Hypersonic Flight and the Re-Entry Problem: The Twenty-First Wright Brothers Lecture

Abstract

Up to this point discussion of problems of rocket vehicles has been confined to effects of phenomena which have in past been important ones for lower speed aircraft and will continue to be important for aircraft of all speeds. Now with considerable extension of both speed and altitude, other phenomena also become important. The nature of some problems will be altered, as a result, and new problems, of course, will be encountered. First, it is well to note that our interest in bluff bodies for ballistic vehicles in particular, and in rounded-nosed bodies generally, has changed our emphasis in aerodynamics. The detached bow waves which occur with such bodies at high supersonic speeds complicate calculations of flow-field characteristics. In present period, much attention is being given to such studies. In addition, at very high altitudes attained by most of rocket-craft, mean free path of air molecules can be of same order, or long, compared to dimensions of Thus, slip-flow and free-molecule-flow studies are of interest, particularly for satellite vehicles. The aerodynatnicist must deal with air having unfamiliar states and properties. Second, at hypersonic speeds where, for example, air is greatly decelerated, it may undergo considerable change in composition, degree of change depending upon many factors. Dissociation of oxygen and nitrogen molecules can occur and, in addition, thermal ionization of many of constituents. It is naturally to be expected that convective heat transfer will, as a result, be altered from what it was for perfect gas, and this has been subject of much recent research effort. Moreover, decelerated gas becomes capable of radiating energy and radiative heat transfer must generally be considered for hypersonic vehicles, particularly for long-range ballistic rockets. It is not only aerodynamic heating problems that are affected. The fact that at very high air temperature gas becomes electrically conductive introduces new problems in radio wave transmission and reception. In addition, a conducting gas flow can, of course, be influenced by a magnetic field. The study of such flows, which has been termed magneto gas dynamics, is still in too primitive a state to indicate how important a role it can play, but many interesting possibilities suggest themselves. Third, our experience with airplanes powered by air-breathing engines has naturally been restricted to stratosphere, or lower. Our ignorance increases with altitude. For rockets, literally, the sky's limit, and it is not surprising that a great emphasis has now been placed on obtaining a more thorough understanding of whole atmosphere. These studies are not aimed at an understanding of chemical and physical characteristics alone, but also of occurrence of high-energy particles, from meteors to cosmic rays, and nature of problems they will promote.