This paper focuses on the conceptual design optimization of liquid hydrogen aircraft and their performance in terms of climate impact, cash operating cost, and energy consumption. An automated, multidisciplinary design framework for kerosene-powered aircraft is extended to design liquid hydrogen-powered aircraft at a conceptual level. A hydrogen tank is integrated into the aft section of the fuselage, increasing the operating empty mass and wetted area. Furthermore, the gas model of the engine is adapted to account for the hydrogen combustion products. It is concluded that for medium-range, narrow-body aircraft using hydrogen technology, the climate impact can be minimized by flying at an altitude of 6.0 km at which contrails are eliminated and the impact due to NO_x emissions is expected to be small. However, this leads to a deteriorated cruise performance in terms of energy and operating cost due to the lower lift-to-drag ratio (– 11%) and lower engine overall efficiency (– 10%) compared to the energy-optimal solutions. Compared to cost-optimal kerosene aircraft, the average temperature response can be reduced by 73–99% by employing liquid hydrogen, depending on the design objective. However, this reduction in climate impact leads to an increase in cash operating cost of 28–39% when considering 2030 hydrogen price estimates. Nevertheless, an analysis of future kerosene and hydrogen prices shows that this cost difference can be significantly decreased beyond 2030.

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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.@en

To reduce the climate impact of aviation, researchers are studying the replacement of fossil kerosene with liquid hydrogen and/or drop-in sustainable aviation fuel (SAF). These fuels can bring significant reductions in CO2 emissions and can offer savings in terms of non-CO2 climate effects. In addition, tube-and-wing aircraft can be optimized to decrease the global-warming impact by using a climate metric as a design objective rather than the operating costs. Previous research has shown that airplanes designed for minimal climate impact have a reduced cruise speed and fly at a lower altitude. This paper suggests a multidisciplinary, multi-level approach the evaluate the consequences of such design and fuels choices at the network level. Following the aircraft design step, a dynamic programming routine allocates the fleet and schedules the flights to maximize the network profit. We consider a hub-and-spoke network operating from Atlanta, with demand for domestic and international destinations. Compared to the reference cost-optimal kerosene fleet, a fleet consisting of climate-optimized kerosene aircraft can reduce the climate impact by 61% at a loss in network profit of approximately 21%. This design choice requires allocating an additional five aircraft. A fleet operating climate-optimal, hydrogen aircraft minimizes the climate impact. However, the high operating cost of long-range, hydrogen aircraft lowers the achievable profit. Aircraft powered by drop-in SAF provides Pareto-optimal solutions. These insights can be used to make decisions about the allocation of future aviation fuels in a network and the payload-range requirements of future aircraft.@en

Electrification of aviation is regarded as one of the means to make aircraft operations less polluting and to have lower climate impact. Yet, air transportation's environmental impact depends on power train technologies and novel designs and aircraft operations within airline networks. Fully- or hybrid-electric aircraft may enter existing air transport networks through fleet replacement yet require airlines to adapt in order to operate electrified aircraft strategically. This research studies how airlines can strategically adjust their network and fleet composition when considering electrified aircraft. The novelty of this approach is to provide a direct feedback coupling between fleet planning, conceptual hybrid-electric aircraft design and climate impact minimization. Therefore, a strategic airline planning model, consisting of fleet and network analysis, is coupled to a hybrid-electric aircraft design model. A case study on the sensitivity of a regional airline network is presented to demonstrate the framework and assess the impact of trying to design aircraft and fleets with minimal climate footprint. A decrease in emissions with respect to a kerosene fleet of 11% can be achieved when a hybrid-electric fleet is designed particularly for the specified network, at the penalty of a profit decrease of 13%. Limiting fleet diversity to three types results in only 7% emissions decrease. Increasing the battery-specific energy shows an expected beneficial effect on emissions.@en

