Freedom involves transports. In the current society people want to be able to travel around the world. Tourism, student exchange programs, visits to family or friends living abroad, business, and many other facts could be a reason to move by plane. The air traffic experimented a progressive grow during the last years, and the prediction is that it will still continue increasing. However, there are two main problems which could act as hindrances against such process of development. On the one hand, the majority of engines that are being used to propel aircraft nowadays work burning fossil oil derivative fuels. Oil is a limited raw material the value of which is currently rising due to the fact that it is being exhausted. On the other hand, the other concern is the emissions produced by the combustion of such fuels by aircraft engines. The fumes produced remain in the atmosphere contributing to the greenhouse effect and the global warming, which are two of the ecological problems that the society must fight against. These reasons cause the need to research and develop Future Propulsion Systems. In this project a study of the further possibilities to solve such problems is presented, as well as a comparison and discussion trying to forecast what could be the best path to follow, in order to continue propelling aircraft in a greener, more economical and sustainable way.