In This Issue
Summer Bridge Issue on Aeronautics
June 26, 2020 Volume 50 Issue 2
The articles in this issue present the scope of progress and possibility in modern aviation. Challenges are being addressed through innovative developments that will support and enhance air travel in the decades to come.

Supersonic Flight and Sustainability: A New Horizon

Thursday, June 25, 2020

Author: Raymond Russell, Lourdes Maurice, and Rachel Devine

Nearly two decades ago the age of supersonic commercial aviation appeared to come to a close. The Anglo-French Concorde, which flew p­assengers at Mach 2 for 27 years, was retired in 2003.

Concorde was a technological marvel but plagued by high operating costs. The limited production run prevented carriers from achieving economies of scale, and Concorde failed to attain widespread commercial success. Factors such as increases in maintenance costs for the aging Concorde airframes, the fatal accident of July 2000 in France, and the slump in air travel after the terrorist attacks of September 11, 2001, all contributed to Concorde’s relegation to the world’s air museums (Learmount and MacKenzie 2003).

Several manufacturers are now working to ensure that the end of Concorde was not the end of the supersonic era. More than 50 years of technological progress enable quieter, more efficient supersonic aircraft. Indeed, current commercial aircraft are 80 percent more fuel efficient than the first jet airliners. According to the Aerospace Industries Association, a flight today produces 50 percent less CO2 than the same flight in 1990 and aircraft noise footprints have shrunk up to 90 percent in the past 50 years.[1]

The subsonic fleet’s fuel efficiency and noise footprints have steadily improved, but the advances enabling these improvements—in computational design, propulsion systems, materials, route optimization, and others—have yet to be applied to a civil supersonic aircraft. Nonetheless, important progress is being made on supersonic applications, as reviewed in this article.

Rationale for Building Supersonic Commercial Aircraft

Despite the limited success of Concorde, the past decade has seen a rebirth of interest in commercial and business supersonic flight. New materials, advanced engines, and innovation in aerodynamics can render civil supersonic flight economically viable. Companies driven by the energy and enthusiasm of a new generation of engineers are building on the work of legacy manufacturers and the National Aeronautics and Space Administration (NASA) to bring back supersonic travel (e.g., figure 1).

Figure 1 

From the first scheduled passenger flight in the ­United States in 1914 to today’s transcontinental network, tourism, business, and the economy have thrived (Sharp 2018), and the tremendous benefits of aviation (IHLG 2017) will be compounded at higher speeds (ICAO 2019). Time is a valuable commodity in the 21st century, and supersonic aircraft will allow humans to travel farther in less time, making the world more accessible to new business connections, greater cross-cultural understanding, and stronger bonds between distant families.

What about Environmental Concerns?

The aviation sector produces about 2 percent of worldwide greenhouse gas emissions (CO2) and accounts for 12 percent of all transportation-related emissions, although this share could grow as more people fly and other modes decarbonize[2] (Lee et al. 2010).[3] These figures are small relative to other modes of transportation, but aviation is a high-profile target of the climate movement, especially as the rate of air travel is growing[4]—and outpacing the rate of improvement in aircraft fuel efficiency. Thus greenhouse gas emissions attributed to aviation are rising. But it is important to keep these figures in context: Passenger and cargo aircraft carry 35 percent of the value of world trade, transport more than 4 billion passengers per year, and support over 65 million jobs (IHLG 2017).[5]

Given society’s increasing emphasis on environ­mental stewardship and concerns about the effect of air ­travel on climate change, can commercial supersonic flight become a reality in the 21st century? There are reasons to believe that sustainable supersonic flight is indeed viable—and in fact inevitable. In many ways, the focus of manufacturers on sustainability and reducing climate impacts may be the very thing that secures the return of supersonic transportation.

In this article we explore the environmental challenges and opportunities for commercial supersonic transport, focusing on noise and climate effects. The industry is leveraging technological progress to deliver improvements in supersonic aircraft community noise footprints, fuel efficiency, and carbon emissions, as well as operational changes and an international industry agreement to help mitigate environmental effects.

Community Noise

Successive generations of subsonic aircraft have become steadily quieter, thanks to continued improvements in propulsion technology as well as government-mandated phaseouts of noisier aircraft (FAA 2018). A key technological factor in this “quieting” of aviation is the move from turbojet engines to turbofan engines, and then from low-bypass turbofans to higher-bypass turbofans.

From Turbojet Engines to Turbofans

Concorde was an extraordinary technological achievement for its time. Its variable engine intakes and fly-by-wire system were industry firsts, and its Olympus 593 engines were the most thermodynamically efficient machines built to date (Leyman 1986).

