The Virtual Propulsion Expert: Application of a Hybrid Surrogate-Based Rubber Engine Model in Aircraft Design

An aircraft is a complex system of systems. The engine, as a subsystem, strongly influences the design of other subsystems in a complex interplay by its performance characteristics, dimensions and mass. The ability to reliably model the overall aircraft and to find optimal designs within a short period of time is critical in the conceptual design phase to provide the basis for sound decision making. However, this is difficult to achieve since simplified engine models may lead to unreasonable results and more sophisticated models usually require the consultation of a propulsion expert. Therefore, a hybrid surrogate-based rubber engine approach is demonstrated that facilitates the exchange of disciplinary knowledge and enables the convenient integration of detailed engine models into multidisciplinary processes for overall aircraft design. A rubberized generic geared turbofan with an entry into service in 2035 is created to equip a long-haul, wide-body aircraft with different suitably sized engines from a multidimensional design space. In order to generate the training data for the surrogate-based rubber engine model, a multidisciplinary process for conceptual engine design is employed, which combines a multi point thermodynamic cycle analysis with flow path sizing, a basic aerodynamic analysis of turbomachinery, mass estimation on the level of single engine parts and a model to predict engine emissions. With the rubber engine model integrated into the aircraft design process, the off-design performance of individual engines is provided on-demand via tabulated maps, which are calculated in-the-loop. For a long-haul aircraft configuration, a bypass ratio of 14 is identified as optimal in terms of mission fuel considering snow ball effects. For growing bypass ratios, the thrust lapse increases leading to higher combustor inlet temperature and pressure at cruise operation. As a result, nitrogen oxide emissions increase with BPR for an assumed rich-burn quick-quench lean-burn (RQL) combustor and counteract savings in carbon dioxide and water emissions leading to minimum climate impact for a bypass ratio of 11. The minimum direct operating costs are realized for a bypass ratio of 12.