A significant part of the current aviation climate impact is caused by non-carbon-dioxide emissions, mainly nitrogen oxides (NO_x) and contrails. It is, therefore, important to have a holistic view on climate metrics. Today’s conventional, but already well-developed, aero-engines are based on the Joule–Brayton cycle, and leave only limited room for improvement in climate impact. The revolutionary Water-Enhanced Turbofan (WET) concept represents a technical step change addressing all relevant emissions by implementing the Cheng cycle, which combines the gas turbine cycle with a Clausius–Rankine steam cycle. This paper builds upon previous publications regarding the WET concept, and outlines the evolution since then. Promising WET configurations are evaluated according to their ability to reduce global warming potential compared to an evolutionarily advanced turbofan engine. A quantitative approach to estimate reduction of NO_x emissions through steam injection is presented. The impact on the creation of contrails is considered using the Schmidt-Appleman criterion. In conclusion, all three climate-relevant emissions can be reduced with the WET concept compared to a technologically similar turbofan in terms of CO₂ (up to 10%), NO_x (more than 90%), and contrails (more than 50%). The resulting in-flight climate impact can be reduced by more than 40% when using fossil kerosene, paving the way to climate-neutral aviation.

Recognizing the attention currently devoted to the environmental impact of aviation, this three-part publication series introduces two new aircraft propulsion concepts for the timeframe beyond 2030. The first part focuses on the novel steam injecting and recovering aero engine concept. In the second part, the free-piston composite cycle engine (FP-CCE) concept is presented. Complementary to the two technical publications, this third part describes the cooperative project, which was initiated by an interdisciplinary consortium, aiming at the demonstration and the proof-of-concept of both aforementioned aero engine concepts. At the beginning of the project, simulations on propulsion, aircraft system, and test bench level will be conducted. On this basis, preliminary tests and fundamental experiments are planned in order to establish a solid basis for the demonstration. Finally, a system demonstration will be carried out at the laboratory level. Thus, the project allows for a final judgment on both the feasibility of the new concepts and the attainability of the requirements for future aircraft propulsion systems.

Pursuing the targets of the Flightpath 2050 program, i.e. a reduction of 75% in CO~2~ and 90% in NO~x~ emissions, the aviation industry must look beyond the incremental improvement of the conventional gas turbine cycle and consider completely new propulsion concepts. Therefore, a consortium of industrial, academic and national research institutions was formed to investigate and

Recognizing the attention currently devoted to the environmental impact of aviation, this two-part publication introduces two new aircraft propulsion concepts for the timeframe beyond 2030. Part one focuses on the exhaust heat generated steam injection engine concept featuring in-flight condensation and recovery of the used water. In the second part, the free-piston composite cycle