The once-iconic supersonic jet, Concorde, a testament to human innovation and hailed as a technological marvel, concluded its 27-year service with a final landing at Filton runway in November 2003. However, its commercial viability ultimately faltered due to a confluence of factors. Public concerns about the plane’s noise and sonic boom followed by the environmental ramifications, the supersonic transport ultimately met its demise.
Envisioned as a groundbreaking revolution in aviation, the Concorde was constructed to reduce flight duration drastically. Driven by a quest for speed and efficiency, it was a collaborative effort between Britain and France, designed to showcase technological prowess for rapid transatlantic travel appeaseing a specific niche of travellers.
With a total of 20 Concordes built, only 14 were used for commercial purposes. The remaining six were prototype, pre-production, and developmental models. These 14 aircraft made 500,000 flights since its first transatlantic crossing in September 1973. Travelling at a speed faster than sound, the supersonic jet flew over 2.5 million passengers to their destination.
A Triumph of Engineering for a Privileged Few
Born in an era defined by flickering black-and-white television, the Concorde soared as a monument to human achievement in aviation. Capable of supersonic speeds exceeding Mach 2, it represented the pinnacle of engineering for a diverse range of stakeholders. From the scientists and engineers who poured their expertise into its creation to the privileged passengers who experienced the thrill of high-speed travel, the Concorde embodied a future far beyond the limitations of its time. With a total of 20 Concordes built, only 14 ever flew commercially, carrying over 2.5 million passengers across the Atlantic in opulent comfort with limited seating (around 100) and small windows.
Queen Elizabeth II used Concorde to travel home from Barbados in 1977.
CNN’s Richard Quest (2019), who flew Concorde five times, stated:
“The flight attendants loved being on it; the passengers loved being on it, you were aware of being part of a very small group of people that were privileged enough to be on Concorde.”
“Concorde was extremely small, only about 100 seats. It had more like office chairs, bucket seats, and very small windows. It was noisy, extremely noisy, but I challenge anybody not to have a smile from ear to ear when they got on it.”
Former chief engineer John Britton (2023), a designer on the Concorde engine, describes his ‘Fantastic experience’ —
“In 1969 I had the privilege, we were all allowed out of the design office out onto the runway because Concorde had been doing some high-speed taxiing and it was thought that on this particular day, it might take off.”
“It went hurtling down the runway and we all held our breath and we all cheered and clapped and you could really feel the heat and the paraffin engines, you could actually feel it in your chest.”
“That was a fantastic experience as a young engineer when that happened.”
Rolls-Royce/Snecma Olympus 593, an Anglo-French turbojet
The Environmental Shadow of a Supersonic Jet
However, the Concorde’s reign was cut short, leaving many to lament its discontinuation, not just for its luxurious passenger experience but also for the engineering marvel it represented. Yet, lurking beneath the surface of this technological marvel lay a significant environmental concern. Despite being the final supersonic jet ever used for commercial flights, the Concorde generated three times more harmful emissions like nitrogen oxides (NOx) and CO2 compared to modern subsonic planes. This environmental impact was even worse due to the high altitude at which it released these emissions, contributing five times more to global warming.
Researchers conducted emission measurements from a Concorde aircraft operating at supersonic speeds within the stratosphere. As anticipated, the study identified NOx emissions of the plane. However, the findings revealed lower-than-expected levels of nitric acid. D W Fahey et. al. noted,
“Measurements of NO, and HO, indicate a limited role for nitric acid in the plume. The large number of submicrometer particles measured implies efficient conversion of fuel sulfur to sulfuric acid in the engine or at emission.”
This suggests the engine efficiently converts fuel sulfur into sulfuric acid.
A future fleet of comparable supersonic aircraft could lead to a significant increase in stratospheric fine particles, potentially causing a greater degree of ozone depletion than solely from the emitted nitrogen oxides.
British Airways Concorde G-BOAC, Heathrow, London, England, between 1980 and 1985.
