Misunderstanding Hypersonic Weapons

“Hypersonic” has become an empty word, which means almost nothing at this point. People use it as a general, all embracing term in order to create some kind of a hype. In that context, “hypersonic” is meaningless as a definition that provides some understanding and context to the subject matter.

The German WWII-era V-2 ballistic missile had a max. velocity of 5,760 km/s (from wiki), which translates to Mach 4.7. So it is practically a “hypersonic missile” (since it achieves speed close to Mach 5 or more, which is considered to be the hypersonic domain), technically speaking. But obviously, this is not what is meant by the term. So what does the term “hypersonic” means, and when it is applicable?

First of all, only cruise of air-breathing missiles should be called “hypersonic”, because almost all ballistic missiles of certain range (let’s say: 300 km) and above are hypersonic by nature. Cruise missiles differ from the ballistic missiles in that they are:

  1. Use air-breathing engine;
  2. Use powered flight throughout all (or almost all) of their flight path;
  3. Use aerodynamic surfaces to create the “lift” force (like aircraft).

This is reflected in their name. For example, the English term “cruise” implies sustained propelling force generated by the engine. As opposed to ballistic missile, which uses (relatively) short-burst engine to provide some initial speed, which is translated into altitude (kinetic energy converted into potential energy) and then back into speed (the reverse conversion). Simply put, to launch a ballistic missile, is akin to throwing a stone. The stone doesn’t have any propelling forces acting on it, but only the initial impulse of the throw. The more powerful the throw, the larger is the distance a stone would travel.

Another example is the Russian term for cruise missile, which is “winged”. It implies the aerodynamic nature of the missile, which is the same as that of an aircraft. It uses the lifting power generated by wings to keep the object in the air. As opposed to ballistic missile, which uses the translation of initial kinetic energy into altitude (i.e. potential energy), and will stay in the air as long as this initial kinetic energy isn’t exhausted by the opposing forces of gravity and air-resistance (aerodynamic drag force).

So both definitions, the “cruise” and the “winged”, refer to different aspects of the missile physics, but both are intended to made a definition for meaningful separation between cruise and ballistic missiles.

In aerodynamics, air behaves differently in subsonic, trans-sonic, supersonic, and hypersonic speeds of an object moving through it. For the ballistic missile, this makes less difference. For example, a rifle bullet is designed to have a stable trajectory at supersonic speeds. But as soon as its speed drop below supersonic, it will start to behave erratically, and will rapidly become less precise as a result. So there are still consideration, but they are relatively minor.

Regarding cruise missiles, which use (mostly) air-breathing engine, the effect of the conditions of supersonic or subsonic or other regimes of flight are vastly different. Every gas turbine engine is designed to operate at specific velocities. For example, passenger jet’s gas turbine engines will not work at supersonic regime. Fighter jets may have engines optimized for supersonic flight, which will come at a cost of worse performance at lower speed, than that of a civilian aircraft designed for subsonic flight. At hypersonic speeds, those engines will not work at all, so there is a need to design an engine specifically for hypersonic speed of flight. This is the origin of “supersonic” term — it reflects the special design of the engine.

If so, any mention of “hypersonic” in relation to “Kinzhal” of “Iskander” missiles is erroneous and shouldn’t be made. Those are not cruise missiles. In Russian lingo, they are described are “quasi-ballistic” or “aero-ballistic” missiles, which denotes their special flight trajectory. Similarly, the reference to glide vehicles as hypersonic is also problematic. They are indeed designed to be able to achieve controlled glide (i.e. unpowered flight) at very high speeds. But the main point is their ability to change their trajectory inside Earth’s atmosphere using aerodynamic surfaces. Before that, ballistic missiles warhead were achieving the similar or greater hypersonic speeds, but weren’t able to glide — only to slightly correct their point of impact. The main trajectory adjustment was made outside of the atmosphere, using rocket thrusters.

So, to reiterate, the term “hypersonic” should only be used in relations to missiles such as “Zirkon” (aka “Tsirkon”). Rocket propelled missiles, or glide reentry vehicles are almost all hypersonic by their nature. The breakthrough is in the field of cruise, air-breathing missiles, there the term “hypersonic” has entirely different meaning. This requires a new scientific and industrial base to design and produce such engines.

The other problem with the “hypersonic” meme usage is the implied meaning of not being able to intercept such targets. So “hypersonic” became synonymous with “un-interceptable”. It is true to some degree, but only then related to low flying cruise missiles. Modern (and some outdated as well) medium and long-range air-defense systems have no problem of intercepting hypersonic but high-flying and unmaneuvering target. They do it to ballistic missile without much problem at all.

The “un-intercetpability” comes from the low altitude flight + maneuvering capabilities + hypersonic speed. All combined, those measures make it extremely difficult to detect, calculate a fire solution, launch an interceptor missile and have it successfully destroy the target by existing air-defense systems. High speed, or hypersonic part of the equation, is mainly intended to shorten the reaction time. By itself, if only hypersonic speed of flight is used, without other measures, the intercept will be more difficult, but not impossible at all. Very much as the supersonic cruise missiles there intended to make the intercept harder, hypersonic missiles just take is farther along the same line.

In relation to aero-ballistic or quasi-ballistic missiles such as “Kinzhal” or “Iskander”, the invulnerability part comes not as much from the hypersonic speed (as mentioned before, this is just a natural attribute of most of ballistic missiles), but from its ability to stay at altitudes which are above the engagement envelope of some air-defense systems, and below of the others. This is a result of two-sided approach to air-intercept: first is relying on aerodynamically controlled missiles, and the other on thruster controlled missiles.

Aerodynamically controlled missiles use their control surfaces which interact with the air to change the direction of flight. At altitudes above 40-50 km, the air becomes so thin, that those surfaces lose their effectiveness. So they are only designed at intercepting maneuvering targets at altitudes below that.

Thruster controlled interceptor missiles use rocket thrusters to make manoeuvres outside of the Earth’s atmosphere. They are designed for this almost airless environment, and will be useless at lower altitudes, such as 40-50 km.

If so, the main idea behind those Russian missiles is to keep them flying at altitudes which are behind the effective envelope of existing air or missile defense systems. The know-how is in the ability to keep the missile which is ballistic in nature to fly at flat trajectory, which lies in the cruise missile domain. Hence the name “aero-ballistic” or “quasi-ballistic” — a middle ground between the classic ballistic missile rocket propulsion and classic cruise missile flat trajectory.

As we can see, it has not much to do with the “hypersonic” part. Naturally, the higher the velocity, the harder it is to intercept something. But not impossible. Exo-atmospheric intercepts are made at velocities almost order of magnitude greater than Mach 5. So the “hypersonic” part by itself is never the silver bullet it is made to be in the discussions on the Internet.

Understanding those reasons and limitations of “hypersonic” should make discussions and analytics much more mature and realistic than they are at the moment.

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