hypersonic flight[¦hī·pər′sän·ik ′flīt]
Flight at speeds well above the local velocity of sound. By convention, hypersonic flight starts at about Mach 5 (five times the speed of sound) and extends upward in speed indefinitely. Supersonic vehicles also fly at speeds greater than the local speed of sound. However, when the Mach number is high, the flow field around an object exhibits a special behavior, which is worth studying separately from supersonic flight. This behavior is characteristic of hypersonic flight.
A body entering the Earth's atmosphere from space (for example, a meteorite, a ballistic reentry vehicle, or a spacecraft) has high velocity and hence large kinetic and potential energy. During reentry, drag forces act upon a reentry vehicle or spacecraft and cause it to decelerate, thus dissipating its kinetic and potential energy. The energy lost by the reentry vehicle is then transferred to the air within the flow field around the reentry vehicle. The flow field around the forward portion of a blunt-nosed vehicle (body of revolution or leading edge of a wing) generally exhibits (1) a distinct bow shock wave, (2) a shock layer of highly compressed hot gas, and (3) a highly sheared boundary layer over the surface. The flow field is defined as the region of disturbed air between the body surface and the shock wave. The temperature of the shock layer is so high that molecules begin to dissociate during collisions at about 4000°F (2500 K), or at Mach 7. At about Mach 12, gas in the stagnation region reaches a temperature of 6700°F (4000 K) and becomes ionized.
In general, at high hypersonic flight speed the characteristic temperature in the shock layer of a blunted body and in the boundary layer of a slender body is proportional to the square of the Mach number. Heat energy is then transferred from the hot gases to the vehicle surface by conduction and diffusion of chemical species in the boundary layer and by radiation from the shock layer near the nose. Heat energy is also radiated from the vehicle surface to space or to adjacent objects. One important problem confronting the designer of reentry vehicles or spacecraft is therefore to design a minimum-weight vehicle able to withstand large heat loads from adjacent hot-gas layers during reentry while retaining the ability to carry a given useful payload.