As climate change aggravates, the aviation sector strives to minimize its climate footprint. To this end, international organizations, such as ICAO and ACARE, are promoting mitigation measures including novel technologies, operations, and energy carriers to reduce aircraft emissions significantly. Hydrogen (H2) as an alternative fuel has the advantage of eliminating CO2 and soot emissions and the potential to reduce NOx emission substantially. Nevertheless, burning H2 emits more H2O and increases the contrail formation probability. Therefore, the actual climate impact of hydrogen aircraft is still uncertain. This paper intents to evaluate the climate impact of a hydrogen powered aircraft considering the effects of H2O, NOx , and contrails . To frame the contribution of each individual climate agent, the research compares a hydrogen and a kerosene aircraft with similar mission capabilities. To assess the climate impact, a modeling chain was developed including network selection, flight routes calculation, aircraft and propulsion performance, emissions prediction, and climate impact assessment. In total, 2.24 million flights covering 1128 city pairs were analyzed. The energy consumption of hydrogen aircraft is about 10% higher than that of the kerosene aircraft due to the larger wetted area for hydrogen storage. However, the average atmospheric temperature response caused by the hydrogen aircraft is 67% lower compared to the kerosene aircraft due to the absence of CO2, the lower radiative forcing of hydrogen contrails, and the reduction in NOx emissions when assuming advanced hydrogen combustion technology. It was also observed that climate impact from hydrogen aircraft is more sensitive to flights over the tropics than to flights over the poles.@en

Zero-carbon-dioxide-emitting hydrogen-powered aircraft have, in recent decades, come back on the stage as promising protagonists in the fight against global warming. The main cause for the reduced performance of liquid hydrogen aircraft lays in the fuel storage, which demands the use of voluminous and heavy tanks. Literature on the topic shows that the optimal fuel storage solution depends on the aircraft range category, but most studies disagree on which solution is optimal for each category. The objective of this research was to identify and compare possible solutions to the integration of the hydrogen fuel containment system on regional, short/medium- and large passenger aircraft, and to understand why and how the optimal tank integration strategy depends on the aircraft category. This objective was pursued by creating a design and analysis framework for CS-25 aircraft capable of appreciating the effects that different combinations of tank structure, fuselage diameter, tank layout, shape, venting pressure and pressure control generate at aircraft level. Despite that no large differences among categories were found, the following main observations were made: (1) using an integral tank structure was found to be increasingly more beneficial with increasing aircraft range/size. (2) The use of a forward tank in combination with the aft one appeared to be always beneficial in terms of energy consumption. (3) The increase in fuselage diameter is detrimental, especially when an extra aisle is not required and a double-deck cabin is not feasible. (4) Direct venting has, when done efficiently, a small positive effect. (5) The optimal venting pressure varies with the aircraft configuration, performance, and mission. The impact on performance from sizing the tank for missions longer than the harmonic one was also quantified.@en

This paper presents a method to assess the key performance indicators of aircraft designed for minimum direct operating costs and aircraft designed for minimum global warming impact. The method comprises a multidisciplinary aircraft optimization algorithm capable of changing wing, engine, and mission design variables while including constraints on flight and field performance. The presented methodology uses traditional class-I methods augmented with dedicated class-II models to increase the sensitivity of the performance indicators to relevant design variables. The global warming impact is measured through the average temperature response caused by several emission species (including carbon dioxide, nitrogen oxides, and contrail formation) over a prolonged period of 100 years. The analysis routines are verified against experimental data or higher-order methods. The design algorithm is subsequently applied to a single-aisle medium-range aircraft, demonstrating that a 57% reduction in average temperature response can be achieved as compared to an aircraft optimized for minimal operating costs. This reduction is realized by flying at 7.6 km and Mach 0.60, and by lowering the engine overall pressure ratio to approximately 37. However, to compensate for the lower productivity, it is estimated that 13% more climate-optimized aircraft have to be operated for the hypothetical fleet under consideration.@en

Sustainable aviation fuel (SAF) and liquid hydrogen are currently being studied to replace kerosene in commercial aviation to reduce global warming. In this study, the question is how do the airplane design variables change when minimizing the global warming impact of aircraft powered by SAF or LH2? Secondly, how do these aircraft compare in terms of climate impact and operating costs, considering regional, medium-, and long-range categories? A multidisciplinary design optimization process varies airframe, turbofan engine and mission design variables to obtain the cost- and climate-optimal design solutions. A linearized temperature response model evaluates the average temperature response over 100 years considering both CO2 and non-CO2 effects. The trade-off between climate impact reduction on the one hand and operating cost, on the other hand, is studied for each fuel type and aircraft category. We conclude that LH2 can achieve the largest reduction in temperature response in all categories. The maximum reduction of 98% compared to the cost-optimal kerosene aircraft comes at an estimated increase of 30, 42, or 69% in operating costs for regional, medium-, and long-range missions. The SAF aircraft can reduce the climate impact by 86, 82, and 72% for regional, medium-range, and long-range aircraft. These savings lead to an 8, 14, and 26% increase in operating costs. The analysis shows that the SAF-powered aircraft are preferred over the cost-optimal hydrogen aircraft for the regional and medium-range categories. Hydrogen does provide a Pareto-optimal solution for long-range aircraft, albeit at a significant in-flight energy and cost penalty.@en

