{"id":9368,"date":"2022-02-07T15:04:38","date_gmt":"2022-02-07T18:04:38","guid":{"rendered":"https:\/\/www.fie.undef.edu.ar\/ceptm\/?p=9368"},"modified":"2022-02-07T15:04:38","modified_gmt":"2022-02-07T18:04:38","slug":"misiles-hipersonicos-algo-mas-que-una-cuestion-de-velocidad","status":"publish","type":"post","link":"https:\/\/www.fie.undef.edu.ar\/ceptm\/?p=9368","title":{"rendered":"Misiles hipers\u00f3nicos, algo m\u00e1s que una cuesti\u00f3n de velocidad"},"content":{"rendered":"

Recientes anuncios realizados por Corea del Norte, acerca de ensayos de lanzamiento de vectores de empleo militar, afirmando \u201cavances exitosos\u201d en su Programa de Armas Hipers\u00f3nicas, volvi\u00f3 a colocar el tema en las portadas de medios de comunicaci\u00f3n. Lo cierto es que hasta ahora, s\u00f3lo las grandes potencias como EUA, China y Rusia, han podido demostrar cierto grado de avance en estas tecnolog\u00edas verdaderamente disruptivas, luego de d\u00e9cadas de avances y retrocesos, con un enorme esfuerzo de I&D e inversiones espec\u00edficas. La informaci\u00f3n disponible es variada, \u00a0confusa y los desaf\u00edos que se presentan son enormes, por lo que el trabajo publicado por SIPRI\u00a0(Stockholm International Peace Research Institute)<\/em>, realiza un valioso aporte a la comprensi\u00f3n de los diferentes aspectos t\u00e9cnicos claves de estos sistemas de armas y sus\u00a0 capacidades de empleo militar.<\/p>\n

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Over the past weeks and months, \u2018hypersonic missiles\u2019 have again made headlines in global defence news. On\u00a05<\/a>\u00a0and\u00a011 January<\/a>, North Korea performed test flights of what it claims is a \u2018hypersonic missile\u2019. The announcement, the released pictures and the flight path suggest North Korea tested a rotational symmetric glide vehicle atop a rocket booster that performed pull-up and cross-range manoeuvres during its flights. However, many analysts have argued that in this case, the label of\u00a0manoeuvrable re-entry vehicle<\/a>\u00a0would be more appropriate. In September 2020, North Korea tested what it called its first \u2018hypersonic missile\u2019, a ballistic missile booster with a wedge-shaped hypersonic glide vehicle. Another example where the term \u2018hypersonic missile\u2019 was used by media outlets was in August 2020, when China was reported to have tested a \u2018hypersonic glider\u2019.<\/p>\n

These examples demonstrate how confusing\u2014and potentially distorting\u2014the term \u2018hypersonic\u2019 is when it is applied to a whole range of missile systems by a range of different actors. Some of the\u00a0news coverage<\/a>\u00a0of\u00a0these events<\/a>\u00a0also indicated a lack of understanding of the different types of \u2018hypersonic missiles\u2019, the role of their speed and manoeuvring capabilities, the physics behind them, and their military capabilities and missions. This SIPRI Topical Backgrounder seeks to improve the understanding of hypersonic speed, the nature of hypersonic missile systems, as well as their key subsystems and technologies. Improving the understanding among policymakers and journalists could help inform political and public discourses and identify opportunities for applying targeted non-proliferation and arms control measures to reduce the risks associated with them.<\/p>\n

Understanding hypersonic speed<\/strong><\/p>\n

The term \u2018hypersonic speed\u2019 is widely defined as any speed beyond Mach 5, meaning five times faster than the speed of sound. This threshold is used to define a subset of air vehicles because a range of physical effects start becoming a significant engineering challenge at that speed. Specifically, the object endures a massive heat flux when it flies through dense layers of the earth\u2019s atmosphere at hypersonic speeds. This and other physical effects make developing aerial vehicles for hypersonic flight particularly difficult and costly. However, there are also some problems linked to the definition of \u2018hypersonic speed\u2019, and with that, part of the definition of \u2018hypersonic missiles\u2019.<\/p>\n

First, defining hypersonic speed as anything beyond Mach 5 actually results in a variation of the object\u2019s speed, depending on its altitude. The speed of sound, which defines the speed of Mach 1, depends not only on the chemical composition of the gas that the sound is moving through (in this case, the air in the earth\u2019s atmosphere) but also on its temperature. The most common standard model of the earth\u2019s atmosphere, the US Standard Atmosphere, shows a significant change of temperature with altitude. This results in a different measurement of what the speed\u2014in kilometres per hour\u2014of a missile moving with Mach 5 is, simply depending on the altitude that the missile is flying at (see figure 1).<\/p>\n

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There is also another factor to add to this variation. Since the Mach number strongly depends on the surrounding gas that the object is moving through, using a Mach number for definitions becomes more and more difficult once the surrounding gas gets thinner and thinner\u2014and disappears completely at higher altitudes. While scientists and engineers agree that it still makes sense to talk about Mach numbers at altitudes around 30 km\u2014which weather balloons and special aeroplanes can reach\u2014there certainly is not enough atmosphere at an altitude of 300 km\u2014where satellites are already orbiting the earth. Due to the nature of the earth\u2019s atmosphere, which becomes exponentially thinner with altitude, it is difficult to agree upon a clear altitude limit where Mach numbers should still be used as a measurement of speed, and with that, where hypersonic missiles can sensibly be defined by stating that such missiles travel faster than Mach 5.<\/p>\n

