This thesis presents the first systematic experimental study of boattail radius effects on Hammerhead Fairings (HHFs) in transonic flow, using the Coe and Nute Model 11 as a representative platform. Boattail radii were selected based on ratios of radius to boundary-layer thickness, and tested in two campaigns: high-speed schlieren imaging, oil-flow visualisation, and PIV at selected cases. Results show that increased radius generates a third expansion region, often with a lambda-shock, reducing separation length by 10–25% but increasing reattachment-point oscillations. Strong radius effects promoted shock-induced separation and lifted the shear layer at the shock foot. Two dominant unsteady modes were identified: a flapping mode linked to reattachment motion and shock features, and an undulation mode related to momentum injection and ejection from the separation bubble. Findings hint at trade-offs between separation reduction and increased unsteadiness, with implications for launch vehicle stability and structural loading.