The aerodynamic properties of a high-slenderness finned projectile with and without a threadlike roughness element on its shaft are evaluated by force and moment balance measurements in a supersonic blow-down tunnel. The test conditions cover a Mach number range from 2.5 to 4.1 at diameter-based Reynolds numbers in the order of 8 × 10^5. The forebody axial force is increased by approximately 15 % by the roughness element at zero angle of attack. Further measurements at non-zero angles of attack show that this axial force increase growths over-proportionately with the angle of attack. Whereas a minor influence by the roughness element is observed for the normal force, the pitching moment is either decreased by about 7 % or increased by about 13 % depending on the flow condition and angle of attack. Flow visualization by a high-resolution schlieren setup gives insight into the flow physics responsible for these phenomena. The schlieren setup also enables the detection of the laminar-turbulent boundary layer transition.