Hypersonic flight
Hypersonic flight is flight through the atmosphere below altitudes of about at speeds greater than Mach 5, a speed where dissociation of air begins to become significant and heat loads become high. Speeds over Mach 25 had been achieved below the thermosphere as of 2020.
File:General_Bernard_A._Schriever_ICBM_Test_Vehicle_USAF_1959.jpg|thumb|right|400px|Reentry vehicle after an flight, 1959. Note the blackened tip of the RV due to aerodynamic heating. Compare to the aerodynamic heating effect on the iron meteorite on the right.
History
The first manufactured object to achieve hypersonic flight was the two-stage Bumper rocket, consisting of a WAC Corporal second stage set on top of a V-2 first stage. In February 1949, at White Sands, the rocket reached a speed of, or about Mach 6.7. The vehicle burned up on re-entry, and only charred remnants survived.In April 1961, Russian Yuri Gagarin became the first human to travel at hypersonic speed, during the world's first piloted orbital flight. Soon after, in May 1961, American Alan Shepard became the first American and second human to fly hypersonic when his capsule reentered the atmosphere at a speed above Mach 5 at the end of his suborbital flight over the Atlantic Ocean.
In November 1961, American Robert White flew the X-15 research aircraft at speeds over Mach 6. On 3 October 1967, in California, an X-15 reached Mach 6.7.
A key technology for hypersonic flight is the Scramjet. The NASA X-43A flew on scramjet for 10 seconds, and then glided for 10 minutes on its last flight in 2004. The Boeing X-51 Waverider flew on scramjet for 210 seconds in 2013, reaching Mach 5.1 on its fourth flight test.
Space vehicle reentry was extensively studied. The hypersonic regime is the subject of development during the 21st century, amid [|strategic competition] between the United States, India, Russia, and China.
Physics
Stagnation point
The stagnation point of air flowing around a body is a point where its local velocity is zero. At this spot, moving 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. This can affect the lifting ability of a flight surface, needed to counteract its drag and subsequent free fall.In order to maneuver in the atmosphere at beyond supersonic speeds, propulsion can still use airbreathing systems, but a ramjet is not sufficient to attain Mach 5, as a ramjet slows the airflow to subsonic speed. Systems such as waveriders use a rocket to boost a body into the hypersonic regime. Boost-glide vehicles use scramjets after their initial boost, in which the speed of the air passing through the scramjet remains supersonic. Munitions typically use a cannon for their initial boost.
High temperature effect
Hypersonic flow is a high energy flow. 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, high viscous effects appear due to the friction between air and the speeding object. In this case, the 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 significant. For example, for air at T >, dissociation of diatomic oxygen into oxygen radicals is active: O2 → 2O) For T >, dissociation of diatomic nitrogen into N radicals is active: N2 → 2N Consequently, in this temperature range, a plasma forms: —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−Low density flow
At standard sea-level condition for air, the mean free path of air molecules is about. At an altitude of, where the air is thinner, the mean free path is. Because of this, large free mean path aerodynamic concepts, equations, and results based on the assumption of a continuum, begin to break down, forcing consideration of aerodynamics from kinetic theory. This regime of aerodynamics is called low-density flow. For a given aerodynamic condition low-density effects depend on the value of a nondimensional parameter called the Knudsen number, defined as where is the typical length scale of the object considered. The value of the Knudsen number based on nose radius,, can be near one.Hypersonic vehicles frequently fly at 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.
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 as where a is the speed of the sound wave and v is the flow velocity. Since M=v/a, the equation becomes. Higher Mach numbers position the shock wave closer to the body surface, thus at hypersonic speeds, the shock wave lies close to the body surface, resulting in a thin shock layer. At low Reynolds number, the boundary layer is thick and merges with the shock wave, leading to a viscous shock layer.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 impact of 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 much higher skin friction than normal, causing greater surface heat flow. Additionally, 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.
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 body surface heating. 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.Propulsion
Controlled detonation
Researchers in China used shock waves in a detonation chamber to compress ionized argon plasma waves moving at Mach 14. The waves were directed into magnetohydrodynamic generators to create a current pulse that could be increased to gigawatt scale, given enough argon gas.Rotating detonation
Hybrid
Companies such as Hermeus, Venus Aerospace, and AstroMechanica are developing hybrid engines capable of operating from subsonic to hypersonic speeds.| Company | 0-3 | 3-4 | 5+ |
| Hermeus | Precooled P&W F100 turbojet | Dive | Ramjet |
| Venus Aerospace | Rotating detonation | Ramjet | Ramjet |
| AstroMechanica | Turbofan | Turbojet | Ramjet |
Applications
Shipping
Transport consumes energy for three purposes: overcoming gravity, overcoming air/water friction, and achieving terminal velocity. Hypersonics addresses all three. Proponents claim that the net energy costs of hypersonic transport can be lower than those of conventional transport while slashing journey times.Stratolaunch Roc has been used to test hypersonic aircraft.
Hermeus demonstrated transition from turbojet operation to ramjet operation on 17 November 2022, without using a rocket or scramjet.
Weapons
Two main types of hypersonic weapons are hypersonic cruise missiles and hypersonic glide vehicles. Scramjet-powered hypersonic cruise missiles are limited to below ; while hypersonic glide vehicles can travel higher.Hypersonic vehicles travel much slower than ballistic missiles, because they travel in the atmosphere, while ballistic missiles travel in the vacuum above the atmosphere. However, they can use the atmosphere to manoeuvre, enabling large-angle deviations from a ballistic trajectory. Hypersonic glide vehicles are typically launched with a ballistic first stage, then deploys wings and switch to hypersonic flight upon re-entering the atmosphere, allowing the final stage to evade missile defense systems that were designed for purely ballistic missiles.
National efforts
Russia and China lead in hypersonic weapon development, trailed by the United States and other countries.China
China's XingKong-2 waverider first flew on 3 August 2018.In August 2021 China launched a boost-glide vehicle to low-earth orbit, circling Earth before maneuvering toward its target location, missing by two dozen miles. However China claimed that the vehicle was a spacecraft, and not a missile.
On July 2021 China tested a spaceplane. An orbital trajectory would take 90 minutes for a spaceplane to circle Earth. The Pentagon reported in October 2021 that two such hypersonic launches had occurred; one did not demonstrate the accuracy needed for a precision weapon; the second demonstrated its ability to change trajectories.
In 2022, China unveiled two more hypersonic models. An AI simulation reported that a Mach 11 aircraft can outrun a Mach 1.3 fighter attempting to engage it, while firing its missile at the "pursuing" fighter. This strategy entails a fire control system to accomplish an over-the-shoulder missile launch, which did not exist as of 2023.
In February 2023, the DF-27 covered in 12 minutes, according to leaked secret documents. The capability directly threatens Guam, and US Navy aircraft carriers.