lishes a conical separated flow zone when it is of a proper length. When a certain critical spike length is exceeded, the separation moves from the spike tip to some position on its length, and the effectiveness of the spike is greatly reduced. A method for estimating this critical spike length is presented in this Note. Some studies5'810 have shown that the effect of the spike on the nose drag is diminished as the angle of attack a. increases. The effect of the spike on the static stability is very small.11 In the present investigation these effects are studied at Mach numbers 1.5 and 2.25 at various Reynolds numbers and spike lengths. The results provide a better understanding of these effects. The experiments were done in the 30 cm X 30 cm super- sonic blow down wind tunnel at the Aerodynamic Laboratory of the Department of Aeronautical Engineering. The models are cylindrical bodies 28 mm in diameter by 180 mm long, hemispheric and ogive with a hemispherical nose of 11 mm radius. The spike is installed in the nose center. Its diameter (a = 1.75 mm) is T^ of the model's body diameter (d = 28 mm) and its tip is conical with half-angle of 10°. The spike length I is varied to give l/d values of 0, 0.5, 1.0, 1.5, and 2.0. The aerodynamic forces and moments are measured by a 3-component, strain-gage balance. The flow is photographed by a Schlieren or a shadow optical system using movie photography (24 to 80 frames/sec) and by a Polaroid camera.