbofan) engine, no Exceedance –37% +38% —b
further adjustments were made. LTO Maximum takeoff mass (kg) 77,000
emissions for the best-case engine SST (kg/km) 1.21 1.33 1.72
CO2 CAEP/10 2020
(i.e., clean-sheet turbofan) were esti- Standard (kg/km) 0.80
mated by correcting for the lower Exceedance +52% +67% +115%
emissions of the LEAP engine family
relative to current CFM56 engines.
a
ICAO’s environmental standards are referenced to the meeting at which they were agreed. ICAO’s
current CAEP/8 (NOX) and CAEP/10 (CO2) standards were finalized in 2010 and 2016, respectively.
b
NOX emission estimates were unavailable for this configuration.
Building a sophisticated noise model
was beyond the scope of this work,
6 The ICAO Chapter 3 noise standard, applicable
since 1978, is the current operational noise
5 This fit line is generally consistent with standard in many ICAO member states
typical NOX correlation equations, such as including the United States and Europe. This
the one found in the GasTurb manual, which means that airplanes that do not comply with
is a widely used program for calculating the Chapter 3 noise standard are not allowed
engine cycle parameters (GasTurb, n.d.). This to fly. The Chapter 4 noise standard, applicable
approach has the added benefit of providing from 2006 to 2017, is 10 EPNdB (cumulative)
NOX data for all of the required mode settings, quieter than Chapter 3. The current applicable
whereas the SUAVE model cannot handle noise standard, Chapter 14, is 17 EPNdB
off-design conditions. (cumulative) more stringent than Chapter 3.
8 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION WORKING PAPER 2018-12ENVIRONMENTAL PERFORMANCE OF EMERGING SUPERSONIC TRANSPORT AIRCRAFT
estimated to comply with the latest SST Subsonic average Subsonic economy Subsonic business
subsonic NOX standards. This finding
is consistent with the view that staged
Average
lean-burn combustors can consider- JFK-LHR
A321LR
ably reduce NOX emissions. Note that By subsonic class
the CO2MV results are more certain
than the NOX estimates.
A detailed assessment of noise cer- SFO-NRT
tification levels is beyond the scope B787-9
of this work. Some simple observa-
tions can be made, however. Relative
to the jet exit velocity limit of 1,100
ft/s identified in Welge et al. (2010),
LAX-SYD
SUAVE’s engine model predicts a jet B787-9
exit velocity of 1,350 ft/s in the most
likely configuration, and 1,550 and
3,400 ft/s for the best- and worst- 0 300 600 900 1200 1500 1800
case configurations, respectively. Mission fuel (kg/passenger)
This implies that the aircraft would Figure 2. One-way mission fuel consumption per passenger by route and class.
not meet existing (Chapter 14/Stage
5) standards. Engine derating, com-
consumed between 5 and 7 times as to enable refueling and a high share of
bined with modified landing and
much fuel per passenger relative to belly freight carriage.
takeoff procedures, is believed to be
comparable subsonic aircraft. Divided
needed to bring new SST aircraft into These values are for the most likely SST
by class, the SST burned between 3
compliance with the 2006 Chapter 4 configuration. Taking into account the
and 4 times as much fuel per passen-
noise standards (Welge et al., 2010). full range of uncertainty, the per-pas-
ger for premium (business) service,
Cer tification to current subsonic senger fuel intensity of the SST varied
and between 6 and 8 times as much
noise standards is likely to require from 3 times (best configuration rela-
fuel per economy passenger. 7 The
additional technological solutions— tive to subsonic business class, LAX-
lower multiples were associated with
for example, a clean-sheet advanced SYD) to 9 times (worst configuration
the New York (JFK)–London (Heath-
variable-cycle engine—that are cur- relative to subsonic economy class,
row) and Los Angeles–Sydney routes,
rently not being considered for near- SFO-NRT) that of its subsonic equiva-
which largely followed great-circle
term SSTs. lent. Estimated travel times were 30
distances with relatively little belly
to 50% shorter for the Mach 2.2 SST
Figure 2 summarizes the expected freight carriage. The higher multiples
relative to the subsonic aircraft, which
mission fuel performance of the refer- were seen for the SFO-NRT route,
typically operate near Mach 0.85.
