THE BASICS OF GUNNERY
"Gunnery - the practical application of the science of ballistics"
Linchpin of the artillery system - the Artillery Board
Indirect firing data
When the guns can't see their target - indirect fire - then their firing data has to be calculated. This data has to be in a form that the guns can use, they need to know:
Horizontal aiming can be relative to anything, but in World War 2 the western allies always used an arbitrary line that the UK called a 'zero line', and angles relative to it. The zero line itself was a bearing relative to grid North and could be different for every battery position. The vertical aim is an angle relative to the horizontal plane, an 'elevation angle'.
The elevation angle and the amount of propellant determines how far the shell goes, its range. In WW2 no UK guns were true 'guns', they all had propelling charges comprising several bags of propellant - 'increments'. There was usually more than one charge that could be used for a particular target, more increments propelled a shell further so a lower elevation angle could be used for a particular range. If the target was at a different height above sea level to the gun then allowance had to be made for the angle of sight between them.
For a given charge, range increases with elevation angle until the angle reaches about 45º. It then starts to decrease. 45º is the dividing line between lower (below) and upper (above) register, today called 'low' and 'high' angle fire.
Orienting the guns
The first task on any position was to align all a battery's guns with the battery's zero line. This meant that their barrels were all parallel. Once a gun's barrel was pointing in the zero line the dial sight's angle to an aiming point was recorded. Each gun selected several aiming points for its own use, each aiming point had a different angle. They included distant objects, aiming posts or parallelescope, and an illuminated 'night picket'. Figure 1 illustrates the geometry of this.
Figure 1 - The geometry of orienting the guns
The director was the preferred method of 'passing line' to - 'orienting' - to the guns, although there were other methods.
Calculating firing data
The first step in calculating firing data is to produce 'map data' - basically a 'switch' from the zero line, the distance and the angle of sight from the pivot gun to the target. Sometimes this was sufficient to fire with. This calculation involves conversion from map references (rectangular coordinates) used by observers and target acquisition systems into bearing and distance (polar coordinates) used by the guns was usually made on an 'artillery board'. However, it could be measured from a map, or it could be calculated using trigonometry if special accuracy was required. In any event it this of course meant knowing the whereabouts, map references, of guns and target, the former being a matter of survey.
The next step was to adjust the map data for 'non-standard' conditions and produce firing data. This was done by combining the corrections data provided in the Range Tables Part 1 for the type of gun being used with the data for the current conditions in the field. These calculations concern the internal ballistics and external ballistic performance of the shell. The former determine the velocity at which the shell left the barrel, the latter what happens to it in flight.
The standard conditions used by UK in WW2 were:
These adjustments provided corrections to range and switch, using data provided in the Range Table for the type of gun being used. The calculations were done in the Battery and/or Troop Command Post (CP).
For some Range Table page examples see:
Introductory page for Charge 3 - this page provides details about the charge
A page of data - this page provides most of the data for non-standard conditions.
Other pages include:
Non-rigidity Table - this table gives the corrections to range for targets above or below the height of the gun position, see Figure 2 below.
Wind Correction Graph - this graph gives corrections to range and line per 10 feet/second for wind. Wind details were provided in the Meteor Telegram. The CP used this data to calculate a correction or with temperature correction data to set up a 'correction of the moment graph' for the current meteor period.
Crest Clearance Table - this table was used to ensure that the firing data provided sufficient clearance when firing over crests occupied by friendly troops.
Although Range Tables also provided data to convert ranges to elevation angles and fuze settings for each charge this data was not normally derived in the troop or battery CP because most British guns had guns rules and fuze indicators for this conversion.
Figure 2 shows the effect of angle of sight - when the guns and target were at different heights - on the trajectory.
Figure 2 - Effect of Angle of Sight
Internal ballistics
For a particular elevation angle a higher muzzle velocity (MV) makes the shell go further, and lower one means it goes less far, therefore when the MV varies from the Range Table standard the elevation has to be modified. The variations from standard MV arose from:
External ballistics
In-flight shells were are affected by meteorological conditions, - changes from standard in air temperature and pressure, and the direction and speed of the wind. This information was provided in a periodic meteor telegram and used to produce a simplified 'correction of the moment' graph that was in turn used to find the correction needed for a particular target..
An alternative method was to use data derived from firing - 'correction of the moment by ranging', either at an accurately known 'datum point' or airbursts that were accurately observed by cross-observation. This process derived a correction to map data (range and switch) that could be used for targets within a certain distance of the datum point up to a few hours after it was fired at. As well as correcting for non-standard conditions affecting both internal and external ballistics it also corrected for errors in survey at the guns. However, it was usually only applicable to the type of gun that fired and the charge that was used.
The output of these calculations to find the map data and correct it for all the non-standard conditions was a selected charge, a range adjusted for non-standard conditions and non-rigidity, an angle of sight, and a switch from the zero line also adjusted for non-standard conditions. If airburst fuzes were being used then there was a corrector setting to adjust the range dependent fuze length for non-standard conditions.
If the fall of shot was ranged by an observer ordering corrections, then a difference in range and switch for the correction was calculated, without allowance for non-standard conditions, and applied to the last fired range and switch. This calculation was done by plotting the ordered correction on an artillery board and measuring the new range and switch.
Laying the guns
Firing data was ordered by a CP to the guns, this data comprised:
Technically, Elevation = Angle of Projection ± Angle of Sight. However, British guns had a separate sight clinometer for angle of sight, so this was ordered separately, although if the shoot was being observed and ranged they would use zero unless angle of sight was significant.
The switch was applied to the dial sight (which moved it away from its aiming point), the angle of sight was set on the sight clinometer, and the gun rule cursor was set to the charge and the range. If airburst fuzes were used the corrector and range were set on the fuze indicator, which was then read to find the fuze setting, this was then applied to the fuze using a fuze setter. The barrel was then moved in vertical and horizontal planes until all leveling bubbles were centred and the dial sight was again pointing at its aiming point.
Figure 3 - Laying a Gun in the Horizontal Plane
The guns of a battery usually fired with their barrels parallel to one another and the same range set on their sights. The thing that differed between guns was their MV because every gun had a different amount of wear. Most British guns had a fitted gun rule that converted the range in yards to the elevation angle in degrees and minutes taking account of and corrected for the difference between the gun's calibrated and the Range Table MVs. For guns that did not have a gun rule the CP had to calculate a different elevation angle for each gun, and a fuze length if they did not have a fuze indicator. For certain types of target, such as a smoke screen or barrage, each gun required an individual range and switch from the battery zero line. This substantially increased the amount of calculation that had to be done in the CP, and took much longer.
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Copyright © 2001 Nigel F Evans. All Rights Reserved.
