the hypersonic regime. Other systems (boost-glide vehicles) use scramjets after their initial boost, in which the speed of the air passing through the scramjet remains supersonic. Other systems (munitions) use a cannon for their initial boost. High temperature effect Hypersonic flow is a high energy flow. The ratio of kinetic energy to the internal energy of the gas increases as the square of the Mach number. When this flow enters a boundary layer, there are high viscous effects due to the friction between air and the high-speed object. In this case, the high kinetic energy is converted in part to internal energy and gas energy is proportional to the internal energy. Therefore, hypersonic boundary layers are high temperature regions due to the viscous dissipation of the flow's kinetic energy. Another region of high temperature flow is the shock layer behind the strong bow shock wave. In the case of the shock layer, the flow's velocity decreases discontinuously as it passes through the shock wave. This results in a loss of kinetic energy and a gain of internal energy behind the shock wave. Due to high temperatures behind the shock wave, dissociation of molecules in the air becomes thermally active. For example, for air at T > 2000 K, dissociation of diatomic oxygen into oxygen radicals is active: O2 → 2O For T > 4000 K, dissociation of diatomic nitrogen into N radicals is active: N2 → 2N Consequently, in this temperature range, a plasma forms: —molecular dissociation followed by recombination of oxygen and nitrogen radicals produces nitric oxide: N2 + O2 → 2NO, which then dissociates and recombines to form ions: N + O → NO+ + e− Low density flow At standard sea-level condition for air, the mean free path of air molecules is about . Low density air is much thinner. At an altitude of the mean free path is . Because of this large free mean path aerodynamic concepts, equations, and results based on the assumption of a continuum begin to break down, therefore aerodynamics must be considered from kinetic theory. This regime of aerodynamics is called low-density flow. For a given aerodynamic condition low-density effects depends on the value of a nondimensional parameter called the Knudsen number , defined as where is the typical length scale of the object considered. The value of the Knudsen number based on nose radius, , can be near one. Hypersonic