Hypersonic missiles:
Evolution or revolution for missile non-proliferation and arms control instruments?
HCOC RESEARCH PAPERS NO. 12
The development of new weapons combining high speed and manoeuvring ability, referred to as ‘hypersonic weapons’, has become a priority in many countries that tend to see these systems as game-changers. On the other hand, it is sometimes assessed that hypersonic missiles are more an evolution than a revolution and may not deeply modify the strategic balance between states.
After listing major programmes and key drivers beyond the acquisition of these technologies, this paper considers their development under the prism of arms control, and analyses whether current mechanisms (non-proliferation arrangements, bilateral arms control treaties and confidence-building measures) dealing with missiles are adapted to these weapons.
It notes that export control mechanisms are largely taking into account hypersonic cruise gliders and hypersonic gliders, while confidence-building measures such as the HCoC may require adaptations to fully cover these delivery vehicles. Most fundamentally, the ways these systems are regulated will continue to depend mostly on whether they are designed to carry WMDs. The global missile arms control architecture may remain ill-equipped to limit the spread of conventional hypersonic weapons, raising questions on what is perceived as strategic weapons.
FEBRUARY 2023
Emmanuelle Maitre and Stéphane Delory
INTRODUCTION AND DEFINITION
“Export control mechanisms are largely taking into account hypersonic cruise gliders and hypersonic gliders, while confidence-building measures such as the HCoC may require adaptations to fully cover these delivery vehicles. Most fundamentally, the ways these systems are regulated will continue to depend mostly on whether they are designed to carry WMDs. The global missile arms control architecture may remain ill-equipped to limit the spread of conventional hypersonic weapons, raising questions on what is perceived as strategic weapons.“
Hypersonic missiles are defined as devices that spend most of their trajectory in the atmosphere at speeds above Mach 5, i.e., more than 1.5 km/s, and which are able to manoeuvre. ‘Hypersonic missiles’ are thus distinguished from purely ballistic missiles, which spend most of their flight outside the atmosphere, at speeds systematically greater than 1.5 km/s for missiles with a range of over 300 km. The notion of hypersonic has so far covered technological developments in propulsion and trajectory, leading to a distinction between two major classes of so-called ‘hypersonic’ systems: hypersonic cruise missiles and hypersonic gliders.
Hypersonic cruise missiles
Generally speaking, cruise missiles are missiles that are propelled throughout their trajectory. They are said to be hypersonic when their maximum speed exceeds 1.5 km/s. The best way to achieve this level of performance currently is to use a type of propulsion called ‘scramjet’ (supersonic combustion ramjet). Current programmes aiming at using this technology seek to reach speeds of between 1.5 and 2.6 km/s, or Mach 5 to Mach 8. They are achievable at altitudes of between 20 and 35 km, which allows the vehicle to manoeuvre over the entire trajectory.
The super-ramjet (or scramjet) is a type of aerobic propulsion – in other words, these engines use oxygen from the atmosphere as an oxidiser and only carry fuel to achieve combustion (solid or liquid propellant is composed of a fuel and an oxidiser). Moreover, in a ramjet or a scramjet, the compression of the air is obtained by the speed of the airflow in the inlet, enabling smaller and lighter engines with no mobile part (compressor, turbine). Nonetheless, this reaction has to be initiated at over Mach 5, which means that the scramjet has to be propelled with a booster to get the minimal speed. Nowadays, the main focus of development is to combine a ramjet and a scramjet in order to lower the initial speed needed for igniting the combustion.
At over 1.5 km/s, the air flows to the combustion chamber at supersonic speed, which generates specific constraints around the nose of the engine (shock wave), in the compression of the air in front of the air inlet and inside it, in the air flow to the combustion chamber, and in the combustion itself. One of the greatest difficulties in the operation of the scramjet is to achieve the combustion of the fuel and the oxidiser (the air) at supersonic speeds and to manage the resulting thermal stresses in the combustion chamber. The structural integrity of the latter can only be maintained over a relatively short period of time – nowadays, around a few minutes, but likely to be extended thanks to the development of adapted materials. Compared to subsonic cruise missiles, scramjets are propelled for a rather short time, but, due to the very high speeds they are able to attain for several minutes, they can fly hundreds of kilometres and probably more than a thousand in the years to come. The load of propellant needed to reach these speeds is far lower than in any other type of propulsion, which is attractive for military propulsion but also for commercial projects.
The numerous difficulties linked to the development of an operational scramjet
explain why missiles propelled by scramjets are still largely in the development phase. To date, the closest to operational deployment is the Tsirkon/Zircon missile, developed by Russia. The ranges of these systems are currently estimated at around 500 to 1,000 km and should gradually increase.
Hypersonic gliders
The second major family of hypersonic missiles is that of hypersonic gliders, which are, schematically, manoeuvring re-entry vehicles reentering into the atmosphere at an altitude of approximately 100 to 120 km after a launch by either a booster derived from a space launcher or a ballistic missile. The trajectory of the launch means that the re-entry vehicle rapidly falls back into the atmosphere, at a speed and an angle of attack that allows it to ‘glide’ within it and also to bounce on its dense layers. Although the bounces cause a deceleration, as most of the trajectory is in the upper atmosphere, the gliders keep a good part of the energy obtained upon injection, which allows them to maintain very high speeds over a large portion of their flight.
The speed and range of the glider are directly dependent on the launcher. Thus, a glider launched by an intercontinental ballistic missile (ICBM), such as the Russian Avangard deployed on the SS-19 intercontinental ballistic missile, could have an injection speed of more than 7 km/s, a range of more than 10,000 km, and, at the end of its trajectory, would still have a significant residual speed. Gliders designed to operate over ranges of the order of 1,000 to 2,000 km associated with medium range ballistic missiles (MRBM) type launchers, such as the Chinese DF-17, are beginning to be deployed, while the United States is developing vehicles flying over ranges of between 1,500 and 3,000 km.
When it reaches the denser layers of the atmosphere, the aerodynamic support of the glider is sufficient to allow it to bounce and then, as the altitude decreases, to manoeuvre by modifying its trajectory and/or direction. This ability to manoeuvre, performed at high altitudes and high speeds, greatly complicates any interception attempt. Manoeuvring also makes it more difficult to predict the potential target.
Gliders currently represent the less challenging technology, in particular because this technology derives from work carried out for many years on manoeuvring warheads. However, their development remains complex, because of the aerothermal effects generated at very high speeds and the constraints of navigation and guidance.1
Besides the two major technological families, aero-ballistic missiles such as the Russian Kinzhal or quasi-ballistic missiles such as the North Korean KN-23 can be defined as hybrid systems, responding to the criteria of a hypersonic weapon (speed, flight in the atmosphere at over 1.5 km/s, manoeuvrability, ability to bounce) but using traditional chemical propulsion. They can be categorised as a kind of tactical glider and are very likely to proliferate in the near term. Their main advantage is that they derive from well-known technologies and can be relatively easily designed, even by emerging ballistic powers such as Iran or North Korea. Their main drawback is the mass of propellant needed to achieve long-range flight and their inability to maintain a high speed throughout their trajectory.
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1 Stéphane Delory and Christian Maire, ‘Missiles hypersoniques: le cas du Kh-47M2 Kinjal,’ Note de la FRS n°17/2022, 6 April 2022.
