Novel aircraft propulsion technologies are being demonstrated on unmanned vehicles vying to reach general and commercial aviation. Each technology offers significant advantages and challenges. For example, Li-ion batteries enabled the development of new electric propulsion concepts, yet they pose a fire hazard. Liquid hydrogen and fuel cells will expand the range of electric propulsion, but innovative integration of this cryogen with the vehicle is needed to protect against accidental leaks and hydrogen embrittlement. Unducted fans will increase thrust, but center wing tanks will need protection against a fan burst. This talk will discuss these challenges and their mitigation methods.alb