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{{Short description|British anti-aircraft fire-control system}}
{{Use dmy dates|date=October 2020}}
{{Use British English|date=November 2024}}
[[File:HMS Scylla 1942 IWM FL 2932.jpg|thumb|300px|Anti-aircraft cruiser {{HMS|Scylla|98|6}}.  Her four twin [[QF 4.5-inch Mk I – V naval gun|4.5-inch gun]] mountings are controlled by the two High Angle Director Towers, one sited behind the bridge and the other abaft the after funnel.]]

'''High Angle Control System''' ('''HACS''') was a British [[anti-aircraft]] [[fire-control system]] employed by the [[Royal Navy]] from 1931 and used widely during [[World War II]]. HACS calculated the necessary [[Deflection (ballistics)|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 [[Royal Navy]] (RN) ships in January 1930, when HACS I went to sea in {{HMS|Valiant|1914|6}}.<ref>Weapon Control in the Royal Navy 1935-45, Pout, from The Application of Radar and other Electronic Systems in the Royal Navy in WW2, p87 (Kingsley-editor)</ref> HACS I did not have any stabilisation or power assist for director training. HACS III which appeared in 1935, had provision for stabilisation, was hydraulically driven, featured much improved data transmission and it introduced the HACS III Table.<ref>Appendix one, Classification of Director Instruments, see external links.</ref><ref>Weapon Control in the Royal Navy 1935-45, Pout, from ''The Application of Radar and other Electronic Systems in the Royal Navy in WW2'', p87,99 (ed Kingsley)</ref> The HACS III table (computer) had numerous improvements including raising maximum target speed to {{cvt|350|kn}}, continuous automatic fuze prediction, improved geometry in the deflection Screen, and provisions for gyro inputs to provide stabilisation of data received from the director.<ref>HACS III Operating Manual Part 1, paragraph 2 (a-o)</ref> 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 [[anti-aircraft]] (AA) guns.

==Development==
===Operation===
[[File:HACS deflectionscreen in operation.jpg|thumb|right|HACS deflection screen and [[List of British ordnance terms#Table|table]] with the deflection screen operator in the foreground. The solid line through the centre of the ellipse shows the wire aligned with the aircraft's course (moving right to left) of approximately 295 degrees; the Deflection Screen operator has his right hand on the Lateral Deflection Control which is aligning the vertical line with the ellipse, and the aircraft track wire, and he is also using his left hand to align the Vertical Deflection Control and a horizontal wire, (which cannot be seen) so that it also intersects the aircraft track wire at the edge of the ellipse.]]

The bearing and altitude of the target was measured directly on the ''UD4 Height Finder/Range Finder'', a [[coincidence rangefinder]] located in the ''High Angle Director Tower'' (HADT). The direction of travel was measured by aligning a [[Binoculars|binocular]] [[Reticle#Graticule|graticule]] with the target aircraft fuselage. The early versions of HACS, Mk. I through IV, did not measure target speed directly, but estimated this value based on the target type. All of these values were sent via [[selsyn]] to the HACS in the ''High Angle Calculating Position'' (HACP) located below decks.<ref>''British Mechanical Gunnery Computers of World War II'', Bromley, p19</ref> The HACS used these values to calculate the ''range rate'' (often called ''rate along'' in RN parlance), which is the apparent target motion along the line of sight. This was also printed on a paper plot so that a range rate officer could assess its accuracy.<ref>The Gunnery Pocket Book, B.R. 224/45, 1945, paragraphs 429-431. See external links</ref>

This calculated range rate was fed back to the UD4 where it powered a motor to move prisms within the UD4. If all the measurements were correct, this movement would track the target, making it appear motionless in the sights.<ref>HACS III Operating Manual Part 2, paragraph 300,301</ref><ref>HACS III Operating Manual Part 1, paragraph 2 (a)</ref> If the target had apparent movement, the UD4 operator would adjust the range and height, and in so doing would update the generated range rate, thereby creating a feedback loop which could establish an estimate of the target's true speed and direction.<ref>Weapon Control in the Royal Navy 1935-45, Pout</ref><ref>British Mechanical Gunnery Computers of World War II, Bromley, p22</ref> The HACS also displayed the predicted bearing and elevation of the target on indicators in the Director tower, or on later variants,<ref>''Naval Weapons of World War Two'', Campbell, p. 30</ref> the HACS could move the entire Director through [[List of British ordnance terms#RPC|Remote Power Control]] so that it could continue to track the target if the target became obscured.<ref>HACS III Operating Manual Part 2, paragraph 174 (a)</ref>

