The
first part of this article explained what big guns actually do and how,
and more importantly the crew protection systems. This part describes
various fire control systems
To shoot a target, the gun of a tank or self-propelled gun (SPG) has to be oriented at a particular angle with respect to horizontal and vertical planes. These two angles are called ‘firing solutions’ which are highly dependent on various factors and need to be calculated mathematically. Fire control system (FCS) further alleviates this complication.
Before knowing how FCS alleviates the complication, it is necessary to first understand the complications present in shooting. Primary target for a tank is an enemy tank. A shell fired from a tank’s gun will not travel in a straight line but will lose altitude due to gravity as it travels. So a gunner must know the distance of the target, and accordingly shoot higher to compensate for the fall. But if the target is on an elevated or depressed ground with respect to the tank, the compensations differs.
 Fig. 12: Components of FCS and tank (Courtesy: http://en.wikipedia.org) |
To further complicate matters, a moving target presents a problem in a different league. The shell should be fired such that the target will come into the shell’s path and take the hit, in what is called ‘lead shooting’—synonymous to a fielder running to catch the ball in cricket. To lead shoot, the gunner must know the speed of the target and its bearing. If the target is moving perpendicular to the gun then shooting it will be relatively easier. But if the target is making a different angle then shooting it will not be easy.
When shooting a moving target complicates the matter, shooting a stationary target while the tank itself is on the move is equally complicated. For this, the speed and angle the tank is making with respect to the stationary target should be known, and accordingly the shell has to be fired. Shooting a moving tank from a moving tank was a fascination and was taken for granted as impossible during World War II.
Because, for targeting, the earlier tanks had only telescopic sights engraved with stadiometric scales and crosshairs, the tank commander had a commander’s sight—a traversing periscope. It had to be rotated manually through a wheel to get a 360° view. The tank commander would search for targets rotating this sight. He would select a target and command the gunner to shoot it. The gunners were trained to use the stadiometric scales for estimating the range of the target and shoot it. To shoot a target, the tanks had to halt and thus became a sitting duck for other enemy tanks.
But today, with the introduction of electronic fire control system (FCS), which calculates the firing solution, the contemporary tanks can shoot moving targets accurately even while on the move. What is inside an FCS? The FCS has a ballistic computer, electronic sights and an array of sensors.
Ballistic computer. FCS is based on a ballistic computer which chiefly derives inputs from various sensors fitted in the tank. But it derives primary data from the electronic sights.
Electronic sights. Electronic sights have replaced the telescopic sights of the earlier tanks. Electronic sights are technically nothing but cameras hooked to interactive LCD screens. These sights are capable of showing images even during night time due to their low-light image capturing capability through thermal imaging. The sights are technically called forward looking infra red (FLIR) imagers. These are generally capable of detecting IR wavelengths in the 8 to 14 micron band and have their lineage from the targeting sights of combat aircraft. These imagers actually see (sense) the heat signature of a target rather than its visual image; even a target well-camouflaged inside foliages cannot escape from it. Because of these sights, contemporary tanks are as active in the nights as they are during day time. But tanks of the past were night-blind.
Crosshairs are generated electronically and are not engraved crosshairs; the ballistic computer keeps track of the alignment of the crosshairs. The gun gets heated heavily due to high pressure build up inside the barrel during firing process. The gun barrels tend to slightly bend due to this heating. This miniscule bending can lead to pronounced deviation at longer range. Often the gunner, using a system called muzzle reference system, can adjust the crosshair’s position in his sight based on the barrel’s degree of bending.
The commander’s and gunner’s sights are almost equally capable and independent. The tank commander’s cumbersome opto-mechanical sights of the past have been replaced by a stabilised panoramic sight for day and night observation. Commanded through a joystick, these panoramic sights can rotate 360°. The gunner’s sight is also night-imaging capable but not panoramic. The gunner’s sight can be enslaved to the commander’s sight by the commander to see what the gunner sees. These sights find the range, speed and the bearing of a target!
