With the aviation sector growing each year, the need for a reduced climate impact is becoming increasingly important. Electrification of the propulsion system is believed to offer promising avenues in achieving this reduction. Additionally, airlines operating these aircraft have to adapt their operations and network to optimally utilize these aircraft. This research presents a methodology for the coupled design of a hybrid-electric aircraft fleet with strategic airline planning to optimally serve a specific network. The objective is to maximize the airline profit and minimize the network CO2 emissions. Aircraft design trade-offs in payload, range and runway length will guide the creation of new aircraft until the optimal aircraft fleet is determined. The methodology is tested in a case study for the regional airline network of SATA Air Acores. The study investigates the impact of introducing new hybrid-electric aircraft designs in the fleet on the creation of new aircraft, the aircraft allocation and the network performance. By directly integrating hybrid-electric aircraft design (having a parallel hybrid architecture) with strategic airline planning, it is possible to reduce the network CO2emissions by -11% at the cost of an airline profit decrease of -13%. When including a climate optimization, an additional reduction of network CO2 emissions is achieved of -27% with a small additional decrease in profit of -1%. Network profitability and climate impact are mainly dictated by the fleet diversity and the assumed technology level of the batteries employed in the aircraft. This research highlights the importance of including climate optimization in the design of new aircraft and the need for more advanced hybrid-electric propulsion architectures (such as distributed propulsion systems) to further contribute to climate impact reduction.

To reduce the climate impact of the turbofan dominated aviation industry, a conceptual propeller aircraft study has been performed to see the a possible reduction in climate impact between the two. The results analysed showed great promise for the reduction of the climate impact at the cost of block time. Additionally, a loss in productivity or fleet size is observed for the climate impact seen. The change in productivity or fleet size is remedied with an decrease in block time, however this results in an increase in climate impact, especially when compared to the turbofan aircraft. Overall it is safe to say that the climate impact can be reduced by utilising propeller aircraft, however either the productivity or fleet size must be changed. Additionally, the costs per flight potentially go up. All for a more sustainable and greener aviation industry.

This report covers the investigation of impact of uncertainties on the multidisciplinary design optimization of a medium-range single-aisle turbofan aircraft for minimum global warming impact. The employed workflow for the investigation is a five step process, starting with the implementation of the deterministic climate impact model and carrying out of the design optimization for minimal climate impact. The second step involves the characterisation of uncertainties, where the uncertainties within the climate impact model are identified and quantified. The third step involves the uncertainty analysis, where Monte Carlo simulations are performed to estimate the variability in the average temperature reduction potential of the climate-optimized aircraft with respect to the cost-optimized aircraft. In the fourth step, a robust design optimization is carried out using a non-sorted genetic algorithm to minimize both the average temperature response and variability in average temperature response potential. The sensitivity analysis is carried as the last step using the Morris and variance-based Sobol methods, to identify what the key uncertain parameters are towards the uncertainty in climate impact of the aircraft designs. Scientific uncertainty is identified within the linear climate impact model for the carbon impulse response function parameters, species radiative efficiencies, the NOx and contrail altitude forcing factors, methane lifetime, species efficacies, and are all assigned a probabilistic description. Scenario uncertainty is identified in the future average global CO2 atmospheric concentration projection, for which different realistic future scenarios are characterised. Carrying out the uncertainty analysis has shown that the average temperature response reduction potential of the climate-optimized aircraft is highly uncertain, having a 90% likelihood ranging between 17 and 98 % of the average temperature response of the cost optimized aircraft. This is primarily due to large variability in the estimation of contrail average temperature response. Although the robustness-based optimization did not allow to find any significant improvement in robustness for the climate-optimized aircraft, it did allow to identify an array of robust climate-optimized design solutions. From the sensitivity analysis, it was found that the uncertain parameters showing predominant influence on the output variability are the contrail-related radiaitive efficiency and forcing factors. Additionally, a variability of ±50% in average temperature response apportioned to CO2 emissions was identified due to uncertainty related to future average global CO2 concentration projections.

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. Nevertheless, most recent studies agree that hydrogen aircraft would underperform their kerosene counterparts in terms of operative empty mass and specific energy consumption. The main cause for the drop in performance lays in the fuel storage, as not only the liquid hydrogen has to be kept in cryogenic conditions and pressurised, but for the same energy content, it has four times the volume of kerosene. The inevitable consequences are an increase in fuselage size, which adds mass and drag to the aircraft, and the addition of an heavy fuel storage and distribution system. On the other side, hydrogen has 2.8 times higher specific energy, and the consequent reduction in fuel mass could balance the previously mentioned drawbacks. Literature on the topic shows that the optimal fuel storage solution depends on the aircraft mission, but most studies disagree on what solutions are optimal for each aircraft range category.The objective of this research was to identify and compare possible solutions to the integration of the hydrogen fuel containment system on short, medium and long-range airliners. The capabilities of an automated synthesis program for CS-25 aircraft have been expanded with validated structural and thermodynamic physics-based tank design models, to allow for the design and analysis of liquid hydrogen aircraft. Studies were performed on several design options. The effect of using an integral tank structure was found to be negligible for short-range aircraft, but increasingly more beneficial for medium and long-range aircraft. The effect of increasing the fuselage diameter was found to be favourable, especially when seats abreast could be added without the addition of one aisle. The effect of using a combination of an aft and a forward tank was found to be detrimental in terms of operational empty mass, beneficial in terms of specific energy consumption and negligible in terms of maximum take-off mass. The use of spherical tanks was found to be slightly beneficial, but only when compared to a non-spherical tank version using the same tank layout, non-integral tank structure, and same cabin layout. The study on the venting pressure revealed that with increasing aircraft size the optimal venting pressure in terms of main aircraft performance decreases whereas the sensitivity to those same parameters to the choice of venting pressure increases. The use of direct gas venting as a means to contain the pressure rise did not appear to provide significant performance improvements. The optimal designs, in terms of operational empty mass, maximum take-off mass and specific energy consumption, feature increased fuselage diameters, the use of the aft & forward tank layout, non-spherical tanks and no direct venting. The short-range aircraft uses non-integral tanks and high venting pressure, while the medium and the long-range aircraft benefit from an integral tank structure and a lower venting pressure. Nevertheless, the sensitivity to these design choices is not significant, meaning that with a different set of assumptions and/or requirements different design choices may become optimal. The overall best performing LH2 aircraft for the short, medium and long-range categories were found to have respectively 8%, 24% and 22% higher operative empty mass, -2%, 1% and -5% higher maximum take-off mass and 5%, 13% and 5% higher specific energy consumption than their kerosene versions.