Factors affecting Indirect Fire

Engaging targets at indirect fire - i.e. targets that can not be seen from the gun position - is quite a complex business. In an ideal situation, it should be possible simply to determine where a target is in relation to a gun in terms of a compass bearing and a distance, to set these on an appropriate sighting system on the gun and to hit the target with the first round fired. Unfortunately for gunners, situations are seldom ideal!

Let us look at some of the factors that affect indirect fire.

The gunner has first to know exactly where on the map his gun is sited, and to ensure that when his gun is pointed on a given compass bearing, it is truly pointing on that bearing. It is surprisingly easy to be wrong on both accounts, which can obviously result in missing the target. It is of course important for the target's position to be accurately located on that same map grid, since the bearing and range to it are determined by comparing the two locations. In effect, a line drawn on the map between the gun and target provides the bearing (the angle between that line and grid north), while the length of the line is the scale of the distance, or range.

But even when those pieces of information are known, the gun has to be able to shoot 'to the map' - i.e. perform to a set of standards that are laid down as 'normal conditions'. When a gun is calibrated, the gunner determines to what extent its performance matches the standard set for that gun. There are many factors that can make it differ in practice and these have to be taken into account in setting target information on the sights. The main non-standard condition on the gun itself is any difference in muzzle velocity for the charge being fired. Gun barrels get worn by the corrosive gases and the passage of shells, and as they wear they become less able to achieve the standard muzzle velocity. A lower than standard velocity means the shell will not go as far as it should when the range is set at a given distance (angle of elevation), so that non-standard muzzle velocity has to be taken into account by increasing the range set.

Another important factor at the gun is the ambient temperature of the propelling charge. Propellant burns faster when the ambient temperature is above standard, leading to higher muzzle velocity, so again, compensation is needed. Projectiles can vary in weight, and although the weight differences from round to round are not large, they nevertheless have an effect on the range achieved and need compensation.

It is obvious that the shell is affected in flight by atmospheric conditions - wind speed and direction are obvious examples, but perhaps it is not as well known that these can vary at the different levels along the trajectory and require compensation in the settings on the gun before firing. Barometric pressure affects air density and that determines what air resistance is offered to the shell in flight. On a long trajectory, even the rotation of the Earth can become a significant factor and need compensating adjustments.

Some factors, such as 'drift', are built in to all situations. Drift is the tendency of a spinning projectile to build up pressure on one side and hence drift away from that pressure. That effect is well-known and range tables automatically include it.

These are the most common factors. There are others, but this will be sufficient to make the point that engaging targets at indirect fire is a complex business!

Firepower - A Quantum Leap

In having the most dominant firepower, artillery has long been the "Queen of the Battlefield", but its reign began to falter towards the end of the 19th Century, when infantry weapons made enormous steps forward in their rate of fire, range and accuracy. Up to that point, artillery had by far the longer range and, in addition to cannonballs and explosive shells, it could deliver punishing salvos of cased shot (canister) at infantry and cavalry targets long before they could retaliate with their own weapons. It remained vulnerable to enemy artillery, of course, but that was true of both sides, and artillery tended to devote most of its effort towards reducing the enemy's infantry and cavalry forces.

This new infantry firepower had the effect of driving field artillery back from the front line: it was too valuable a resource to leave it in positions where gunners and horses could be picked off by skilled riflemen. After moving out of range of the infantry weapons, it still had the dominating firepower, but the field guns of that period had to be able to see their targets. Frequently these targets were out of sight and guns had to be layed for indirect fire: this brought about a whole new era of change.

But the gunner first needed to know where the target was, so someone had to be far enough forward to be able to see it and to pass that information to the gun position. In addition, he had to act as an observer of the fall of shot when the target was engaged, so that corrections to the aim could be given. Some means had to be found of pointing the guns at their 'invisible' target. This was resolved by using maps with a grid system that allowed guns and their targets to be plotted accurately. This enabled bearing (direction) and range to be measured, so that this information could be used to point the guns and to set the correct elevation.

A forward observer was not able to communicate with the gun position unaided, so signalling systems had to be developed to pass orders and corrections between them. Signal flags (semaphore), heliographs and telephones were stages in the progress towards radio communications - all unnecessary, of course, in the days when guns were in the front line.

