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Inaerodynamics, ahypersonic speed is one that exceeds five times thespeed of sound, often stated as starting at speeds ofMach 5 and above.[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 moleculardissociation andionization) occur at different speeds; these effects collectively become important around Mach 5–10. The hypersonic regime can also be alternatively defined as speeds wherespecific heat capacity changes with the temperature of the flow askinetic energy of the moving object is converted into heat.[2]
While the definition of hypersonic flow can be quite vague and is generally debatable (especially because of the absence 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.[citation needed] The peculiarities in hypersonic flows are as follows:[citation needed]
As a body's Mach number increases, the density behind abow shock generated by the body also increases, which corresponds to a decrease in volume behind the shock due toconservation of mass. Consequently, the distance between the bow shock and the body decreases at higher Mach numbers.[3]
As Mach numbers increase, theentropy change across the shock also increases, which results in a strongentropy gradient and highlyvortical flow that mixes with theboundary layer.
A portion of the largekinetic energy associated with flow at high Mach numbers transforms intointernal 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.[citation needed]
High temperatures due to a manifestation of viscous dissipation cause non-equilibrium chemical flow properties such as vibrational excitation anddissociation andionization of molecules resulting inconvective andradiative heat-flux.[citation needed]
Although "subsonic" and "supersonic" usually refer to speeds below and above the localspeed of sound respectively, aerodynamicists often use these terms to refer to particular ranges of Mach values. When an aircraft approachestransonic speeds (aroundMach 1), it enters a special regime. The usual approximations based on theNavier–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.[4]
The "supersonic regime" usually refers to the set of Mach numbers for which linearised theory may be used; for example, where the (air) flow is not chemically reacting and whereheat 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 returningSoyuz andDragonspace capsules; the previously-operatedSpace Shuttle; various reusable spacecraft in development such asSpaceXStarship andRocket LabElectron; 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".[citation needed]
| Regime | Mach No | Speed | General characteristics | Aircraft | Missiles/warheads |
|---|---|---|---|---|---|
| Subsonic | [0–0.8) | <614 mph (988 km/h; 274 m/s) | Most often propeller-driven and commercialturbofan aircraft with high-aspect-ratio (slender) wings, and rounded features like the nose and leading edges. The subsonic speed range is that range of speeds within which, all of the airflow over an aircraft is less than Mach 1. The critical Mach number (Mcrit) is lowest free stream Mach number at which airflow over any part of the aircraft first reaches Mach 1. So the subsonic speed range includes all speeds that are less than Mcrit. | All commercial aircraft | — |
| Transonic | [0.8–1.2) | 614–921 mph (988–1,482 km/h; 274–412 m/s) | Transonic aircraft nearly always haveswept wings that delay drag-divergence andsupercritical wings to delay the onset of wave drag and often feature designs adhering to the principles of the Whitcombarea rule. The transonic speed range is that range of speeds within which the airflow over different parts of an aircraft is between subsonic and supersonic. So the regime of flight from Mcrit up to Mach 1.3 is called the transonic range.[citation needed] |
| — |
| Supersonic | [1.2–5) | 921–3,836 mph (1,482–6,173 km/h; 412–1,715 m/s) | The supersonic speed range is that range of speeds within which all of the airflow over an aircraft is supersonic (more than Mach 1). But airflow meeting the leading edges is initially decelerated, so the free stream speed must be slightly greater than Mach 1 to ensure that all of the flow over the aircraft is supersonic. It is commonly accepted that the supersonic speed range starts at a free stream speed greater than Mach 1.3. Aircraft designed to fly at supersonic speeds show large differences in their aerodynamic design because of the radical differences in the behavior of flows above Mach 1. Sharp edges, thinaerofoil-sections, and all-movingtailplane/canards are common. Moderncombat aircraft must compromise in order to maintain low-speed handling; "true" supersonic designs, generally incorporating delta wings, are rarer. |
| — |
| Hypersonic | [5–10) | 3,836–7,673 mph (6,173–12,348 km/h; 1,715–3,430 m/s) | Coolednickel ortitanium skin; small wings. The design is highly integrated, instead of assembled from separate independently-designed components, due to the domination of interference effects, where small changes in any one component will cause large changes in air flow around all other components, which in turn affects their behavior. The result is that no one component can be designed without knowing how all other components will affect all of the air flows around the craft, and any changes to any one component may require a redesign of all other components simultaneously[citation needed]. |
|
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| High-Hypersonic | [10–25) | 7,673–19,180 mph (12,348–30,867 km/h; 3,430–8,574 m/s) | Thermal control becomes a dominant design consideration. Structure must either be designed to operate hot, or be protected by specialsilicate tiles or similar. Chemically reacting flow can also cause corrosion of the vehicle's skin, with free-atomicoxygen featuring in very high-speed flows. Hypersonic designs are often forced intoblunt configurations because of theaerodynamic heating rising with a reducedradius of curvature. | — | |
| Re-entry speeds | ≥25 | ≥19,180 mph (30,870 km/h; 8,570 m/s) | Ablative heat shield; small or no wings; blunt shape. Seereentry capsule. |
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The categorization of airflow relies on a number ofsimilarity parameters, which allow the simplification of a nearly infinite number of test cases into groups of similarity. For transonic andcompressible flow, theMach andReynolds numbers alone allow good categorization of many flow cases.[citation needed]
Hypersonic flows, however, require other similarity parameters. First, theanalytic equations for theoblique shock angle become nearly independent of Mach number at high (~>10) Mach numbers. Second, the formation of strong shocks around aerodynamic bodies means that the freestreamReynolds number is less useful as an estimate of the behavior of theboundary layer over a body (although it is still important). Finally, the increased temperature of hypersonic flow mean thatreal gas effects become important. Research in hypersonics is therefore often calledaerothermodynamics, rather thanaerodynamics.[5]
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 aKnudsen number above 0.1) do not follow theNavier–Stokes equations.[citation needed]
Hypersonic flows are typically categorized by their total energy, expressed as totalenthalpy (MJ/kg), total pressure (kPa-MPa), stagnation pressure (kPa-MPa),stagnation temperature (K), or flow velocity (km/s).[citation needed]
Wallace D. Hayes developed a similarity parameter, similar to the Whitcombarea rule, which allowed similar configurations to be compared.[citation needed] In the study of hypersonic flow over slender bodies, the product of the freestream Mach number and the flow deflection angle, known as the hypersonic similarity parameter:is considered to be an important governing parameter.[5] The slenderness ratio of a vehicle, where is the diameter and is the length, is often substituted for.
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.[citation needed]
In this regime, the gas can be regarded as anideal 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, whereramjets become inefficient, and the upper border around Mach 10–12.[citation needed]
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.[citation needed]
In this regime, diatomic or polyatomic gases (the gases found in most atmospheres) begin todissociate as they come into contact with thebow shock generated by the body.Surface catalysis 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). At the upper border of this regime, the effects ofionization start to have an effect on the flow.[citation needed]
In this regime theionized 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 flow velocities around 3–4 km/s. Gases in this region are modeled as non-radiatingplasmas.[citation needed]
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:[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.
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