HACS



High Angle Control System (HACS) was a British anti-aircraft fire-control system employed by the Royal Navy from 1931 onwards and used widely during World War II. HACS calculated the necessary deflection required to place an explosive shell in the location of a target flying at a known height, bearing and speed.

Early history
The HACS was first proposed in the 1920s and began to appear on RN ships in January 1930, when HACS I went to sea in HMS Valiant. HACS I did not have any stabilization or power assist for director training. HACS III which appeared in 1935, had provision for stabilization, was hydraulically driven, featured much improved data transmission and it introduced the HACS III Table. The HACS III table (computer) had numerous improvements including raising maximum target speed to 350 knots, continuous automatic fuze prediction, improved geometry in the deflection Screen, and provisions for gyro inputs to provide stabilization of data received from the director. The HACS was a control system and was made possible by an effective data transmission network between an external gun director, a below decks fire control computer, and the ship's medium calibre AA guns.

Development
Operation

The earliest versions of the HACS could not generate true target motion, by computer prediction, through direct measurement of target speed, bearing, and altitude, so they were not "tachymetric" systems, and made the assumption that the target speed, direction and altitude would remain unchanged from the time of prediction until the fired shell reached the target, which was a flaw common to most pre-WW2 AA computers. Instead the HACS Mk I through Mk IV generated predicted target motion based upon estimates of target speed by target type, and target direction by aligning a binocular graticule with the target aircraft fuselage, and the measured values of target bearing, target range and target altitude. The HACS would use this information to create a predicted target motion or Range Rate (often called Rate Across in RN parlance), which is the apparent target motion across the line of sight. The predicted Range Rate was then used to move the High Angle Director Tower (HADT) UD4 Height Finder/Range Finder  prisms via electric motor so that the UD4 operator would see the target being held apparently motionless in his instrument eyepiece. Additionally, the HACS would generate the predicted bearing and elevation of the target and feed this information back to the Director via training and elevation indicators in the Director, or on later variants, the HACS could move the Director through Remote Power Control so that it could continue to track the target if the target became obscured. If the target had apparent movement, the UD4 operator would adjust the range/height and in so doing would correct the generated Range Rate, thereby creating a feedback loop which could establish an estimate of the target's true speed and direction, within the limits of optical ranging accuracy. Target ranging output also generated a paper plot of the range on the computer itself in the High Angle Calculating Position (HACP) located below decks, so that a range rate officer could access its accuracy. The HACS predicted target position for gun orders by modelling target position and movement using the "ellipse method of calculating deflection". A 2D analogue of 3D target position and direction was created by projecting an ellipse onto a ground glass screen. The shape of the ellipse would vary with target range, altitude and speed. The intersection of the ellipse and the target direction was used as a basis for calculating elevation and training of the guns. The ellipse method had the advantage of requiring very little in the way of mechanical computation and essentially modelled target position in real-time with a consequent rapid solution time. Information flow:

The HADT provides target direction, range, speed, altitude and bearing data to the HACP, which transmits direction and fuse timing orders to the guns. The HACP transmits the computer generated range rate and generated bearing back to the HADT, creating a feedback loop between the HADT and HACP, so that the fire control solution generated by the computer becomes more accurate over time if the target maintains a straight line course. The HADT also observes the accuracy of the resulting shell bursts and uses these bursts to correct target speed and direction estimates, creating another feed back loop from the guns to the HADT and thence to the HACP, again increasing the accuracy of the solution, if the target maintains a straight line course. Most guns controlled by the HACS had Fuze Setting Pedestals or Fuze Setting Trays where the correct fuze timing was set on a clockwork mechanism within the AA shell warhead, so that the shell would explode in the vicinity of the target aircraft.