The angle measured by the graticule also caused a metal wire to rotate around the face of a large circular display on one side of the HACS, known as the ''Deflection Display''. The measured value of altitude and range, and estimated value of target speed, caused optics to focus a lamp onto a ground glass screen behind the wire, displaying an ellipse whose shape changed based on these measures. The deflection operator used two controls to move additional wire indicators so they lay on top of the intersection of the outer edge of the ellipse where it was crossed by the rotating metal wire.<ref>The Gunnery Pocket Book, B.R. 224/45, 1945, paragraphs 424. See external links.</ref> 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.<ref>British Mechanical Gunnery Computers of World War II, Bromley, p19, figure 18</ref>

===Information flow===
[[File:The Royal Navy during the Second World War A3883.jpg|thumb|left|HADT on {{HMS|King George V|41|6}}. The control officer is shown looking through his binoculars, while the rangetaker's face is hidden.]]
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.<ref>British Mechanical Gunnery Computers of World War II, Bromley</ref> 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 [[Target drone|radio controlled aircraft]], and achieved the first AA kill against these targets in 1933.<ref>Unmanned Aviation, Newcombe, p47</ref> In March 1936, six [[Queen bee (aircraft)|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.<ref>The British Defence of Egypt, 1935–40, Morewood.P70-71</ref> Target practice against target drones was carried out using special shells which were designed to minimise the possibility of destroying expensive targets.<ref>UK Hansard, Queen Bee Aircraft (shooting practice). HC Debate07 June 1939 volume 348 column 427W: <!-- ''"Mr. Perkins: asked the Parliamentary Secretary to the Admiralty whether he will state the total number of Queen Bee aircraft shot down on 22nd May during the practice shoot by "Nelson", "Rodney", "Sheffield" and "Ark Royal," respectively, and the total number of shells fired by all ships at the Queen Bee during that afternoon? Mr. Shakespeare: 509 rounds of innocuous shell were fired in the practice referred to by my hon. Friend. This shell is specially designed to avoid damage to an expensive Queen Bee target, which was not shot down on the occasion in question."'' --></ref><ref>''The Naval Review, A Misleading Success'', July 1978, Jones, Basil, p254:<!-- ''"I feared that the target would be shot down before we could have a go, since we were sixth ship in the line; although the general use of 'practice' shell, whose small bursting charge and fine fragmentation had been ordered, reducing the possibility of lethal damage to the aircraft."  ''The author was the Gunnery Officer aboard HMS Achilles, and the target shoot occurred in the Spring of 1935. --></ref> The RN allowed media coverage of AA target practice and a 1936 [[newsreel]] has footage of a shoot.<ref>[https://www.youtube.com/watch?v=MSPmr-7MLrc HACS Target practice video]</ref> In 1935 the RN also began to practice HACS controlled shoots of target aircraft at night.<ref>ADM 186/339: Progress in naval gunnery, 1914-1936, p132. See external links</ref>

===Tachometric and radar additions===
[[File:HACS MK IV Table on board HMS Duke of York.jpg|thumb|High Angle Control System fire control computer (table) Mk IV, aboard {{HMS|Duke of York|17|6}}. The Deflection operator is seated in front of the deflection screen. The range plot operator stands directly opposite.]]
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 World War II-era mechanical AA fire control system still had severe limitations as even the highly advanced [[United States Navy]] (USN) [[Ship Gun Fire Control Systems|Mk 37 system]] in 1944 needed an average of 1,000 rounds of {{convert|5|in|mm|adj=on|0}} ammunition fired per kill.<ref>Naval Weapons of WW2, Campbell, P106. This data in turn was based upon USN wartime AA kill claims, and so probably overstates the system's effectiveness.</ref> In 1940 the [[Gyro Rate Unit]] (GRU) was added to the HACS system, an [[analogue computer]] capable of directly calculating target speed and direction,<ref>HACS III Operating Manual Part 1, paragraph 56,61</ref> converting the HACS into a [[tachymetric]] system.<ref>Weapon Control in the Royal Navy 1935-45, Pout, p104, from The Application of Radar and other Electronic Systems in the Royal Navy in WW2 (Kingsley-editor)</ref><ref>''British Battleships of World War Two'', Raven & Roberts, p378: <!--''"The design of this last item, known as the GRU was begun in 1937, and was intended for use in the HACS, although it was also fitted in pom-pom and barrage directors. By means of a [[gyroscope]], it could measure vertical and lateral movements of a target which, when provided with radar ranging, provided a fast and accurate system of fire-control".'' --></ref> Also in 1940, radar ranging was added to the HACS.<ref>Weapon Control in the Royal Navy 1935-45, Pout, p97</ref> 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 {{convert|360|kn|adj=on|lk=in}} crossing target at a range of {{convert|2000|yd}}.<ref>Naval Weapons of WW2, Campbell, pp. 17–18</ref>

