Climate-Optimal Aircraft Design and Fleet Allocation

Abstract

Research into aviation-induced global warming has highlighted the significant impact of non-CO2 effects, such as NOx emissions, water vapor emissions, and contrail formation. Unlike CO2 emissions, which scale directly with fuel consumption, non-CO2 effects depend on location, have varying lifetimes, and do not scale purely with fuel burn. Minimizing these effects often conflicts with traditional design objectives like cost, mass, and energy consumption. This necessitates incorporating non-CO2 effects into the conceptual evaluation and optimization of new aircraft and fuels.

The goal of this research is to explore how aircraft design optimization and fleet allocation can reduce global warming impacts while balancing changes in energy consumption and financial costs. A multidisciplinary, multi-objective approach is used to investigate the design space of aircraft, considering airframe, engine, and mission variables. A linear temperature response model estimates the climate impact based on a multi-year emission scenario, including CO2, NOx, water vapor, soot, sulfate, and contrails, and evaluates the average temperature response over 100 years (ATR100) as an objective function.

Initial optimization of medium-range, kerosene-powered aircraft shows that ATR100 can be reduced by up to 64% with a 17% increase in cash operating costs. This reduction targets the radiative impacts of contrails and short-term ozone by flying at lower altitudes and reducing the engine's overall pressure ratio (OPR) to lower NOx emissions. A lower cruise Mach number also maintains an optimal lift-to-drag ratio. Turboprop engines, with higher propulsive efficiency at lower speeds, further reduce ATR100, potentially by up to 71%.

Multi-objective optimization reveals that targeting contrails can cut ATR100 by 53% for medium-range, kerosene aircraft with a 1% cost increase. However, these aircraft still significantly impact due to CO2 emissions. Consequently, the research also examines liquid hydrogen and sustainable aviation fuels (SAF). Hydrogen-powered aircraft can reduce ATR100 by 73% at a 28% cost increase, while climate-optimized designs achieve a 99% reduction at a 39% cost increase. SAF-powered aircraft offer ATR100 reductions of 47% to 83%, with cost increases of 4% to 21%.

Changes in aircraft design impact operations by increasing mission block times and reducing productivity, affecting profitability. A network analysis shows that a climate-optimal kerosene fleet can decrease climate impact by 55% but reduce network profit by 24%. A hydrogen aircraft fleet offers the lowest climate impact with a 35% profit penalty. SAF-powered fleets provide intermediate solutions with climate impact reductions of 47% to 78% and profit decreases of 3% to 27%.

Although the findings offer valuable insights, further analysis on different networks is necessary. Climate-optimal aircraft also perform better in terms of local and global air quality but may have higher noise levels at take-off and departure. Uncertainty remains in the climate impact assessment due to incomplete understanding of all climate effects and the lower fidelity of the linear temperature response model. Future research should include air quality and noise disciplines in the optimization framework and further study propeller-based propulsion and contrail avoidance technologies for better sustainability.