105mm, 155mm or Something In-between?

60 pounder Cape Helles June 1915.jpg. A 60-pounder Mk I at full recoil

A guest post from Obsvr

The defining characteristic of field artillery for the last century has been ‘firepower mobility’ of indirect fire.

Any target within range can be engaged, providing the effective command and control arrangements are in place.

This gives each battery a very large area of influence.

Many widely separate batteries can engage the same target or several targets simultaneously in the same area.

This mobile firepower is used to deliver weapons (the shell is the weapon of artillery) to suppress, cause casualties and damage, and if precision guided munitions are used, destroy.

Suppression is generally caused by the explosion, the size of the explosion is of no great consequence and the number required per unit of area per unit of time is modest, but time could be lengthy if dismounted infantry are being supported.

Casualties and damage are caused by shell fragments.

Fragments weighing less than a gram are lethal because of their high velocity, large fragments from a burst within about 10 metres of a lightly armoured vehicle may penetrate and fragments far further from a burst will damage optics, antennae and other external fittings of armoured fighting vehicles, tyres are likely to suffer significantly.

The probability of a fragment hit depends on fragment density which decreases with distance from the burst.

Crudely, the number of fragments is a function of shell size and the amount of HE.

For good fragmentation around 25% or more by weight of HE is required.

This threshold was first achieved in the 1970s by the UK designed 155mm L15 shell (45kg with over 11kg of HE).

The new 105mm L50 shell (15kg with just under 3kg) is better than most but still inferior to L15.

The distance from a burst at which there is a 50% chance of a person in the fragmentation zone  being hit by a groundburst fragment is about 30 metres for L50 and 55 metres L15, this increases a bit for 105mm airburst and lot for 155mm airburst.  The shape and area of this zone depend on the height of burst and angle of descent.

It is important to note that multi-function fuzes are now standard in some western armies, including the UK, giving choices of proximity height of burst, point detonating or delay.

There is an important reverse side to this coin, safe distances for own troops will be greater with 155mm than 105mm and this can be particularly significant when suppressive fire is being delivered to cover assaulting troops.

While bigger is better for damage and casualties, smaller is better for suppression.

The risks to own troops can be significantly reduced by using delayed action fuzes (providing the ground is suitable).

Course Correcting Fuzes would reduce the risks still further.

Course Correcting Fuse
Course Correcting Fuze

Another factor favouring 155mm is exotic cargo munitions such as electronic jamming payloads, SADARM sub-munitions able to destroy AFVs, even moving ones, and anti-armour mines.

155mm Ballistic Sensor Fuzed Munition
155mm Ballistic Sensor Fuzed Munition

105mm is adequate for more traditional natures such as smoke and illuminating.

Next consideration is rate of fire where it is impossible to beat self-propelled guns with a useful degree of automation.

The first point to note is that most casualties occur in the first 10 seconds. The modern availability of an airburst fuze with every round may alter this unless the target has immediate access to overhead protection. Guns like AS90 deliver 3 rounds in 10 seconds, FH70, a towed 155mm, achieved 3 in 15 seconds (like AS90 it had a primer magazine) and a towed 105mm won’t improve much on this.

AS90
AS90
FH70
FH70

The 155mm M777, lacking a primer magazine and relying on manual shell ramming seems to be about 2 rounds in 15 seconds.

On-board ammunition in SPs is limited, particularly if there are several ammunition types.

This makes ease and speed  of getting ammunition into the gun an important consideration.

Tracked SPs are best because, providing they have a rear hull door,  their floors are lower to the ground hence ammunition handling is quicker and easier.

British troops of D Battery, 3 Royal Horse Artillery, fire on enemy positions in Basra.  28 March 2003 Location: Iraq OP TELIC
British troops of D Battery, 3 Royal Horse Artillery, fire on enemy positions in Basra. 28 March 2003 Location: Iraq OP TELIC

Tracks are also less likely to be damaged than tyred wheels if a battery receives counter-battery fire.

The modern trend for wheeled artillery is a reminder of an artillery maxim, all too frequently forgotten, that ‘in peace the cry is for mobility, in war for weight of shell’.

This problem is a consequence of peacetime training where ammunition is limited and its real effects are not apparent, but there is a lot of scope for impressive rushing around.

