Analysis of the transition process in dual bell nozzles

Dual bell nozzles may represent a possible alternative to replace conventional bell nozzles in future launcher first stages. Their main feature is the peculiar shape of the divergent section, displaying two bells jointed by an inflection point. Although this is a non-optimized shape, it allows two different operating modes that provide at high altitude larger expansion ratios than conventional nozzles, while avoiding side-loads at sea-level. The present study addresses the main critical aspects of the design of the nozzle shape. In particular, numerical tests have been carried out to understand the role of the inflection point region and of the second bell profile on the nozzle behavior during the phase of transition between the two operating modes. As a result suggestions are provided to be considered when designing the second bell profile.