thumb|[[Computational fluid dynamics|CFD image of the NASA X-43A at Mach 7]]
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 vehicles to fly above Mach 5.
Characteristics of flow
thumb|Simulation of hypersonic speed (Mach 5)
While the definition of hypersonic flow can be quite vague
The peculiarities in hypersonic flows are as follows:
- Shock layer aerodynamic heating
- 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. Consequently, the distance between the bow shock and the body decreases at higher Mach numbers.
Entropy layer
As Mach numbers increase, the entropy change across the shock also increases, which results in a strong entropy gradient and highly vortical flow that mixes with the boundary layer.
Viscous interaction
A portion of the large kinetic energy associated with flow at high Mach numbers transforms into internal energy in the fluid due to viscous effects. The increase in internal energy is realized as an increase in temperature. 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. This causes the bottom of the boundary layer to expand, so that the boundary layer over the body grows thicker and can often merge with the shock wave near the body leading edge.
High-temperature flow
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.
Classification of Mach regimes
Although "subsonic" and "supersonic" usually refer to speeds below and above the local speed of sound respectively, aerodynamicists often use these terms to refer to particular ranges of Mach values. 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. So, more sophisticated methods are needed to handle this complex behavior.
The "supersonic regime" usually refers to the set of Mach numbers for which linearized theory may be used; for example, where the (air) flow is not chemically reacting and where heat transfer between air and vehicle may be reasonably neglected in calculations. Generally, NASA defines "high" hypersonic as any Mach number from 10 to 25, and re-entry speeds as anything greater than Mach 25. 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.
In the following table, the "regimes" or "ranges of Mach values" are referenced instead of the usual meanings of "subsonic" and "supersonic".
{| class="wikitable sortable"
! Regime
! Mach No
! Speed
! General characteristics
!style="width: 280px;"| Aircraft
!style="width: 280px;"| Missiles/warheads
|-
! style="background:#FFFFFF;" | Subsonic
| data-sort-value=0 | < 1
The introduction of real gas effects means that more variables are required to describe the full state of a gas. Whereas a stationary gas can be described by three variables (pressure, temperature, adiabatic index), and a moving gas by four (flow velocity), a hot gas in chemical equilibrium also requires state equations for the chemical components of the gas, and a gas in nonequilibrium solves those state equations using time as an extra variable. 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.
Hypersonic flows are typically categorized by their total energy, expressed as total enthalpy (MJ/kg), total pressure (kPa-MPa), stagnation pressure (kPa-MPa), stagnation temperature (K), or flow velocity (km/s).
Wallace D. Hayes developed a similarity parameter, similar to the Whitcomb area rule, which allowed similar configurations to be compared. In the study of hypersonic flow over slender bodies, the product of the freestream Mach number <math>M_{\infty}</math> and the flow deflection angle <math>\theta</math>, known as the hypersonic similarity parameter:<math display="block">K = M_{\infty}\theta</math>is considered to be an important governing parameter.