In reality guns’ cross country mobility is limited by their loaded ammunition vehicles.

DROPS (Image Credit  - Plain Military)
DROPS (Image Credit – Plain Military)

Fortunately, built-up areas are well endowed with suitable gun positions and in most parts of the world there is seldom a need to deploy too far from solid surfaces. Nevertheless ‘into action time’ is a relevant issue, and if it is really fast it means that batteries can quickly respond to calls for fire when on the move.

Generally, towed 105mm will be faster than 155mm but spadeless self-propelled guns will always be the fastest, not least because they have ammunition onboard and ready to fire.

One final issue, traverse.

In conventional operations most targets can be engaged with an arc of fire of about 90 degrees.

Typically towed guns have top-traverse (ie saddle on carriage) of about 30 degrees left and right although M777 is limited to 23 degrees.

Targets outside the top traverse arc require the carriage to be man-handled, this takes time and delays the response to a call for fire, by some minutes with 155mm.

The M777 155mm howitzer
The M777 155mm howitzer

 

This is not a problem with turreted SP’s, nor is it an issue of any significance with a well balanced gun with a box-trail and platform.

These were introduced in 1918, later used with 25-pr and today with L118, but is probably impractical with 155mm.

All Terrain Mobility Platform 1 (ATMP) towing a Light Gun
All Terrain Mobility Platform 1 (ATMP) towing a Light Gun

 

But it is not clear that it has ever been tried, although sole plates have been used and undoubtedly facilitate wide traversing.

If there is a real need for movement by helicopter then towed guns are the only option and they need to be light.

If any sort of air movement is required then the lighter the gun the better.

Merlin Helicopter Light Gun Royal Artillery
Merlin Helicopter Light Gun Royal Artillery

It is useful to remember that L118 with essential stores and camouflage is about the same weight as two ammunition pallets (72 rounds) or about a hundred unboxed rounds.

Logistically, a given delivery weight will provide about twice as many packed  105mm shells as 155mm, although stripped of pallets, ULCs and boxes (leaving fibreboard tubes and plastic containers) it increases because 105mm uses more packaging.

From an effects perspective, 105mm is more efficient than 155mm for suppression.

For casualties and damage 155mm is probably three or more times as effective as 105mm.

For benchmark rates of fire (to inflict maximum casualties) a self-propelled gun with extensive automation is required. Importantly, the longer range of 155mm gives a greater area of influence than 105mm and places more batteries in range of a target area.

If the situation is counter-insurgency, then speed of all round traverse becomes an issue, the pecking order is simple, turreted SPs are best, then box-trail and platform, lastly anything else.

So, which is best, the only answer is ‘it depends on what you want’.

155mm gives the best coverage and lethality but 105mm is logistically more efficient for suppression and more suited to air movement and associated ammunition supply.

What about something that combines the qualities of both?

The problem here is that neither 105mm nor 155mm are particularly suitable as general purpose non-armoured artillery.

That suggests it’s time to think about alternatives.

Both came about because they were what the US used in WW2 and NATO decided to ‘standardise’ on them.

This 1950s standardisation was nonsense, there was no agreement of any characteristics except calibre, and a few years later the UK adopted their own standard of 105mm  which was a considerable improvement on what the US had been using, albeit with totally different ballistic characteristics.

Subsequently, there was ballistic standardisation agreed by a handful of nations for 155mm with 39 and 52 calibre length barrels.

The basic requirements are, perhaps, the best possible maximum range, a gun weighing no more than M777’s 3400 kg, but with rapid traversing over a large arc.

To keep ammunition weight down it should not use a metal cartridge case, so a sliding block breach with a primer magazine is required with the goal of 3 rounds in no more than 12 seconds.

The technology for this only exists in UK and Germany, although France produced a primer magazine for the 155mm AUF2.

Splitting the difference between 155mm and 105mm gives 130 mm and seems a good starting point.

The Soviet 130mm M46 was a WW2 vintage gun, reputedly naval in origin.

At some 8.5 tonnes it was distinctly ‘chunky’ and had a very long 55 calibre barrel with a high maximum muzzle velocity of some 930 m/s giving a maximum range of 27 km.

M46 130mm
M46 130mm

 

Projectiles were about 33kg, a tad more than the 105 – 155 midpoint of 30kg.

