2 August 2021 — Origin: Moon of Alabama
A Technical Look at the Science Behind the Headlines
The Americans are now crying ‘uncle’ about Russia’s hypersonic weapons. After the most recent flight test of the scramjet-powered Zircon cruise missile, the Washington Post on July 11 carried a Nato statement of complaint:
“Russia’s new hypersonic missiles are highly destabilizing and pose significant risks to security and stability across the Euro-Atlantic area,” the statement said.
At the same time, talks have begun on the ‘strategic dialog’ between the US and Russia, as agreed at the June 16 Geneva Summit of the two presidents. The two sides had already agreed to extend the START treaty on strategic weapons that has been in effect for a decade, but, notably, it was the US side that initiated the summit—perhaps spurred by the deployment of the hypersonic, intercontinental-range Avangard missile back in 2019, when US weapons inspectors were present, as per START, to inspect the Avangard as it was lowered into its missile silos.
But what exactly is a hypersonic missile—and why is it suddenly such a big deal?
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.
Consider that a modern combat rifle with a high-velocity cartridge can fire a bullet at a speed of about 1,200 meters per second [1.2 km/s]. That is barely one fifth the speed of an ICBM warhead, and only about half the speed of a short or intermediate-range ballistic missile. Clearly, intercepting anything that flies double or even five times the speed of a rifle bullet is going to be a daunting task. [Note from our previous discussion on the space race and the technicalities of orbital flight, that the ICBM does not reach orbital velocity, but flies on a suborbital trajectory—although it does exit the atmosphere].
Between the two, speed and maneuvering, the latter is much more effective in evading defensive interception.
We know this from many actual battlefield results. When the US launched large salvoes of subsonic Tomahawk cruise missiles at Syria in 2017 and again in 2018, a number of them were intercepted by Syrian air defenses. But not nearly all. Many did get through despite the T-Hawk’s relatively slow speed of about 500 mph, which is only about M 0.7. But the cruise missile’s ability to fly low to the ground and maneuver in flight, changing direction constantly, make it a tough target to hit. Likewise in the Falklands War, the Argentines used subsonic and fairly short-range, French-made Exocet sea-skimming cruise missiles to sink several large British warships, including a then-state-of-the-art Royal Navy destroyer, HMS Sheffield.
Even bird hunters know this, and will use a shotgun that scatters many pellets over a wide area rather than a bullet-firing rifle to take down slow-flying, but maneuvering, land and waterfowl! Obviously, if you combine high speed WITH maneuvering, you will have a missile that is going to be very difficult to stop. [If not impossible, with something like the Avangard, which reaches ICBM speeds of up to M 25!].
But let’s lower our sights a little from ICBMs and IRBMs [and even subsonic cruise missiles] to a quite ancient missile technology, the Soviet-era Scud, first introduced into service in 1957! A recent case with a Houthi Scud missile fired at Saudi Arabia in December 2017 shows just how difficult missile interception really is:
At around 9 p.m…a loud bang shook the domestic terminal at Riyadh’s King Khalid International Airport.
‘There was an explosion at the airport,’ a man said in a video taken moments after the bang. He and others rushed to the windows as emergency vehicles streamed onto the runway.
Another video, taken from the tarmac, shows the emergency vehicles at the end of the runway. Just beyond them is a plume of smoke, confirming the blast and indicating a likely point of impact.
The Houthi missile, identified as an Iranian-made Burqan-2 [a copy of a North Korean Scud, itself a copy of a Chinese copy of the original Russian Scud from the 1960s], flew over 600 miles before hitting the Riyadh international airport. The US-made Patriot missile defense system fired FIVE interceptor shots at the missile—all of them missed!
Laura Grego, a missile expert at the Union of Concerned Scientists, expressed alarm that Saudi defense batteries had fired five times at the incoming missile.
‘You shoot five times at this missile and they all miss? That’s shocking,’ she said. ‘That’s shocking because this system is supposed to work.’
Ms Grego knows what she’s talking about—she holds a physics doctorate from Caltech and has worked in missile technology for many years. Not surprisingly, American officials first claimed the Patriot missiles had done their job and shot the Scud down. This was convincingly debunked in the extensive expert analysis that ran in the NYT: Did American Missile Defense Fail in Saudi Arabia?
This was not the first time that Patriot ‘missile defense’ against this supposedly obsolete missile failed spectacularly:
On February 25, 1991, an Iraqi Scud hit the barracks in Dharan, Saudi Arabia, killing 28 soldiers from the U.S. Army’s 14’th Quartermaster Detachment.
