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Looking ahead to the infantry mortar of the 1990s; the trend towards larger calibres and smarter munitions.

Looking Ahead to the Infantry Mortar of the 1990s

Not so many years ago the infantry mortar was simply regarded as the poor man's artillery - a cheap weapon firing cheap ammunition, with rudimentary fire control and equally rudimentary accuracy. Today, however, when artillery weapons are becoming almost as expensive as main battle tanks, the mortar can afford some degree of sophistication and still be a cost-effective weapon. As a result, we are beginning to see some very interesting developments in the infantry mortar world, and the coming decade will see the mortar assuming far greater importance.

Broadly speaking, mortars fall into three distinct groups: the light 60 mm, the medium 81/82 mm and the heavy 120 mm. There is a fourth group, the superheavy 160 mm used by a few nations, but this is an artillery weapon and we need not consider it further. Neat as this classification is, the activities of designers are now beginning to merge the classes together, so that we now have 60 mm mortars which can range as far as 81 mm models with bombs that are almost as effective, and 81 mm weapons capable of keeping up with the 120 mm class.

The Calibre Issue

In the past the employment of these groups could virtually be forecast; a continental European army would have 120 mm weapons, the British and Americans and countries under their influence would stop at 81 or 107 mm. This may well have been a reflection of the different methods of integration of field artillery in the infantry structure: where quick-firing and ample artillery was available, infantry were happy to abandon the idea of using heavy mortars and utilise their men for other tasks. But today the artillery tendency is towards 155 mm weapons and the engagement of targets deep inside enemy territory. Moreover there is less artillery available than in the past, and priorities are having to be laid down for artillery engagements. This means that the infantry are becoming more concerned with having the ability to protect themselves and to project their power onto the battlefield, all of which argue in favour of more and better heavy mortars. The Canadian, American and British armies have all, in the past three or four years, begun looking hard at 120 mm mortars and conducting trials.

Another factor which has stimulated interest in this calibre has been the development of more sophisticated projectiles for this class of weapon. Hitherto the mortar has used simple and inexpensive bombs relying entirely upon blast and fragmentation on impact, and their steep angle of descent and almost vertical attitude on impact meant that the distribution of fragments was almost circular and generally much better than a comparable artillery shell. But artillery scored in being able to use time fuzes for airbursts, giving better downward fragmentation against troops behind cover. Time fuzes for mortars were considered an expensive luxury and, moreover, the infantry were not very enthusiastic about the idea since it meant carrying fuze setters and firing tables and generally building up a small command structure for what they felt ought to be essentially a simple weapon system.

Projectiles

Today, the cheapness and ready availability of solid-state electronic chips capable of performing timing operations, and the utility of chips to permit the use of hand-held fire-control computers, have removed the obstacles to time-fuzed mortar fire. The mortarman merely keys in his own coordinates and those of the target, presses another key and is given the elevation, azimuth and fuze setting. The actual setting of fuzes is now greatly simplified; it may be mechanical or electrical, but the device is generally less cumbersome than the heavy mechanical setters used with artillery thirty years ago. In addition, should the use of setters still be looked upon with disfavour, there are a number of fuze options which will deliver airburst or impact burst at the flick of a switch and without the need to determine precise settings. Proximity/point detonating (PPD) fuzes have become relatively common and will deliver either type of fire on demand.

However, it is the promise of projectiles which will produce more than a simple detonation which offers the medium and heavy mortars a more versatile role. The first of these to appear was the anti-armour bomb with a seeking capability, such as the FFV Strix, the Diehl Bussard and the British Aerospace Merlin. The first two are in 120 mm calibre, the latter in 81 mm calibre, simply because these are the basic mortars of the countries of origin; but there is no doubt that, like any other munition, a good big one is always better than a good little one, and the 120 mm types offer the designers more leeway in fitting everything into the calibre limitation. A 120 mm shaped-charge warhead landing on the upper surfaces of a tank will do a satisfactory amount of damage, and the average 120 mm mortar should be able to fire this type of projectile to ranges of five or six kilometres quite easily. Various methods of guidance are being studied, including infrared homing and millimetric-wave guidance; the technology of both of these is sufficiently well advanced to make it probable that bombs of this type will be in troop service in the early 1990s.