==The Fuze Keeping Clock==
{{main|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 [[QF 4.7-inch Mk IX & XII naval gun|{{convert|4.7|in|mm|adj=on|0}} guns]] equipping such ships, which were consequently unable to engage directly attacking [[dive bomber]]s, although they could provide "barrage" and "predicted fire" to protect other ships from such attacks.<ref>Tribal Class Destroyers, Hodges, p27</ref> Destroyers did not use HACS, but rather the [[Fuze Keeping Clock]] (FKC), a simplified version of HACS.<ref>''Naval Weapons of WW2'', Campbell, p19</ref> Starting in 1938 all new RN destroyers, from the {{sclass2|Tribal|destroyer|4||1936}} onwards, were fitted with a FKC and continuous prediction fuse setting trays for each main armament gun.<ref>Tribal Class Destroyers, Hodges, p27</ref> WWII experience from all navies showed that dive bombers could not be engaged successfully by any remote computer-predictive AA system using mechanical fuzes<ref>Summary of USN and RN gunnery reports. For example, the USS ''Enterprise'' reported her Mk 33 AAFC system as "ineffective" against dive bombers. USS ENTERPRISE CV6/A16-3/(10-My)</ref><ref>US Destroyers-An Illustrated Design History, Friedman, p203: <!-- "In theory, the 5in gun could counter either horizontal or torpedo bombers; it could not fire nearly fast enough to present any threat to dive bombers, which, ironically, were probably the most lethal threat to fast manoeuvrable craft such as destroyers." --></ref> due to the lag time in the computer and the minimum range of optical rangefinders.<ref>HACS III Operating Manual Part 2</ref> 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 specialised gunnery computer and radar ranging system that used [[List of World War II British naval radar#Type 283|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.<ref>[https://maritime.org/doc/br224/part4.htm#par446 The Gunnery Pocket Book, paragraph 448]</ref> The ABU was also used with guns that were nominally controlled by the HACS to provide a limited blind fire capability.<ref>Campbell, ''Naval Weapons of World War Two'', p16.</ref><ref>Victor Humphries, HMS Renown WWII: "I was a radar RP3 rating on board HMS Renown during WW2. During which time I operated the ABU, which was very reliable; the set itself was not very big, the CRT being about 9 inches in diameter. The screen showed two green lines, the top one straight across the screen and the lower line had about a 1/4-inch step which was moved across the screen with a steering wheel situated below and to the left, which had a smaller steering wheel attached to it so that on moving the larger steering wheel to keep the step steady on the left side of the echo. Once I was satisfied that the aircraft was moving in at a steady speed, I could LOCK on with the small steering wheel and the stepped line would close in with the echo. There was an indicator on the screen at 5000 yards; when the aircraft was inside this range I could see by a green light that all guns were loaded, cocked, and ready to fire; all I then had to do was depress a foot pedal which fired ten 4.5-inch guns, either on the port or starboard sides. If the aircraft echo produced a downward echo every so many seconds (the number of seconds decided just before the operation) it would indicate IFF = identification friend or foe, which meant it was our own aircraft, if there was no IFF and the pilot did not break radio silence he could be shoot down. We also had long range aircraft warning Radar Type 281, which would warn of approaching enemy aircraft 150 miles away, and at WW2 aircraft speeds we had time to prepare. The types 273 & 284 are another story."</ref>

==Wartime experience==
By May 1941, RN cruisers, such as {{HMS|Fiji|58|6}}, were engaging the Luftwaffe with stabilised 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 sunk in the [[Battle of Crete]] after running out of AA ammunition but her HACS IV directed 4-inch AA gun battery fended off Luftwaffe attacks for many hours.<ref>''The true experiences of Mr Leonard Charles Eades during the Second World War'', from the HMS Fiji Association.</ref>

Demonstrating the RN's rapid strides in naval AA gunnery, in May 1941, [[HMS Prince of Wales (53)|HMS ''Prince of Wales'']] went to sea with HACS IVGB, with full radar ranging systems, and nine AA associated fire control radars: four [[Type 285 radar]], one on each ''High Angle Director Tower'' (HADT) and four [[List of World War II British naval radar#Type 282|Type 282 radar]], one on each [[Pom-Pom director|Mk IV director]] for the [[QF 2 pounder naval gun|QF 2 pdr (40mm) "pom pom" mounts]], and a long range [[Type 281 radar]] Warning Air (WA) radar which also had precision ranging panels for aerial and surface targets.<ref>''The Development of Radar Equipments for the Royal Navy'', Kingsley, p383.</ref> 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]].<ref>''The Royal Navy and the Mediterranean Convoys''. A Naval Staff History, p26</ref> Although the shortcomings of HACS are often blamed for the loss of [[Force Z]], the scope of the Japanese attack far exceeded anything the HACS had been designed to handle in terms of aircraft numbers and performance.  The failure of anti-aircraft gunnery to deter the Japanese bombers was also influenced by unique circumstances. The HACS was originally designed with Atlantic conditions in mind and ''Prince of Wales''{{'}}s AA FC radars had become unserviceable in the extreme heat and humidity in [[British Malaya|Malayan]] waters and her 2-pdr ammunition had deteriorated badly as well.<ref>''Battleship: The Loss of the Prince of Wales and the Repulse'', Middlebrook</ref>