A government investigation revealed that the failed intercept at Dhahran had been caused by a software error in the system’s handling of timestamps. The Patriot missile battery at Dhahran had been in operation for 100 hours, by which time the system’s internal clock had drifted by one-third of a second. Due to the missile’s speed this was equivalent to a miss distance of 600 meters.
Whether this explanation is factual or not, the Americans’ initial claims of wild success in downing nearly all of the 80 Iraqi Scuds launched, was debunked by MIT physicist Theodore Postol, who concluded that no missiles were in fact intercepted!
As the missile experts in the NYT point out:
Shooting down Scud missiles is difficult, and governments have wrongly claimed success against them in the past.
Governments have overstated the effectiveness of missile defenses in the past, including against Scuds. During the first Gulf War, the United States claimed a near-perfect record in shooting down Iraqi variants of the Scud. Subsequent analyses found that nearly all the interceptions had failed.
Why is shooting down Scuds so difficult? Because this was arguably the world’s first hypersonic missile [it flies at M 5 and does MANEUVER]!
If we take a closer look at this missile, we see that it is propelled nearly throughout its entire flight. This is the key. The warhead only separates from the missile body a few miles [mere seconds], before reaching its target. That missile body contains a means for maneuvering the missile, by means of thrust vector—using graphite paddles that move into and out of the rocket engine exhaust stream, as seen here. So it will be jinking and jibing as it enters the terminal phase of flight—making it a very hard target to radar track and shoot down!
Once the warhead separates, the spent missile body falls harmlessly to the ground, as it did just outside the Riyadh airport, landing on a nearby street. It is this now uselessly falling body that could be locked onto by air defense radars and hit by interceptor missiles—while the warhead itself sails unobstructed overhead.
The only real problem with those ancient Scuds was their accuracy. They could be off by hundreds of meters. But of course, accuracy and missile guidance systems have come a long way since then. The modern successor to the Scud, the Russian truck-launched Iskander, has an accuracy of about 5 meters! It too, is really a hypersonic missile that reaches M 7, but has a range of only 500 km—which was dictated by the now-defunct INF treaty, from which the Trump administration unilaterally withdrew.
The Russian Iskander-M cruises at hypersonic speed of 2,100–2,600 m/s [Mach 6–7] at a height of 50 km. The Iskander-M weighs 4,615 kg carries a warhead of 710–800 kg, has a range of 480 km and achieves a CEP [circular error probable] of 5–7 meters. During flight it can maneuver at different altitudes and trajectories to evade anti-ballistic missiles.
Iskander is generally described, at least in the west, as a ‘quasi-ballistic’ missile. But ‘quasi’ or not, the US considers the Iskander a very dangerous weapon, and a type of weapon which it does not yet possess. In fact, the US’ attempts to develop its very first hypersonic missile have been rather slow out of the blocks. Its first flight test attempt with the proposed Lockheed-Martin AGM183 [aka ARRW] in April of this year, did not even manage to release the rocket from the wing of the B52 carrier! The second attempt, on July 29, managed to get the rocket to release, but the engine failed to fire!
Clearly the US is many years away from fielding a workinghypersonic missile. These early tests were only supposed to test the rocket, and carried a dummy ‘glide vehicle’ which is supposed to separate from the rocket once it reaches a speed of about M 6 or so, and then glide to its target while maneuvering.
The prototype missile would carry a frangible surrogate for that [glide] vehicle that would disintegrate after release.
However, it is unclear how an unpowered gliding body is going to accomplish aerodynamic maneuvering INSIDE the atmosphere. The concept of boost-glide, which is used by Avangard, works by hoisting the glide vehicle up above the atmosphere, at ICBM speed, where the ‘glider’ can then skip off the upper layers of the atmosphere like a flat pebble skipping over the surface of a still pond.
The overall flight range of AGM183 is a claimed 1,000 miles [1,600 km]. Clearly such a short-range missile, and reaching a speed of only about M 8 at most [based on statements of reaching its target in a flight time of 10 to 12 minutes] is not going to be able to use the boost-glide means of maneuvering, which requires exiting the atmosphere.
The Technical Deep Dive (If you are not inclined to follow technical details jump to the conclusions.)