 Fig. 13: Block diagram of a tank’s FCS |
With an embedded laser range finder (LRF) present in the sights, the ballistic computer gets the range of the target. The LFR flashes a narrow pulse-coded IR laser beam for a microsecond on the target area covered by the crosshairs. With laser sensors associated with the LRF, the reflected laser beam from the target is sensed. The time duration between the flashing and sensing gives the round-trip time between the tank and target. From this the ballistic computer calculates the range. The typical maximum range of laser rangefinders is 10 kilometres with an error of 10 to 20 metres at the extremities of the range.
Similarly, for a moving target, the gunner has to keep the crosshairs on the target for a second or two. During this time period, the computer flashes the LRF on the target repeatedly and extrapolates the target’s course. Now the ballistic computer has the data regarding the target’s range, speed and angle.
Sensors. Using wind sensors situated on the top of the turrets, the ballistic computer measures the crosswind speed. Using pendulum static cantilever sensors located at the centre of the turret roof, the ballistic computer calculates how much the tank is tilted due to uneven ground.
Similarly, the ballistic computer takes the speed of the tank and its course from a system called inertial navigation system (INS). (This INS is a navigation system that helps ships, aircraft and missiles to find their position in the open water and airspace, respectively. But a tank getting them is one of the strangest aspects of battle dynamics.) From the gunner’s console, the gunner has to manually input the data regarding the ammunition type and some miscellaneous data.
Additional functions of FCS
The FCS has some additional functions. For instance, FCS lays and stabilises the gun and produces symbology for the gunner/commander.
Laying the gun. From the data derived from the sensors and sights, the firing solution is derived in no time. Accordingly, the ballistic computer actuates amplidynes that move the turret in the required position. Similarly, the gun is also actuated through the amplidynes. A constant feedback is maintained to ensure that the turret and gun are moved according to the desired angles.
If the tank is moving fast and the gun is kept at the required angle rigidly, even a slight bump on the road can take the gun off the correct angle. If at that particular instant a shot is fired, even a slight bump will reduce the accuracy. So the ballistic computer holds the gun loosely on the target through stabilisation. To sense the tank’s movements, accelerometers are used as sensors that give an electrical output whenever the tank jerks. Immediately the FCS correspondingly controls the amplidynes and maintains the gun in the required orientation—all in just a fraction of second.
Symbology. Apart from these sensing and controlling operations, the ballistic computer generates symbologies on the display regarding various parameters, like range of the target. By displaying a ready symbology on the sight, it also indicates to the gunner that he can shoot.
Autoloader and shell loading. The firing solution has been derived, the gun is laid, what about the ammunition? Generally, it will be loaded even before deriving the firing solution. Who does it? Western tanks require a soldier known as loader who, under the command of the gunner, picks up the appropriate shell from ammunition rack and loads it in the gun. A slight mistiming can result in amputation for him.
The Soviet Union/Russian-origin tanks use an important system called autoloader. The autoloader is a separate system but loosely interacts with the FCS. The autoloader selects the shell according to the gunner’s command and loads it in the gun. During combat, different types of shells are required to tackle various situations. For this, the gunner inputs the required shell type in his console and the autoloader loads accordingly. The FCS checks whether the autoloader has loaded the shell into the gun or not.
Engagement sequence
In a typical combat, the tank commander (with his panaromic sight) searches for a suitable target, detects it and presses a switch called align switch. The FCS slews the turret to face the target. The gunner then takes over the engagement of the target. The gunner aligns the reticle on the target and then the FCS takes over the engagement from him. The FCS starts performing the sequence of events from ranging and tachometric process (detecting the speed of the target), lays the gun and when all are set, the gunner can fire the gun.
 Fig. 14: Block diagram of SPG’s FCS |
The commander, after having assigned a target to the gunner, uses his panoramic sight to locate the next target. If he has located a target even before the current firing, it is stored in the memory of the FCS. After firing and damage assessment of the target is over, the gunner can take up this new target from the FCS memory. The commander can keep on looking for new targets through his sight and, if required, he can take over the engagement from the gunner and himself shoot on sudden threats. In the past such an engagement sequence would have been a dream! The modern tanks are said to be capable of typically engaging around six different targets within 35 seconds while on the move.