The other option, which has appeared in the last three years, is the sub-munition carrier bomb. Artillery projectiles carrying sub-munitions are well understood, and scaling this technology down to fit the mortar calibres has been difficult but possible. The first of these to be announced was the Espin, from Esperanza y Cia, the well-known Spanish mortar specialists. Espin is a parallel-walled bomb containing a number of small sub-munitions; Espin 15 carries 15 bomblets and ranges to 5 500 metres, while Espin 21 carries 21 bomblets and ranges to 4 300 metres. The carrier bomb is fitted with a time fuze which opens the casing and distributes the bomblets; depending upon the height of burst the bomblets can cover an area of up to 4 000 square metres. Each bomblet has a shaped charge with a fragmentation sleeve giving both anti-armour and anti-personnel effect.

There is, of course, no guidance in these bombs; they are simply fired like any other mortar bomb, but the bursting of the bomb releases the submunitions so as to cover an area beneath the point of burst, thus making it highly probable that a proportion of the bomblets will find targets: even those which do not find a specific target will still have a useful fragmenting effect. Bearing in mind the rate of fire that a well-trained 120 mm mortar squad can produce, and multiplying that by the usual four mortars of the platoon, it is obvious that a minute of mortar fire is going to place a very high number of submunitions into the target area.

Shortly after the announcement of Espin, the Greek Powder & Cartridge Company, not previously known for developing mortar ammunition, announced their Pyrkal GRM-20, developed for the ex-American 107 mm mortars used by the Greek Army. The 107 mm is an odd calibre, and an odd design. It is one of the few rifled mortars, and uses a quite unique projectile which resembles a flat-based artillery shell. At the rear end is a copper driving band formed on the circumference of a dished plate surrounding the cartridge container. The bomb can be drop-loaded in the normal way, since the driving band's diameter is less than that of the bore; but the explosion of the propelling charge flattens out the dished-plate and so forces the driving band into engagement with the rifling, and the bomb spins its way out of the bore. The cylindrical bomb gives the maximum space and there are a total of 20 sub-munitions packed inside. As with Espin, these sub-munitions have a shaped-charge head surrounded by a fragmentation sleeve. They are dragstabilised and will penetrate up to 60 mm of armour. The parent bomb can be fitted with either a time or proximity fuze and upon bursting the bomblets are scattered over an area of about 400 metres diameter. An advantage of the spun projectile is that it can be fitted with a number of conventional artillery fuzes and does not demand a special non-spin fuze configuration as do most other mortars.

It was only a matter of time before some designer put the sub-munition bomb and the guided bomb together and produced a guided sub-munition carrier. The first of these to be made public was announced in South Africa in 1988 and has been developed for the 120 mm mortar. The bomb carries 21 shaped-charge/fragmentation bomblets in its cylindrical body and is fitted with a millimetric wave seeker head. After being fired from the mortar in the conventional way, the bomb deploys four moveable fins into the airstream after passing the vertex of its trajectory and starting on the downward leg. The seeker switches itself on some 2 500 metres from the target and has a 300 metres radius of sensitivity within which it will detect a target and steer the bomb towards it. At the optimum height above the target the bomb is disrupted and the sub-munitions distributed. Laser guidance has also been tried with this bomb, and trial firings showed a consistent accuracy, the bombs all homing to within seven metres of their targets. Either system of guidance could be employed in a service bomb, depending solely upon cost-effectiveness.

Looking further ahead, there seems no reason why carrier bombs loaded with anti-personnel mines should not be produced. This would give the infantry battalion the ability to deny possible approach or concentration areas at long range and set up a series of obstacles and barriers so as to channel an attack into areas which could then be covered by firing anti-personnel sub-munition bombs and conventional bombs. Moreover, the dual-purpose sub-munitions would also be able to deal with any armoured force accompanying the attack. Medium mortars armed with these types of ammunition would thus provide the infantry with a total defensive system.

In the attack these munitions would also be useful, but the greater attraction in this phase of the battle is to have a mortar with longer range. Study of current designs of 120 mm conventional mortars and bombs shows that the average bomb weighs about 14 kg and the average maximum range is 6 850 metres. If this can be stepped up to, say, 10 km maximum range, then the infantry commander has an extremely valuable weapon in his hands.