So let’s look at Russian hypersonic technology in a little more detail, so that we may understand more than just what the technically-challenged media are telling us. From what the Russian military has already fielded, we can see that hypersonic missiles come in all shapes and sizes. Some, like Avangard, are launched by powerful ICBM rockets and have ICBM-like striking range. Others, like Zircon, are more like a Tomahawk or Kalibr cruise missile, powered by an air-breathing engine, and able to aerodynamically maneuver throughout their flight to the target—but flying about ten times faster.
Others, like Kinzhal, which appears to be an evolution of the Iskander [itself an evolution of the Scud] are powered by relatively small rockets and are designed to maneuver gas-dynamically [thrust vectoring], again, during all phases of flight, right up to the target.
These are the three primary types for purposes of basic classification. They all fly very fast [up to M 25 for Avangard], but they use different propulsion systems, and different means of maneuvering. Let’s begin with the Kinzhal, since we already understand the basics of how a Scud or Iskander works. In the case of Kinzhal, it is launched from a very high speed and height by a MiG31 interceptor aircraft, which is designed to fly up to 1,500 km at a cruising speed of M 2.4, at a height of about 20 km.
By carrying even an unmodified Iskander up to this speed and height, its range could easily double, to about 1,000 km—since the rocket chemical energy required to reach that height and speed would be saved, and could be expended on increasing its flight range.
The range given for Kinzhal is 2,000 km, but it is not clear if that includes the flight range of the MiG31 carrier aircraft. My guess would be that it does. The MiG has a combat radius of over 700 km at its M 2.4 cruise speed. That means that after release, the Kinzhal would need to fly for about 1,300 km before hitting its target—for an overall system range of 2,000 km. In fact, the MiG could fly a significant portion of its flight subsonically, saving fuel, and accelerate up to supersonic cruise speed, or even its top speed of M 2.8, only in the last couple of hundred km, before launching Kinzhal. It would then circle back and return to base subsonically again. This would increase range even more.
Either way, it is a safe bet that the overall range to a target, say a US aircraft carrier, from the takeoff point of the MiG [now deployed in Syria], is realistically going to be no less than the stated 2,000 km, if not more. This is certainly a game-changer for US naval dominance! Carrier-based aircraft would have no chance to fly far enough from their floating airfield to intercept a MiG31 launching a Kinzhal at 1,000 km or more distance from the ship. The F/A-18 has a combat radius for air-to-air missions of only 740 km. Obviously, it is not going to be able to reach the MiG launching from outside of 1,000 km.
Now let us look at the Zircon cruise missile that Nato is complaining about. So far, this missile has been successfully test-flown at target distances of up to about 450 km. The Russian MoD says its range is actually in excess of 1,000 km, and that flight tests to maximum range will be forthcoming.
This too is a game-changer. The Zircon will be carried by Russia’s new class of surface warships in the frigate or ‘small destroyer’ size, as well as on the new Yasen-class cruise missile nuclear subs that are now coming into service. These state-of-the-art subs will also carry subsonic Kalibr cruise missiles with a maximum range of 4,500 km! Combined with the air-launched Kinzhal, the US Navy will face some very stiff challenges—from the air, from the sea, and even from under the sea. It should be noted that both the Zircon and Kinzhal are not exclusively anti-ship missiles. They can just as readily target land objects, including Nato command and control centers—which Putin has said Russia will do, in the event of any kind of western aggression!
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.
Not surprisingly, the Russians flew the world’s first scramjet prototype back in 1991—the Kholod, which means ‘cold’ in Russian. Remarkably, in the Yeltsin détente atmosphere of the early nineties, the Russian developers of the world’s first functional scramjet engine, the Central Institute of Aviation Motors [CIAM] invited Nasa to participate in the flight tests at the Sary Shagan test range in Kazakhstan. The results were published in the US professional literature, here, and here.
But despite this technology boost from Russia, the US has not been able to keep up. Its experiments with scramjet engines, although wildly hyped in the media, have been dormant for several years. It appears that the US has given up on the idea of building a working scramjet engine for the time being—much as they gave up, decades ago, on the idea of building a closed-cycle rocket engine, having deemed the technology ‘impossible.’
So what is a scramjet engine anyway? To fully understand this, let’s first look at how a turbojet engine works. Here is a picture that is worth a thousand words. Air enters the front of the engine and is then compressed by a number of rotating blades on a series of wheels, similar to a fan or propeller. The compressed air is then passed into the burner, or combustion chamber, where fuel is squirted in and the result is a high temperature and high-pressure gas that then drives the turbine wheels—which are bladed in a way similar to the compressor wheels up front.
