Hypersonic speed

In aerodynamics, hypersonic speed refers to speeds much faster than the speed of sound, usually more than approximately Mach 5.

The precise Mach number at which a craft can be said to be flying at hypersonic speed varies, since individual physical changes in the airflow (like molecular dissociation and ionization) occur at different speeds; these effects collectively become important around Mach 5–10. The hypersonic regime can also be alternatively defined as speeds where specific heat capacity changes with the temperature of the flow as the kinetic energy of the moving object is converted into heat.

Hypersonic weapons are typically boost-glide vehicles or cruise missiles designed for aerodynamic flight and maneuvering above Mach 5.

High hypersonic speeds are experienced during atmospheric entry. Spaceplanes are designed to be capable of flight in this regime. The North American X-15 and the Space Shuttle orbiter are the only crewed spaceplanes to fly faster than Mach 5.

Characteristics of flow

While the definition of hypersonic flow can be quite vague a hypersonic flow may be characterized by certain physical phenomena at very fast supersonic flow.

The peculiarities in hypersonic flows are as follows:

Shock layer

Shock interaction - aerothermal: aerodynamic heating of the fuselage

Entropy layer

Real gas effects

Low density effects

Independence of aerodynamic coefficients with Mach number.

Small shock stand-off distance

As a body's Mach number increases, the density behind a bow shock generated by the body also increases, which corresponds to a decrease in volume behind the shock due to conservation of mass.