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Boeing X-43 at Mach 7

In aerodynamics, hypersonic speeds are those that are highly supersonic. Since the 1970s, the term has generally been assumed to refer to speeds of Mach 5 (5 times the speed of sound) and above. The hypersonic regime is a subset of the supersonic regime.

The precise Mach number at which a craft can be said to be fully hypersonic is elusive, especially since physical changes in the airflow (molecular dissociation, ionization) occur at quite different speeds. Generally, a combination of effects become important "as a whole" around Mach 5. The hypersonic regime is often defined as speeds where ramjets do not produce net thrust. This is a nebulous definition in itself, as there exists a proposed change to allow them to operate in the hypersonic regime (the Scramjet).


Characteristics of flow

While the definition of hypersonic flow can be quite vague and is generally debatable (especially due to the lack of discontinuity between supersonic and hypersonic flows), a hypersonic flow may be characterized by certain physical phenomena that can no longer be analytically discounted as in supersonic flow. These phenomena include:

Small shock stand-off distance

As Mach numbers increase, the density behind the shock also increases, which corresponds to a decrease in volume behind the shock wave due to conservation of mass. Consequently, the distance between the shock and the body generating it reduces at high 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. Thus, the boundary layer over the body grows and can often merge with the thin shock layer.

High temperature flow

High temperatures discussed previously as a manifestation of viscous dissipation cause non-equilibrium chemical flow properties such as dissociation and ionization of molecules resulting in convective and radiative heat flux.


The hypersonic flow regime is characterized by a number of effects which are not found in typical aircraft operating at low subsonic Mach numbers. The effects depend strongly on the speed and type of vehicle under investigation.


Generally, NASA defines "high" hypersonic as any Mach number from 10 to 25, and re-entry speeds as anything greater than Mach 25. Aircraft operating in this regime include the Space Shuttle and various space planes in development.

Comparison of regimes
Regime Mach Mph km/h m/s General Plane Characteristics
Subsonic <1.0 <768 <1,230 <340 Most often propeller-driven and commercial turbofan aircraft with straight wings
Transonic 0.8-1.2 610-768 980-1,475 270-410 Sharp intakes; compressibility becomes noticeable; slightly swept wings
Supersonic 1.0-5.0 768-3,840 1,230-6,150 340-1,710 Sharper edges; tailplane is a stabilator
Hypersonic 5.0-10.0 3,840-7,680 6,150-12,300 1,710-3,415 Cooled nickel-titanium skin; highly integrated, small wings
High-hypersonic 10.0-25.0 7,680-16,250 12,300-30,740 3,415-8,465 Silicon thermal tiles, blunt wings
Re-entry speeds >25.0 >16,250 >30,740 >8,465 Ablative heat shield; no wings; blunt capsule shape

Similarity parameters

The categorization of airflow relies on a number of similarity parameters, which allow the simplification of a nearly infinite number of test cases into groups of similarity. For transonic and compressible flow, the Mach and Reynolds numbers alone allow good categorization of many flow cases.

Hypersonic flows, however, require other similarity parameters. Firstly, the analytic equations for the Oblique shock angle become nearly independent of Mach number at high (~>10) Mach numbers. Secondly, the formation of strong shocks around aerodynamic bodies mean that the freestream Reynolds number is less useful as an estimate of the behavior of the boundary layer over a body (although it is still important). Finally, the increased temperature of hypersonic flows mean that real gas effects become important. For this reason, research in hypersonics is often referred to as aerothermodynamics, rather than aerodynamics.

The introduction of real gas effects mean 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 (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 a 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 velocity (km/s).

Wallace D. Hayes developed a similarity parameter, similar to the Whitcomb area rule, which allowed similar configurations to be compared.


Hypersonic flow can be approximately separated into a number of regimes. The selection of these regimes is rough, due to the blurring of the boundaries where a particular effect can be found.

Perfect gas

In this regime, the gas can be regarded as an ideal gas. Flow in this regime is still Mach number dependent. Simulations start to depend on the use of a constant-temperature wall, rather than the adiabatic wall typically used at lower speeds. The lower border of this region is around Mach 5, where Ramjets become inefficient, and the upper border around Mach 10-12.

Two-temperature ideal gas

This is a subset of the perfect gas regime, where the gas can be considered chemically perfect, but the rotational and vibrational temperatures of the gas must be considered separately, leading to two temperature models. See particularly the modeling of supersonic nozzles, where vibrational freezing becomes important.

Dissociated gas

In this regime, diatomic or polyatomic gases (the gases found in most atmospheres) begin to dissociate as they come into contact with the bow shock generated by the body. Surface catalycity plays a role in the calculation of surface heating, meaning that the type of surface material also has an effect on the flow. 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 upper border of this regime is where the effects of ionization start to have an effect on the flow.

Ionized gas

In this regime the ionized electron population of the stagnated flow becomes significant, and the electrons must be modeled separately. Often the electron temperature is handled separately from the temperature of the remaining gas components. This region occurs for freestream velocities around 10-12 km/s. Gases in this region are modeled as non-radiating plasmas.

Radiation-dominated regime

Above around 12 km/s, the heat transfer to a vehicle changes from being conductively dominated to radiatively dominated. The modeling of gases in this regime is split into two classes:

  1. Optically thin: where the gas does not re-absorb radiation emitted from other parts of the gas
  2. Optically thick: where the radiation must be considered as a separate source of energy.

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.

See also

Other flow regimes


  • Anderson, John (2006). Hypersonic and High-Temperature Gas Dynamics Second Edition. AIAA Education Series. ISBN 1563477807.  

External links

Simple English

In aerodynamics, hypersonic speeds are speeds that are highly supersonic. The word hypersonic came from a word used in the 1970s, Mach 5, or 5 times the speed of sound. Everything about the plane changes greatly when the airplane that is flying at those speeds reaches supersonic speeds.


Characteristics of Hypersonic Flow

Though the definition for hypersonic speeds is not very clear and is a subject of debate among scientists, they have come up with a possible definition. A hypersonic flow may be characterized by certain things that can no longer be discounted in analysis. Some of these things include:

Thin Shock Layer

As the plane goes faster and mach numbers increase, the density behind the shock also goes up. This happens in concert with a decrease in volume behind the shock wave because of the conservation of mass theory. Because of this, the shock layer (the volume between the body and the shock wave) is thin at high mach numbers.

Entropy Layer

As a plane goes faster and mach numbers go up, the entropy change across the shock also goes up. This results in a strong entropy gradient and highly vortical flow that mixes with the boundary layer.

Viscous Interaction

Some of the big kinetic energy found with flow at large mach numbers becomes internal energy in the fluid because of viscosity. The increase in internal energy makes the temperature go up. Because the pressure gradient normal to the flow within a boundary layer is zero, the increase of temperature through the boundary layer causes the density to fall. This causes the boundary layer over the body of the airplane to grow. This, in turn, causes the temperature to go up.

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