This paper studies the climate impact of propeller aircraft which are optimized for either minimum direct operating costs, minimum fuel mass, or minimum average temperature response (ATR100). The latter parameter provides a measure of the global warming impact of the aircraft design, considering both CO2 and non-CO2 effects. We study turboprop-powered aircraft in particular because these offer higher propulsive efficiency than turbofan aircraft at low altitudes and low Mach numbers. The propeller aircraft are designed for medium-range top-level requirements, employing a multidisciplinary design optimization framework. This framework uses a combination of statistical, empirical, and physics-based methods, which are verified using existing engine and aircraft data. For this medium-range design case, a climate impact reduction of 16% can be realized when shifting from the cost design objective to the climate objective. The optimal solutions for the fuel mass and climate objectives are nearly identical as CO2 and other fuel proportional climate effects are the main contributors. The effects of NOx and contrails are lower than for the turbofan aircraft due to the lower cruise altitude of the propeller aircraft. Compared to turbofan data, propeller-powered aircraft can achieve a further 33% reduction in climate impact, comparing both climate-optimal designs. This reduction is lessened to 23% when the propeller aircraft is constrained to achieve the same mission block time as the turbofan aircraft. Note that these reductions in ATR100 require a propeller efficiency of 88%. Overall, the results show that the utilization of propeller-powered aircraft in the medium-range category can further reduce the climate impact compared to climate-optimal turbofan aircraft designs. @en

The aircraft’s environmental performance on fleet level is so far completely decoupled from the design process. The climate impact from aviation arising from non-CO2 effects are largely independent from CO2 emissions, but rather depend on the atmospheric state. Previously complex climate-chemistry models were used to evaluate the non-CO2 emissions impact on climate. This is far too computationally demanding for a multidisciplinary design optimisation (MDO) process, requiring a multitude of climate impact evaluations. The question then is, how to efficiently design the next generation climate optimal aircraft? In this paper, a new concept for designing aircraft with minimum climate impact using Climate Functions for Aircraft Design (CFAD) is presented. The content of this paper provides an overview of the development of these innovative CFAD and demonstrates the ability to be integrated in an existing MDO framework. The mitigation potential by optimising aircraft design using CFAD is analysed with respect to different cruise conditions and by minimizing the overall climate impact. To validate the CFAD, a higher fidelity assessment is carried out. Finally, the key performance indicators, i.e. fuel consumption, flight time and operating cost, of the optimised aircraft design are compared to that of the reference aircraft.@en

This paper presents a method to assess the key performance indicators of aircraft designed for minimum direct operating cost and aircraft designed for minimum global-warming impact. The method comprises a multidisciplinary aircraft optimization algorithm capable of changing wing, engine and mission design variables while including constraints on flight and field performance. The presented methodology uses traditional Class-I methods augmented with dedicated Class-II models to increase the sensitivity of the performance indicators to relevant design variables. The global-warming impact is measured through the average temperature response caused by several emission species, including CO2, NOx and contrail formation, over a prolonged period of one hundred years. The analysis routines are verified against experimental data or higher-order methods. The design algorithm is subsequently applied to a single-aisle, medium-range aircraft, demonstrating that a 45% reduction in average temperature response can be achieved by flying at 8.64 km and Mach 0.61, and by reducing the engine overall pressure ratio to 34 when compared to an aircraft optimized for minimal operating costs or fuel burn. However, if the total productivity of the aircraft fleet is to be maintained, the potential reduction shrinks to 38%.@en