The Effects on the Ozone — A Major Issue
The supersonic Concorde, a marvel of speed, fell victim to its environmental toll. Compared to modern planes, it generated three times more harmful emissions and contributed five times more to warming due to its high-altitude flight. However, the Concorde’s environmental impact went beyond just air pollution. A key concern was its impact on the ozone layer, the shield that protects us from harmful ultraviolet (UV) radiation from the sun.
Hypersonic aircraft, like the Concorde, fly at high altitudes (16-23 km) where the ozone layer resides (18-45 km). These aircraft emit nitrogen oxides (NOx), which are known to contribute to ozone depletion in the lower stratosphere. A depleted ozone layer can have severe consequences, UV radiation increases, for human health, including skin cancer and eye damage, and can also harm plant life.
Research revealed that Concorde’s engines emitted significantly higher quantities of sulfuric acid particles than the exhaust produced by subsonic aircraft. These sulfuric acid particles were subsequently determined to be a contributing factor in the depletion of the ozone layer.
The World Meteorological Organization (WMO) warns widespread supersonic travel could reduce ozone by 10%. New supersonic and hypersonic designs raise similar concerns due to emissions impacting the ozone layer. Dr. Paul Newman (NASA) emphasizes the potential danger of a large-scale supersonic fleet.
The evolution of the ozone hole over the past four decades.
NOx Emissions and Aerosols: A Complex Influence
Both conventional and supersonic aircraft emit NOx, but the impact on ozone depends on altitude. While the influence of the Concorde on ozone depletion may be less significant compared to dominant factors like halocarbons, its contribution should not be entirely disregarded. The complex interplay between NOx emissions and other pollutants in the stratosphere necessitates further investigation. For instance, the water vapour in the Concorde’s exhaust plume could modulate the impact of NOx on ozone. Despite these remaining uncertainties, mitigating NOx emissions from supersonic aircraft, alongside regulations targeting other pollutants, could be a contributing factor in safeguarding the ozone layer.
Aircraft emissions impact ozone levels depending on altitude. At high altitudes, ozone depletion dominates. In the lower UT/LS region (around 17 km), NOx emissions can increase ozone, but this effect depends on existing ozone concentrations. Seasonal variations in ozone concentrations also influence total ozone levels
Supersonic aeroplanes using traditional fuels create two climate-affecting aerosols: black carbon (warming) and sulfates (cooling). Studies suggest the cooling effect from sulfates might outweigh the warming from black carbon, but uncertainty remains. These aerosols can also indirectly affect ozone levels in the stratosphere, both positively and negatively. Predicting the impact is difficult because it depends on future engine technology, fuel types, and how well we can model how these tiny particles move through the atmosphere. While water vapor emissions from supersonic flight have a clearer impact, the effect of aerosols remains a mystery demanding further investigation. However, factors like flight altitude and particle lifetime can influence this effect.
Aerosols also indirectly impact ozone through complex chemical reactions. They can both increase ozone production and enhance ozone loss under specific conditions. Recent research suggests these indirect effects might be significant for ozone depletion.
Concerns and Uncertainties
NOx emissions at high altitudes deplete ozone, while their effect at lower altitudes is dependent on various factors. Aerosol emissions can have both warming and cooling effects, and may also indirectly impact ozone levels. The combined effect of direct and indirect aerosol impacts on climate and ozone remains complex. While potentially significant, the overall impact may be comparable in magnitude but opposite in direction to the warming caused by water vapour emissions from aircraft. A major challenge in quantifying these effects is the difficulty of accurately calculating them.
In 1971, Professor H. Johnston of Berkeley University, California raised concerns about potential ozone depletion from hypersonic aircraft. His theoretical analysis suggested a fleet of 500 daily-operating aircraft could halve ozone levels. However, uncertainties in reaction rates and pre-existing stratospheric NOx limited these predictions’ conclusiveness.
Estimations made by researchers D W Fahey et. al. suggest and reinforce that a fleet of 500 tISCT aircraft operating at Mach 2.4 (20 km) in 2015 would lead to a maximum ozone column reduction of less than 1% between 40° and 70° latitude. This assumes an emission index (EI) for NOx below 15 grams of NO2 per kilogram of fuel. Higher ozone losses are anticipated with increasing NOx emissions or flight altitudes.