Distinguishing hypersonic weapon systems<\/strong><\/p>\n

Hypersonic speed is often pointed to as one of the key factors\u2014if not the key factor\u2014that set \u2018hypersonic\u2019 missiles apart from other missiles. However, the speed of ballistic missiles (predating the current hype around hypersonic missiles by almost a century) in many cases far exceeds that of today\u2019s \u2018hypersonic missiles\u2019. The ancestor of ballistic missiles, Germany\u2019s A-4 (which later became commonly known as the V-2) was first launched in the 1940s. During ascent, it could reach a speed greater than Mach 5 (although only for a brief period) and could do so again momentarily on its way back down. But, no one would claim that the V-2 was a hypersonic missile. In a similar vein, should one apply this label to modern intercontinental ballistic missiles that reach speeds beyond Mach 20 at ascent and re-entry?<\/p>\n

Certainly not, and there are other characteristics commonly cited when defining \u2018hypersonic missiles\u2019. However, while a combination of defining characteristics is increasingly adopted among experts, hypersonic missiles are often not well understood within public discussions in politics and the media. Wikipedia, a common starting point for those new to the topic,\u00a0defines \u2018hypersonic flight\u2019<\/a>\u00a0as \u2018flight through the atmosphere below about 90 km at speeds ranging between Mach 5\u201310, a speed where dissociation of air begins to become significant and high heat loads exist.\u2019 However, a V-2 missile would be classified as a hypersonic missile under this definition. The US-based Missile Defense Advocacy Alliance states that \u2018hypersonic weapons refer to weapons that travel faster than Mach 5 (~3800mph) and have the capability to maneuver during the entire flight.\u2019 An\u00a0article published by the Russian International Affairs Council<\/a>\u00a0states that \u2018there are two major defining characteristics [that] are prerequisite[s] to label a weapon \u201chypersonic\u201d: Speed exceeding Mach 5 [and the] capability to make maneuvers (both vertical and horizontal) while traveling at this speed inside the atmosphere.\u2019 Many ballistic missiles fall outside of the definition since they do not meet these prerequisites.<\/p>\n

Given the speed and manoeuvrability characteristics, hypersonic weapons are further subdivided into two different types of missile systems: hypersonic cruise missiles (HCMs) and hypersonic glide vehicles (HGVs). HCMs keep a constant hypersonic speed (and usually altitude) and are powered over the entire course of their flight. In contrast, HGVs are usually launched on tops of ballistic missiles (often referred to as a boost-glide system) and then glide back through the atmosphere to their target at hypersonic speeds. There are also\u00a0hybrid cases<\/a>\u00a0that do not match either of these categories, but exploring these is beyond the scope of this backgrounder.<\/p>\n

It is worth considering the speeds of hypersonic missiles (in this case HCMs) and comparing them with those of ballistic missiles (which may or may not carry an HGV as a re-entry vehicle) to get an idea of how long it takes for both types to reach targets at different distances (see figure 2).<\/p>\n

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The distance a ballistic missile can fly depends on the speed which it accelerates\u2014the faster it flies, the further it goes, just like throwing a rock. In contrast, HCMs travel at an (almost) constant speed, which is independent of how far away their target is. As illustrated by figure 2, ballistic missiles reach their target quicker than hypersonic missiles at distances beyond roughly 600\u2013800 km for HCMs constantly travelling at Mach 5 (depending on the altitude). Conversely, hypersonic missiles reach their targets faster at ranges of more than roughly 1900\u20132500 km for HCMs constantly travelling at Mach 8, which is currently seen as the maximum speed they may travel using current technology. This means that HCMs usually take longer than ballistic missiles to reach distant targets. As HGVs are usually carried by ballistic missiles, the time they require to reach a target depends on the trajectory the booster is launched at, the distance covered, and the manoeuvres performed while gliding towards the target.<\/p>\n

Speed, manoeuvrability and the characteristics of each of these types of hypersonic missile systems make them more or less suitable for specific military missions and present challenges for missile defence systems. This also influences\u00a0their possible impact on strategic stability<\/a>. However, reaching the desired performance characteristics often implied when talking about HGVs and HCMs requires overcoming a range of significant technical challenges.<\/p>\n

Subsystems and technology challenges for hypersonic glide vehicles<\/strong><\/p>\n

Any HGV, whether it carries a conventional, nuclear, or no weapon payload, is designed so it can independently perform the necessary manoeuvres to fly precisely into a given target. Sensors and computational capabilities are required to enable the vehicle to maintain a certain degree of autonomy. Consequently, an HGV requires many of the same (or at least similar) basic subsystems a ballistic missile requires, with the exception of the propulsion system. Because an HGV is launched on a rocket booster, it is usually intended to glide towards its target and does not need a main engine.<\/p>\n

The main subsystems of an HGV are:<\/p>\n