The design of a new launcher involves a full set of disciplines -propulsion, thermal and structural sizing, aerodynamics, mission analysis, flight control, transient phases analyses, stages layout -with strong interactions between each other and large impact on the quality-price ratio of the launch service offered to the customer with the resulting vehicle. The overall activity is therefore a typical Multi-Disciplinary Analysis and Optimisation (MDAO) design process. The classical approach to solve the problem is of sequential and iterative type. It includes a large number of loops, and generally leads to solutions very much dependent on simplified initial assessments. It does not guarantee to achieve the best compromise, and may even end into deadlocks. The paper describes an alternative methodology implemented for the integrated design of a new expendable launch vehicle, and the results obtained, in the frame of the prospective activities about post-ARIANE5 European launchers family performed by EADS Space Transportation under internal R&D funding. The different disciplines considered are stage loading, propulsion complete design (parameters and mass budget), trajectory optimisation, aerodynamic drag coefficient assessment, and non propulsive stage mass budget. Except for trajectory computation, based on "exact" dynamic equations, the tools used for each discipline are simplified models, which are described in the paper.The whole problem -about 30 parameters, linked to both the architecture and the command (trajectory optimisation), 10 constraints -is solved by Genetic Algorithm thanks to the EO core optimiser, an open source freeware evolutionary computational library, including a large number of selection/reproduction mechanisms. The optimisation under trajectory constraints is done by incorporating a penalty function for unsatisfied constraints into the fitness function. The interest of the method at preliminary design development step is shown through the illustrative resolution of two kind of problems: first a pure trajectory optimisation problem (maximise the injected payload mass), the architecture of the launcher being frozen. An adequate adaptative strategy for the penalty weight all along the optimisation process proves to be accurate and efficient for intermediate and final constraints, classically difficult to take into account with this type of approach. A second application is performed taking into account architecture parameters, but freezing the type of architecture itself to the ARIANE 5 case: assuming a 3 stage launcher with 2 solid boosters, a main cryogenic first stage and a storable liquid propellant upper stage, the best staging configuration and propulsion parameters are assessed for a given class of payload into orbit. The best solution is assessed for both the lift-off minimum mass and launcher recurrent cost criteria. In a third step, not fully completed here, the "global" problem will be solved from scratch, with no preconceived idea on the number of stages, characteristics of the pro...
