Stagnation point

The stagnation point of air flowing around a body is a point where its local velocity is zero.^[6] At this point the air flows around this location. A shock wave forms, which deflects the air from the stagnation point and insulates the flight body from the atmosphere.^[6] This can affect the lifting ability of a flight surface to counteract its drag and subsequent free fall.^{[8][lower-alpha 1]}

In order to maneuver in the atmosphere at faster speeds than supersonic, the forms of propulsion can still be airbreathing systems, but a ramjet does not suffice for a system to attain Mach 5, as a ramjet slows down the airflow to subsonic.^[10] Some systems (waveriders) use a first stage rocket to boost a body into the hypersonic regime. Other systems (boost-glide vehicles) use scramjets after their initial boost, in which the speed of the air passing through the scramjet remains supersonic. Other systems (munitions) use a cannon for their initial boost.^[11]

High temperature effect

Hypersonic flow is a high energy flow.^[12] The ratio of kinetic energy to the internal energy of the gas increases as the square of the Mach number. When this flow enters a boundary layer, there are high viscous effects due to the friction between air and the high-speed object. In this case, the high kinetic energy is converted in part to internal energy and gas energy is proportional to the internal energy. Therefore, hypersonic boundary layers are high temperature regions due to the viscous dissipation of the flow's kinetic energy. Another region of high temperature flow is the shock layer behind the strong bow shock wave. In the case of the shock layer, the flow's velocity decreases discontinuously as it passes through the shock wave. This results in a loss of kinetic energy and a gain of internal energy behind the shock wave. Due to high temperatures behind the shock wave, dissociation of molecules in the air becomes thermally active. For example, for air at T > 2000 K, dissociation of diatomic oxygen into oxygen radicals is active: O₂ → 2O^{[13]: 41 [14][15]} For T > 4000 K, dissociation of diatomic nitrogen into N radicals is active: N₂ → 2N^[13]: 39 Consequently, in this temperature range, a plasma forms:^[16] —molecular dissociation followed by recombination of oxygen and nitrogen radicals produces nitric oxide: N₂ + O₂ → 2NO, which then dissociates and recombines to form ions: N + O → NO⁺ + e^{−[13]: 39  [17]}

Low density flow

At standard sea-level condition for air, the mean free path of air molecules is about ${\displaystyle \lambda =68\,\mathrm {nm} }$. Low density air is much thinner. At an altitude of 104 km (342,000 ft) the mean free path is ${\displaystyle \lambda =1\,\mathrm {ft} =0.305\,\mathrm {m} }$. Because of this large free mean path aerodynamic concepts, equations, and results based on the assumption of a continuum begin to break down, therefore aerodynamics must be considered from kinetic theory. This regime of aerodynamics is called low-density flow. For a given aerodynamic condition low-density effects depends on the value of a nondimensional parameter called the Knudsen number ${\displaystyle \mathrm {Kn} }$, defined as ${\displaystyle \mathrm {Kn} ={\frac {\lambda }{l}}}$ where ${\displaystyle l}$ is the typical length scale of the object considered. The value of the Knudsen number based on nose radius, ${\displaystyle \mathrm {Kn} ={\frac {\lambda }{R}}}$, can be near one.

Hypersonic vehicles frequently fly at very high altitudes and therefore encounter low-density conditions. Hence, the design and analysis of hypersonic vehicles sometimes require consideration of low-density flow. New generations of hypersonic airplanes may spend a considerable portion of their mission at high altitudes, and for these vehicles, low-density effects will become more significant.^[12]

Thin shock layer

The flow field between the shock wave and the body surface is called the shock layer. As the Mach number M increases, the angle of the resulting shock wave decreases. This Mach angle is described by the equation ${\displaystyle \mu =\sin ^{-1}(a/v)}$ where a is the speed of the sound wave and v is the flow velocity. Since M=v/a, the equation becomes ${\displaystyle \mu =\sin ^{-1}(1/M)}$. Higher Mach numbers position the shock wave closer to the body surface, thus at hypersonic speeds, the shock wave lies extremely close to the body surface, resulting in a thin shock layer. At low Reynolds number, the boundary layer grows quite thick and merges with the shock wave, leading to a fully viscous shock layer.^[18]

Viscous interaction

The compressible flow boundary layer increases proportionately to the square of the Mach number, and inversely to the square root of the Reynolds number.

At hypersonic speeds, this effect becomes much more pronounced, due to the exponential reliance on the Mach number. Since the boundary layer becomes so large, it interacts more viscously with the surrounding flow. The overall effect of this interaction is to create a much higher skin friction than normal, causing greater surface heat flow. Additionally, the surface pressure spikes, which results in a much larger aerodynamic drag coefficient. This effect is extreme at the leading edge and decreases as a function of length along the surface.^[12]

Entropy layer

The entropy layer is a region of large velocity gradients caused by the strong curvature of the shock wave. The entropy layer begins at the nose of the aircraft and extends downstream close to the body surface. Downstream of the nose, the entropy layer interacts with the boundary layer which causes an increase in aerodynamic heating at the body surface. Although the shock wave at the nose at supersonic speeds is also curved, the entropy layer is only observed at hypersonic speeds because the magnitude of the curve is far greater at hypersonic speeds.^[12]

