Reducing climate-damaging emissions is a key objective of today’s aviation industry. Suitable approaches include emission-optimized flight profiles and the use of hybrid-electric propulsion technologies. In order to quickly identify and quantify their potential, powerful 0D engine models are needed to calculate the performance of conventional thermal and hybrid-electric propulsion systems at the design point and in off-design conditions.This paper presents the model Maple (“Modular Aircraft Engine Performance for Sustainable Aviation”) for such performance calculation, emission and mass estimation. Special emphasis is laid on a flexible, modular design, so that the framework can be extended by new configurations and additional components, e.g. electric subsystems or heat exchangers. Established turbojet, turbofan, and turboprop configurations as well as various (hybrid-)electric configurations are implemented. Validation against an established commercial reference shows deviations well below one percent. For the analysis along entire flight missions, off-design operating points are computed quasi-steadily depending on atmospheric boundary conditions and thrust requirements. The latter are determined using the open source tool OpenAP. Thereby, the influence of additional mass due to more sophisticated engine configurations is taken into account.To illustrate the capabilities of the performance tool, it is used to compare and discuss the performance of a conventional turbofan engine and a parallel hybrid-electric engine by calculating two exemplary flight missions with two battery technology assumptions. The benefits and challenges of using hybrid-electric engines are highlighted. These include the need for careful matching of conventional and electric components, gas turbine retrofit, and advanced thermal management.Overall, the strengths of the developed tool are demonstrated. The modular structure and easy adaptability are ideal for the efficient design and calculation of state-of-the-art and future aircraft engines. The tool allows estimating their impact on flight mission fuel consumption and emissions, and thus on the climate.