Hypersonic weapons are those that can travel at Mach 5 or higher, which is at least five times faster than the speed of sound, or around one mile (1.6km) per second. For reference, commercial airliners fly subsonically, just below Mach 1 whereas modern fighter jets can travel supersonically at Mach 2 or Mach 3. A traditional jet engine could operate up to Mach 3 or Mach 4, and so anything travelling faster would need an altered system. The answer would then be a supersonic combustion ramjet (SCRAMJET), which can operate between Mach 5 and Mach 15. In order to maintain sustained hypersonic flight, a vehicle must also endure the extreme temperatures of flying at such speeds.
Hypersonic cruise missiles come in two flavours: Hypersonic cruise missiles and hypersonic boost-glide weapons Hypersonic cruise missiles are powered by rockets or jets throughout their flight and they can fly at altitudes up to 100,000 feet. They are simply faster versions of existing cruise missiles. Hypersonic boost-glide weapons are different. They are launched into the upper atmosphere in the normal fashion atop existing ballistic missiles, but then release hypersonic glide vehicles (HGVs) which fly lower, faster and – to an adversary – much more unpredictably than old-fashioned re-entry vehicles. Though some are intended to carry nuclear warheads, others can use their high speed and accuracy to destroy targets with the kinetic energy of impact alone. At ten times the speed of sound, a kilogram of anything has more kinetic energy than you get from exploding a kilogram of TNT. Current ballistic weapons are very fast, but not manoeuvrable; current cruise missiles are very manoeuvrable, but not fast. Hypersonic cruise missiles and HGVs are novel because they fuse these qualities of speed and agility.
Hypersonic missiles built in large numbers could pose a serious challenge to missile defences. The low-altitude path of HGVs combined with the curvature of the Earth helps them hide from radar. Their speed gives adversaries less time to respond. And their manoeuvrability makes them harder to intercept. “Point” defence systems like America’s Terminal High-Altitude Area Defence, which protects fixed sites from incoming missiles, might remain effective. But the unpredictable trajectory of HGVs for most of their flight allows them to hold a huge area at risk, even switching target midcourse. It will completely change the balance between offence and defence. A missile travelling 1,000km at ten times the speed of sound would reduce response times to six minutes, notes a RAND study. It could be mere seconds between the time the target is known for certain and the moment of impact. That could force jittery leaders to devolve control of their own weapons to military commanders or even to launch on the mere warning of an attack. And since HGVs are so much harder to spot, a larger number of lower-altitude satellites will have to be deployed; they, in turn, will become juicy targets in wartime.
The United States, Russia and China are ahead of other nations in developing hypersonic weapons while France, India, and Australia are also developing military uses of hypersonic technology. Japan and various European countries are working on civilian uses of the technology, such as space launch vehicles or civilian airliners, but civilian uses can be adapted for military purposes.