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\u003cp\u003eAmong 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.\u003c/p\u003e\n\u003cp\u003eThe 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.\u003c/p\u003e\n\u003cp\u003eThe 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.\u003c/p\u003e\n\u003cp\u003eThe 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.\u003c/p\u003e\n\u003ch3\u003eTechnical challenges\u003c/h3\u003e\n\u003cp\u003eHypersonic 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.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eSpeed\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eWhile 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.\u003c/p\u003e\n\u003cp\u003eHowever, 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.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eHeat and guidance\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eAs 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 nozzle\u003csup\u003e1\u003c/sup\u003e. 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 high-accuracy inertial-navigation-system requirement on any hypersonic craft.\u003c/p\u003e\n\u003cp\u003eNot only does the high level of heat lead to chemical dissociation in the surrounding airflow, it can also lead to physical deformation. Thin, metallic elements, such as wings, could potentially be deformed during flight if the temperatures and pressures become too great. Active cooling and heat-dissipative ceramics are two areas being pursued to address this.\u003c/p\u003e\n\u003cp\u003eTechnical issues notwithstanding, China, Russia and the United States are known to be developing hypersonic weapons. While the US has been the world leader in hypersonic technology, having developed and demonstrated several scramjet engines, there is concern in Washington about China and Russia’s growing capabilities in this area as part of their respective military-modernisation initiatives.\u003c/p\u003e\n\u003cp\u003e\u003csup\u003e1\u003c/sup\u003eMichael E. White and Walter R. Price, \u0027Affordable Hypersonic Missiles for Long-Range Precision Strike\u0027, in John Hopkins APL Technical Digest, Vol 20, No 3, 1999. \n\u003c/p\u003e\n\u003chr /\u003e\n\u003cp\u003e\u003cstrong\u003eThis analysis originally featured on the \u003ca href=\u0022http://go.iiss.org/2gnVMys\u0022\u003eIISS Military Balance+\u003c/a\u003e, the online database that provides indispensable information and analysis for users in government, the armed forces, the private sector, academia, the media and more. Customise, view, compare and download data instantly, anywhere, anytime.\u003c/strong\u003e\u003c/p\u003e","className":"richtext reading--content font-secondary"}), document.getElementById("react_BIUpClLUOXfQ8pcib9HQ"))});
Very high speed cruise missiles are a long-held ambition for the US, Russia, and China, but progress toward them has been slow. Technological barriers are only now being overcome.
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