New instruments were needed on the guns, too. A means of sighting the gun, using a simple form of theodolite and compass, meant that guns could be reliably pointed at any required bearing. The correct range was set using data from range tables coupled with a form of gunner's quadrant. Sighting systems that were initially rudimentary soon became quite advanced, thanks to the technical skills developed in other branches of artillery, such as coast guns.

The combination of these changes freed field artillery from the space constraints of the front line, where there was insufficient room for more than a few guns per battalion of infantry. Suddenly there was an enormous amount of room, since guns soon had enough range not only to clear their own front line and hit the enemy, but also to hit targets well behind the enemy's front lines.

By the time of the Great War in 1914, most of the problems of indirect fire had been ironed out and artillery was being massed to provide devastating firepower, vastly more than had ever been envisaged in the days of direct fire. It was truly a quantum leap and maintained the old tradition of "Queen of the Battlefield".

Muzzle loading versus Breech loading

The idea of loading an artillery piece at the breech end is not a 19th Century development. It is the logical place to load and it was practised in 15th Century wrought iron guns (see separate article) for sound reasons. On a ship, for example, a long cannon that has to be loaded at the muzzle end has to be pulled right back in order to get at the muzzle. It would not only be difficult to put a heavy cannonball into the muzzle if it was sticking out over the side of a ship, especially in heavy seas: it would be virtually impossible to ram it all the way down the bore to the breech end. It was much easier to leave the gun in place and simply open up the back end to load the ammunition.

Unfortunately, it was hard to make an opening that could be tightly sealed before firing the gun. Gas under high pressure will escape out of every tiny crevice and since it was both extremely hot and highly corrosive, it wore away metal and made the holes larger. It also made the gun very dangerous to operate! It soon became apparent that it would be better to stay with the original idea of a tube with only one opening (at the muzzle) and to load the ammunition from that end. This required only one small hole at the breech end in order to let in the flame that would light the propellant and fire the gun. This 'vent' was subject to the same problems of corrosive wear, of course, and needed to be repaired from time to time.



Muzzle loading had its dangers, too. After firing there was almost always some hot residue in the chamber - i.e. the breech end of the barrel, where the ammunition is placed. There was a risk that the next charge of propellant pushed down to the chamber would ignite prematurely and injure the gunner who was loading it. There were two basic precautions taken against this: one was to swab the bore with a wet sponge; the second was to cover the vent hole with a large thumb while the propellant was being loaded. This prevented a flow of air being created by the ramming of the propellant and the fanning of any residue of spark into flame.

Breech loading was resurrected in British artillery in the 19th Century when multi-groove rifling made it very difficult to load ammunition from the muzzle end. Various designs were tried, but it was the design by William Armstrong that was adopted in 1858. This had a 'breech block' that could be lifted out to allow ammunition to be loaded into the chamber of the gun. When replaced, a screw mechanism pushed it tight against the rear face of the barrel and sealed it - a process known as 'obturation'. This block contained the vent for firing.

Later designs also used a breech block, but they were sliding blocks, sometimes operated horizontally, but mostly vertically. These incorporate the firing mechanism, usually a percussion mechanism like that used in a small arm. The striker in the mechanism either fires a small cartridge that sends hot gases into the propellant, or activates a priming tube in the base of a cartridge that holds the gun's propellant. (Note that, depending on the design of the gun, propellant can be loaded either in bags or contained in a large 'brass' cartridge case.)

Despite Armstrong's achievements, the higher cost of rifled breech loaders (RBL) caused the authorities to revert to muzzle loaders (RML) for some 20 years, albeit with a rudimentary rifling system based on fewer grooves and projectiles with projecting studs. (See a later article on aspects of rifling.) Eventually, designers reverted to RBL and that is how most guns in service use are designed today.

Smoothbore to Rifling

For the better part of 600 years, artillery weapons were smoothbores: in other words, the inside lining of the barrels was smooth, with no means of influencing the spin of the projectiles. Nevertheless, projectiles almost always spun because, as they rattled up the bore, bouncing from one contact with the bore to another on the way, the final contact before emerging into free air imparted a slight drag at that point and thus made the projectile spin. It was pure chance which direction the ball spun, and since a spinning ball (like a sliced golf or tennis ball) will veer off line, this built in an element of unpredictable behaviour. It was less apparent in a high speed shot from a cannon than in a lower speed shot from a howitzer, but it was still an unwanted effect.

