Analysis of cost-efficient launcher family composition under consideration of uncertain launch market scenarios

Abstract A modular family of launch vehicles based on the Prometheus engine, currently in development, is among the concepts being explored for future European launch systems. This modular approach enables the sharing of major components across different configurations, potentially reducing development costs and lowering recurring launch expenses through increased production efficiency. However, the optimal composition and sizing of such modular launch families remain insufficiently addressed in existing literature. This study provides an initial investigation into the recurring and non-recurring costs associated with two modular launch vehicle families employing reusable Vertical Takeoff and Vertical Landing first stages. It examines how the number and selection of family members influence overall costs and evaluates the sensitivity of these outcomes to uncertainties in the future launch market. The analysis reveals that the largest family is not the most cost-effective composition for partially reusable launch vehicle systems. Thanks to the recurring cost reduction enabled by reusing the first stage, the additional cost of developing the entire family is not amortized across a broad spectrum of future possible launch markets. Instead, a streamlined family consisting of two members with significant component overlap minimizes total costs. In contrast, for a launch vehicle family utilizing expendable first stages, the larger family compositions amortize the additional development costs, resulting in lower total costs compared to family compositions with fewer members. As an additional observation, the cost-optimal family composition with reusable first stages costs approximately one-third less than its expendable counterpart.