But Concorde’s Olympus engines were noisy turbojets, and afterburners were used to produce the high thrust needed at takeoff and again to achieve supersonic speeds. Afterburners inject additional fuel into an engine’s jet exhaust downstream of the turbine, increasing jet noise. Fortunately, they are now unnecessary in this application.

Because airport noise is an important concern, and many people associate supersonic travel with ­Concorde’s disruptive airport noise, eliminating the need for afterburners is a critical prerequisite for widespread adoption of civil supersonic aviation. New supersonic aircraft designs are likely to employ low- to medium-bypass turbo­fans, rather than pure turbojets, substantially lessen­ing noise impacts.

In a turbojet, air is compressed to a significantly higher pressure than the freestream. Fuel is then introduced and ignited, adding energy to the air. The flow loses a small amount of this energy to the following turbine stage. Finally, the flow exits the engine, producing thrust as a high-temperature, high-velocity stream of jet exhaust.

A turbofan’s engine core is essentially a turbojet, with a second thrust-producing pathway. The turbofan diverts a large amount of incoming air around the core, where a large fan slightly accelerates the flow to produce thrust. Because driving the fan requires relatively little energy from the turbine, a turbofan engine can produce much more thrust than a turbojet for the same fuel consumption.

The fast stream of air exiting the engine core is the dominant contributor to engine noise, and an ancillary benefit of relying more on bypass flow for thrust is a decrease in this jet velocity. Lighthill’s acoustic ­­analogy relates the noise produced by a turbulent source to the eighth power of the velocity of that source (­Goldstein 2003). As a result, subsonic engines have moved toward higher bypass ratios. Modern engines, such as the ­Airbus A320neo’s CFM LEAP[6] or the forthcoming Boeing 777x’s GE9X,[7] have bypass ratios of 11:1–12:1, meaning that 91–92 percent of air entering the engine flows around the core to be accelerated by the fan.

Noise Challenges for Supersonic Aircraft

Figure 2 

Engineers continue to strive to build quieter engines. Those designing subsonic engines find that incentives to reduce both noise and fuel consumption are aligned by moving to larger-diameter, higher-bypass engines. However, this convenient parallel does not hold for supersonic applications. Supersonic engines will rely more on thrust from the jet core than do contemporary, high-bypass subsonic engines; figure 2 shows a comparison of a subsonic and a notional supersonic engine. To further reduce noise, designers can leverage operational changes, discussed below.

As parts of an airframe begin to experience local supersonic flow, the corresponding shock waves add an additional term to the drag equation: wave drag, the cost of pushing a body faster than the speed of sound.

Drag considerations above Mach 1 require careful optimization of the cross-sectional area, necessitating lower-bypass engines to improve fuel efficiency at supersonic cruise. But that increase in fuel efficiency comes at the cost of noise performance near the ground, as the engine relies relatively more on thrust from the core’s noisy jet exhaust. Supersonic aircraft thus require more thrust at takeoff and landing, where noise is a chief concern.

Despite these challenges, technology advances and new operational procedures ensure that the overall noise of new supersonic aircraft will not differ substantially from those of the existing subsonic fleet.

Operational Changes

Supersonic engines, sized for transonic acceleration and sustained supersonic cruise, have a greater ­margin of excess thrust at departure than subsonic aircraft and often use nearly all available thrust at takeoff. One proposed strategy to reduce supersonic aircraft noise during departure is programmed lapse rate (PLR), a computer-managed reduction in thrust after the takeoff roll.

Because of their excess thrust, supersonic aircraft should still be able to accelerate during the climb phase even after reducing power. PLR and accelerating climb-outs may offer significant benefits in noise reduction (Berton et al. 2017).

Another approach is to vary the wing’s flaps and slats by computer (Berton et al. 2017), enabling the aircraft to operate closer to its best lift-to-drag ratio at all times. This action maximizes performance and removes the aircraft from the vicinity of communities more quickly.

Whatever technologies are employed, supersonic jets should aim to meet noise standards similar to those for subsonic aircraft,[8] taking into account trade-offs with fuel efficiency (FAA 2017). The technology enabling this compliance exists or is within reach with additional research and technology development.

New Tools to Improve Supersonic Climate Impacts

Supersonic fuel efficiency has improved in part through aircraft-level improvements. In addition, sustainable aviation fuels and the potential to use offsets, facilitated by an international aviation industry agreement to offset and reduce carbon emissions, can help address the climate effects of supersonic flight.