Apart from these commander’s and gunner’s sights, the tank driver also has a day/night-capable sight. With that he can drive the tank as efficiently during nights as he does during day; the driver needs not to even switch on the headlights to drive. In some very latest tanks, rear facing cameras feed imagery to the driver who can easily reverse the tank, or manoeuvre into the confined spaces of a transport aircraft easily.
Fire control system of SPG
The FCS of a self-propelled gun also performs more or less like a tank’s FCS. It executes the gun laying and stabilisation job but does not have the complexity of engaging a moving target or firing the gun on the move. For an SPG, the targets are stationary and the gun is also stationary. But what is important is the number of shells rained accurately on a target situated at long ranges. Because once artillery shells start coming, the enemy will track their trajectory through their weapon locating radars and find out locations of the guns. Then the enemy will not sit idle. They will try to neutralise the SPGs with their own guns in what is called as ‘counter battery fire’ in artillery parlance. So, to evade counter battery fire, the SPGs are required to bombard heavily and then move away rapidly in what is called as ‘shoot and scoot’ tactics.
But laying the gun accurately has not been easy. A fraction of a radian deviation will cause the shell to deviate and land way away from the target. (A trigonometric milliradian (mrad) is the angle formed when the length of a circular arc equals 1/1000 of the radius of the circle.) The place where the SPG stands, its elevation/depression, tilt, etc affect the gun’s alignment. During firing, the gun also moves backwards due to recoil and needs to be readjusted for every shot fired. All these complexities are met by the modern electronic fire control systems.
The first requirement to fire at an unseen target is that the FCS must know its geographical position. So it derives its own position with respect to ‘true north’ through a navigation system based on an inertial navigation system. Often this navigation system’s output is added with a GPS ‘topping’ for accurate derivation of coordinates. When the target’s geographical coordinates, altitude and other environmental parameters are entered into the FCS, it finds out how the target is aligned with it and in which direction. It also takes other inputs like altitude of the current position, tilt angles if any, wind speed, etc. With that, it transforms the positional information into firing solution, which tells how the gun has to be aligned. Accordingly, it actuates the electrical drives and lays the gun. In World War II era photographs we often see the gunners working with a bare chest, beaten by sun and dirt. To an extent, we can comfortably say that modern-day guns can be fired without losing the crease of gunners’ uniforms. Not only that, the enemy is at the MRSI of these guns. We know mercy, but what is this MRSI?
MRSI stands for multiple round simultaneous impacts. As these SPGs fire at a long-range target, the shell reaches the target in a parabolic trajectory. If a shell is fired by slightly depressing the gun, the shell’s trajectory will have a shallow angle and it will reach the target quicker than the previous shell. Using this technique, it is possible for the SPG to rapidly fire some five shells by progressively depressing the gun for each firing. Thus the shells will land on the target at almost the same time and can pepper the target. This is what MRSI is for.
Though easily said, this technique is impossible to execute without modern FCS and automatic loaders. The FCS calculates the time interval between successive firings and lays the gun at the required firing angle. The time duration between the successive firings may be at a maximum of 2-3 seconds. So, within 20-30 seconds, the entire firing can be completed and the SPG can move even before the enemy recovers from the shell shock. Typically, if four SPGs fire on a single target in this fashion, it is possible that a total of twenty shells land on the target at the same time to annihilate it in no time.
Apart from this indirect fire, the SPG also retains some direct fire capability. For this it has an electronic sight, as in the case of a tank, but with limited functions. When the direct fire mode is selected, the gunner puts the target on the crosshairs and the FCS accordingly lays the gun. This direct fire mode is very rarely used. For direct firing the gun will be almost parallel to the ground. Engaging a moving tank in this position will be out the limits of the FCS of SPGs.
Having understood what big guns actually do and how, and crew protection systems in the first part, this part described various fire control systems. The concluding part next month will cover various types of smart shells and anti-tank missiles.
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