# The Water-Enhanced Turbofan as Enabler for Climate-Neutral Aviation
## 1 Introduction  
Since the first gas turbine-powered commercial jet airliner was deployed about 70 years ago, aviation transport efficiency has improved by approximately eight-fold [1]. Both aircraft and engines are constantly being further developed and technologically improved. The latest turbofan engine generation, the geared-turbofan PW1100G, introduced in 2016, achieves an efficiency improvement of approximately 16% compared to previous generation technology [2]. With respect to fuel burn and CO\({}_{2}\) emissions, this evolutionary progress was not sufficient to compensate for the increase in air travel, as the sector has grown steadily ever since the 1960s and up until the outbreak of the COVID-19 pandemic [3]. With an average growth rate of worldwide air traffic of approximately 5% per year (pre-pandemic), and an overall aircraft efficiency improvement of approximately 1.5% per year, the global aviation kerosene consumption increases by approximately 3.5% annually [4]. Globalization, low-cost airlines, cheap fuel, low interest rates, and growing prosperity--all of these factors have also made flying affordable for increasing portions of the population. With this increase, CO\({}_{2}\) emissions have continued to increase.  
With respect to sustainability, aviation is in the spotlight of politics and society. In recent years, the focus has expanded beyond CO\({}_{2}\) and local air quality around airports towards climate impact. Following the Paris Agreement and the 1.5 \({}^{\circ}\)C target [5], aviation-specific targets have been derived. The European Green Deal targets climate neutralityuntil 2050 [6], and was further broken down into the \"Fit for 55\" plan, targeting a 55% reduction by 2030 (compared to year 1990 level) [7]. With respect to aviation, this includes phasing out of free emission allowances and inclusion into the European Trading System (ETS) for greenhouse gas emissions, as well as a ramp-up of Sustainable Aviation Fuels (SAF) to 6% by 2030 and 63% by 2050. The industry has begun to adopt this paradigm shift to climate impact. For example, MTU published a Clean Air Agenda (Claire) with targets for climate impact and mission energy consumption [8].  
The increasing growth in air traffic and the approaching physical limits of the known gas turbine cycle require revolutionary ideas for novel aero engine concepts. In 2020, [PERSON] et al. published a paper series on innovative aero engine concepts [9; 10; 11], including the novel Water-Enhanced Turbofan (WET) concept (at that time, still referred to as \"the steam injecting and recovering aero engine (SIRA)\"). It addresses the three most climate-relevant emissions. The new dual-fluid propulsion concept was described, and, based on preliminary thermodynamic studies, the potential to reduce climate-effective emissions was derived. The present article outlines the evolution since then. The most promising WET configuration is evaluated regarding its ability to reduce global warming potential compared to an evolutionarily advanced turbofan engine at the same technology level. The climate impact assessment was refined beyond the last state published [12], with a quantitative approach to estimate reduction of NOx emissions through steam injection. The potential to reduce contrails is outlined, applying the Schmidt-Appleman criterion.