Extending the Range

This question of maximum range can be easily misunderstood. In real life few mortar commanders are particularly interested in targets deep inside enemy territory; these are the province of artillery and tactical air support. What is more important is that with a greater maximum range the mortar needs to be moved less often. When 4-5 km was the maximum range, once the infantry advanced they soon ran out of mortar support until the mortars were moved forward. But with 10 km range the mortars can stay in place and continue to give supporting fire for a much longer time before needing to redeploy.

Another attraction of a long-range weapon is that it allows a greater latitude of siting within one's own area while still retaining the same amount of command over enemy territory. A mortar with 5 km range, required to cover an area 4 km deep within enemy lines, has no more than a belt 1 km deep in which to find positions. A mortar with a maximum range of 8 km, given the same task, has a 4 km deep area in which to deploy and redeploy, offering far better opportunities for concealment and alternate positions.

There are three current approaches to obtaining more range: to employ a lighter bomb with better ballistic shape with the standard mortar; to employ the same bomb with a longer-barrelled mortar with improved propelling charge; or to add rocket propulsion to the bomb. A fourth option open to artillery is to employ base-bleed techniques, but this is not applicable to a mortar bomb. However, a system which might prove workable and profitable would be exploit the "boundary layer" effect or "Chilowsky Effect". In this a suitable substance is burned in the nose of the bomb and the gas allowed to flow over the surface of the bomb during flight to produce a friction-free layer. This system first appeared in 1918, and again in 1940; in neither case did it work well, but advances in modern technology might well make it feasible today.

The "long-range bomb" frequently provided for existing mortars is usually somewhat smaller and lighter than the standard bomb. It uses a better grade of steel (to obtain thinner walls) and a more powerful explosive so as to provide the optimum fragmentation from the smaller payload, is well-shaped and provided with an efficient obturating ring which ensures that the propelling gas stays behind the bomb and gives it velocity instead of leaking past into the barrel. The combined effect of these measures can vary widely. Some bombs designed to this formula produce a range increase of 10% above the standard bomb, others achieve as much as 20%. If the improved bomb is used in conjunction with a longer barrel, then the increase in range can reach as much as 27-30%.

Rocket assistance has not been widely used in mortar bombs. The idea was pioneered by Thomson Brandt, who introduced the 120 mm PEPA rocket-assisted bomb in the 1960s. This increased the maximum range from 4 250 metres to 6 550 metres, a 54% improvement which was quite impressive; but since then the range of conventional mortars has increased, and there are now simple mortars which can outrange the PEPA bomb. Brandt followed the PEPA with the PRPA for their rifled mortar; this lifted the range from 8 135 to 13 000 metres, a 58% improvement.

The most recent arrival in this field is the Yugoslav 120 mm M77 rocket-assisted bomb. As with the Brandt designs, this can be fired with or without rocket assistance, and switching in the rocket lifts the maximum range from 5 300 metres to 9 400 metres, a remarkable 77% improvement. The bomb weighs 13.42 kg in firing condition and carries a payload of 2.91 kg, so that the charge/weight ratio is approximately 22%.

One of the generalised objections to rocket-boosting is that finding space for the rocket motor takes away space from the payload, so that while you might be able to fire a lot further, you are actually sending less explosive to the target. It is up to the individual user to decide what importance to attach to these conflicting factors. In the case of the Yugoslav M77 bomb, it is interesting to compare values with the standard M62 bomb. This is a conventional teardrop-shaped bomb weighing 12.6 kg and with an explosive loading of 2.25 kg, a charge-weight ratio of about 18%, so in this case the generalised objection is no longer valid. This is mainly because for the rocket-boosted bomb the Yugoslavs have moved away from the teardrop shape and produced a flat-based cylindrical bomb, giving better capacity and thus giving them the best of both worlds.

The more certain objection to rocket assistance is the reduction in accuracy. Rocket assistance for mortar bombs takes effect shortly after launch and on the upward leg of the trajectory; the rocket is all-burnt by the vertex, which is thus moved further away from the mortar. Once over the vertex the bomb follows a conventional ballistic path. But mortar bombs tend to yaw alarmingly as they leave the barrel; displacements of up to 40 (Degrees) from the trajectory are not uncommon in the first few hundred metres of flight, and although the fins damp down this yaw very rapidly, there is still some unsteadiness for a considerable distance. And if the rocket ignites and begins thrusting while the bomb is yawing off-trajectory, then it will find a new trajectory for itself and land off-target. As somebody once said, "It is in the nature of mortars to be inaccurate; the measure of a designer's skill is how accurate he can make one". However, too much should not be read into the question of accuracy. Nobody expects a mortar to be a pinpoint weapon, and the fire of four 120 mm mortars at 8 km range, using rocket-assisted bombs, would be accurate enough for all practical purposes.