The turbine wheels and compressor are on a single shaft and rotate at the same speed—so it is the energy of the gas driving the turbines, that drives the compressors. The remaining energy in the gas is squeezed out through a nozzle, which accelerates the gas flow, which, in turn, creates thrust—on the principle of Newton’s Third Law, action-reaction. The force of the fast-moving mass flow of gas out the nozzle, must be compensated by a REACTION force in the opposite direction [forward thrust], as per the conservation of momentum principle. Hence all jet engines, whether air-breathing or rocket, are called reaction engines.
[Incidentally, the heart of any liquid-fuel rocket engine is a turbopump, which is basically a gas turbine engine. It has a burner, where some amount of the fuel and oxidizer are burned, supplying gas to drive a turbine wheel or wheels, which then drive two ‘compressor’ pumps [also wheels], that pressurize the oxidizer and fuel, which is then delivered to the main combustion chamber under great pressure.]
Now what happens when you want to go very fast with a turbojet engine? Well, you basically hit a wall, due to the physics of airflow]. The faster you go, the greater the ram pressure on the front of the engine. This ram pressure [technically called dynamic pressure, or ‘Q’] is like kinetic energy—it increases by the square of speed. [KE = M x V^2 / 2; Q = rho x V^2 / 2; they are the same except mass is replaced by density, rho, since we are dealing with a flowing fluid instead of a solid particle!]
In simple terms, dynamic pressure [aka ram pressure] is what you feel on your hand when you stick your hand out the window of your car while driving on the highway.
The results of this quadratic pressure rise with speed are profound! At a typical passenger jet cruise speed of 450 knots, or M 0.8, the pressure increase from ram effect, at the front of the engine fan, is about 1.5. Also, the engine inlet must SLOW the airflow down to about M 0.5, so that the rotating blades can work efficiently.
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.
But the ramjet hits a speed limit too, just like the turbojet. In both cases it has to do with the falling efficiency of the engine inlet at higher speeds: more of the kinetic energy of the high-speed airflow is converted into heat, rather than usable pressure. In a turbojet, the heat limit is reached by about Mach 3, when the heat of that incoming air exceeds the materials limit of the compressor blades. In the ramjet, eliminating those unneeded blades and all the other moving parts raises the temperature limit to a much higher value—so flight up to about Mach 5 is possible.
Above those speeds, the Ramjet faces a different kind of problem. As flight speeds continue to increase, the efficiency of turning that kinetic energy into pressure continues to decrease steeply. This pressure loss is due to a series of shockwaves generated by slowing down the airflow in the engine inlet passage, upstream of the combustion chamber. The biggest shockwave and biggest pressure loss happens when the flow finally transitions to below sonic velocity. This is called the normalshockwave, because it is perpendicular [normal] to the inlet wall, as seen in this illustration of a supersonic inlet and its shockwaves.
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].
In a previous discussion here, the technically-inclined commenter and longtime gyroplane pilot PeterAU1, dug up some interesting material about ‘doping’ kerosene with certain additives to enhance flame front speed. But the technicalities of that subject are beyond the scope of this relatively brief introductory discussion. [Although I’m sure we may hear more in the comments section!]
The bottom line is that the Zircon represents not only a formidable and very deadly weapon—but it is indicative of the engineering capabilities of the Russian aerospace industry. It is an impressive achievement that is in fact groundbreaking. As mentioned already, Zircon is only the beginning of scramjet engine use by the Russian military. The next generation of such missiles, like the already mentioned Gremlin, will be even smaller and more capable in range and speed. At some point in the future, we may even see scramjet engines on superfast civil aircraft—but that is probably a long way off yet.
An even bigger engineering accomplishment is the astonishing Avangard boost-glide vehicle. But I will leave that remarkable story for another discussion.
The bottom line is that these new Russian technologies are in fact tilting the global military balance going forward. They are game-changing because they are UNSTOPPABLE with today’s air defense technology. Just like the Plains Indians couldn’t hope to stop, with their bows and arrows, the US cavalry with their repeating rifles.
Even more profound may be the psychological effect that Russia’s engineering accomplishments must be exerting on the American psyche, which is used to assuming that they have the smartest engineers and make the best military hardware.
That is demonstrably NOT the case anymore.
And that may be the biggest game-changer of all!