Rate of fire is usually given as up to 8 rds/min but this gun was not capable of high angle fire and its top traverse was only slightly better than M777.

Nevertheless, being designed some 70 years ago it provides food for thought.

Dropping down we reach 127mm, taking us back a century to the even older the 60-pr.

60 pounder Cape Helles June 1915.jpg. A 60-pounder Mk I at full recoil
60 pounder Cape Helles June 1915.jpg. A 60-pounder Mk I at full recoil

With a 37 calibre barrel this delivered various shells between 54 and 60 lbs (27 kg) using 4 kg propelling charge with a MV of 650 m/s to about 14 km.

Maximum elevation was limited to 37 degrees.

For comparison, a 5 inch (127mm) naval gun with a 38 calibre barrel, firing 25 kg shells with an MV of 790 m/s gave a range of almost 16 km. The 55 calibre version, with an MV of about 805 m/s gives 24 km range.

Finally 122mm with a 22kg shell.

The M1931 Gun, MV 800 m/s and 46 calibre barrel gave 20.4 km albeit with a total weight of 7900 kg, the M1955 (D74) was a considerable improvement, 5500 kg, 900 m/s MV, 47 calibre barrel and 23.9 km.

D30 weighing 3,200 kg with a 35 calibre barrel and MV 690 m/s gives 15.3 km.

It might be a politically astute move to adopt 122mm as a NATO calibre and in recognition of the newer members.

D30 Howitzer
D30 Howitzer

 

122mm is 48 lines in pre-1917 terms, ie 4.8 inches.

For comparison, 39 calibre 155mm has an MV of about 820 m/s for 24 km range and 105mm L118 with 37 calibre length, MV 810m/s gives 17.2 km

From this it seems reasonable to seek an ordnance of about 125mm, 40+ calibres, 800+ m/s MV firing 25 kg projectiles (with 5 – 6 kg of HE) to 20+ km with an in-action weight no more than M777.

The tricky bit is the carriage, with the need for rapid and wide traverse as well as the usual high angle fire and direct fire capabilities.

The basic requirement for such a carriage is to provide stability, this requires trails or stabilisers and the longer they are the greater the overall weight.  Trail length is determined by the recoil forces and the height of the trunnions. Minimising this height requires the trunnions to be as far back on the ordnance as possible (or severely limiting maximum elevation).

The shortest trail is a single pole trail, the problem is that this limits maximum elevation because it is in the way of the recoiling breach.  Split trails are the most common, but they have to be far apart to permit wide top traverse, and the wider the open trail angle the longer and hence heavier the trails need to be – this explains M777’s relatively short stabilisers and limited top traverse.

Lifting a pair of long and heavy trails, particularly when the spades at their ends have’ dug- in’ with firing, is never quick and not always easy.  The need is a carriage or mounting that can be quickly traversed over a large arc of fire.

Guns like the 122mm D30 that can traverse 360 degrees on its three legged mounting (a mounting because the wheels are not on the ground when firing) have long legs, they’re 120 degrees apart.

This configuration has another constraint, barrel elevation angle and hence range are limited whenever the barrel partially aligns with a leg, there is only some 48 degrees of full elevation traverse between each pair of legs.

Clearly this is not a suitable design.

The obvious solution is a box trail, as used with 25-pr and L118, or sole plate.

The latter requires lowering the sole plate to lift the wheels clear of the ground and still requires the trails to be lifted for a wide traverse.

The former rotates on its platform but invites the question as to whether it is doable with a gun of 3000+ kg or so.

The key to this is the balance of the gun so that once the trail is lifted the load is equally distributed forward and rear of the wheels, trunnions well to the rear facilitates this.

The ease of lifting the trail to the position of balance for a 3000+kg gun may still be a challenge. 

As an indication of the problem, the L119 has a shorter barrel than L118, it has a notably heavier muzzle brake to provide the necessary balance.

In summary, what is currently missing is a towed gun, light enough for tactical air movement, offering a wide and rapid arc of fire, and delivering a shell with a significant improvement over 105mm. 

155mm is not the solution. 

Something like 125mm seems reasonable but 122mm might be politically attractive although the existing 122mm ammunition would need much improvement

 

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