HACS

British anti aircraft fire control system

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 required to place an explosive shell in the location of a target flying at a known height, bearing and speed.

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 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 anti aircraft (AA) guns.

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 binocular 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. 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. 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. 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. The HACS also displayed the predicted bearing and elevation of the target on indicators in the Director tower, or on later variants, the HACS could move the entire Director through Remote Power Control so that it could continue to track the target if the target became obscured. 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. 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.

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.

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 carried out 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.

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) Mk 37 system in 1944 needed an average of 1,000 rounds of 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 crossing target at a range of .

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 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 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 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.

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 Type 275 radar. 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.