The carbon is deep rooted in our energy ecosystems in general and specifically it is the primary source for current aero-propulsion. Alternative energy sources are being explored across the globe to address and mitigate the environmental issues such as climate change arising from the use of carbon energy. This exploration is likely to accelerate further because of the COP28 agreement to “transition away from fossil fuels”, time will tell whether this is conceivable or not. Renewables are catching on for electricity production but it is impractical for aero applications at this time. Aero-engine industries are exploring the use of hydrogen, biofuels and also electric propulsion. Each of these brings their own challenges and scientific opportunities to develop the required technologies. This talk is aimed to give a broad overview on the future directions, to discuss the associated major scientific and technological challenges and opportunities. The role of high performance computing in the envisaged scientific journeys will be highlighted.