We all remember when Vladimir Putin announced these wonder weapons in his March 2018 address to his nation [and the world]. The response from the US media was loud guffaws about ‘CGI’ cartoons and Russian ‘wishcasting.’ Well, neither Nato nor the Biden team are guffawing now. Like the five stages of grief, the initial denial phase has slowly given way to acceptance of reality—as Russia continues deploying already operational missiles, like the Avangard and the air-launched Kinzhal, now in Syria, as well as finishing up successful state trials of the Zircon, which is to be operationally deployed aboard surface ships and submarines, starting in early 2022. And in fact, there are a whole slew of new Russian hypersonic missiles in the pipeline, some of them much smaller and able to be carried by ordinary fighter jets, like the Gremlin aka GZUR.
The word hypersonic itself means a flight regime above the speed of Mach 5. That is simple enough, but it is not only about speed. More important is the ability to MANEUVER at those high speeds, in order to avoid being shot down by the opponent’s air defenses. A ballistic missile can go much faster—an ICBM flies at about 6 to 7 km/s, which is about 15,000 mph, about M 25 high in the atmosphere. [Mach number varies with temperature, so it is not an absolute measure of speed. The same 15,000 mph would only equal M 20 at sea level, where the temperature is higher and the speed of sound is also higher.]
But a ballistic missile flies on a straightforward trajectory, just like a bullet fired from a barrel of a gun—it cannot change direction at all, hence the word ballistic.
This means that ballistic missiles can, in theory, be tracked by radar and shot down with an interceptor missile. It should be noted here that even this is a very tough task, despite the straight-line ballistic trajectory. Such an interception has never been demonstrated in combat, not even with intermediate-range ballistic missiles [IRBMs], of the kind that the DPRK fired off numerous times, sailing above the heads of the US Pacific Fleet in the Sea of Japan, consisting of over a dozen Aegis-class Ballistic Missile Defense ships, designed specifically for the very purpose of shooting down IRBMs.
Such an interception would have been a historic demonstration of military technology—on the level of the shock and awe of Hiroshima! But no interception was ever attempted by those ‘ballistic missile defense’ ships, spectating as they were, right under the flight paths of the North Korean rockets!
The bottom line is that hitting even a straight-line ballistic missile has never been successfully demonstrated in actual practice. It is a very hard thing to do.
But Zircon is also a technological tour de force. The unique feature of the Zircon is its scramjet engine. This is the first time that the world has a production engine of this type—something which has long been a goal for both the US and Russia.
If you increase flight speed to M 2, the pressure rise at the engine face due to ram effect is seven-fold! At this speed, you don’t even need a compressor or turbines.
This is the idea of the ramjet engine—you need no moving parts, just an air inlet that is designed to slow down the airflow to below sonic velocity, turning kinetic energy into pressure energy. The combustion chamber is simply a pipe with fuel squirters, where that compressed air is burned with fuel, and then expelled through a nozzle, exactly as on the turbojet. In fact the afterburner on supersonic fighter jets works exactly like a ramjet engine—fuel is squirted in and combusts with air that was used for cooling the combustion chamber walls upstream [only a small amount of air is burned in a turbojet engine, with air to fuel ratios of over 50, compared to about 15 for a car engine.] An illustration of an afterburner shows the simple basic geometry.
So the speed limit comes because most of that ram pressure is not recoverable—it is simply dissipated into heat by the inlet shockwaves.
Enter the scramjet. Here, the flow is never actually slowed to below sonic velocity. That’s why it’s called a SCramjet, for supersonic combustion—the airflow through the combustion chamber is well above Mach 1, perhaps closer to Mach 2. By comparison, the flow in a turbojet enters the burner at just M 0.2, ten times slower—and in the afterburner and ramjet, it is about M 0.5.
This solves the speed limit issue of not having any more pressure energy available. But it comes with HUGE challenges. At a flight speed of M 6 or 7, the craft is moving at a speed of about 2,000 m/s. The main challenge is the flame front speed of combustion. Even if it took only one hundredth of a second to combust the air-fuel mixture, it would require a combustion chamber 20 meters long! That is hardly practical of course, but is in line with the flame propagation speed of aviation kerosene. That is why the afterburner jetpipes on supersonic aircraft are several meters long.
So we see that each type of airbreathing engine, turbojet, ramjet and scramjet, has its own speed limit, as shown graphically here. Even the scramjet will run into a wall at some point. The vertical measure is specific impulse [ISP], which is engine efficiency, per mass of fuel burned. We see that ISP decreases the faster we go, in any type of engine—it simply means that fuel use rises much faster than flight speed!
But back to the main challenge of the scramjet, which is flame speed. This is strictly a limit of the chemical physics of fuel combustion. Hydrogen burns ten times as fast as kerosene, but is not a practical fuel—it must be cooled to near absolute zero to be liquid, and so is not storable, and cannot be launched at will without time-consuming fueling. All of the previous scramjet experimental prototypes, both US and Russian, used cryogenic liquid hydrogen fuel. But the Zircon uses a kerosene-based fuel innovation that the Russians call Detsilin-M.
The exact means by which the Russians have achieved this fuel chemistry is of course a tightly held secret, but it is clearly a remarkable breakthrough in chemical engineering—comparable to the breakthrough in materials science that led to the closed-cycle, oxygen-rich staged combustion rocket engine in the 1960s [which the US still has not demonstrated].https://www.moonofalabama.org/2021/08/hypersonic-missiles-are-they-a-gamechanger-by-gordog.html