Aviation ensures mobility for both passengers and goods. It is important as a transport sector for connections on and between continents. Nevertheless, aviation also contributes to anthropogenic climate change. The effects are usually divided in CO2 and non-CO2 effects and therefore not only CO2 emissions but also other emissions (e.g., NOx , water vapour or soot) and contrails are covered. To reduce the effects of aviation’s climate impact, several mitigation options are applied. One approach are climate change functions, which will be addressed here. The concept of climate change functions was used in previous projects, e.g. REACT4C, WeCare, ATM4E. The goal of these functions was to optimize the aircraft routes regarding the calculated climate impact. Climate change functions measure the climate impact per unit emission for a specific day, which considers the current meteorological conditions. Climate change functions were previously used to optimize the aircraft routings. The concept should now be applied for the optimization of the aircraft design as well since the promising concept is currently missing for the application of aircraft design optimization. The climate functions for aircraft design will connect the aircraft design with the climate impact of various emission in order to be able to optimize the aircraft design. For the calculation of the functions, it is necessary to define a specific application. This application results from a combination of aircraft design parameters. Aircraft design parameters can be for example flight altitude, climb rate, speed or range. Based on a resulting emission inventory, the temperature response can be calculated with the model “AirClim”. This model calculates with the input, first, the radiative forcing and based on that the temperature change. The final development step is the verification of the climate functions.@en

Given the comparably high impact of aircraft emissions, especially their non-CO2 effects, on climate in the order of 5%, aviation stakeholders are required to act to reduce the warming effects of air traffic. Besides new operational procedures, like e.g. climate-optimized routing, this demands the development of completely new global-warming optimized aircraft by aircraft manufacturers. The European Clean Sky 2 project "Global-Warming Optimized Aircraft Design" (GLOWOPT) aims at providing aircraft designers an innovative tool to perform aircraft design studies for minimum climate impact, which we call Climate Functions for Aircraft Design (CFAD). The CFAD will substantially change the way aircraft are designed, while maintaining compatibility to existing Multidisciplinary Design Optimization (MDO) methods. The functions need to integrate a lot of information on the typical aircraft usage, including the routes the aircraft will be operated on. This is because, besides the amount of emissions, the impact of aviation non-CO2 effects, such as NOx, H2O as well as contrails, on climate is highly dependent on location (i.e. latitude, longitude) and altitude. So, the representative operating profile of the aircraft needs to be considered in a characteristic route and fleet model. This work will present the interdisciplinary GLOWOPT approach, which comprises expertise on aircraft design, operations, atmospheric physics and climate. Conceptual thoughts on how the complexity of the operating profile in combination with the geographically variable climate impact of aircraft emissions will be reduced such that it can be used in an aircraft design process are given. @en

Aviation is a highly necessary transport sector in our modern society. It guarantees mobility on a short- and long-range spectrum and is still a growing sector. However, aviation also contributes signi_cantly to the anthropogenic climate change via CO2 and non-CO2 e_ects. One possibility to reduce the climate impact of aviation would be to optimize the aircraft at the design level. Another possibility is to optimize the operations, e.g. to avoid climate sensitive regions in the ight route. To derive modelling capabilities, we review the climate impact of aviation with a focus on climate optimization of aircraft operations and design. The overall climate impact of aviation based on CO2 and non-CO2 e_ects is analyzed under consideration of contrails and di_erent emissions like CO2, NOx, and H2O. The connection to the related temperature change is shown via the climate sensitivity for each species. An overview over the most common climate metrics, including radiative forcing, global warming potential, global temperature potential, and the average temperature response is given in order to _nd the most suitable climate metric for aircraft design purposes. During previous studies within various projects, e.g. WeCare, REACT4C, and ATM4E, climate optimization strategies for aircraft operations were investigated. The aircraft routes regarding the ight path or altitude can be adjusted in regard to climate considerations, also in dependence on the current weather situation. In these projects, climate change functions and algorithmic climate change functions were developed which could potentially facilitate the climate optimized routings. The aircraft design for climate optimization di_ers from the approach to optimize the design for reduced cost or reduced fuel burn. For the climate impact, ying slow and low is bene_cial which was shown in the project CATS, but this is not reected in the current aircraft design. Therefore, previous studies propose redesigned aircraft. The relation between climate, aircraft operations and aircraft design is used to point out the requirements for modelling resulting from that. The focus is on the connection between climate and operations on one side, and on the connection between climate and design considerations on the other side. Currently, the model capacity for aircraft design does not support the climate optimized design. Therefore, deriving climate functions for aircraft design is highly important which will be one of the main goals in the Clean Sky 2 project GLOWOPT (Global-Warming-Optimized Aircraft Design). @en