Enhanced Fuel Efficiency

Fuel efficiency is as much a financial imperative for operators as it is a climate goal, as fuel outlays are a significant share of airline operating costs (IAG 2018).[9] Manufacturers of supersonic aircraft today enjoy ­advances that were not available to Concorde’s engineers.

Modern engines consume much less fuel per unit thrust than their predecessors. Over the past few decades subsonic aircraft have become on average roughly 1.5 percent more fuel efficient each year, and applying improvements in combustor and engine cycle design to new supersonic engines may correspondingly improve efficiency.[10]

In addition, modern materials and manufacturing techniques allow for greater flexibility in shaping and weight reduction. Concepts such as area ruling (avoiding abrupt changes in cross-sectional area to manage wave drag and sonic boom) were understood in the 1960s, but engineers faced manufacturing limitations. For example, aluminum is difficult to form into curved, area-ruled shapes, so Concorde’s aerodynamicists could not take full advantage of what they understood to be true at the time. Carbon composites, on the other hand, can be easily molded into the desired shape, so the next generation of civil supersonic aircraft will be much more area ruled, reducing drag at transonic and supersonic speeds.

Sustainable Aviation Fuels

While many other industries and modes of transportation are rapidly electrifying, long-haul air travel will likely rely on liquid hydrocarbons for years to come as jet fuel maintains a considerable energy-density advantage over the best available batteries. The industry must therefore try to reduce the net emissions of the fuel consumed during flight. Sustainable aviation fuels (SAFs) are a key solution and a growing—albeit still small—share of airline fuel use.

These fuels are considered “drop in,” conforming to the same specifications as Jet A. However, by employing renewable sources, SAFs can reduce emissions over the fuel lifecycle, accounting for emissions from production, transportation, and use. The best SAFs today offer up to an 80 percent reduction in lifecycle CO2 emissions (Hileman et al. 2009).

While commercial use of SAF remains small, demand is rapidly increasing, and both government and industry are responding. Each year, more potential fuels are approved by regulators for use, more processing facilities come online, and more investment flows into the sector. An ASTM specification exists for qualification of new SAF projects; six “pathways” are approved for producing SAF,[11] and ASTM is evaluating several more (ASTM International 2019).

This process has led to a number of interesting jet fuel sources. Fulcrum BioEnergy,[12] with investment from BP and United Airlines, uses municipal waste as a feedstock for fuel. LanzaTech,[13] a partner of Virgin ­Atlantic, uses genetically modified bacteria to synthesize fuel from waste gases at steel, cement, and other industrial plants. Others, such as Carbon Engineering[14] and Prometheus (Brustein 2019), turn directly to ­atmospheric carbon capture, recycling CO2 from ambient air into hydrocarbon fuels. These novel methods add to the existing supply of more traditional biofuels, produced from corn, soy, forest residues, algae, and other sources.

Importantly, the industry is committed to ensuring that SAF substitution for fossil fuel will not result in other social ills: Demand for SAF crops will not incentivize adverse land-use changes (e.g., the conversion of virgin forests into cropland) or generate pressure on food crops, and production will not offset the environmental benefits of use.

SAF will be an important contributor to the environmental performance of supersonic aircraft. The incorporation of sustainability in aircraft design concepts may be precisely what enables the global acceptance and successful return of supersonic transportation. Manufacturers intend new aircraft to accommodate alternative fuels in much higher blends than currently possible or permitted. With greater market penetration, SAF should build a track record of safe operation and gain a reputation for safety that will allow the use of higher blends or pure SAF.[15]

Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA)

Even when a flight operator uses SAF, some emissions remain. These must be offset to earn a license to fly. The International Civil Aviation Organization (ICAO) has undertaken CORSIA, a globally coordinated, ­market-based effort to mitigate aviation’s ­climate impacts by requiring that growth in international aviation—­supersonic or subsonic—be carbon neutral after 2020.[16]

Established in 2016, CORSIA is, in effect, a global climate accord for international air travel, and aviation was the first industry to have such a sectoral program. Because supersonic commercial service is most profitable on long-distance international routes, most of it will fall under CORSIA jurisdiction.

Any such offsetting program comes with challenging accounting questions, such as ensuring the quality of offset credits and avoiding double-counting of emission reductions (i.e., both in the generating country and under CORSIA). Proper implementation is critical to the program’s effectiveness, and ICAO and its members have set up a centralized auditing process for CORSIA. The scheme can ultimately help ensure that commercial supersonic jets do not add to the carbon burden from commercial aviation.

