The Aerodynamic Performance of Small Spheres from Subsonic to High Supersonic Velocities

Smooth spheres, 9 / i6 in. in diameter, were fired through a group of photographic stations spaced at intervals along the trajectory. Each station recorded an image of the sphere and of the pattern of the surrounding shock waves. The successive times of operation of each station were recorded on a precision chronograph and were combined with the distances to determine the retardation and, hence, the drag. The firings were carried out at Mach Numbers from 0.29 to 3.96 and at the corresponding Reynolds Numbers from 9.3 X 10 to 1.3 X 10. The results show that the drag coefficient is essentially constant a t subsonic velocities, rises rapidly through a relatively broad transonic region, and decreases slowly with further increase in velocity in the supersonic region. At subsonic velocities, the drags agree well with the measurements made elsewhere below the critical Reynolds Number. However, a close examination of drags and separation points showed no evidence of a critical Reynolds Number for this size sphere. It is believed tha t separation phenomena on small spheres at supersonic velocities are controlled by compressibility effects rather than boundary-layer conditions. Additional firings were carried out with rough /i6-m. spheres, with smooth /32-in. spheres, and with smooth iy 2 in . spheres to study the effects of roughness and size. At supersonic velocities these effects change the drag but little. At low transsonic velocities the drag coefficients of the P/Vin. spheres fall well below the Vie-in. sphere curve, and this difference, combined with changes in the wake flow pattern, demonstrates the occurrence of a critical Reynolds Number for the P/Vin. sphere.