Naval Anti-Aircraft Guns and Gunnery. Norman Friedman
The director was the eyes of the system. It incorporated a coincidence rangefinder (heightfinder), which was set horizontally to provide its crew with weather protection. Moreover, setting it vertically would have limited its size (base length), hence its accuracy or its effective range (as the US Navy found with its vertically-mounted ‘altiscopes’ at this time). The director also carried telescopes for layer and trainer, control officer’s glasses and transmitting devices. It was manned by a control officer, layer, trainer, heightfinder operator and communications number. The control officer provided the two key estimates (target course and speed), and he was responsible for corrections based on feedback. When on the target, the layer and trainer had the target in their telescopes. Their view was ‘undisturbed:’ no mechanism threw it off the target so that they could make corrections.21 The operators provided the below-decks HACT with angle of sight (layer), bearing (trainer) and height/range (heightfinder). The below-decks computer generated estimated target bearing and elevation, power-driving the director accordingly. There were manual back-ups. Different versions of the director had different means of stabilising it against the ship’s roll, none of them precise enough to make tachymetric operation possible. The Mk III version introduced a cross-roll periscope operated by the communications number.
The HA director tower as shown in the 1930 handbook for HACS I and II.
HACS was conceived as a more automated and integrated evolution of the STS, with the same inputs: range or sight angle from a heightfinder and estimated target course and speed from the control officer. The difference was that the elements were tied together. Range was computed and therefore could be predicted. However, because it seemed the STS worked to some extent, HACS was designed so that it could be de-integrated into something more like the earlier system.
Target course was expressed as angle of presentation, the angle between target plan course and the line across the line of sight.22 It was equivalent to inclination (of a surface target), which was so difficult to estimate that the Royal Navy used special inclinometers for that purpose.
The control officer transmitted it by placing the graticule of his glasses on the target. Angle of presentation cannot have been easy to estimate, but the way it changed could be sensed, and that was related to how close the aircraft came to the ship. Ease of measurement may have been over-estimated before the war because anti-aircraft practice was conducted against aircraft towing sleeve targets. The combination of tower and sleeve was quite long, and that probably made its apparent course relatively obvious. The problem did not become evident until the Royal Navy began experimenting with radio-controlled targets in the late 1930s.
This drawing of the Mk IVG director was used as a wall chart at HMS Excellent, the Royal Navy gunnery establishment. It was dated 31 March 1946. Mk IV was designed to work with 5.25in guns (in King George V class battleships and Dido class cruisers). Compared to earlier directors, it incorporated many more electrical circuits, many of them for Magslip data transmitters.
The trial system was tested on board Tiger in 1926.23 Instead of the AA Dumaresq and a deflection calculator, it featured a new graphic approach to calculating deflection which became characteristic of later versions of the HACT. Without any precise means of measuring angular rates, speed across (which gave horizontal deflection) could not be derived directly from measurement. Angle of presentation was far too imprecise. The unusual and elegant new approach used the image of a circular tilting ring projected onto a flat screen. The circular ring (deflection circle) represented all the points from which an aircraft flying at constant speed could reach the centre of the ring, the point at which it would meet a shell. The circle measured deflection, because a gun had to aim off by the angular distance between circle and centre to hit an aircraft flying from the circle.
As seen from a ship, the circle was tipped over at the sight angle at which the shell burst. Deflections were measured in a plane perpendicular to the line of sight (the presentation plane, which of course was not parallel to the deflection circle). This projected figure was nearly an ellipse, whose near (upper) side was slightly larger than its far side. The appropriate deflections, vertical and horizontal, were given by the point on the projected circle corresponding to that which the aircraft occupied at the outset, which in turn was set by the aircraft’s course (angle of presentation, the course projected onto the near-ellipse).
A Mk IV director is shown aboard the cruiser Ajax at New York Navy Yard, 16 October 1943. The antenna on top is for its Type 285 range-only radar.
The Mk V DCT was installed on board the later King George V class battleships (Duke of York, Howe and Anson) and the later fleet carriers (Implacable, Indefatigable and Indomitable, of which the first two had RPC for their 4.5in guns). There were two versions, of which Mk V* (for the carriers) had its rangefinder high rather than low.
The radius of the circle was defined by aircraft speed (u) and the time of flight of the shell (t). The circle could be imagined as the upper edge of a cone, the height of which was R, the range to the aircraft when the shell burst. R in turn was time of flight multiplied by the average projectile velocity (APV). The tangent of the half-angle which defined the deflection circle was u/APV. Given enemy speed (u), APV was found by positioning a pointer on an APV drum marked with curves indicating average shell velocity at various ranges. If the combination of u and angle of presentation seemed to be wrong, the table operator could change both until the director seemed to follow the target.
An arrow on the screen was set at the angle of presentation (in production HACTs it was generally controlled by the graticule in the control officer’s binoculars). That gave an estimated present position along the ellipse. Deflection was the angle between present and future positions – between the centre of the ellipse and the present position along the ellipse. It was split into vertical and lateral components using vertical and horizontal cross-wires moved by handwheels which in turn fed vertical and horizontal deflection into the table (the computer). Two operators moved the wires until they intersected at the estimated target position on the ellipse.
Range did not enter directly, although APV certainly depended on it. The elegant graphic technique was not exact. Errors were imposed by the fixed cone angle of the projection unit (about 18½°); by assuming that lines of equal angle on the screen were perpendicular to the axis of the ellipse; and by substituting present angle of presentation for future angle of presentation. Note that there was no direct feedback to correct u, which was an estimate.
HACS I used present sight angle to calculate deflection, meaning that the range-calculating part of the system was divorced from deflection. The appropriate input was future angle of sight, which was given by future range. The I* version (1930) substituted it (as derived from the integrator described below) in both the deflection unit and the own-course and speed gear. This connection made it impossible for the HACS I* to deal with a climbing or diving target.24
The trial installation seems not to have computed future range, but the production HACT incorporated an integrator which calculated it. To do that the computer needed both target speed and inclination, because for a target with any component of speed across (due to inclination) the speed along the line of sight varied with range because inclination (hence the components of speed across and along) varied with range. Although far too crude for calculating deflection, angle of presentation data seem to have been good enough for range calculations. Range in turn was needed both to set gun elevation in combination with vertical deflection and for fuse calculations (using a Hill predictor). For fuse calculation, the operator had to find the intersection between the range plot and a fuse curve. The curve of range against time up to the moment of firing had to be extrapolated to take account of dead time between