As regards this question of long-range capability there is one aspect which must be borne in mind: the potential of counter-mortar radar systems. Protagonists of these devices have long been assuring everybody that the day of the mortar was over, that radar could detect the bomb in flight and computers could reconstruct the trajectory and determine the firing point before the first bomb landed on its target. Although experiments have shown that this worked, practical considerations suggest that in a shooting war things might not be so easy. In war mortars will not fire in isolation, which is generally the case when counter-mortar radars are being exercised in peacetime; and it seems probable that the radar will be offered such a profusion of bombs that it is quite likely to make a mistake and produce a wrong answer in actual combat.

Fire-control

The last area of debate lies in that of fire-control. Short-range mortars needed little beyond a pair of binoculars and a map, their target invariably being within easy view. But mortars capable of ranges in excess of 10 km will demand some form of observation, communications and a fire-control organisation. As we have already said, the provision of simple hand-held computers to do the calculations had removed the need for any sort of fire direction centre for technical calculation. All that is needed is tactical direction, instructing the mortar on target priorities and zones of responsibility. With modern data communication systems the observer can speak directly to the mortar controller, and the computer will do all the necessary arithmetic, so that the infrastructure is kept to a minimum and there is no need to tie up numbers of men in duplicating an artillery control system. We have also noted that modern fuzes, particularly the proximity/point detonating option, make the minimum demands upon time and calculation. So it would seem that the most significant factor in the development of modern and effective mortar fire is not smokeless powder or high explosive or rocket-assistance, but the humble microchip; it is the electronic potential which makes all else possible.

What, therefore, might we expect to see in the infantry mortars of the 1990s? Many signs show that we shall see a move towards 120 mm, to the point where it will become the standard calibre, much as the 81 mm has been the standard for the past forty years. There will be a gradual adoption of sophisticated sub-munition bombs and guided bombs, giving the infantry commander greater flexibility in his fire-planning and greater offensive potential against armour. The adoption of computers, already widespread, will refine the fire direction systems so that there need not be any increase in manpower to control this improved firepower - something which, in the past, has tended to discourage the utilisation of the mortar to its maximum. Tactical handling will not change very much, though the infantry commander will now have available to him a very flexible and versatile arm which will expand his area of influence and offer him a variety of responses. From being the poor man's artillery, mortars are going to assume a far greater importance in the coming decade.

PHOTO : 120 mm ammunition from Spanish mortar specialist Esperanza, showing the proportions of a typical modern long-range bomb among a selection of older bombs.

PHOTO : One attraction of the 120 mm mortar (here the famous Thomson Brandt rifled version) is its ability to provide heavy fire support to airmobile forces.

PHOTO : The Lambda electronic time fuze, from Reshef of Israel, uses an air-driven turbine to generate its own power.

PHOTO : The terminally-guided Strix anti-armour bomb; the shaped-charge is aft, whilethe guidance system in the front surrounds a channel for the armour-piercing jet.

PHOTO : Greece's Powder & Cartridge Co. has now produced its own mortar bomb, the Pyrkal GRM-20.

PHOTO : One of the Pyrkal GRM-20 submunitions developed for the odd 107 mm calibre American mortar.

PHOTO : The South African 81 mm M3 mortar, a typical long-barrelled mortar intended to extract the utmost from a modern bomb design.

PHOTO : A 120 mm mortar in an APC (here the 120 mm Soltam RML-6) becomes a versatile and mobile source of firepower. This one uses a special mounting to absorb recoil.

PHOTO : The Spanish Seimor fire-control computer, typical of several now in use in various armies around the world.

PHOTO : Electronic computing makes fire-control simple and economical in manpower; here one man is seen handling both communications and computing.
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Copyright 1989, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Stone, Walter
Publication:Armada International
Date:Aug 1, 1989
Words:3494
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