# The Water-Enhanced Turbofan as Enabler for Climate-Neutral Aviation
## 6 Conclusion and Outlook  
This paper provides a comprehensive description of the WET configuration and cycle. Compared to a technologically similar turbofan, the WET concept features wet combustion, two heat exchangers in the exhaust (vaporizer and condenser), a water recovery device, a water pump, and a steam turbine. The complete configuration was adapted so that it could fit entirely into the engine nacelle. The resulting cycle improvement through heat recovery from the exhaust added power extraction in a steam turbine, and increased specific work in the turbines improved SFC by 13%. The resulting fuel burn, considering efficiency improvement, weight penalty, and added nacelle drag, reduces by up to 10%.  
The paper introduces two new aspects for the quantification of climate impact of the WET concept. First, an approach to quantify the NOx reduction potential through steam addition is presented. Second, reduced impact of contrail formation is motivated through application of the Schmidt-Appleman criterion to show that the WET concept is less inclined to form contrails. As a result, all three main drivers of climate warming from aviation are significantly reduced, paving the way to climate-neutral aviation. CO2 reduces proportionately to fuel burn by up to 10%. NOx can be reduced by more than 90% through steam addition and cycle adaptation. contrails could be reduced by more than 50%. The resulting in-flight climate impact is reduced by more than 40%.  
The WET concept is now in a proof-of-concept phase. All new components, i.e., wet combustion, condenser, vaporizer, water separation, and steam turbine, will be experimentally demonstrated, in order to improve the prediction of target performance of all components under given boundary conditions, or to reveal unknown challenges or even show-stoppers.  
Additionally, the concept will still be further optimized, in particular, with respect to weight and heat exchanger design. Advanced heat exchanger designs optimized for the given boundary conditions promise significant reductions in size and weight, which will benefit fuel burn, reduce nacelle drag, and thus simplify integration. Further improvement potentials might also be achieved by optimizing the heat recovery in the vaporizer and condenser, minimizing the required temperature differences.  
Figure 7: Absolute Pulse Global Warming Potential (APGWP) over one hundred years of the WET concept relative to TF2030+ at same technology level. We assume 10% fuel burn reduction, 50% contrail reduction, and 90% NOx reduction. For simplicity, only the cruise segment at 35,000 ft is considered. Concerning the climate impact, scientific uncertainties are omitted.  
**Author Contributions:** Conceptualization, S.K., O.S. and P.Z.; methodology, S.K., O.S. and P.Z.; software, S.K. and P.Z.; validation, S.K. and P.Z.; formal analysis, S.K., O.S. and P.Z.; investigation, S.K. and P.Z.; writing--original draft preparation, S.K., O.S. and P.Z.; writing--review and editing, S.K., O.S., P.Z. and H.K.; visualization, S.K., O.S., P.Z. and H.K.; supervision, S.K., O.S. and H.K.; project administration, O.S. and S.K.; funding acquisition, O.S. All authors have read and agreed to the published version of the manuscript.  
This research was supported by the German Federal Ministry for Economic Affairs and Climate Action (BMWK) under the DLR-contract D/442/67294977.  
Not applicable.  
We: the authors, would like to thank MTU Aero Engines for permission to publish this research. We would like to thank the WET team, the performance department and the combustor team for fruitful discussions and contributions to the concept, in simulating the concept and assessing climate impact. We thank colleagues from Bauhaus Luftfahrt for their expertise in assessing aircraft integrated effects.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.