UpdateBot at 21:08, 26 May 2022

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{{short description|Speed that is highly supersonic}}
{{Redirect|Hypersonic}}
{{mdy|date=October 2014}}
[[File:X-43A (Hyper - X) Mach 7 computational fluid dynamic (CFD).jpg|thumb|[[Computational fluid dynamics|CFD]] image of the [[NASA X-43|NASA X-43A]] at Mach 7]]
In [[aerodynamics]], a '''hypersonic speed''' is one that exceeds 5 times the [[speed of sound]], often stated as starting at speeds of [[speed of sound|Mach]] 5 and above.<ref name="Galison">{{Cite book |url=https://books.google.com/books?id=qfrOBgAAQBAJ |title=Atmospheric Flight in the Twentieth Century |publisher=Springer |year=2000 |isbn=978-94-011-4379-0 |editor-last=Galison |editor-first=P. |page=90 |editor-last2=Roland |editor-first2=A.}}</ref>

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 (chemistry)|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 kinetic energy of the moving object is converted into heat.<ref name="Glenn">{{Cite web |title=Specific Heat Capacity, Calorically Imperfect Gas |url=https://www.grc.nasa.gov/WWW/BGH/realspec.html |access-date=2019-12-27 |website=Glenn Research Center |publisher=[[NASA]]}}</ref>

==Characteristics of flow==
[[File:Dopplereffectsourcemovingrightatmach5.0.gif|thumb|Simulation of hypersonic speed (Mach 5)]]
While the definition of hypersonic flow can be quite vague and is generally debatable (especially due to 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.{{cn|date=October 2014}} The peculiarity in hypersonic flows are as follows:{{cn|date=October 2014}}
# 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 [[Shock wave#Bow shocks|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.{{cn|date=October 2014}}

===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.{{cn|date=October 2014}}

===High-temperature flow===
High temperatures due to a manifestation of viscous dissipation cause non-equilibrium chemical flow properties such as vibrational excitation and [[dissociation (chemistry)|dissociation]] and [[ionization]] of molecules resulting in [[convection|convective]] and [[Atmospheric reentry#Real (non-equilibrium) gas model|radiative heat-flux]].{{cn|date=October 2014}}

== Classification of Mach regimes ==
{{expand section|addition of "General spacecraft characteristics" to the existing airplane characteristics in the table|date=June 2021}}
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. This occurs because a "[[transonic]] regime" exists around M=1 where approximations of the [[Navier–Stokes equations]] used for subsonic design no longer apply, partly because the flow locally exceeds M=1 even when the freestream{{clarify|date=March 2018}} Mach number is below this value.{{clarify|seems odd: some Mach numbers are obviously above, and others below|date=June 2021}}

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 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 (spacecraft)|Soyuz]] and [[SpaceX Dragon|Dragon]] [[space capsule]]s; the previously-operated [[Space Shuttle]]; various reusable spacecraft in development such as [[SpaceX]] [[SpaceX Starship|Starship]] and [[Rocket Lab]] [[Electron (rocket)|Electron]]; as well as (theoretical) [[spaceplane]]s.{{cn|date=October 2014}}

In the following table, the "regimes" or "ranges of Mach values" are referenced instead of the usual meanings of "subsonic" and "supersonic".{{cn|date=October 2014}}

