HACS - AbsoluteAstronomy.com

HACS, an acronym of High Angle Control System, was a British

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

anti-aircraft fire-control system

Fire-control system

A fire-control system is a number of components working together, usually a gun data computer, a director, and radar, which is designed to assist a weapon system in hitting its target. It performs the same task as a human gunner firing a weapon, but attempts to do so faster and more...

employed by the Royal Navy

Royal Navy

The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

from 1931 onwards and used widely during World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

. HACS calculated the necessary deflection

Deflection (military)

Deflection is a technique used for effectively firing a ranged weapon at a moving target, that describes "leading the target"; that is, shooting ahead of a moving target so that the target and projectile will collide...

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 HACSI went to sea in HMS Valiant

HMS Valiant (1914)

HMS Valiant was a Queen Elizabeth-class battleship of the British Royal Navy. She was laid down at the Fairfield Shipbuilding and Engineering Company, Govan on 31 January 1913 and launched on 4 November 1914...

. HACSI did not have any stabilization or power assist for director training. HACSIII which appeared 1937, 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

Tachymetric

A tachymetric anti-aircraft fire control system refers to a method of generating target position, speed, direction, and rate of target range change, by computing these parameters directly from measured data....

" 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

Binoculars

Binoculars, field glasses or binocular telescopes are a pair of identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes when viewing distant objects...

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
Coincidence rangefinder
A coincidence rangefinder is a type of rangefinder that uses mechanical and optical principles to allow an operator to determine the distance to a visible object....

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

Target drone

A target drone is an unmanned, remote controlled aerial vehicle, usually used in the training of anti-aircraft crews.In their simplest form, target drones often resemble radio controlled model 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 drone

Target drone

s 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

Ship Gun Fire Control Systems

Ship gun fire-control systems enable remote and automatic targeting of guns against ships, aircraft, and shore targets, with or without the aid of radar or optical sighting. Most US ships destroyers or larger employed GFCS for 5 inch and larger guns, up to battleships such as the USS Iowa...

in 1944 needed an average of 1,000 rounds of 5-inch ammunition fired per kill. In 1940 the Gyro Rate Unit

Gyro Rate Unit

-History:The Royal Navy, after World War I, became increasingly concerned with the threat posed by aerial attack. In 1930 the RN began equipping ships with the High Angle Control System, a non-tachymetric anti-aircraft fire control system, that would compute the gun laying orders and the time fuze...

(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

Destroyer

In naval terminology, a destroyer is a fast and maneuverable yet long-endurance warship intended to escort larger vessels in a fleet, convoy or battle group and defend them against smaller, powerful, short-range attackers. Destroyers, originally called torpedo-boat destroyers in 1892, evolved from...

size; for much of the war 40° was the maximum elevation of the 4.7 inch gun

4.7 inch gun

4.7 inch Gun may refer to a number of British-built 120 mm naval guns, and others:*QF 4.7 inch Gun Mk I - IV British naval gun deployed on cruisers 1888 to 1918, also as a field gun in World War I...

s equipping such ships, which were consequently unable to engage directly attacking dive bomber

Dive bomber

A dive bomber is a bomber aircraft that dives directly at its targets in order to provide greater accuracy for the bomb it drops. Diving towards the target reduces the distance the bomb has to fall, which is the primary factor in determining the accuracy of the drop...

s, 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

Fuze Keeping Clock

The Fuze Keeping Clock was a simplified version of the Royal Navy's High Angle Control System analogue fire control computer. It first appeared as the FKC Mk1 in destroyers of the 1938 Tribal class, while later variants were used on sloops, frigates, destroyers, aircraft carriers and several...

(FKC), a simplified version of HACS. Starting in 1938 all new RN destroyers, from the Tribal class

Tribal class destroyer (1936)

The Tribal class, or Afridi class, were a class of destroyers built for the Royal Navy, Royal Canadian Navy and Royal Australian Navy that saw service in World War II...

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"

QF 2 pounder naval gun

The 2-pounder gun, officially designated the QF 2-pounder and universally known as the pom-pom, was a 1.575 inch British autocannon, used famously as an anti-aircraft gun by the Royal Navy. The name came from the sound that the original models make when firing...

director

Pom-Pom Director

-History:The Vickers 40mm "Pom-Pom" Antiaircraft mounting was introduced to the Royal Navy in the early 1930s. The mounting was capable of a tremendous volume of fire but the crew had great difficulty in aiming the mounting due to the smoke and vibration created by the guns...

, 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

Operation Halberd

-Summary:During World War II, Operation Halberd was a British naval operation in September 1941 to escort a convoy from Gibraltar to Malta....

. However, by December 1941, PoW's AA FC radars had become unserviceable due to the extreme heat and humidity in Malayan

British Malaya

British Malaya loosely described a set of states on the Malay Peninsula and the Island of Singapore that were brought under British control between the 18th and the 20th centuries...

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 March 28 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 Dec 31 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 antenna

Yagi antenna

A Yagi-Uda array, commonly known simply as a Yagi antenna, is a directional antenna consisting of a driven element and additional parasitic elements...

s that could take ranges of targets, and take accurate readings of bearing using a technique known as "lobe switching

Lobe switching

Lobe switching is a method used on early radar sets to improve tracking accuracy. It used two slightly separated antenna elements to send the beam slightly to either side of the midline of the antenna, switching between the two to find which one gave the stronger return, thereby indicating which...

" 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":

HACS III: ABC transmission, AV cradle for 15 ft HF/RF. Introduced Mk III table.

HMS Ajax, Galatea, Arethusa, Coventry, HMAS Hobart, Sydney, Perth

HACS III*: Similar to MarkIII but with larger windscreen and space for a rate officer.

HMS Penelope, Southampton, Newcastle, Malaya, Hood*, Australia*, Nelson*, Royal Sovereign*, Barham*, Resolution*, Cairo*, Excellent (gunnery training school)*, Revenge*, Calcutta*, Carlisle*, Curacoa

HMS Curacoa

Four ships of the Royal Navy have been named HMS Curacoa, after the island in the Caribbean Sea more usually spelled Curaçao:, a 36-gun fifth rate launched in 1809. She was reduced to 24 guns in 1831 and broken up in 1849., a wood screw frigate launched in 1854...

*, Exeter*, Adventure*, Warspite*.
Ships marked with * had roll stabilization for layer.

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.

HACS IV G: Mk IV with Gyro rate unit.

Dido class

Dido class cruiser

The Dido class was a class of sixteen light cruisers built for the Royal Navy. The design was influenced by the Arethusa class light cruisers. The first group of three ships was commissioned in 1940, the second group and third group were commissioned in 1941–1942...

and Fiji classes.

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.

HACS V* :As Mk V but single HF/RF and raised HF/RF compared to Mk V.

HMS Indomitable, Implacable and Indefatigable.

External links

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