Hypersonic speed
[1] 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.
[citation needed] The peculiarities in hypersonic flows are as follows:[citation needed] 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.
Consequently, the distance between the bow shock and the body decreases at higher Mach numbers.
Since the pressure gradient normal to the flow within a boundary layer is approximately zero for low to moderate hypersonic Mach numbers, the increase of temperature through the boundary layer coincides with a decrease in density.
[citation needed] High temperatures due to a manifestation of viscous dissipation cause non-equilibrium chemical flow properties such as vibrational excitation and dissociation and ionization of molecules resulting in convective and radiative heat-flux.
When an aircraft approaches transonic speeds (around Mach 1), it enters a special regime.
The usual approximations based on the Navier–Stokes equations, which work well for subsonic designs, start to break down because, even in the freestream, some parts of the flow locally exceed Mach 1.
Among the spacecraft operating in these regimes are returning Soyuz and Dragon space capsules; the previously-operated Space Shuttle; various reusable spacecraft in development such as SpaceX Starship and Rocket Lab Electron; and (theoretical) spaceplanes.
[citation needed] In the following table, the "regimes" or "ranges of Mach values" are referenced instead of the usual meanings of "subsonic" and "supersonic".
Modern combat aircraft must compromise in order to maintain low-speed handling; "true" supersonic designs, generally incorporating delta wings, are rarer.
Finally, the increased temperature of hypersonic flow mean that real gas effects become important.
This means that for nonequilibrium flow, something between 10 and 100 variables may be required to describe the state of the gas at any given time.
Additionally, rarefied hypersonic flows (usually defined as those with a Knudsen number above 0.1) do not follow the Navier–Stokes equations.
[citation needed] In the study of hypersonic flow over slender bodies, the product of the freestream Mach number
The selection of these regimes is rough, due to the blurring of the boundaries where a particular effect can be found.
The lower border of this regime is where any component of a gas mixture first begins to dissociate in the stagnation point of a flow (which for nitrogen is around 2000 K).
The modeling of gases in this regime is split into two classes:[citation needed] The modeling of optically thick gases is extremely difficult, since, due to the calculation of the radiation at each point, the computation load theoretically expands exponentially as the number of points considered increases.