Rifling was a system of grooves cut in a spiral along the lining of the barrel. The projectile's surface was forced into following the spiralling grooves, emerging with a constant, defined spin. This had two advantages: first it provided a predictable effect that could be compensated for; second it provided stability along the trajectory due to the gyroscopic effect of the spin. This became particularly important for artillery when it moved from firing spherical shot and shells to cylindrical munitions.



Unlike a spherical shape which presents the same aspect to the air all along its trajectory, unspun cylindrical projectiles from a smoothbore are quickly forced by air pressure to present their largest surface to any resistance and hence tumble in flight, severely affecting both range and accuracy. To avoid this they need to be gyroscopically stabilized by spinning, or else fitted with fins. Tail fins, like the feathers on arrows, can stabilize a projectile in flight. However, while there are occasions when fitting fins is a useful option, it is not regarded as a good solution for the bulk of "tube" artillery because the fins add to design problems inside the barrel.

Attempts were made to introduce a form of rifling quite early (as far back as the middle of the 15th Century), and by the 17th Century it was shown to be effective in sporting weapons. However, it was considered to be too expensive for military use until the end of the 18th Century, and even then it was only used for specialized troops like sharpshooters. It was still not taken up in the artillery world because it was not considered necessary, but all that changed in the 19th Century.

One of the major factors driving the change was the recognition that the weight of fire from a given calibre of gun could be significantly increased by moving from a spherical munition to a cylindrical shape: it meant that the projectile could contain a much larger amount of explosive. To get that amount of explosive inside a spherical shape would mean a larger diameter ball and in turn a larger diameter barrel and a heavier gun.

But rifling had a significant disadvantage: it was much harder to load a gun by pushing the projectile down the barrel from the muzzle end, as had been the practice with smoothbores. That meant opening the back (breech) end of the barrel and then having to close it with sufficient strength in the design to cope with the very high pressure in the chamber on firing. Solving this problem is a big subject that I will tackle in another article.

Cannons, Mortars & Howitzers - "horses for courses!"

While the term 'gun' is a catch-all, there are other names in the artillery world that have a more specific meaning. Cannons, mortars and howitzers are specific types of artillery and their roles are markedly different from one another. It may be useful to explain these roles as an aid to understanding how artillery developed.

This article will briefly set out the roles in the period of smoothbore artillery - i.e. before the middle of the 19th Century, when there was a major upheaval in the development of artillery. (Smoothbore versus rifled artillery and the meaning of those two terms will appear in another article in due course.)

Cannons and mortars were the earliest of the artillery weapons. The essential difference between the two lay in their trajectories. The cannon fired a solid cannonball with a large charge of gunpowder to propel it out of the barrel at a high speed: the cannonball gradually slowed down as it travelled along its very 'flat' trajectory. This slowing down produces the curved shape of a trajectory as gravity gradually takes a greater effect and pulls the projectile downwards.

The cannon's role on the battlefield was like that of the bowler in a bowling alley, knocking down skittles, only these were soldiers or horses. In siege warfare, the cannon's task was to breach the walls of a fortress so that the infantry could get inside. These walls tended to be thick and high, and needed heavy shot fired at a high velocity. Clearly, then, a cannon was a 'direct fire' weapon - it engaged targets it could 'see', just like a tank gun does today.



The mortar, on the other hand, was almost exactly the opposite. It lobbed shells in a trajectory that was like that of a tennis player's lob over his opponent's head - going steeply up and descending equally steeply. A mortar's maximum range (distance fired) tended to be much less than that of a cannon, but it could project its shells over intervening obstacles like fortress walls, small hills or woods. Mortar ammunition consequently developed along different lines. A mortar could deliver heavier projectiles and in the early days, they often fired a charge of large stones, showering these down on the those inside the fortress. Another much-used projectile was an incendiary that would cause fires in the timber constructions inside the fortress. Later it became easier to cast hollow shells and these usually had a filling of gunpowder to provide an explosion at the target end.