Conclusion

The aircraft design process now includes environ­mental considerations alongside payload, range, and other familiar requirements. Challenges remain, but today’s technology allows for widespread supersonic passenger travel in the near future. Global acceptance will be key for reintroduction, and will require that manufacturers be vigilant about addressing environmental concerns through design, production, and operation.

Building a new aircraft is an opportunity to improve the sustainability position of air travel, and it may be that very concern for the environment that will make supersonic air travel successful.

References

ASTM International. 2019. ASTM D7566-19b: Standard specification for aviation turbine fuel containing synthesized hydrocarbons. West Conshohocken PA.

Berton JJ, Jones SM, Seidel JA, Huff DL. 2017. Advanced noise abatement procedures for a supersonic business jet. Aeronautical Journal 122(1250):1–16.

Brustein J. 2019. In Silicon Valley, the quest to make gasoline out of thin air. Bloomberg, Apr 30.

FAA [Federal Aviation Administration]. 2017. Stage 5 airplane noise standards. Federal Register 82(191):46123.

FAA. 2018. History of noise. Washington.

Goldstein ME. 2003. A generalized acoustic analogy. Journal of Fluid Mechanics 488:315–33.

Hileman JI, Ortiz DS, Bartis JT, Wong HM, Donohoo PE, Weiss MA, Waitz IA. 2009. Near-Term Feasibility of Alternative Jet Fuels. Santa Monica: RAND Corporation.

IAG [International Airlines Group]. 2018. 2018 Annual Report. Madrid.

ICAO [International Civil Aviation Organization]. 2019. Environmental Report: Destination Green—The Next Chapter. Montreal.

IHLG [Industry High Level Group]. 2017. Benefits of Aviation. Montreal.

IPCC [Intergovernmental Panel on Climate Change]. 1999. Aviation and the Global Atmosphere, ed Penner JE, Lister DH, Griggs DJ, Dokken DJ, McFarland M. Cambridge UK: Cambridge University Press.

Learmount D, MacKenzie C. 2003. High costs clip Concorde’s wings. FlightGlobal, April 14.

Lee DS, Pitari G, Grewe V, Gierens K, Penner JE, Petzold A, Prather MJ, Schumann U, Bais A, Berntsen T, and 3 ­others. 2010. Transport impacts on atmosphere and climate: ­Aviation. Atmospheric Environment 44(37):4678–734.

Leyman CS. 1986. A review of technical development of Concorde. Progress in Aerospace Sciences 23(3):185–238.

Sharp T. 2018. World’s first commercial airline: The greatest moments in flight. Space.com, May 22.

 

[1]  AIA Environment, https://www.aia-aerospace.org/issue/­environment/

[2]  Facts and figures, Air Transport Action Group, https://www.atag.org/facts-figures.html

[3]  We are aware of concerns about the effects of altitude ­emissions; this complex subject could not be accommodated within the length limits of this article. While many scientific ­uncertainties remain, it may be possible to avoid sensitive parts of the ­atmosphere while still realizing the benefits of commercial supersonic flight; for a discussion of advances and uncertainties, see IPCC (1999).

[4]  https://www.atag.org/facts-figures.html. This article was drafted before the devastating impacts of covid-19 on commercial aviation and around the globe. We believe the industry will recover but recognize that it will be some time before air travel service and growth return to 2019 levels.

[5]  https://www.atag.org/facts-figures.html

[6]  https://www.safran-aircraft-engines.com/commercial-engines/ single-aisle-commercial-jets/leap/leap-1a

[7]  https://www.geaviation.com/commercial/engines/ge9x-­ commercial-aircraft-engine

[8]  The most recent ICAO noise standard is Chapter 14, referred to as Stage 5 in the United States.

[9]  For IAG Holdings (which include British Airways, Aer Lingus, and Iberia) fuel accounts for around 25 percent of total annual costs (IAG 2018).

[10]  https://www.atag.org/facts-figures.html

[11]  The standards for the origin of an SAF require that it be chemically identical to conventional jet fuel and specify a maximum permitted blending ratio (50 percent) with conventional fuel.

[12]  http://fulcrum-bioenergy.com/

[13]  https://www.lanzatech.com/

[14]  https://carbonengineering.com/

[15]  Commercial Aviation Alternative Fuels Initiative, www.caafi.org/focus_areas/deployment.html

[16]  https://www.icao.int/environmental-protection/­CORSIA/ Pages/default.as

About the Author:Raymond Russell is head of sustainability, Lourdes Maurice is an independent consultant and advisor, and Rachel Devine is head of US policy, all at Boom Supersonic