{| class="wikitable"
|-
! rowspan="2" | Regime
! colspan="4" | Velocity
! rowspan="2" | General airplane characteristics
|-
! [[Mach number|Mach No]]
! mph
! km/h
! m/s
|-
! style="background:#FFFFFF;" | [[Speed of sound|Subsonic]]
| < 0.8
| < 614
| < 988
| < 274
| Most often propeller-driven and commercial [[turbofan]] aircraft with high aspect-ratio (slender) wings, and rounded features like the nose and leading edges.
|-
! style="background:#FFD0D0;" | [[Transonic]]
| 0.8–1.2
| 614–921
| 988–1482
| 274–412
| Transonic aircraft nearly always have [[swept wing]]s that delay drag-divergence, [[supercritical airfoil|supercritical wings]] to delay the onset of wave drag, and often feature designs adhering to the principles of the Whitcomb [[area rule]].{{cn|date=June 2021}}
|-
! style="background:#FFA0A0;" | [[Supersonic]]
| 1.2–5
| 921–3836
| 1482–6174
| 412–1715
| Aircraft designed to fly at supersonic speeds show large differences in their aerodynamic design because of the radical differences in the behaviour of [[fluid flow]]s above Mach 1. Sharp edges, thin [[airfoil]]-sections, and all-moving [[tailplane]]/[[canard (aeronautics)|canards]] are common. Modern [[combat aircraft]] must compromise in order to maintain low-speed handling.{{cn|date=June 2021}} "True" supersonic designs include the [[F-104 Starfighter]] and BAC/Aérospatiale [[Concorde]].
|-
! style="background-color:#FF7070;" | Hypersonic
| 5–10
| 3836–7673
| 6174–12350
| 1715–3430
| Cooled [[nickel]] or [[titanium]] skin; 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{{cn|date=June 2021}}; small wings. See [[Boeing X-51 Waverider]], [[BrahMos-II]], [[X-41 Common Aero Vehicle]], [[DF-ZF]], [[Hypersonic Technology Demonstrator Vehicle]], [[Hypersonic Air-breathing Weapon Concept]] (HAWC, pronounced Hawk), [[Shaurya (missile)|Shaurya missile]].
|-
! style="background:#FF0000;" | High-Hypersonic
| 10–25
| 7673–19180
| 12350–30870
| 3430–8507
| Thermal control becomes a dominant design consideration. Structure must either be designed to operate hot, or be protected by special [[silicate]] tiles or similar. Chemically reacting flow can also cause corrosion of the vehicle's skin, with free-atomic [[oxygen]] featuring in very high-speed flows. Examples include the [[53T6]] (Mach 17), [[Hypersonic Technology Vehicle 2]] (Mach 20), [[LGM-30 Minuteman]] (Mach 23), [[Agni-V]] (Mach 24), [[DF-41]] (Mach 25), and [[Avangard (hypersonic glide vehicle)|Avangard]] (Mach 20-27). Hypersonic designs are often forced into [[Atmospheric entry#Blunt body entry vehicles|blunt configurations]] because of the [[aerodynamic heating]] rising with a reduced [[Radius of curvature (mathematics)|radius of curvature]].
|-
! style="background:#A00000; color:#fff8f5;" | [[Atmospheric entry|Re-entry speeds]]
| > 25
| > 19030
| > 30870
| > 8575
| Ablative or Thermal Soak heat shield; small or no wings; blunt shape. See [[Reentry capsule|Reentry Capsule]].
|}

==Similarity parameters==
The categorization of airflow relies on a number of [[Dimensionless numbers|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 number|Mach]] and [[Reynolds number]]s alone allow good categorization of many flow cases.{{cn|date=October 2014}}

Hypersonic flows, however, require other similarity parameters. First, the [[analytic equation]]s for the [[shock wave|oblique 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 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]].<ref name="Anderson">{{Cite book |last=Anderson |first=John |title=Hypersonic and High-Temperature Gas Dynamics |publisher=AIAA Education Series |year=2006 |isbn=1-56347-780-7 |edition=Second}}</ref>

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 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]].{{cn|date=October 2014}}

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).{{cn|date=October 2014}}

[[Wallace D. Hayes]] developed a similarity parameter, similar to the [[area rule|Whitcomb area rule]], which allowed similar configurations to be compared.{{cn|date=October 2014}}

==Regimes==
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.{{cn|date=October 2014}}

===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 [[ramjet]]s become inefficient, and the upper border around Mach 10-12.{{cn|date=October 2014}}

