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, XNO , noise, ICAO SUMMARY and route. In the best-case scenario, international agreements that call for the modeled SST burned 3 times as reductions in CO , the International Three U.S.-based startup companies 2 much fuel per business-class passen- Civil Aviation Organization (ICAO) are working to develop new super- ger relative to recently certificated projects that CO emissions from sonic transport (SST) aircraft for 2 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 CO emissions This paper provides a preliminary 2 ing new aircraft based upon advanced, assessment of the environmental per- after 2020 (ICAO, 2010). In March clean sheet engines. Second, policy- formance of new commercial SSTs. 2017, ICAO formally adopted new makers could establish new environ- global aircraft CO emission standards Results suggest that these aircraft are 2 mental standards specifically for SSTs unlikely to comply with existing stan- for member states to implement start- based upon the performance of poorer dards for subsonic aircraft. The most ing in 2020. ICAO’s Carbon Offsetting performing derivative engines. Such likely configuration of a representative and Reduction Scheme for Interna- standards would allow for increased SST was estimated to exceed limits for tional Aviation (CORSIA) is expected air pollution, noise, and CO relative to nitrogen oxides and carbon dioxide 2 to come into effect around the same new commercial aircraft. time (ICCT, 2017). (CO2) by 40% and 70%, respectively. A noise assessment concludes that Aircraft development is capital- 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 airframe manufacturers, often with carbon dioxide (CO2) emissions and

On average, the modeled SST was 11% of all CO2 emissions from the substantial government support. 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.

designs. Examples include compa- Table 1. SST startup companies. nies developing electric-powered Company Aerion Spike Boom aircraft, such as Zunum Aero and Joby Aviation, and three companies Aircraft type Business jet Business jet Airliner S-512 Quiet in the United States developing new Aircraft name AS2 — Supersonic Jet supersonic transport (SST) aircraft: Boom Supersonic, Spike Aerospace, Target entry into service 2025 2023 2023 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 “Quiet supersonic Low-boom technology? No 1 No on supersonic business jets. flight technology” Virgin Group The potential return of supersonic Corporate customers Flexjet — Japan Airlines flights could have large environmen- CTrip 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 Aérospatiale/BAC Concorde and the Aerion Corporation, Spike’s competi- Aviation Environmental Protection Tupolev Tu-144. Seventeen Tu-144s tor in delivering the first supersonic (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 Concorde, while equally limited in manufacturer: General Electric. analysis of a new commercial SST’s production, had a more substantial Aerion collaborated with Airbus in air- performance in terms of fuel burn, service life. It flew its first scheduled craft design and signed an agreement CO and nitrogen oxide (NO ) emis- 2 X supersonic passenger service in 1976 for co-development with Lockheed sions, and landing and takeoff (LTO) and the last in 2003. Both aircraft Martin, the developer of NASA’s low- noise. Other environmental factors, were powered by turbojet engines boom flight demonstrator experimen- including sonic boom, particulate with afterburners, which led to high tal plane (or X-plane). This business matter, and stratospheric water vapor fuel burn and takeoff noise. jet is targeted to fly at Mach 1.4 using 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: Boom Supersonic is developing 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

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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- three companies and their products. sion standard within ICAO typically 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 istration (FAA, 1973) banned civilian technology development for decades, 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- the human response to sonic boom, and CO2 standards for new subsonic ing a process to develop U.S.-specific and validate the findings using wind- airplanes. Separately, both the eco- standards for civil supersonic aircraft tunnel experiments and flight tests. nomic viability and public acceptabil- noise, including a proposed rule for This is part of the effort by the EU ity of new supersonics will depend in 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,

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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 estimates 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 aircraft 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 Subsonic flight tests are planned near sonic aircraft fuel burn with compara- 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

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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- the mathematics involved. aircraft will use a variable-geometry 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 available information on Boom’s air- design point, which means that climb 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.

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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 40,000 50,000 30,000 with refanning, the engine would 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 market more easily and more cheaply FUEL EFFICIENCY 7.5. We also assume a bypass ratio than one using a turbofan. DETERMINATION 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. takeoff and cruise thrust. Boom two metrics: the ICAO CO2 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 created an advanced engine (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 aircraft (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 × RGF0.24), 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 Uncertainty in the aerodynamic per- engine would be more advanced, 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

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gives the maximum value. In addition to finding the maximum cruise SAR, we also find the maximum SAR at Mach 2.2 and at subsonic conditions. ANC Those values are important because 5500 km 3200 km SSTs will be required to fly over many 5600 km LHR 8200 km countries subsonically and will likely NRT SFO JFK not fly at maximum supersonic SAR if LAX 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 aircraft. To determine this, we use climb and descent segments similar to what Concorde performed, along PPT 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- mium passengers based on Bofinger operating between North America NRT), Los Angeles–Sydney (LAX- and Strand (2013).3 To apportion fuel and East Asia (ICAO, 2018). SYD), and New York–London (JFK- use between premium and economy LHR). Because of the SST’s shorter To determine the aircraft takeoff 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.