The Future of Supersonic Travel: A Balancing Act
The allure of supersonic travel, offering dramatically reduced travel times for long-haul flights, remains a potent force driving innovation within the aviation industry. However, the environmental legacy of the Concorde, the last commercial supersonic aircraft, raises significant concerns regarding the sustainability of this technology.
Supersonic flight generates more emissions than conventional aircraft, posing a challenge to the environmental sustainability of this mode of travel. The aviation industry is actively exploring ways to mitigate this environmental impact. Potential solutions include utilizing alternative fuels and designing more efficient engines to reduce emissions, such as the Sustainable Air Fuel (SAF). Balancing the desire for hypersonic travel with sustainability is a critical challenge requiring collaboration between engineers and policymakers. Only then can supersonic travel become a reality without significant environmental consequences.
NASA’s X-59, its quiet supersonic research aircraft, sits on the ramp at Lockheed Martin Skunk Works in Palmdale, Calif.
However, supersonic travel remains in its early stages of development, and the environmental effects of contrails from these high-altitude aircraft are still under debate. Contrails are condensation trails formed by water vapour in the exhaust of jet engines, and some experts believe that contrails from supersonic aircraft could have a more significant impact on the climate than those from conventional aeroplanes due to the colder temperatures at high altitudes.
The future of supersonic travel hinges on the industry’s ability to address environmental concerns. By implementing technological advancements and exploring alternative fuels, supersonic flight has the potential to become a more sustainable mode of transportation.
Mitigating Uncertainties in Future Research
A comprehensive understanding of the atmospheric effects of future supersonic fleets is crucial before deployment. Current studies rely on assumptions that introduce uncertainties. For instance, these studies often assume a “volcanically clean” atmosphere in 2035. Major volcanic eruptions could significantly increase ozone depletion due to accelerated heterogeneous chemistry. Additionally, plume chemistry, which plays a role in the initial stages of emission, and the dynamic effects of local heating are not currently considered. These factors can influence transport and ozone distribution. Further research is needed to address these uncertainties and evaluate other potential environmental impacts.
Research by the International Council on Clean Transportation (ICCT) suggests that next-generation supersonic business jets may generate even higher pollution levels than the Concorde. These projections highlight the critical environmental challenges that must be tackled before the widespread adoption of supersonic travel becomes a reality. Potential solutions lie in technological advancements and a shift towards sustainable practices, such as developing more efficient engines and utilizing sustainable aviation fuels (SAF). Additionally, exploring alternative flight paths to minimize the impact on the stratospheric ozone layer is crucial.
Despite the Concorde’s retirement, renewed interest in supersonic travel is driving the development of new aircraft. Boom Supersonic’s Overture and NASA’s X-59 program exemplify this trend, with the former aiming for flight tests by the decade’s end and the latter focusing on sonic boom reduction.
However, significant research and development efforts are necessary to ensure the viability and scalability of these solutions. Furthermore, achieving widespread adoption across the industry necessitates collaboration between policymakers, aircraft manufacturers, and fuel producers.
Conclusion
The concept of supersonic travel presents an enticing proposition—vastly accelerated long-haul journeys. However, the environmental ramifications of this technology remain a considerable challenge. Supersonic flight engenders heightened emissions and potential disruption to the ozone layer compared to traditional aircraft.
The future of supersonic travel is contingent upon achieving an equilibrium between technological progress and environmental conscientiousness. Persistent research is imperative to comprehensively grasp the atmospheric implications of extensive supersonic fleets.
Potential resolutions encompass the refinement of propulsion systems, the utilization of sustainable aviation fuels, and the exploration of alternative flight trajectories. Cohesive engagement among engineers, policymakers, and fuel manufacturers is indispensable to ascertain the practicality and scalability of these resolutions. Only through such integration can supersonic travel materialize.