Current unmanned aerial vehicles (UAVs) are limited in operating altitude and endurance by the maximum attainable performance of small gas turbine engines. While improvements in the cycle thermal efficiency can be obtained through the introduction of heat exchangers, the consequent increase in engine weight partially offsets the benefits gained in specific fuel consumption (SFC). Conceptual studies on semi-closed cycles, where part of the engine mass flow rate is recirculated through the engine core, have shown that a significant reduction in engine weight can be obtained in comparison with equivalent open cycle solutions. This thesis presents a preliminary design and optimization study of semi-closed cycle engines for high altitude UAV applications. A detailed thermodynamic and mechanical model has been created to correlate component performance and weight variation as function of thermodynamic design parameters. The model has been coupled with a multi-objective optimization algorithm to optimize various engine cycles and to assess the trade-off between minimum SFC and minimum engine weight. Results have shown that a considerable degree of compactness can be obtained with this novel configuration, leading to an overall engine weight approximately two time lower than conventional open cycles.