This paper investigates the performance of different configurations of gas turbine engines. A full numerical model for the engine is built. This model takes into account the variations in specific heat and the effects of turbine cooling flow. Also, the model considers the efficiencies of all component, effectiveness of heat exchangers and the pressure drop in relevant components. The model is employed to compare the engine performances in cases of employing intercooler, recuperation and reheat on a single spool gas turbine engine. A comparison is made between single-spool engine and two-spool engine with free power turbine. Also, the performance of the engine with inter-stage turbine burner is investigated and compared with engine employing the nominal reheat concept. The engine employing inter-stage turbine burners produces superior improvements in both net work and efficiency over all other configurations. The effects of ignoring the variations on specific heat of gases and turbine cooling flow on engine performance are estimated. Ignoring the variation in specific heat can cause up to 30% difference in net specific work. The optimum locations of the intercooler and the reheat combustor are determined using the numerical model of the engine. The maximum net specific work is obtained if the reheat combustor is placed at 40% of the expansion section. On the other hand, to get maximum efficiency the reheat combustor has to be placed at nearly 10%-20% of the expansion section. The optimum location of the intercooler is almost at 50% of the compression section for both maximum net specific work and efficiency.