The term sustainability became a popular subject both in the automotive industries and in the aerospace industries. Increasing and threatening environmental pollution problems and reduction in limited fuel sources are motivating either industries and academicians to develop alternative power systems to sustain more healthier and economical life in the long term. One innovation that has been researched in the automotive industry is all electric and hybrid electric propulsion concepts. These concepts have also been proposed as alternative solutions for aviation. These novel propulsion technologies are composed of a gas turbine/internal combustion engine structure (necessary for hybrid electric and turboelectric propulsion systems) and/or energy storage components (battery, fuel cell and so on.) with multiple electric motors respectively. In this paper, simulation of a high fidelity turboshaft engine-alternator model for turboelectric propulsion system is derived. To develop an aero-thermal engine model, GE T700 turboshaft engine data is used and constructed model is connected to an alternator model on MATLAB/Simulink environment. Open-loop simulations are carried out and satisfactory results are obtained. Simulation results are compared to the real engine design point data. Results show that there are acceptable differences between the simulation results and the real engine data. The power balances between compressor - high pressure turbine and power turbine - alternator are proven in the mathematical model. It is expected that the proposed model can be easily extended to power system design and power management studies in turboelectric propulsion systems and also in other suitable novel propulsion systems.