Target drones

The HACS was the first Naval AA system to be used against radio controlled aircraft, and achieved the first AA kill against these targets in 1933. In March 1936, six Queen Bee targets were destroyed by the RN Mediterranean Fleet during intensive AA practice at a time of extreme tension between the UK and Italy. Target practice against target drones was done by using special shells which were designed to minimize the possibility of destroying expensive targets. The RN allowed media coverage of AA target practice and a 1936 Newsreel has footage of an actual shoot. In 1935 the RN also began to practice HACS controlled shoots of target aircraft at night.

Tachometric and radar additions

The RN moved quickly to add true tachometric target motion prediction and radar ranging to the HACS by mid 1941. The RN was the first navy to adopt dedicated FC AA radars. However the system, in common with all WW2 era mechanical AA fire control system still had severe limitations as even the highly advanced USN Mk 37 system in 1944 needed an average of 1,000 rounds of 5-inch ammunition fired per kill. In 1940 the Gyro Rate Unit (GRU) was added to the HACS system, an analogue computer capable of directly calculating target speed and direction, converting the HACS into a tachymetric system. Also in 1940, radar ranging was added to the HACS. The GRU and its associated computer, the Gyro Rate Unit Box (GRUB) no longer assumed straight and level flying on the part of the target. GRU/GRUB could generate target speed and position data at angular rates of up to 6 degrees per second, which was sufficient to track a 360 knot crossing target at a range of 2000 yards.

The Fuze Keeping Clock
RN destroyers were hampered by the lack of good dual-purpose weapons suitable for ships of destroyer size; for much of the war 40° was the maximum elevation of the 4.7 inch guns equipping such ships, which were consequently unable to engage directly attacking dive bombers, although they could provide barrage and predicted fire to protect other ships from such attacks. Destroyers did not use HACS, but rather the Fuze Keeping Clock (FKC), a simplified version of HACS. Starting in 1938 all new RN destroyers, from the Tribal class onwards, were fitted with a FKC and continuous prediction fuse setting trays for each main armament gun. WWII experience from all navies showed that dive bombers could not be engaged successfully by any remote computer predictive AA system using mechanical fuzes due to the lag time in the computer and the minimum range of optical rangefinders. In common with other contemporary navies, pre-war designed RN destroyers suffered from a lack of short range, rapid fire, AA with which to engage dive bombers.

The Auto Barrage Unit
The Auto Barrage Unit or ABU, was a specialized gunnery computer and radar ranging system that used Type 283 radar. It was developed to provide computer prediction and radar anti-aircraft fire control to main and secondary armament guns that did not have inherent anti-aircraft capability. The ABU was designed to allow the guns to be pre-loaded with time fused ammunition, and it then tracked incoming enemy aircraft, aimed the guns continuously to track the aircraft, and then fired the guns automatically when the predicted aircraft position reached the preset fuse range of the previously loaded shells. The ABU was also used with guns that were nominally controlled by the HACS to provide a limited blind fire capability.

Wartime experience
By May 1941, RN cruisers, such as HMS Fiji, were engaging the Luftwaffe with stabilized HACS IV systems with GRU/GRUB and Type 279 Radar with the Precision Ranging Panel, which gave +/- 25 yd accuracy out to 14,000 yds. HMS Fiji was finally sunk after running out of AA ammunition but her HACS IV directed 4-inch AA gun battery fended off Luftwaffe attacks for many hours. Demonstrating the RN's rapid strides in naval AA gunnery, in May 1941, HMS Prince of Wales (PoW) went to sea with HACS IVGB, with full radar ranging systems, and no less than 9 AA associated fire control radars: four Radar Type 285, one on each High Angle Director Tower (HADT) and four Radar Type 282, one on each Mk IV QF 2 pdr "pom pom" director, and a long range Radar Type 281 Warning Air (WA) radar which also had precision ranging panels for aerial and surface targets. This placed HMS Prince of Wales in the forefront of naval HA AA fire control systems at that time. In August and September 1941, HMS Prince of Wales demonstrated excellent long range radar directed AA fire during Operation Halberd. However, although the shortcomings of HACS are often blamed for the loss of Force Z, the failure of the anti-aircraft gunnery on intercepting the Japanese bombers were due to bizarre circumstances. The HACS was originally designed on Atlantic conditions on mind, not the tropics, and by December 1941, PoW's AA FC radars had become unserviceable due to the extreme heat and humidity in Malayan waters and her 2 pdr ammunition had deteriorated badly as well.