Controlled detonation

Researchers in China have used shock waves in a detonation chamber to compress ionized argon plasma waves moving at Mach 14. The waves are directed into magnetohydrodynamic (MHD) generators to create a current pulse that could be scaled up to gigawatt scale, given enough argon gas to feed into the MHD generators.^[19]

Rotating detonation

A rotating detonation engine (RDE)^[20] might propel airframes in hypersonic flight; on 14 December 2023 engineers at GE Aerospace demonstrated their test rig, which is to combine an RDE with a ramjet/scramjet, in order to evaluate the regimes of rotating detonation combustion. The goal is to achieve sustainable turbine-based combined cycle (TBCC) propulsion systems, at speeds between Mach 1 and Mach 5.^{[21] [22] [23]}

Shipping

Transport consumes energy for three purposes: overcoming gravity, overcoming air/water friction, and achieving terminal velocity. The reduced trip times and higher flight altitudes reduce the first two, while increasing the third. Proponents claim that the net energy costs of hypersonic transport can be lower than those of conventional transport while slashing journey times.^[24]

Stratolaunch Roc can be used to launch hypersonic aircraft.^[25]

Hermeus demonstrated transition from turbojet aircraft engine operation to ramjet operation on 17 November 2022,^[26] thus avoiding the need to boost aircraft velocities by rocket or scramjet.^[27]

See: SR-72, § Mayhem

Weapons

Two main types of hypersonic weapons are hypersonic cruise missiles and hypersonic glide vehicles.^{[lower-alpha 2][33]} Hypersonic weapons, by definition, travel five or more times the speed of sound. Hypersonic cruise missiles, which are powered by scramjets, are limited to below 100,000 feet (30,000 m);^{[lower-alpha 3]} hypersonic glide vehicles can travel higher.

Hypersonic vehicles are much slower than ballistic (i.e. sub-orbital or fractional orbital) missiles, because they travel in the atmosphere, and ballistic missiles travel in the vacuum above the atmosphere. However, they can use the atmosphere to manoeuvre, making them capable of large-angle deviations from a ballistic trajectory.^[10] A hypersonic glide vehicle is usually launched with a ballistic first stage, then deploys wings and switches to hypersonic flight as it re-enters the atmosphere, allowing the final stage to evade all existing nuclear missile defense systems, which were designed for ballistic-only missiles.^[36]

According to a CNBC July 2019 report (and now in a CNN 2022 report), Russia and China lead in hypersonic weapon development, trailed by the United States,^{[37][38][39][7][40]} and in this case the problem is being addressed in a joint program of the entire Department of Defense.^[41] To meet this development need, the US Army is participating in a joint program with the US Navy and Air Force, to develop a hypersonic glide body.^[49] India is also developing such weapons.^[50] France and Australia may also be pursuing the technology.^[10] Japan is acquiring both scramjet (Hypersonic Cruise Missile), and boost-glide weapons (Hyper Velocity Gliding Projectile).^[51]

Iran

In 2022, Iran was believed to have constructed their first hypersonic missile. Amir Ali Hajizadeh, the commander of the Air Force of the Islamic Republic of Iran's Revolutionary Guards Corps, announced the construction of the Islamic Republic's first hypersonic missile. He noted: "This new missile was produced to counter air defense shields and passes through all missile defense systems and which represents a big leap in the generation of missiles"^[140] and has a speed above Mach 13.^[141] but Col. Rob Lodwick, the spokesman for the Pentagon on Middle East affairs said that there are doubts in this regard.^[142]

In 2021, DoD was codifying flight test guidelines, knowledge gained from Conventional Prompt Strike (CPS) and the other hypersonics programs,^[143] for some 70 hypersonics R&D programs alone, as of 2021.^[144][145] In 2021-2023, Heidi Shyu, the Under Secretary of Defense for Research and Engineering (USD(R&E)) is pursuing a program of annual rapid joint experiments,^[146] including hypersonics capabilities, to bring down their cost of development.^[147][148] A hypersonic test bed aims to bring the frequency of tests to one per week.^[149][150]

Other programs

France,^[125] Australia,^[125] India,^[151] Germany,^[125] Japan,^[125] South Korea^[152] and North Korea^[153] and Iran^[154] also have hypersonic weapon research programs.^[125]

Australia and the US have begun joint development of air-launched hypersonic missiles, as announced by a Pentagon statement on 30 November 2020. The development will build on the $54 million Hypersonic International Flight Research Experimentation (HIFiRE) under which both nations collaborated on over a 15-year period.^[155] Small and large companies will all contribute to the development of these hypersonic missiles,^[156] named SCIFIRE in 2022.^[157][136]

NDSA / PWSA

As part of their Hypersonic vehicle tracking mission, the Space Development Agency (SDA) launched four satellites and the Missile Defense Agency (MDA) launched two satellites on 14 February 2024 (launch USSF-124).^[174][175] The satellites will share the same orbit, which allows the SDA's wide field of view (WFOV) satellites and the MDA's medium field of view (MFOV) downward-looking satellites to traverse the same terrain of Earth. The SDA's four satellites are part of its Tranche 0 tracking layer (T0TL). The MDA's two satellites are HBTSS or Hypersonic and ballistic tracking space sensors.^{[lower-alpha 5]}

Additional capabilities of Tranche 0 of the National defense space architecture (NDSA), also known as the Proliferated warfighting space architecture (PWSA) will be tested over the next two years.^[175][180]