Sublimation of Flat-Nosed Cones with Various Axisymmetric Steps in Supersonic Flow

Abstract

A method of sintering powdered camphor and naphthalene after deaeration under vacuum which is well suited for making models for wind-tunnel experiments on the aerodynamics of subliming bodies is described. Exploratory tests on cones with flat and hemispherical noses and on several complex conical shapes, featuring downstream and upstream facing steps, rectangular grooves and shoulders are presented. The experiments were performed at a Mach number of 3, blowing parameters up to 0.35 and model Reynolds numbers up to 200,000. Emphasis is placed on the general evolution of the profiles, the history of the tips, and the occurrence of grooves, striations, and surface markings. I. Introduction P AST work on ablative heat shields was dominated by thermal and structural problems, and focused mainly on applications to blunt bodies. Recent experiments15 have focused on fine details of the surface features left on the ablated skin after flight, which are of considerable fundamental interest. These include grooves associated with identifiable three-dimensional roughness elements, turbulence wedges, and even more surprising patterns, only recently identified, which look like crosshatching by grooves spiraling in both directions over the surface. In these applications, a relatively thin shell of material ablates without substantially influencing the geometry or the aerodynamic characteristics of the vehicle. However, recent emphasis on low-fineness- ratio re-entry shapes has given prominence to changes in shape of the ablating surface, particularly in the neighborhood of the nose. These changes can seriously affect the drag, the aerodynamic stability parameters of the vehicle, and the amount and aerothermochemistry of observables left in the wake of the body. Some features of complex (composite) initial shapes with corners, steps, etc., are eroded smoothly and stably, whereas others tend to magnify and induce further self-perpetuating nonuniformities. There have been virtually no studies of such phenomena published and the possible uses of the coupling between the trajectory and the history of shape/ drag/ablation thermochemistry of such composite shapes has not been considered. The present report describes a method of sintering camphor and naphthalene which results in very satisfactory models for wind-tunnel investigations of the shape change of ablating bodies, and presents exploratory results on the surface recession history of cones, blunted cones, and composite shapes having expansion corners, upstream and downstream flow separations, and notches. The experiments were performed over a period of three years,6'7 with limited resources. Only photographic records were collected which are not sufficient to allow a detailed and quantitative reduction of data and a comparison with theory. As an exploratory program, however, the tests were quite successful, and further exploitation of the technique seems quite worthwhile.