Iain Boyd, University of Colorado Boulder, explains how hypersonic missiles work:
These new systems pose an important challenge due to their maneuverability all along their trajectory. Because their flight paths can change as they travel, these missiles must be tracked throughout their flight.
A second important challenge stems from the fact that they operate in a different region of the atmosphere from other existing threats. The new hypersonic weapons fly much higher than slower subsonic missiles but much lower than intercontinental ballistic missiles. The U.S. and its allies do not have good tracking coverage for this in-between region, nor does Russia or China.
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Describing a vehicle as hypersonic means that it flies much faster than the speed of sound, which is 761 miles per hour (1,225 kilometers per hour) at sea level and 663 mph (1,067 kph) at 35,000 feet (10,668 meters) where passenger jets fly. Passenger jets travel at just under 600 mph (966 kph), whereas hypersonic systems operate at speeds of 3,500 mph (5,633 kph) — about 1 mile (1.6 kilometers) per second — and higher.
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All of the intercontinental ballistic missiles in the world’s nuclear arsenals are hypersonic, reaching about 15,000 mph (24,140 kph), or about 4 miles (6.4 km) per second at their maximum velocity.
ICBMs are launched on large rockets and then fly on a predictable trajectory that takes them out of the atmosphere into space and then back into the atmosphere again. The new generation of hypersonic missiles fly very fast, but not as fast as ICBMs. They are launched on smaller rockets that keep them within the upper reaches of the atmosphere.
There are three different types of non-ICBM hypersonic weapons: aero-ballistic, glide vehicles and cruise missiles. A hypersonic aero-ballistic system is dropped from an aircraft, accelerated to hypersonic speed using a rocket and then follows a ballistic, meaning unpowered, trajectory. The system Russian forces used to attack Ukraine, the Kinzhal, is an aero-ballistic missile. The technology has been around since about 1980.
A hypersonic glide vehicle is boosted on a rocket to high altitude and then glides to its target, maneuvering along the way. Examples of hypersonic glide vehicles include China’s Dongfeng-17, Russia’s Avangard and the U.S. Navy’s Conventional Prompt Strike system. U.S. officials have expressed concern that China’s hypersonic glide vehicle technology is further advanced than the U.S. system.
A hypersonic cruise missile is boosted by a rocket to hypersonic speed and then uses an air-breathing engine called a scramjet to sustain that speed. Because they ingest air into their engines, hypersonic cruise missiles require smaller launch rockets than hypersonic glide vehicles, which means they can cost less and be launched from more places. Hypersonic cruise missiles are under development by China and the U.S. The U.S. reportedly conducted a test flight of a scramjet hypersonic missile in March 2020.
The primary reason nations are developing these next-generation hypersonic weapons is how difficult they are to defend against due to their speed, maneuverability and flight path.
Well, yes…and no.
All the pluses have minuses.
The high speed means they radiate in the infrared like a small sun: even a small IR sensor will see one coming if its in line-of-sight, possibly hundreds of kilometers, because the flight path will have to be in the stratosphere for any reasonable range.
The same heat means that the missile itself cannot use IR sensors for detecting defensive missiles – only heavy power-hungry and probably limited view radar.
The maneuvering capability is relatively high compared to an ICBM warhead, but not to much else. At Mach 5 the turning radius is huge for reasonable gee loads. The shape is optimized for cruise after all, the weight budget for, say, 50g popout wings, means a comparable loss in fuel, or payload, radar, or anything else because of the sheer weight of structure required.
And, btw, ramjets and scramjets are notoriously finicky with respect to inlet flow angle-of-attack.
High gee means a huge loss in energy: if you start a 10g and 180degree turn at Mach 5 a gliding vehicle will probably be lucky to be supersonic at turns end.
And then there is terminal phase targeting. A whole new can of worms at hypersonic speed for the missile. All weather means you will be using radar, and radar means emitting, and emitting means you can be seen and killed.
If you drop to mere supersonic and below speeds to usr IR and Optical the defenders can use well understood weapons and tactics against you.
Maybe ALL these issues have been solved…but then again they may just remain niche weapons.