A series hybrid-electric propulsion system has been designed for small rapid-response unmanned aircraft systems (UAS) with the goals of improving endurance, providing flexible and responsive electric propulsion, and enabling heavy fuel usage. The series hybrid architecture used a motor-driven propeller powered by a battery bank, which was recharged by an engine-driven generator, similar to other range-extended electric vehicles. The engine design focused on a custom, two-stroke, lean-burn, compression-ignition (CI), heavy-fuel engine, which was coupled with an integrated starter alternator (ISA) to provide electrical power. The heavy-fuel CI engine was designed for high power density, improved fuel efficiency, and compatibility with heavy fuels (e.g., diesel, JP-5, JP-8). Commercially available gasoline spark-ignition engines and heavy-fuel spark-ignition engines were also considered in the trade study. The series hybrid configuration allowed the engine to be mechanically decoupled from propeller, so that the engine could be operated at the load/speed condition for peak fuel-conversion efficiency. An energy-dense rechargeable battery pack was used to store energy and allow the UAS to operate with the engine shut off, which provided an engine-off operating mode. The ISA allowed re-starting the engine in flight without the need for a separate starter motor.Simulation-based design tools were developed, and trade studies were performed for the various system components. The series hybrid UAS outfitted with the custom diesel engine demonstrated endurance improvements, due to additional benefits netted from the improved engine efficiency. Development of the hybrid propulsion system is ongoing, with current efforts focused on reducing system mass, packaging, and gearing up for a future hardware demonstration.