Effect of strap-on geometry on the aerodynamics of multibody launch vehicle

Strap-on boosters play a crucial role in heavy launch vehicles by providing additional liftoff thrust without major changes to the baseline design, enabling launch with existing propulsion systems. However, strap-on boosters introduce additional pressure drag and alters the overall aerodynamics of the vehicle. While efforts have been previously made to derive empirical relationships to predict the aerodynamics of different strap-on configurations, most are case-specific and primarily limited to estimating drag coefficients (CD) [1]. Strap-on geometry has a vital role in flow field interference effects. The present study focuses on primary non-dimensional geometric parameters such as (1) strap-on core diameter ratio (Ds/Dc), (2) strap-on core length ratio (Ls/Lc), and (3) strap-on-core radial gap (Xg/Dc). The results are used to derive an empirical relationship which can be applied during preliminary design stage of a launch vehicle to predict axial force coefficient (CA), normal force coefficient (CN) and pitching moment coefficient (CPM), which are required for mission design and structural load estimation. While high fidelity CFD and wind tunnel tests provides detailed case specific results, they are computationally and experimentally expensive and impractical during design phase of a launch vehicle. Previous studies suggests that results from simple CFD simulation have acceptable accuracy in preliminary aerodynamic design (compared to experimental data) [2]. In the current study, systematic CFD based parametric studies was conducted using Reynolds- averaged Navier-Stokes based in-house solver PARAS 3D. Simulations were performed at subsonic, transonic and mid supersonic regimes in pitching condition at an angle of attack 4°. The study considers a simplified configuration with two parallel strap-on boosters. Parameters were evaluated relative to a clean-core baseline configuration. The results are validated against a typical vehicle configuration. The derived relations provide a rapid and practical tool for preliminary aerodynamic assessment of multibody launch vehicles.