## 4 Discussion
### Resulting Pressure  
In the framework of the VECEP, the Italian Aerospace Research Centre (_Centro Italiano Ricerche Aerospaziali_, CIRA) and the prime contractor, Avio SpA, are involved in the prediction of the aeroacoustic loads at lift-off due to the introduction of the first-stage solid rocket motor P120, a longer version of the currently used P80 motor. In this work, the empirical models used within the design verification process have been fed with the experimental data acquired using a 1/20 scaled mock-up of the VEGA launcher equipped with the first-stage solid rocket motor P80 [12; 20]. Two experimental campaigns have been carried out by Avio, ELV and ONERA in the framework of the VEGA program [20; 21] and by Avio, ELV and CIRA in the framework of the Italian Space Agency-funded project CAST (_Configurazioni AeroTermodinamiche per Sistemi di Trasporto spaziale_, AeroThermodynamic Configuration for space Transport Systems) [12; 22].The launcher mock-up has been conceived to reproduce the same acoustic environment generated by the full-scale VEGA first-stage solid rocket motor P80 at different launcher altitudes, from 0 to 75 m, corresponding to the first 4 s of the VEGA ascent trajectory during lift-off.  
The free-field results achieved with the Eldred standard model and the Eldred-based uncorrelated and correlated models have been compared with the VEGA and CAST experimental results for three launcher altitudes (0, 10 and 75 m) in [14]. The uncorrelated model has shown to provide a better prediction at 0 and 10 m, while the correlated model is better at 75 m, matching the experimental data with an uncertainty of about 5 dB.  
The reason for the different matching between the correlated/uncorrelated models and the experiments' data can be explained reasonably with the different turbulent mechanisms that occur at different altitudes. At 75 m, the launcher is far from the launching pad, and, as a consequence, the jet does not interact with the ground. Indeed, the turbulence structures are still unchanged, and the correlation is preserved. Because the correlated model is developed to reproduce the jet turbulent scales' correlation, it is expected to be more suitable for reproducing the free jet conditions at the altitude of 75 m. Conversely, at the reduced distances of 0 and 10 m between the launcher and the launching pad, the jet turbulent structures are broken and mixed as a result of the interaction with the launching pad, and, as a consequence, the turbulent structures become more uncorrelated.  
The effect of the launcher has thus been investigated with the hybrid approach. A three-dimensional multifrequency computation has been performed up to a maximum frequency of 1000 Hz, and the results have lastly been processed to achieve third-octave spectra. The uncorrelated model was used for 0 and 10 m, while the correlated model was used for 75 m. The output of this process was then passed as input to the hybrid procedure for analyzing the launcher response, as described in the following section.  
The results of the BEM calculation are shown in Figure 4, where they are compared, in terms of SPL, with the standard Eldred results and the data from the two experimental campaigns.  
Figure 4: Sound pressure level (SPL) in third-band octave. Comparison between hybrid model results and experimental data collected at 0 (a), 10 (b) and 75 (c) m of altitude of the launcher interstages 2–3.  
The SPL values have been computed in two sets of points, representing far- and near-field point. Far-field points, distant from the launcher by 15% of the nozzle diameter, have been dubbed in the figure as \"free-field microphones\". Near-field points, selected on the rocket surface itself, are indicated as \"flush-mounted microphones\". The gap observed between the free-field and the flush-mounted microphones resembles that occurring between the two experimental datasets, suggesting a different position of the microphone in the respective experimental set-up.