Inevitably, someone had the idea that it would be useful to combine the roles of a mortar and a cannon in a single weapon that could both lob exploding shells and project them to greater ranges. This new weapon was called a 'howitzer' and, of course, was a compromise: it was not possible to achieve either the greater weight of shot of the mortar or the range and hitting power of the cannon. However, it could do a bit of both roles and was regarded as a useful weapon. In the Royal Artillery at the time of the Napoleonic Wars, howitzers were often incorporated in a battery of cannons so that both capabilities were available on the manoeuvre battlefield.

Propellants and Explosives

There are two basic types of explosive: low explosives and high explosives. Most explosives detonate rather than burn. A detonation is a very rapid chemical reaction using oxygen that is contained in the material rather than in the air. In a detonation, the chemical reaction releases gases that rapidly expand and give off energy as they become hot, creating a pressure wave, or 'blast'.

However, low explosives tend to burn, giving off great heat and intense light, rather than detonate. This is known as 'deflagration'. Because they burn at a slower rate, this creates less pressure than high explosives and they are often used as propellants in guns and rockets: in other words, they provide the 'pushing' power that drives a rocket or sends the shell out of a gun.

From the time of its discovery until the mid-19th Century, black powder - or gun powder as it was usually known - was the most common explosive used throughout the world, and was used both as a propellant and (by enclosing it tightly in a confined space) to produce a detonation. However, it produced a lot of smoke, became ineffective when damp (hence the expression "Keep your powder dry!") and was dangerous to use - all factors that counted against it on the battlefield. Today black powder is still used for fireworks, special effects and other specialized work, but it has been replaced in warfare by more effective smokeless propellants and higher powered, yet safer, explosives.

It's worth spending a moment on how a propellant works in a gun because it was usually propellant improvements over the years that drove changes in gun design.

Gunpowder is a mixture of potassium nitrate (also known as saltpetre), charcoal and sulphur in a ratio of 15:3:2. The mixture varied over the years, but this became the generally accepted best mixture for guns. When a charge of gunpowder was placed in the chamber at the rear of a cannon and a cannonball loaded in front of it, the powder was in effect in a closed container. There was a vent hole immediately above the charge through which a flame was introduced to 'fire' the gun, and there was a small amount of space around the cannonball ("windage"), but there was not enough of these spaces to allow the propellant gases to escape, once the gunpowder was alight. The rapid burning and the release of huge quantities of gas raised the pressure in the chamber of the gun, and as the pressure increased, so did the rate of burning. Something had to give or there would be an explosion!

What gave, of course, was the cannonball. It was pushed at high speed along the barrel, accelerating all the way until it was ejected and sent off on its journey to the target. If the gun had a long barrel - like most cannons - it was accelerated for longer and therefore emerged at a higher speed than it would in a mortar, which all had much shorter barrels than cannons.

Much development went into making gunpowder burn faster, which increased the muzzle velocity, but in turn, this meant that the pressure at the chamber end of the gun became greater and required extra reinforcement. If you look at an early cannon, it is relatively slender compared with later guns, where the chamber end of the gun tends to be very much thicker than the muzzle end, due to the need for stronger, thicker walls to contain the pressure.

When is a 'tank' not a tank?

Quick answer - when it's a self-propelled gun!

It's quite common to hear people, especially in the media, referring to any tracked vehicle with a gun as a 'tank'. They are demonstrating their ignorance! There are so many points of difference that it's perhaps worth taking a moment to look at them.

First, the roles of a tank and a self-propelled gun are completely different. A tank is designed to operate in contact with the enemy and needs to have the mobility, the protection and the firepower for this role. A self-propelled gun is an artillery weapon and has to be capable of engaging the enemy across a wide sector of the front and in depth, working together with other artillery pieces to produce a great weight of fire on the chosen target.

A tank's gun is designed to knock out other tanks and fires high velocity projectiles that can penetrate very thick armour plate. It has other types of ammunition as well, but it is this high velocity role that drives the design of a weapon that has a very flat trajectory and superb sighting systems. Its armour is especially designed for protection against most forms of direct fire and that armour is concentrated on the front of the tank, facing the enemy. As a result, it is normal for tanks to have their engines in the rear, protected by all the frontal armour and out of harm's way. The thick armour means that tanks tend to be very heavy and the amount of space inside the turret very limited, in turn limiting the amount of ammunition that can be carried. However, tanks do not expect to have to fire large amounts of ammunition and their guns wear quite quickly - in this respect they are very different from artillery weapons.

