Hypersonic speed

In aerodynamics, hypersonic speeds are speeds that are significantly faster than the speed of sound. Any speed faster than Mach 5 (five times faster than the speed of sound; 1,715 m/s at sea level) is said to be hypersonic.  Hypersonic speeds are distinguished from supersonic speeds, because air and aircraft act very strangely at hypersonic speeds; even stranger than at supersonic speeds.  

Characteristics of Hypersonic Flow
When an aircraft is moving at hypersonic speeds, it is subject to certain special effects due to its speed. Some of these effects are:

Thin Shock Layer
As the plane goes faster, the density behind the shock wave increases. At the same time, the volume of the air behind the shock wave decreases. Because of this, the shock layer (the space between the aircraft's body and the shock wave) becomes very thin. This is important because it changes the way the plane has to be built.

Entropy Layer
As a plane goes faster, the entropy change across the shock wave increases.  This results in a strong entropy gradient.  This causes air around the aircraft to swirl like a vortex, in what is called "vortical flow."  This is important because it mixes with what is called the "boundary layer" around the aircraft.  A boundary layer is a small area of air around the plane which is moving slower than the air farther away from the aircraft.  When the vortical flow mixes with the boundary layer, the boundary layer is disrupted, and the aircraft becomes very unstable.

Viscous Interaction
At very high speeds, objects that are inside the atmosphere become very hot. This can cause the previously mentioned boundary layer of the plane to expand. When the boundary layer expands, the aircraft gets even hotter. Put simply, planes moving extremely fast experience lots and lots of drag and get very very hot. This process is caused by the viscosity of air, which is why this is called "viscous interaction."

 Fun Fact 
The adjective "sonic" was first used in the 1920s, from the Latin sonus, or "sound." Ever since, it has been used not only to describe things relating to sound, but also things that travel as fast as sound.

Other websites
NASA's Guide to Hypersonics 

Category:Fluid mechanics