Hypersonic flight

 

Reentry vehicle (RV) after an 8,000-kilometre (5,000 mi) 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[edit]

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 8,288.12 km/h (5,150 mph), or approximately Mach 6.7.^[1] 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.^[2]

In November 1961, Air Force Major Robert White flew the X-15 research airplane at speeds over Mach 6.^[3][4] On 3 October 1967, in California, an X-15 reached Mach 6.7.^[5]

The reentry problem of a space vehicle was extensively studied.^[6] 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 the United States, India, Russia, and China.^[7]

Physics[edit]

Stagnation point[edit]

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][a]

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[edit]

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, 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}

Low density flow[edit]

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[edit]

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.^[16]

Viscous interaction[edit]

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[edit]

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]