Hypersonic flight

Hypersonic flight refers to the motion of aircraft, missiles, or spacecraft through the atmosphere below the Karman line at speeds greater than Mach 5, above which thermochemical effects and aerodynamic heat loads become significant.

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 8,290 km/h (5,150 mph), 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 (first stage) 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 > 2,000 K (1,730 °C; 3,140 °F), dissociation of diatomic oxygen into oxygen radicals is active: O2 → 2O) For T > 4,000 K (3,730 °C; 6,740 °F), 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 λ = 68 n m {\displaystyle \lambda =68\,\mathrm {nm} } . At an altitude of 104 km (65 mi), where the air is thinner, the mean free path is λ = 1 f t = 0.305 m {\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, 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 K n {\displaystyle \mathrm {Kn} } , defined as K n = λ l {\displaystyle \mathrm {Kn} ={\frac {\lambda }{l}}} where l {\displaystyle l} is the typical length scale of the object considered. The value of the Knudsen number based on nose radius, K n = λ R {\displaystyle \mathrm {Kn} ={\frac {\lambda }{R}}} , 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.

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