Gun Laying - Anti-aircraft Gun Laying

Anti-aircraft Gun Laying

The need to engage balloons and airships, from both the ground and ships, was recognised at the beginning of the 20th century. Aircraft were soon added to the list and the others fell from significance. Anti-aircraft was direct fire, the layer aiming at the aircraft. However, the target is moving in three dimensions and this makes it a difficult target. The basic issue is that either the layer aims at the target and some mechanism aligns the gun at the future (time of flight) position of the target or the layer aims at the future position of the aircraft. In either case the problem is determining the target's height, speed and direction and being able to 'aim-off' (sometimes called deflection laying) for the anti-aircraft projectile time of flight.

During World War 1 instruments were introduced to provide laying data; typically Height and Range Finders (HRF) were optical rangefinders of the coincident type, for example the Barr & Stroud 2 metre UB2, that also measured the elevation angle and hence produced height. Deflection was found by entering the range into tachymetric devices that tracked the target in range and elevation to determine the rate of change and hence the deflection. The French Brocq instrument produced both and provided a remote display at the guns. The British Wilson-Dalby was two instruments (vertical and horizontal), and its data was plotted in the command post before being ordered to the guns.

Anti-aircraft guns always had a recoil system. They needed relatively rapid traversing over very wide arcs of fire with very high maximum elevation angles and relatively high rates of fire. This meant they had to have a very stable mounting.

Broadly, anti-aircraft aiming developed along two paths, by World War 2 the situation was:

• For targets up to a few thousand yards away, a smaller-calibre automatic gun was used, with simple sights that enabled a layer to judge the lead based on estimates of target range and speed; projectiles had tracers, allowing the layer to observe their flight path.
• For longer-range targets, manually controlled predictors were used to track the target, taking inputs from optical or radar rangefinders, and calculating firing data for the guns, including allowance for wind and temperature. Electrical signals transferred aiming data to the guns, and the gun layers' task was to keep pointers representing the actual gun alignment and required gun alignment together. Usually there were two layers (horizontal and vertical planes), and they did not look at the target.

Increasing automation, particularly on ships, eventually eliminated the need for layers on the guns, and this happened at much the same time as missiles started to replace heavier anti-aircraft guns. After World War II predictors changed from being electro-mechanical analogue computers to digital computers, but by this time heavy anti-aircraft guns had been replaced by missiles, but electronics enabled smaller guns to adopt fully automated laying.