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 alter 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 (C_D). The present study focuses on geometric parameters of strap-on such as length, diameter and radial gap between strap-on and core. 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 (C_A), normal force coefficient (C_N) and pitching moment coefficient (C_PM), which are required for mission design and structural load estimation. In the current study, systematic CFD based parametric studies were conducted using Reynolds-averaged Navier-Stokes based in-house solver PARAS 3D. Simulations were performed at transonic (Mach=1.2) and supersonic (Mach=1.8) 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. An empirical relation between aerodynamic coefficients and strap-on geometry was derived and were validated against different configurations. The derived relations provide a rapid and practical tool for preliminary aerodynamic assessment of multibody launch vehicles.