A STORY OF LOCATORS, LIGHTS, & ACK-ACK|
WW II searchlights formed part of a system of aircraft detection linking (a) locator devices, (b) searchlights, and (c) antiaircraft (AAA) guns. The locators sent electronic information to the lights and guns, which in turn tracked the target in synch with each other. Once a locator of any of the aforementioned types had "locked on" to an aerial target, the concept was for both lights and guns to be trained on the target (via the height and distance data received from the locator) so the target could be nearly simultaneously illuminated and then destroyed. Locators were first based on sound and heat detection, and ultimately radar became the preferred method of target acquisition. Units were generally separate, but advances in radar technology late in the war saw the integration of radar into both searchlight and AAA gun designs. Antiaircraft artillery accuracy was at stake, both from tactical and economic points of view. In 1940, in England, for example, it took an average of 20,000 rounds of ammunition to down a single enemy aircraft! The demand for more accurate methods of engaging, tracking, and destroying aircraft, especially at night, was driven by the need to destroy more targets without expending lots of ammunition.
So just how did each component of the searchlight system work back in WW II?.
THE SOUND LOCATOR
Before radar, the first practical means of detecting airplanes at a distance at night was by listening to the noise of their engines with the aid of horns. The size of the horns served to gather in more of the sound and thus increased the range of detection. The spacing of the horns aided the operator's binaural sense in determining the plane's direction. The photo at left shows one of the last types of sound locator units used during World War II. One operator listened to the left and right horn for direction information, and the other operator listened to the top and bottom horns for elevation information. As the operators pointed this large hearing device, their direction and elevation movement would generate electric signals that would be sent to the searchlight control station or to the searchlight directly, using a selsyn system.
The photo at right shows an pre-war sound locator that probably dates from around 1935. It featured four sound-collecting horns (although square-edged), and like the unit above, two men were required to operate it. One operator listened to the left and right horns for direction information, and the other operator listened to the top and bottom horns for elevation information. These units had been in use by the Army since the early 1920s, were phased out in the late 1930s, and replaced with the conical, rounded horns. These latter locators had a very short lifespan since the outbreak of hostilities in 1939 led to more aggressive advances in detection, including radar. While units like the 225th trained on the late model sound locator for a brief period in 1942, the bulk of their training for war was carried out using radar to direct their searchlights.
THE HEAT DETECTOR
Heat detectors like those pictured at right were also used to locate enemy aircraft in the late 1930s. Resembling a searchlight drum, this device was designed to sweep the sky and detect the heat emitted from the engines of overflying aircraft. Thermopiles and control equipment designed by the U.S. Army Signal Corps Laboratories were mounted in a mechanical structure built by the General Electric Company and mounted on a trailer. Heat detectors relayed directional data to the on-board radio-control equipment and, via cable connections, to a searchlight. Tests conducted by the Coast Artillery Corps in 1936-37 showed that while adequate in detecting ships, heat detectors were inadequate for aircraft detection, and more emphasis was placed on developing radio-beat and pulse-echo methods (the forerunners of radar) for use by the fledgling antiaircraft branch.
THE RADAR LOCATOR
As the war and technology progressed, radar was added to the list of locator devices used to point searchlights to light up enemy aircraft. In the photo at left, the searchlight in the foreground is connected to the SCR-268 gun-laying radar unit behind it. Signals from the radar unit relayed to the light greatly increased the accuracy of the light beam. By the time the 225th began its training in 1942-43, the combination of the SCR-268 and the 60-inch searchlight was the primary method of detecting, tracking, and illuminating aerial targets for the U.S. Army antiaircraft artillery.