ence SST aircraft relative to new sub- which had a 6% excess flight distance
sonic aircraft on comparable missions. Aircraft fuel burn, LTO NOX , and LTO
7 High mission fuel burn is directly related to noise are not the only environmental
The applicable routes, aircraft types,
high CO2 emissions during cruise. The gap
and fuel use (mass per passenger) between the SST’s smaller (70%) exceedance
issues facing SSTs. Some of the issues
are shown for average, premium, and to the CO2 standard and its larger (5 to 7 not considered in this paper stem
times) overall fuel intensity is due to the way from the high cruise altitudes of SSTs,
economy passengers. Best- and worst- that the CO2 standard assigns regulatory
case SST scenarios are indicated as targets to individual aircraft. Supersonic
including cruise NOX , stratospheric
error bars on the blue SST bars. aircraft, which are disproportionately heavy water vapor, and magnified non-
compared to subsonic aircraft carrying the CO2 effects. Sonic boom or en-route
same number of passengers, would receive
As Figure 2 indicates, in its most less stringent targets if subject to the
noise impacts are also important but
likely configuration the modeled SST standard. See ICCT (2016) for further details. beyond the scope of this work.
WORKING PAPER 2018-12 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION 9ENVIRONMENTAL PERFORMANCE OF EMERGING SUPERSONIC TRANSPORT AIRCRAFT
CONCLUSIONS AND based upon designs using poorer a Paris-compatible global carbon bud-
performing derivative engines. Such get by 2075 (Rutherford, 2018). The
NEXT STEPS
standards would allow for increased introduction of new supersonic air-
This working paper provides a prelimi- pollution and nuisance relative to new craft opens up the potential for even
nary assessment of the environmental commercial aircraft. larger increases. The social accept-
performance of emerging commer- ability of this increase, and therefore
cial SST aircraft. Multiple scenarios Independent of standards, the eco- the public’s support for supersonic
representing most likely, best-case, nomic feasibility and social acceptabil-
travel, remains to be determined.
and worst-case configurations were ity of new designs remain to be seen.
developed to bound the range of pos- The representative SST is expected This working paper has provided an
sible uncertainty. Where provided, to burn 5 to 7 times as much fuel per initial assessment of one aspect of
manufacturer claims of airframe and passenger as comparable subsonic SST operations, namely their emis-
engine design parameters were used aircraft. The results were sensitive sions and noise characteristics. Sub-
as modeling inputs. Accordingly, our to seating class, route, and the exact stantial data gaps persist with respect
overall findings are likely optimis- configuration of the aircraft. In its best to the characteristics of the engines
tic; the actual performance of future possible configuration and route, the that may be deployed as well as pre-
supersonic aircraft may be worse. SST burned 3 times as much fuel per cise airframe parameters (e.g., maxi-
business-class passenger relative to mum takeoff weight, empty weight,
This analysis suggests that near-term subsonic aircraft; in the worst con- range, and payload). Further work is
commercial SSTs are unlikely to com- figuration with a refueling stop, the needed in particular to better charac-
ply with existing standards for com- difference would be 9 times as much terize noise levels, both for LTO and
mercial aircraft. The most likely con- fuel for an economy-class passenger. supersonic en-route noise or sonic
figuration of a representative SST was
Fuel is typically an airline’s single larg- boom. LTO NOX emissions estimates
estimated to exceed limits for NOX and
CO2 by 40% and 70%, respectively. A est operating expense, accounting for could be refined fur ther through
qualitative assessment of noise was 20 to 35% of overall airline operating the use of analytical models such as
consistent with the understanding that costs. Current jet fuel prices of about GasTurb. Furthermore, little is known
engine derating and modified LTO pro- $700 per metric tonne (IATA, 2018) about how LTO NOX relates to cruise
cedures would be needed to comply mean that the fuel costs of trans- NOX for these aircraft, and work will be
with older (2006) Chapter 4 noise porting one passenger round-trip needed to establish this relationship.
