## 1 Introduction  
Many factors influence the definition and selection of the structural design concept. In general, in complex systems, structural designing is an iterative process. The process starts with the conceptual design of possible alternatives, which could be considered to satisfy the general performance requirements and are likely to meet the main mission constraints (e.g., mass, interfaces, operation and cost). The system's engineering activities are equally valid and necessary at all levels of decomposition within the space product. In particular, the verification engineering function, which iteratively compares the outputs from other functions with each other, in order to converge upon satisfactory requirements, functional architecture, and physical configuration, defines and implements the processes by which the finalized product design is proved to be compliant with its requirements. According to the European Space Agency (ESA) standards [1], the verification process activities shall be incrementally performed at different levels and in different stages, applying a coherent bottom-up building-block concept and utilizing a suitable combination of different verification methods. To reach the verification objectives, a verification approach shall be defined in an early phase of the project by analyzing the requirements to be verified, taking into account the following:  
* design peculiarities;
* the qualification status of the candidate solution;
* the availability and maturity of verification tools;
* verification and test methodologies;
* programmatic constraints;the cost and schedule. In the mechanical design of launchers, the verification of vibroacoustics requirements is crucial for guaranteeing the preservation of the payloads' functionalities during the preorbital phase. This process is particularly affected by the lack of both the availability and maturity of standardized design testing tools. This work describes the innovative integrated process used at the design-stage level in the verification of the Vega-C rocket project development. The interstages of the new _Vettore Europeo di Generazione Avanzata_ (VEGA, European Advanced Generation Vector) configurations are examined in the framework of the ESA-funded VEGA Consolidation and Evolution Program (VECEP), in order to derive the acceleration level at the equipment locations and the average vibration response of the connecting interfaces. The analysis is needed in the framework of the re-engineering phase aimed at further increasing the performance of the VEGA launcher, and it is part of a preliminary numerical verification necessary to assess the compliance of the designed structure with the acceleration level imposed as a limit for the protection of the equipment and payload. The complexity of the analysis is driven by two critical elements, which have been addressed in this work with an innovative approach. First, the acoustic pressure generated at lift-off cannot be calculated with the resolution needed because of the computational complexity of the simulation. Then, the vibroacoustic analysis driven by the calculated acoustic field on the basis of finite-element/boundary-element method (FEM/BEM) techniques turns out to be unfeasible in a medium to high frequency range, even when resorting to computationally efficient approaches such as the acoustic transfer vector (ATV) or modal ATV (MATV) [2], because of the high modal density of the structure. On the other hand, the employment of a fully energy-based approach in this frequency range is restricted by the need to simulate the equipment as lumped masses, connected to the main structure throughout rigid links.