# A Study on Aeroengine Conceptual Design Considering Multi-Mission Performance Reliability
## 1 Introduction  
It is a truth universally acknowledged--that an aviation vehicle in possession of a good flight performance, must be in want of a reliable propulsion system. Currently, considerable attention has been paid to the candidates of next generation aviation propulsion solution, such as variable cycle aeroengine [1], hypersonic precooled engine [2, 3]. One noteworthy feature of these candidate solutions is the excellent multi-mission adaptability due to their special functions, such as variable geometry schedule, multi-cycle coupling mechanism [4], the regenerative cooling system [5], etc. Through implementing these innovative functions, the new propulsion solutions could combine the specialties of turbojet and turbofan, including the high specific thrust in supersonic penetration and low specific fuel consumption in subsonic cruise. Impressively, these specialties were supposed to be contradictory and could not simultaneously be achieved on the conventional aircraft engine. Despite these considerable advantages mentioned above, one essential issue has not been addressed and would certainly preclude their potential application. That is the worsening overall reliabilityproblems. Obviously, the realization of above functions requires more complex mechanical structures, such as the extra core driven fan stage for airflow pressurization in second bypass, throttle valves for switching thermal cycle mode, and coupled heat transfer system for hypersonic propulsion [6]. As the novel aeroengine mechanical structure becomes more complex, the following overall reliability problems would undoubtedly be even worse than that of the conventional gas turbine engine. Based on the marginal design philosophy, the solution is to maintain a certain level of redundancy by improving design performance indicators [7]. Essentially, the aviation gas turbine engine is a kind of thermal power plant, which exploits the Brayton cycle and employs air as the working medium for repeated compression, heating, expansion, and exothermic processes [8]. Through the basic thermal cycle analysis, there are two ways to boost Brayton cycle performance behavior. One is to increase the cycle pressurization ratio for higher thermal efficiency; while the other is to improve the adding heat for more output cyclic work [9]. Therefore, conclusions are conducted to enhance design performance of the gas turbine engine. Firstly, increasing the pressure ratio of the compression system could result in higher aeroengine thermal efficiency for raising the fuel-economy; secondly, raising the combustor outlet temperature could obtain more cyclic work so that aeroengine has access to more net thrust. Therefore, it is up to the corresponding key design parameters, which determined design performance indicators. To date, the traditional deterministic single-design point methods are still widely applied to redundant performance design in aeroengine conceptual design phase. This methodology only assess single design point then evaluates other critical operating condition in the off-design phase. Thus, it is incapable to integrate of the requirements and constraints at the different operating conditions into the on-design cycle analysis, which is urgently required for future aviation propulsion schemes that noted for excellent multi-mission adaptability. Besides, the decision makers subjectively determine the increments of key design parameters by referring to the deterministic analysis results with safety factors. These situations might cause either insufficient performance redundancy in actual flight mission, or extra technical problems and increase of manufacturing cost due to potential performance waste. On the one hand, a small increase in design parameters leads to insufficient performance redundancy, which results in flight mission failure or threatens flight safety in the extreme environmental conditions.