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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 NO are set as a function of engine thrust, drag, fuel flow, available X 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. NO Cruise speeds were set to allow 99% X ing has determined that a jet velocity maximum specific air range. Flight emission estimates were not avail- of 1,100 ft/s will be sufficient to meet times were estimated by including able for the worst-case configuration 6 20 min of taxi time (both in and out) Chapter 3 minus 10 to 20 EPNdB 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 As Table 4 indicates, the reference the SST (Figure 1). the key determinant of passing noise 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 NO emissions were estimated for the most likely configuration, and CO2MV X RESULTS most likely and best-case scenarios 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 NOX and cruise CO2 (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 NO emission X Configuration indices versus overall pressure ratio Pollutant Standarda Year Parameter Most Best Worst (OPR) for current CFM56 (CFM56-5 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) NO CAEP/8 2014 X b combustor technology. For the most Standard (g/kN) 29 29 — b likely (derivative turbofan) engine, no Exceedance –37% +38% — 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 CO CAEP/10 2020 (i.e., clean-sheet turbofan) were esti- 2 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 NO emission estimates were unavailable for this configuration. Building a sophisticated noise model X 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.

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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 by class, the SST burned between 3 These values are for the most likely SST compliance with the 2006 Chapter 4 and 4 times as much fuel per passen- configuration. Taking into account the noise standards (Welge et al., 2010). ger for premium (business) service, full range of uncertainty, the per-pas- Certification 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 freight carriage. The higher multiples Figure 2 summarizes the expected relative to the subsonic aircraft, which were seen for the SFO-NRT route, mission fuel performance of the refer- 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 The applicable routes, aircraft types, 7 High mission fuel burn is directly related to noise are not the only environmental 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 economy passengers. Best- and worst- from the high cruise altitudes of SSTs, 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 CO effects. Sonic boom or en-route same number of passengers, would receive 2 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.

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CONCLUSIONS AND based upon designs using poorer a Paris-compatible global carbon bud- NEXT STEPS performing derivative engines. Such get by 2075 (Rutherford, 2018). The 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 nomic feasibility and social acceptabil- representing most likely, best-case, 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 boom. LTO NO emissions estimates estimated to exceed limits for NO and Fuel is typically an airline’s single larg- X X could be refined further through CO2 by 40% and 70%, respectively. A est operating expense, accounting for 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 about how LTO NO relates to cruise engine derating and modified LTO pro- $700 per metric tonne (IATA, 2018) X NO for these aircraft, and work will be cedures would be needed to comply mean that the fuel costs of trans- X 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 NO standards. Profitable operation of these aircraft X of commercial SSTs. A route-based would require revenue and yields analysis of how commercial SSTs These findings suggest two pathways 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 fuel 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 CO2 emissions. The share of CO2 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.

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Lukaczyk, T., Wendorff, A., Botero, E., MacDonald, T., Momose, T., Spike Aerospace (2017b). “Spike Aerospace Expands Quiet Variyar, A., Vegh, J. M., Colonno, M., Economon, T. D., Alonso, Supersonic Aircraft Development Efforts”; www.spikeaero- J., Orra, T. H., & da Silva, C. I. (2015, June). SUAVE: An Open- space.com/spike-aerospace-expands-quiet-supersonic- Source Environment for Multi-Fidelity Conceptual Vehicle aircraft-development-effort/. Design. 16th AIAA Multidisciplinary Analysis and Optimiza- Trimble, S. (2017). “Analysis: JAL invests heavily in supersonic tion Conference, Dallas, TX; http://adl.stanford.edu/papers/ Boom.” FlightGlobal; www.flightglobal.com/news/articles/ suave-open-source.pdf. analysis-jal-invests-heavily-in-supersonic-boom-443857. Mattingly, J. D. (2006). Elements of Propulsion: Gas Turbines U.S. Energy Information Administration (EIA) (2018). Annual And Rockets. Reston, VA: American Institute of Aeronautics Energy Outlook 2018 with Projections to 2050; www.eia.gov/ and Astronautics. outlooks/aeo/pdf/AEO2018.pdf. MedCalc Software. (2018). Digimizer. https://www.digimizer. U.S. Government Publishing Office (2018). Federal Aviation com/. Administration Reauthorization Act of 2017. Report of the National Aeronautics and Space Administration (NASA) (2014). Committee on Commerce, Science, and Transportation; NASA Armstrong Fact Sheet: Tu-144LL Supersonic Flying www.congress.gov/115/crpt/srpt243/CRPT-115srpt243.pdf. Laboratory; www.nasa.gov/centers/armstrong/news/Fact- Welge, H., Bonet, J., Magee, T., Chen, D., Hollowell, S., Kutzmann, Sheets/FS-062-DFRC.html. A., Mortlock, A., National Aeronautics and Space Administration (NASA) Stengle, J., Nelson, C., Adamson, E., Baughcum, S., Britt, R., (2018). “NASA awards contract to build quieter supersonic Miller, G., & Tai, J. (2010). N+2 Supersonic Concept Develop- aircraft” [press release]; www.nasa.gov/press-release/ ment and Systems Integration. NASA/CR-2010-216842. nasa-awards-contract-to-build-quieter-supersonic-aircraft. Witkin, R. (1978, June 10). “Repeat Passengers (one has flown Rutherford, D. (2018, June 4). “ICAO, why can’t you be a bit more 63 times) encourage Concorde’s operators despite current like your sister?” ICCT staff blog; www.theicct.org/blog/ losses.” New York Times; www.nytimes.com/1978/06/10/ staff/icao-why-cant-you-be-bit-more-your-sister. archives/new-jersey-pages-repeat-passengers-one-has- Spike Aerospace (2017a). “Spike Aerospace to fly the flown-63-times-encourage.html. first SX-1 demonstrator”; www.spikeaerospace.com/ spike-aerospace-to-fly-the-first-sx-1-demonstrator.

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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 (m2) 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 (m2) 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 (m2) 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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