The SCR-268 was considered a searchlight radar from the outset, a short-range, height-finding unit expressly designed for fixed antiaircraft defenses such as coastal batteries or other static positions. The method for using the SCR-268 would be to use it to pick up the airplanes at night and to synchronize the radar plot with a searchlight through an already developed gun director. The director performed the basic mathematical function of taking the range and angle data out of the radar and aimed the searchlight in that direction. At the appropriate moment, when range and angle to target were known, the contoller would order the searchlight turned on. At that momemnt, the target was illuminated and it could be engaged by guns. A side benefit was that the pilot would be blinded. It was also advantageous to wait as long as possible to turn on the light since the longer the beam remained on, the more vulnerable the light and crew was to retaliatory fire.
THE SEARCHLIGHT CONTROL STATION
The searchlight control station was operated by three men (as in the photo at right). It was used to aim the searchlight by hard-wired remote control. The control station was placed several hundred feet away from a searchlight so that the controllers could see illuminated aircraft better (since the light beam was so bright, vision would be adversely effected if the control station and its operators set up too close to the searchlight; all that would be visible would be the cone of illuminated atmospheric particles in the path of the beam). The controllers were situated far from the light for another reason: safety. Searchlights made easy targets.
Electric signals from the sound, heat, or radar locators were sent to the searchlight control station using a selsyn-type system. The signals went to the zero-locator meters that were on top of the station. Two of the operators monitored the zero-locator meters (there was one for direction and one for elevation) and were charged with keeping the meters on a zero reading using hand cranks on the side of the unit (in the photo above, the men standing to either side of the unit are performing this function). Keeping these meters set at zero kept the control station and searchlight in synch with the locator device (sound, heat, or radar locator) in terms of the direction and elevation readings of the aerial targets.
The third man on the controller crew was the observer (see photo at left). He wore a special harness over his head that physically connected him to the controller and allowed him to look through the controller's binoculars. As the controller was aimed by the other crew-members monitoring the zero-locator meters, the observer was forced to visually look at the point where the target was computed to be (in the same direction and elevation that the locators were aimed at). Once the observer was able to spot the enemy aircraft in the binoculars, he would then take full control of the station using the crank controls located in front of him. These controls were directly connected to the same crank controls the two other operators used to aim the station. Using the control station, he would now send selsyn signals to the searchlight so it would track the observer's actions. The observer could also throw a switch on the control station so the searchlight could accept selsyn control signals directly from the locator device, whether it was a sound, heat, or radar locator.
Finally, the signals from the control station or location-detection device are sent to the searchlight. In the photo at right, the searchlight operator is busy adjusting the light. His duties would also include maintaining all aspects of the light and the generator that supplies the power to it. The operator throws the switch to start the arc burning when commanded to do so by the control station operator. He ensures that the carbons that create the arc are burning correctly and that the beam is focused. In the event of a communication failure, he is able to control the light by using a long, 10-foot-long rod with a wheel on the end. With this rod, he can walk the searchlight in the direction he wants and turn the wheel to raise or lower the light to the elevation he wants. The 10-foot distance gives the operator a better view of the object since he's somewhat out of the direct path of the beam.
THE GUNS, THE MEN, AND THE TACTICS
What was the final piece of the puzzle in U.S. efforts to put all of these technologies together and integrate them tactically? If one had to point to one moment, it surely had to be May 26, 1937. That evening, Mr. Harry Woodring, the Secretary of War, stood with a group of officers and civilian scientists on a field of the U.S. Army Signal Corps Laboratories at Fort Monmouth, New Jersey. Before them, spread over the field, were the transmitter and receivers of a radio detector, the SCR-268 prototype. Connected with the radar were the controls of a standard antiaircraft searchlight. As prearranged, an Army aircraft flew over the field in the darkness. Three enlisted men, viewing oscilloscopes, put the radar "on target" and tracked it across the sky. When tracking was firmly established, the command was given to light the searchlight. As the light pierced the darkness, the aircraft, caught in the beam, became visible to those on the ground. In this experiment lay the nucleus of the basic antiaircraft artillery tactics that would be practiced for the next eight years. Eventually, three radar sets the SCR-268, SCR-270, and SCR-271 were ready for use by the time America entered WW II. These sets formed by the backbone of antiaircraft detection throughout the war, and were deployed in every major theater of war. And it was up to the U.S. Army to train thousands of men to master both the equipment and the tactics, which gave birth to the Antiaircraft Artillery branch of the armed services.