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<ref>March, ''British Destroyers'', p. 434.</ref>

The RN made the following claims for ship borne anti-aircraft fire against enemy aircraft, from September 1939 up to 31 Dec 1942:<ref>United States Navy, ''AntiAircraft Action Summary, July 1942 to Dec 1942 (Information Bulletin No. 22)'', pp. 281–282.</ref>
* 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]]s 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 [[List of World War II British naval radar#Type 275|Type 275 radar]]. Another improvement was the addition of Remote Power Control ([[List of British ordnance terms#RPC|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.<ref>Weapon Control in the Royal Navy 1935-45, Pout, pp. 105-106.</ref>

==HACS systems in use or planned in August 1940==

HACS Directors fitted to ships in a document dated as "revised Aug 1940":<ref>Appendix one, Classification of Director Instruments.</ref>
* HACS III: ABC transmission, AV cradle for 15&nbsp;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''*, [[HMS Excellent (shore establishment)|''Excellent'' (gunnery training school)]]*, ''Revenge''*, ''Calcutta''*, ''Carlisle''*, [[HMS Curacoa|''Curacoa'']]*, ''Exeter''*, ''Adventure''*, ''Warspite''*. Ships marked with * had roll stabilisation for layer.
* HACS III*G as mark III but fitted with GRU and roll stabilisation for the layer.
* HACS IV: Similar to MkIII but with circular screen, magslip transmission and roll stabilisation 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.
:{{sclass|Dido|cruiser|4}} and {{sclass|Fiji|cruiser|4}}es.
* HACS IV GB: Mk IV and fitted with GRU and complete stabilisation 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 stabilisation for elevation and training. Keelavite system of power training, and GRU. Duplex 15&nbsp;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.<ref>Uses FKC AAFC table according to Campbell.</ref>
:HMS ''Indomitable'', ''Implacable'' and ''Indefatigable''.

==See also==
* [[Arthur Pollen#Fire control|Argo clock]]
* [[Ship gun fire-control system]]
* [[Stabilisierter Leitstand]]

==References==
{{reflist|30em}}

==External links==
* [http://www.britishpathe.com/record.php?id=22589 Newsreel video of HACS controlled guns in action] {{Webarchive|url=https://web.archive.org/web/20110611093241/http://www.britishpathe.com/record.php?id=22589 |date=11 June 2011 }}
* [http://www.britishpathe.com/record.php?id=12701 More Newsreel footage of HACS guns engaging high level bombers] {{Webarchive|url=https://web.archive.org/web/20110605231710/http://www.britishpathe.com/record.php?id=12701 |date=5 June 2011 }}
* [http://www.navweaps.com/index_tech/tech-111.php HACS: A Debacle or Just-in-Time?)]
* [http://archive.hnsa.org/doc/br224/img/dia28.jpg Illustration of the HACS Deflection Screen]
* {{usurped|1=[https://web.archive.org/web/20110525081545/http://www.godfreydykes.info/Directors%20Various%20Part%20I.pdf Appendix one, Classification of Director Instruments]}}
* {{usurped|1=[https://web.archive.org/web/20110525081606/http://www.godfreydykes.info/HACS%20III%20AND%20IIIC%20PART%20ONE.pdf HACS III Operating manual Part 1]}}
* {{usurped|1=[https://web.archive.org/web/20110525081647/http://www.godfreydykes.info/HACS%20III%20AND%20IIIc%20PART%20TWO.pdf HACS III Operating manual Part 2]}}
* [http://www.cv6.org/ship/logs/action19421026.htm USS Enterprise Action Log]
* [http://maritime.org/doc/br224/index.htm ''The Gunnery Pocket Book'', B.R. 224/45, 1945]
* [https://www.angelfire.com/ri/georgev/hmsfijiphotos4.html The true experiences of Mr. Leonard Charles]
* [http://jproc.ca/sari/285_system.jpg HACS III*G with Type 285 Radar and the MkIII table]
* [https://web.archive.org/web/20160305194851/http://sydney.edu.au/engineering/it/research/tr/tr223.pdf British Mechanical Gunnery Computers of World War II]
* [https://web.archive.org/web/20120401233556/http://www.admirals.org.uk/records/adm/adm186/adm186-339.pdf Progress in Naval Gunnery, 1914–1936]
* [https://www.world-war.co.uk/scylla_story.php Memoirs of Sub-Lieutenant Robert Hughes, Gunnery Control Officer on HMS Scylla]

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[[Category:Military computers]]
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[[Category:Fire-control computers of World War II]]
[[Category:Military equipment introduced in the 1930s]]