standards. Advanced technologies, from San Francisco to Tokyo via SST
The viability of new commercial SSTs
including variable-cycle engines and would be around $1400, versus about
will depend on more than just pollu-
staged combustion, on a clean-sheet $180 to $360 for subsonic economy
tion and nuisance. Additional work is
engine would likely be needed to meet class and business class, respectively.
needed to understand other aspects
current LTO noise and NOX standards. Profitable operation of these aircraft
of commercial SSTs. A route-based
would require revenue and yields
These findings suggest two pathways analysis of how commercial SSTs
high enough to recover these extra
for further development of commer- fuel costs. may be integrated into current and
cial SSTs. First, manufacturers could future airline networks and air traf-
refocus their development efforts on This increased f uel consumption fic management at airports is recom-
designs with advanced, clean sheet would lead to proportional increases mended. Similarly, an economic anal-
engines. Those are more likely to meet in CO 2 emissions. The share of CO 2 ysis of likely fares, operating costs,
existing subsonic aircraft standards. emissions attributable to international yield, profitability, etc., would help to
Second, policymakers could establish aviation is expected to increase from clarify the business case for commer-
new environmental standards for SSTs 1.4% today to between 7% and 14% of cial supersonics.
10 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION WORKING PAPER 2018-12ENVIRONMENTAL PERFORMANCE OF EMERGING SUPERSONIC TRANSPORT AIRCRAFT
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12 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION WORKING PAPER 2018-12ENVIRONMENTAL PERFORMANCE OF EMERGING SUPERSONIC TRANSPORT AIRCRAFT
Appendix
Table A1. Detailed geometric parameters.
Parameter Value Source
Main wing aspect ratio (span2/reference area) 1.39 Calculated
Main wing thickness/chord ratio 0.0225 Engineering Judgement
Main wing quarter chord sweep (degrees) 55.9 Measured using Digimizer (MedCald Software, 2018).
Main wing span (m) 18.3 Boom website
Main wing root chord (m) 20.8 Measured using Digimizer.
Main wing tip chord (m) 2.8 Measured using Digimizer.
Main wing mean aerodynamic chord (m) 12.0 Calculated
Main wing total length (m) 21.7 Measured using Digimizer.
Main wing reference area (m ) 2
241 Measured using Digimizer.
Main wing wetted area (m2) 344 Calculated with OpenVSP Model
Tail aspect ratio 0.65 Calculated
Tail thickness/chord ratio 0.035 Engineering Judgment
Tail quarter chord sweep (degrees) 60 Measured using Digimizer.
Tail span (m) 4 Measured using Digimizer.
Tail root chord (m) 11.9 Measured using Digimizer.
Tail tip chord (m) 2.1 Measured using Digimizer.
Tail mean aerodynamic chord (m) 9.4 Measured using Digimizer.
Tail total length (m) 12 Measured using Digimizer.
Tail reference area (m ) 2
24.6 Measured using Digimizer.
Tail wetted area (m2) 60.4 Calculated with OpenVSP Model
Fuselage length (m) 51.8 Boom website
Fuselage maximum height (m) 2.7 Measured using Digimizer.
Fuselage width (m) 2.4 Measured using Digimizer.
Fuselage wetted area (m ) 2
332 Calculated with OpenVSP Model
Fuselage front projected area (m2) 5.3 Measured using Digimizer.
Fuselage effective diameter (m) 2.55 Estimated
10.6 (underwing)
Propulsor length (m) Measured using Digimizer.
12.6 (fuselage)
Propulsor nacelle diameter 1.4 Measured (approximate due to square shape)
Propulsor inlet diameter 1.2 Estimated
Propulsor total wetted area (m2) 45 Estimated (ignored in-fuselage propulsor)
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