At the beginning of World War II the U.S. antiaircraft artillery force was very much the poor stepchild of the Coast Artillery Corps. The units were mostly three-battalion (a gun battalion, an automatic weapons battalion, and a searchlight battalion) regiments and separate battalions. They were equipped with a motley mix of obsolescent 3-inch guns and single-barrel, water-cooled, .50-caliber machine guns. The German Blitzkrieg in Europe forced a widespread re-evaluation of the Army's AAA capability and, beginning in 1940-1941 a vast expansion of the arm (it finally achieved an identity separate from the Coast Artillery in 1943). On September 30, 1942, it was proposed that 811 AAA battalions be organized (with a total strength of 619,000 men).
However, this massive buildup of AAA units became largely redundant when another formerly poor relation of the U.S. Army, the Army Air Corps, wrested command of the air from the Luftwaffe in 1943 and 1944. Many AAA battalions were disbanded to provide replacements in 1944, and some were converted to artillery. A total of 258 battalions were inactivated or disbanded between January 1, 1944 and May 8, 1945. Nevertheless, AAA remained a strong component of the army and achieved something of resurgence in late 1944 in Belgium, defending Antwerp from the threat of the V-1 buzz bombs. On December 31, 1944, there was still a total of 347 AAA battalions (with 257,000 men) active in the Army.
In 1943 the AAA regiments were broken up into separate battalions, with the regimental H&H companies becoming new AAA Group Headquarters. The AAA battalions were organized as either gun (equipped with the M1 90-mm AA gun) or automatic weapons (equipped initially with a U.S.-designed M1 37-mm gun, but later almost wholly re-equipped with the famous M1 40-mm Bofors-designed gun, and with the M51 or M55 quad-mount .50-caliber machine gun). Battalions were further classified as mobile (that is, towed), self-propelled (utilizing halftrack-mounted guns, the M16, a quad .50-caliber mounting, and the M15, a combination mounting twin water-cooled, .50-caliber machine guns and a single 37-mm gun), or semi-mobile (with a reduced number of prime movers, designed for the defense of static installations the 225th was designated as semi-mobile). In the photo above, the crew of an M51 "Quad Fifty" scans the low-ceilinged winter sky for attacking German aircraft (photo courtesy U.S. Army Military History Institute).
The automatic weapons battalions of all types were organized with four firing batteries, lettered A to D, an H&H Battery, and a Service Battery. Each battery nominally contained eight towed 40-mm or 37-mm self-propelled guns and eight quad .50-caliber towed or self-propelled machine guns. However, many slight variations existed, some battalions had batteries composed of eight towed 40-mm guns, four quad .50-caliber mounts, and eight single, water-cooled .50-caliber machine guns (there was a shortfall in production of the M51 and M55 mounts). Gun battalions were organized identically, except the batteries were equipped with four 90-mm guns each and, usually, three water-cooled, .50-caliber machine guns.
Normally, an AAA automatic weapons battalion was attached to each division, self-propelled if attached to an armored division and mobile if attached to an infantry division. A corps normally had one or more AAA groups attached. Each AAA group consisted of two or more automatic weapons battalions (usually mobile), although a gun battalion was occasionally attached. In the European Theater, gun battalions were more frequently found in AAA groups attached to the army or army group. Antiaircraft brigades were also formed and were normally attached to armies or to theater commands. In addition, the IX Air Defense Command (in effect, an AAA division, originally organized as a part of the U.S. Ninth Tactical Air Force), with an average of 10 to 15 gun and automatic weapons battalions, formed a powerful AAA reserve for the U.S. 12th Army Group in Europe.
USE YOUR MOUSE TO FIND THE ENEMY BOMBER IN THE SEARCHLIGHT BEAM!
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