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By Laurence Taylor, Visiting Researcher, and Douglas Barrie, Senior Fellow for Military Aerospace Among the array of weapons developments lauded by Russian President Vladimir Putin in his state of the nation speech on 1 March 2018 was a hypersonic glide vehicle (HGV), one of a number of projects aimed at countering US missile-defence capabilities. Despite decades of research into hypersonic aerodynamics, only recent progress promises to overcome remaining technical barriers to full development. The ability to field very-high-speed cruise missiles or glide vehicles is attractive to armed forces, although there is debate about the potentially destabilising effects of such weapons. Concern in part focuses on the use of intercontinental ballistic missiles as launch systems for HGVs, the risk of an opponent misidentifying the type of threat and the consequent risk of escalation. The two most commonly mentioned forms of hypersonic weapon are HGVs and hypersonic cruise missiles. HGVs provide the advantage of high speed without the challenge of designing an engine capable of sustaining speeds of Mach 5 or more. However, as these vehicles are unpowered, their maneuverability is limited. The advantages of hypersonic cruise missiles and glide vehicles include a reduction in the time it takes to reach a target, the difficulty posed to a defender in trying to shoot them down and the amount of kinetic energy inherent in such vehicles. For example, without a warhead, a 500kg Mach 8 hypersonic cruise missile would deliver the destructive power of around the same weight of high explosives, equivalent to the size of the conventional warhead in a Tomahawk cruise missile. Technical challenges Hypersonic aerodynamics concerns itself with objects travelling at speeds above Mach 5, or five times the local speed of sound. At such speeds, objects encounter much higher levels of heat and pressure than experienced at subsonic, or indeed supersonic, flight, leading to unique engineering challenges. Speed While rocket motors using solid-based fuels can accelerate vehicles and missiles to hypersonic speeds, they are inefficient at maintaining them. Once approaching hypersonic speeds, it is more efficient to use an air-breathing scramjet (‘supersonic combustion ramjet’). Here, the high speed compresses the incoming air through a converging inlet. The supersonic, highly compressed air is then mixed with fuel and ignited, before passing through a diverging nozzle. This accelerates the heated flow, thereby providing thrust. However, scramjets rely on there being enough oxygen in the airflow for ignition, which limits their operation to within the atmosphere at high altitude. Furthermore, injecting, mixing, igniting and burning the fuel within a scramjet must take place in milliseconds, presenting additional engineering challenges. Heat and guidance As vehicles and missiles accelerate to higher speeds, the incoming airflow is decelerated due to frictional forces, which in turn heat the surrounding region. At Mach 6 and above, skin temperatures can exceed 530°C, and above 2,220°C at the nozzle1. At these temperatures, chemical dissociation of atoms and molecules in the airflow results in an ionised plasma enveloping part of the object. Plasmas are difficult to penetrate with radio waves, making active seeking and guidance-system updates difficult. This in turn places a...

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