## 4 Conclusions  
The current study offers potential pathways for significantly reducing the climate impact of future air-vehicle concepts. It also provides the direct comparison of synthetic kerosene and LH2 powered future representative aircraft concepts in terms of climate impact with all relevant disciplines and fidelity levels involved. The main takeaways are listed below:  
* The block-energy for the design mission of the liquid hydrogen (LH2) concept is just 3% higher than the synthetic kerosene one. This is due to the increased overall turbofan engine efficiency of about 7% which counteracts the aerodynamic and structural penalties. * The increased engine efficiency results from the higher water content in the exhaust. This leads to a higher energy density due to the increased heat capacity which results in more powerful turbines for the same mass flow. * The LH2 concept shows relaxed, low-speed thrust requirements leading to lower turbine entry temperatures at take-off engine rating compared with the kerosene concepts. This is due to the smaller difference between MTOM and MLM which results from the higher energy density of LH2. The wing area and high-lift system is sized for Maximum Landing Mass (MLM) condition leading to lower wing loadings at Maximum Take-Off Mass (MTOM). * Compared to the reference concept, the advanced combustion technology based on the Flameless Oxidation (FLOX) concept achieves 91% and 99.8% global NO\({}_{\text{x}}\) emission reduction for kerosene and hydrogen, respectively. * The biggest lever in climate impact reduction is the advanced low-NO\({}_{\text{x}}\) and low-soot combustion chamber technology. The difference between the D261+ which is an  
Figure 16: Relative change in climate impact (ATR100) with conventional and advanced low-NO\({}_{\text{x}}\) and low-soot combustion. The grey cells indicate the changes of the new scenario compared with the preceding neighbour. The uncertainties are visualized by the black whisker lines. evolutionary baseline concept for entry into service (EIS) 2040 and fossil fuel with conventional combustion and the one with advanced combustion chambers (FLOX) is 67%. This reduction is due to the strongly decreased NOx and particles emission, which causes a reduction in the total NOx and contrail induced cloudiness (CiC) impact. In other words, the rigorously implementation of advanced low-NOx and low-soot combustion chamber technology leads to 67% impact reduction. * The second biggest lever is the implementation of flying in lower altitudes to effectively reduce the non-CO2 effects. In this study, a pure altitude constraint of flight level 290 and an reduced Mach number of 0.75 is applied leading to significant non-CO2 impact reductions, but also to 5% higher global fleet level Direct Operating Costs. This impact reduction can potentially be further increased if additional, more precise measures, such as flexible no-fly zones for very sensitive airspaces, are employed. * The third biggest lever is the use of sustainable energy carriers. Together with the previous two measures and the assumption of green hydrogen and synthetic kerosene with renewable carbon sources, a combined climate impact reduction of 85% and 93% for the LH2 and the synthetic kerosene case can be achieved, respectively. Although the impact in terms of Average Temperature Response (ATR) is only the third biggest, the reduction in CO2 emissions is very important, on the one hand due to the very long lifetime of CO2 in the atmosphere, and on the other hand due to the additional fuel consumption for flying in lower altitudes for avoidance of non-CO2 effects. * Significant climate impact reductions with pure efficiency improvements are not possible. However, combined with the described additional measures, it plays an important role to counteract the increasing costs of renewable energy carriers. * Due to the small database for the prediction of contrils by hydrogen burning turbofan engines, there is a significant uncertainty which will be tackled in future work.