Indirect fire shot dispersion

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I noticed some of the things I wrote make more sense with some background knowledge about shot dispersion. Well, this is what the dispersion of artillery shells (and rockets or mortar bombs) looks like in practice, without trajectory correction or guidance:

shot dispersion pattern diagram from U.S.Army FM 6-40

"CEP" (circular error probable) figures pretend that the dispersion  pattern is circular instead of approximately oval. It would be more interesting to have two figures; one for length and one for width of the dispersion.

Shot dispersion depends on range and elevation angle. An example dispersion of a 155 mm howitzer (M777):

"Range Precision PE: .35% (assisted), .30% (unassisted) of range
Deflection Precision PE: 1 mil (low angle), 2 mils (high angle)"

Depending on the fragmentation characteristics, there should be no friendly troops within 300-900 m to the left or right  (wartime safety distance), somewhat less upper and lower side (of this diagram as shown here). This is usually defined by a combination of inaccuracy (mean point of impact deviation from the intended one), dispersion and how far the biggest fragments travel. Modern software-based artillery fire control systems can take into account that the wartime safety distance depends on direction of shot, range, elevation angle and munition and could thus correctly give a "go" for salvoes that would not have been permissible with the old simplistic rules.

Modern HE munitions with pre-formed fragments have less if any large fragments that are dangerous far from the explosion and thus have lower wartime safety distance requirements (see here, for example).

A further possibility is to reduce this wartime safety distance with the use of trajectory correcting munitions. These get aimed a bit too far, and then an airbrake deployed by the fuse at the right time to reduce the dispersion in range by very much (SPACIDO seems to be an extraordinarily versatile and GPS-independent example, but it depends on the use of a muzzle velocity radar and a directional radio data-uplink):

reduction of shell dispersion in range by trajectory correcting fuse

The aiming at a too far point ensures that overshot friendly troops are safe even if the airbrake doesn't engage.

Another possibility for firing more close to friendly troops is to not overshoot them, but to use flanking fires - to shoot at something in front of them from the side. This makes the use of multiple firing positions particularly useful, and thus the concept of horizontal cooperation in which one battalion battlegroup provides support fires to another one if its guns are better positioned. The dispersion in range can (occasionally) be exploited to good effect when firing smoke rounds in order to create a smoke wall at long ranges without adjusting the aim.

The Soviets used (among other artillery tactics) a barrage technique during WW2 in which they left a (relatively) safe path through the battlefield for their assaulting troops, which were protected against flank shots by the suppressive effect and dust thrown up by the shell explosions to the left and right. This required them to set up batteries of suitable types at the right places, so the path would really be quite safe. The knowledge of the ordnance's dispersion patterns was of great utility for this (and so was the knowledge about which batteries had the least worn-out barrels).

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Artillery topics gain less attention than tank or infantry topics, but modern warfare (WW2 and later) has shown that more often than not indirect fires (by howitzers, rockets, mortars, cannons) are the biggest killer, and often more decisive than the supposedly decisive manoeuvre of tanks. The implementation of indirect fires in exercises has been difficult (albeit it should be easier now since radios and GPS receivers are widespread and infantry or tanks caught at the wrong spot at the wrong time could receive a "you were killed by indirect fires" message). To our (NATO, EU) disadvantage, the Russians were and still are very fond of artillery and indirect fires in general. Reports from the Ukraine indicate a 80-95% share of kills by indirect fires, for example. They didn't need to use many guided rounds or even only cluster munitions for this effect. Old-school steel-walled HE rounds without guidance or trajectory correction, often even without muzzle velocity measurement, appear to provide the most volume of fire in the Eastern Ukraine.
 

It's important for our deterrence to get indirect fires right, and this should also have much effect on procurement and force structures. We shouldn't want to use this firepower, but to have it is useful for deterrence. Moreover, the smarter we spend on indirect fires, the less we need to spend for the same military power and thus deterrence.

related:
2013-08 Soft recoil artillery, and electronics-coined artillery in general
2011-10 An article about artillery
 

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defence_and_freedom@gmx.de

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