===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.{{cn|date=October 2014}}

===Dissociated gas===
In this regime, diatomic or polyatomic gases (the gases found in most atmospheres) begin to [[dissociation (chemistry)|dissociate]] as they come into contact with the [[Shock wave|bow shock]] generated by the body. [[catalysis|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 of [[ionization]] start to have an effect on the flow.{{cn|date=October 2014}}

===Ionized gas===
In this regime the [[ionization|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 flow velocities around 3-4 km/s. Gases in this region are modeled as non-radiating [[Plasma (physics)|plasmas]].{{cn|date=October 2014}}

===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:{{cn|date=October 2014}}
#[[Optical depth|Optically thin]]: where the gas does not re-absorb radiation emitted from other parts of the gas
#Optically thick: where the radiation must be considered 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==
{{colbegin}}
*[[Supersonic transport]]
*[[Lifting body]]
*[[Atmospheric entry]]
*[[Hypersonic flight]]
*[[DARPA Falcon Project]]
*[[Reaction Engines Skylon]] (design study)
*[[Reaction Engines A2]] (design study)
*[[HyperSoar]] (concept)
*[[WaveRider|X-51 A Waverider]]
*[[X-20 Dyna-Soar]] (cancelled)
*[[Rockwell X-30]] (cancelled)
*[[Avatar (spacecraft)|Avatar RLV]] (2001 Indian concept study)
*[[Hypersonic Technology Demonstrator Vehicle]] (Indian project)
*[[Ayaks]] (Russian wave rider project from the 1990s)
*[[Avangard (hypersonic glide vehicle)|Avangard]] (Russian hypersonic glide vehicle, in service)
*[[DF-ZF]] (Chinese hypersonic glide vehicle, operational)
*[[Lockheed Martin SR-72]] (Planned)
{{colend}}

;Engines
*[[Rocket engine]]
*[[Ramjet]]
*[[Scramjet]]
*[[Reaction Engines SABRE]], [[LAPCAT]] (design studies)

;Missiles
*[[Shaurya (missile)]] Ballistic Missile - {{flagicon|India}} [[India]] (Entered Production)
*[[BrahMos-II]] Cruise Missile - {{flagicon|India}} {{flagicon|Russia}} (Under Development)
*[[9K720 Iskander]] Short-range ballistic missile {{flagicon|Russia}} [[Russia]] (Currently In Service)
*[[3M22 Zircon]] Anti-ship hypersonic cruise missile {{flagicon|Russia}} (in production)
*[[R-37 (missile)]] Hypersonic air-to-air missile {{flagicon|Russia}} (in service)
*[[Kh-47M2 Kinzhal]] Hypersonic air-launched ballistic missile {{flagicon|Russia}} (in service)

;Other flow regimes
* [[Subsonic flight]]
* [[Transonic]]
* [[Supersonic speed]]

==References==
{{Reflist}}

==External links==
{{Commons category|Hypersonics}}
*[https://web.archive.org/web/20160412050110/http://www.grc.nasa.gov/WWW/BGH/index.html NASA's Guide to Hypersonics]
*[https://web.archive.org/web/20121011210111/http://www3.imperial.ac.uk/hypersonics Hypersonics Group at Imperial College]
*[https://mechmining.uq.edu.au/research/hypersonics University of Queensland Centre for Hypersonics]
*[http://seit.unsw.adfa.edu.au/research/activity.php?activity_id=42 High Speed Flow Group at University of New South Wales]
*[http://oti.eng.ox.ac.uk/research-groups/osney-hypersonics-group Hypersonics Group at the University of Oxford]

{{DEFAULTSORT:Hypersonic Speed}}
[[Category:Aerodynamics]]
[[Category:Aerospace engineering]]
[[Category:Airspeed]]
[[Category:Spacecraft propulsion]]

Hypersonic speed - Revision history

UpdateBot at 21:08, 26 May 2022