A self-propelled (SP) gun does not need heavy frontal armour because it is not in direct contact with the enemy, but sited some 3 - 6 Km behind the front lines where its lighter armour provides adequate protection from shell fragments and small arms fire. Its artillery role means that it is required to be able to fire a lot of ammunition, so its carrying capacity is much greater than that of a tank. It may well fire in excess of 300 rounds during a single day and feeding ammunition into the turret is a major factor in its operation. It is a great help to be able to open the rear of the chassis to allow easy access, but that cannot be achieved with a rear-mounted engine, so that's usually in the front of the chassis, unlike the tank.

Both types of vehicle require good mobility, but in a tank it can be particularly important for its effectiveness and, sometimes, for its survival. Tanks tend therefore to have very powerful engines, not only for speed but also to cope with their weight in cross-country conditions.

The list could go on, but perhaps I could end with a fundamental difference - their sighting systems are completely different. Where tank sights are designed to deal with targets at direct fire, SP gun sights are intended to deal with targets at long range and out of sight.

Trajectories and Hoses!

When or why would you want to shape a trajectory?

Well, you almost certainly know how to do that already! If you have ever lobbed a ball over a wall to someone, you did it because you couldn't throw it directly - the wall was in the way. In lobbing it, you chose a trajectory that would clear the wall.

If there's nothing in the way, a soldier with a rifle simply aims directly at the target. The rifle can 'see' it and the bullet will follow a very 'flat' trajectory to the target. The same applies to a tank gunner: the gun can 'see' the target directly and the tank shell also follows a very flat trajectory to the target.

In the world of artillery, you frequently have to select a trajectory that will deliver a shell to a target when there's a hill in the way - in effect, you have to lob it over the hill. You may also have to deliver a shell to a target that is tucked away in a gully, where a normal trajectory is unable to reach it, so you need a high trajectory with a steep angle of descent. An ability to shape the trajectory to fit the conditions becomes an essential tool.

Have you ever handled a garden hose? If so, you will know how to shape trajectories using the same basic tools that an artilleryman uses - angle of elevation and pressure. You know that as you point the hose upwards, the water goes further down the garden - until you reach an angle of 45 degrees! After that, if you go on tilting the hose upwards, the watering point will start coming back towards you. If you point it at 90 degrees, which is directly upwards, you start to get wet!

Well, exactly the same happens with an artillery weapon. The range (distance) goes on increasing as you elevate the barrel of the gun until you reach that 45 degree mark. This part of the gun's performance is known as "low angle" firing. Above 45 degrees, the range starts to decrease just as it does with a hose. This is known as "high angle" firing.

But using a hose, you can also change the distance the water goes, even if you maintain the same angle of elevation. You simply turn the tap and change the water pressure. The higher the pressure, the further the water goes. In a gun, you change the pressure by adding or subtracting to the propelling charge. The larger the charge, the bigger the kick given to the shell and the faster it goes; so here, too, you can change the range without changing the angle of elevation.

With these two basic tools - angle of elevation and variation of charge - you can shape the trajectory to fit almost any situation. But note that I said "almost". There are often situations where a target cannot be reached from a given firing position and then you have to move the gun.

Welcome

With a long background of military service and a particular interest in military history, I became involved with the setting up of the Royal Artillery museum now sited in the Royal Arsenal, Woolwich. I chose the title "Pieces of History" to use for my consultancy work and I continue to take an interest in the particular subject of artillery.

For those not aware of the pun, I should explain that the word "Piece" has a particular meaning in the world of artillery: a "piece of artillery" is a gun and, although the term is not often used now, it was common practice to refer to guns as "pieces". Since my interest is in historical artillery, the title is now, I hope, clear.

I intend to write a series of short articles about the history of artillery, but I do not intend to write a "history", as such: it would be a major task and one I don't regard as appropriate in a blog! Instead I shall simply pick out odd items that have interested me and post them when the mood takes me. Among these will be the occasional explanation of artillery that seems to be worth putting into this form.

For example, I have often had to explain the importance of trajectories in the context of artillery. Any professional artilleryman has to understand how to shape trajectories, yet for most people, the concept of shaping trajectories sounds weird and somewhat unlikely. Well, perhaps I should tackle that one as my next blog: it could set the tone and allow you to decide whether this is a blog that you want to read!