The RN made the following claims for ship borne anti-aircraft fire against enemy aircraft, from September 1939 up to 28 March 1941:
 * Certain kills: 234
 * Probable kills: 116
 * Damage claims: 134

The RN made the following claims for ship borne anti-aircraft fire against enemy aircraft, from September 1939 up to 31 Dec 1942:
 * Major warships (ships likely to have HACS or FKC fire control systems)
 * Certain kills: 524.
 * Probable kills: 183.
 * Damage claims: 271.


 * Minor warships and merchant vessels (most having no AA fire control systems)
 * Certain kills: 216.
 * Probable kills: 83.
 * Damage claims: 177.


 * Total kill claims: 740.
 * Total probable claims: 266.
 * Total damage claims: 448

Radar and the Mark VI Director
HACS used various director towers that were generally equipped with Type 285 as it became available. This metric wavelength system employed six yagi antennas that could take ranges of targets, and take accurate readings of bearing using a technique known as "lobe switching" but only crude estimates of altitude. It could not, therefore, "lock on" to aerial targets and was unable to provide true blindfire capabilities, which no other navy was able to do until the USN developed advanced radars in 1944 using technology transfers from the UK. This situation was not remedied until the introduction of the HACS Mark VI director in 1944 that was fitted with centimetric Radar Type 275. Another improvement was the addition of Remote Power Control (RPC), in which the anti-aircraft guns automatically trained with the director tower, with the necessary changes in bearing and elevation to allow for convergent fire. Previously the gun crews had to follow mechanical pointers that indicated where the director tower wanted the guns to train.

HACS systems in use or planned in August 1940
HACS Directors fitted to ships in a document dated as "revised Aug 1940":

HMS Ajax, Galatea, Arethusa, Coventry, HMAS Hobart, Sydney, Perth
 * HACS III: ABC transmission, AV cradle for 15 ft HF/RF. Introduced Mk III table.

HMS Penelope, Southampton, Newcastle, Malaya, Hood*, Australia*, Nelson*, Royal Sovereign*, Barham*, Resolution*, Cairo*, Excellent (gunnery training school)*, Revenge*, Calcutta*, Carlisle*, Curacoa*, Exeter*, Adventure*, Warspite*. Ships marked with * had roll stabilization for layer.
 * HACS III*: Similar to MarkIII but with larger windscreen and space for a rate officer.


 * HACS III*G as mark III but fitted with GRU and roll stabilization for the layer. (probably mentioned as planned upgrades for previous Mk III directors).


 * HACS IV: Similar to MkIII but with circular screen, magslip transmission and roll stabilization for the layer. Introduced Mk IV table.

HMS Birmingham, Sheffield, Glasgow, Aurora, Liverpool, Manchester, Gloucester, Dido, and Fiji classes, Forth, Maidstone, Renown, Valiant, Illustrious, Formidable and Ark Royal.

Dido class and Fiji classes.
 * HACS IV G: Mk IV with Gyro rate unit.
 * HACS IV GB: Mk IV and fitted with GRU and complete stabilization in laying and training, Keelavite system of power training.

HMS King George V and Prince of Wales, Dido and Fiji classes.


 * HACS V: Improved design, partially enclosed, complete stabilization for elevation and training. Keelavite system of power training, and GRU. Duplex 15 ft HF/RF.  Uses Mk IV table.

HMS Duke Of York, Anson and Howe.

HMS Indomitable, Implacable and Indefatigable.
 * HACS V* :As Mk V but single HF/RF and raised HF/RF compared to Mk V.