Hypersonic flight

Hypersonic flight is flight through the atmosphere below about 90 km at speeds ranging between Mach 5-10, a speed where dissociation of air begins to become significant and high heat loads exist.
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 approximately Mach 6.7. The vehicle, however, burned on atmospheric re-entry, and only charred remnants were found. In April 1961, Russian Major Yuri Gagarin became the first human to travel at hypersonic speed, during the world's first piloted orbital flight. Soon after, in May 1961, Alan Shepard became the first American and second person to achieve hypersonic flight 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, Air Force Major Robert White flew the X-15 research airplane at speeds over Mach 6.
On 3 October 1967, in California, a X-15 reached Mach 6.7, but by the time the vehicle approached Edwards Air Force Base, intense heating associated with shock waves around the vehicle had partially melted the pylon that attached the ramjet engine to the fuselage.
The reentry problem of a space vehicle was extensively studied. 
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, finally reaching Mach 5.1 on its fourth flight test.
The hypersonic regime has since become the subject for further study during the 21st century, and strategic competition between China, India, Russia, and the U.S.
Physics.
The stagnation point of air flowing around a body is a point where its local velocity is zero. 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. This can affect the lifting ability of a flight surface to counteract its drag and subsequent free fall. Ning describes a method for interrelating Reynolds number with Mach number.
In order to maneuver in the atmosphere at faster speeds than supersonic, the forms of propulsion can still be airbreathing systems, but a ramjet no longer suffices for a system to attain Mach 5, as a ramjet slows down the airflow to subsonic. 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.
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, 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 flows 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
For T > 4000 K, dissociation of diatomic nitrogen into N radicals is active: N2 → 2N
Consequently, in this temperature range, 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 formula_1. Low density air is much thinner. At an altitude of the mean free path is formula_2. 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 formula_3, defined as formula_4 where formula_5 is the typical length scale of the object considered. The value of the Knudsen number based on nose radius, formula_6, 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.
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 formula_7 where a is the speed of the sound wave and v is the flow velocity. Since M=v/a, the equation becomes formula_8. 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.
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.
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.
Hypersonic weapons development.
Two main types of hypersonic weapons are hypersonic cruise missiles and hypersonic glide vehicles. Hypersonic weapons, by definition, travel five or more times the speed of sound. Hypersonic cruise missiles, which are powered by scramjet, are restricted below 100,000 feet; hypersonic glide vehicles can travel higher. Compared to a ballistic (parabolic) trajectory, a hypersonic vehicle would be capable of large-angle deviations from a parabolic trajectory. According to a July 2019 report by CNBC, Russia and China lead in hypersonic weapon development, trailed by the United States, and in this case the problem is being addressed in a joint program of the entire Department of Defense. To meet this development need, the Army is participating in a joint program with the Navy and Air Force, to develop a hypersonic glide body. India is also developing such weapons. France and Australia may also be pursuing the technology. Japan is acquiring both scramjet (Hypersonic Cruise Missile), and boost-glide weapons (Hyper Velocity Gliding Projectile).
Waverider hypersonic weapons delivery is an avenue of development. 
China's XingKong-2 (星空二号, "Starry-sky-2"), a waverider, had its first flight 3 August 2018.
In 2016, Russia is believed to have conducted two successful tests of Avangard, a hypersonic glide vehicle. The third known test, in 2017, failed. In 2018, an Avangard was launched at the Dombarovskiy missile base, reaching its target at the Kura shooting range, a distance of 3700 miles (5955 km). Avangard uses new composite materials which are to withstand temperatures of up to 2,000 degrees Celsius (3,632 degrees Fahrenheit). The Avangard's environment at hypersonic speeds reaches such temperatures. Russia considered its carbon fiber solution to be unreliable, and replaced it with composite materials. Two Avangard hypersonic glide vehicles (HGVs) will first be mounted on SS-19 ICBMs; on 27 December 2019 the weapon was first fielded to the Yasnensky Missile Division, a unit in the Orenburg Oblast. In an earlier report, Franz-Stefan Gady named the unit as the 13th Regiment/Dombarovskiy Division (Strategic Missile Force).
These tests have prompted US responses in weapons development per John Hyten's USSTRATCOM statement 05:03, 8 August 2018 (UTC). At least one vendor is developing ceramics to handle the temperatures of hypersonics systems. There are over a dozen US hypersonics projects as of 2018, notes the commander of USSTRATCOM; from which a future hypersonic cruise missile is sought, perhaps by Q4 FY2021.The Long range precision fires (LRPF) CFT is supporting Space and Missile Defense Command's pursuit of hypersonics. Joint programs in hypersonics are informed by Army work; however, at the strategic level, the bulk of the hypersonics work remains at the Joint level. Long Range Precision Fires (LRPF) is an Army priority, and also a DoD joint effort. The Army and Navy's Common Hypersonic Glide Body (C-HGB) had a successful test of a prototype in March 2020. A wind tunnel for testing hypersonic vehicles will be built in Texas (2019). The Army's Land-based Hypersonic Missile "is intended to have a range of 1,400 miles". By adding rocket propulsion to a shell or glide body, the joint effort shaved five years off the likely fielding time for hypersonic weapon systems. Countermeasures against hypersonics will require sensor data fusion: both radar and infrared sensor tracking data will be required to capture the signature of a hypersonic vehicle in the atmosphere. There are also privately developed hypersonic systems. 
DoD tested a Common Hypersonic Glide Body (C-HGB) in 2020.
According to Air Force chief scientist, Dr. Greg Zacharias, the US anticipates having hypersonic weapons by the 2020s, hypersonic drones by the 2030s, and recoverable hypersonic drone aircraft by the 2040s. The focus of DoD development will be on air-breathing boost-glide hypersonics systems. Countering hypersonic weapons during their cruise phase will require radar with longer range, as well as space-based sensors, and systems for tracking and fire control.
Rand Corporation (28 September 2017) estimates there is less than a decade to prevent Hypersonic Missile proliferation. 
In the same way that anti-ballistic missiles were developed as countermeasures to ballistic missiles, counter-countermeasures to hypersonics systems were not yet in development, as of 2019. But by 2019, $157.4 million was allocated in the FY2020 Pentagon budget for hypersonic defense, out of $2.6 billion for all hypersonic-related research. $207 million of the FY2021 budget was allocated to defensive hypersonics, up from the FY2020 budget allocation of $157 million. Both the US and Russia withdrew from the Intermediate-Range Nuclear Forces (INF) Treaty in February 2019. This will spur arms development, including hypersonic weapons, in FY2021 and forward.
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. Small and large companies will all contribute to the development of these hypersonic missiles.
In 2021 DoD is codifying flight test guidelines, knowledge gained from Conventional prompt strike (CPS) and the other hypersonics programs.