Plasma Physics and Hypersonic Flight

W an object travels through air at altitudes below something like 200,000 ft and at Mach numbers greater than 12 to 15, the production of electrons in the shocked layer of air which surrounds the object has a significant effect on some of the properties of the shocked air. Above some altitude like 200,000 ft the influence of the electrons is felt at even lower Mach numbers, particularly in the ionosphere where the ambient electron density is no longer negligible. The influence of electrons is felt not only on aerodynamic quantities such as heat transfer, drag and flow field, but also on physical quantities such as transport properties, radiative emission and absorption, and electromagnetic signal interaction. The main thesis of the present paper is to show how, and under what conditions, the electrons produced by the passage of a high speed missile through the atmosphere significantly affect both aerodynamic and physical quantities. A plasma may be defined as a gas containing electrons in sufficient quantity to seriously affect the physical properties of the gas. Plasma physics then is simply the physics of such a gas. For example, the air in the shock layer surrounding a hypersonic missile may be in a plasma state if the missile is traveling at an extremely high velocity. We will first discuss in general terms the structure of a hypersonic axially symmetric flow field. This will be followed by a more detailed discussion of the shock front and boundary layer with emphasis on the electron distribution. We will then show an example of an analysis of the Boltzmann equation and the relationship of the electron scattering cross section and the electron velocity distribution function.