DEMONSTRATION OF AN ELECTRICAL DIRECTOR FOR
ANTI-AIRCRAFT GUNS
BELL TELEPHONE LABORATORIES
MURRAY HILL, N.J., NOVEMBER 9, 1943
 

WHAT

     The M9 Electrical Gun Director aims an anti-aircraft gun and sets the shell's fuse so that the burst will occur within lethal distance of a hostile aircraft. It differs from other directors, which employ mechanical movements, in using electrical circuits to make its calculations.
 

WHO

     In Bell Telephone Laboratories, Dr. David B. Parkinson conceived the idea of ack-ack control through an electrical computer. With his associate Dr. Clarence A. Lovell he showed on paper that it could be done.

     National Defense Research Committee, an organization which has the benefit of some of the most famous scientists in the United States, allocated funds to the Laboratories to build a model; then

     U.S. Army Ordnance Department, after endorsement by the Anti-aircraft Artillery Board, collaborated with the Laboratories in the final design and gave an order to

     Western Electric Company, affiliate of the Laboratories, to go ahead. As manufacturer of such complicated things as dial telephone systems, Western Electric had the know how necessary to make electrical directors. Numbers of these instruments have been turned over to the Army.

     An epoch-making development in Army ordnance was shown to newsmen and others (yesterday) when the Army and Bell Telephone Laboratories jointly demonstrated the Electrical Gun-Director on the Laboratories Grounds at Murray Hill, New Jersey. As an Army plane overhead simulated a bombing run, observers noted that four guns of an anti-aircraft battery were following the plane, without manual operation by their gunners, and that they were trained, not at the plane but at a point ahead of and above it. Seated at a sizable metal box mounted on a pedestal, observers with eyes glued to telescopes kept the cross-hairs trained on the plane. Both telescopes turned vertically; and also along with the box and the
observers, they turned horizontally. By another instrument, the plane's distance was measured. After the plane had been "tracked" for a few seconds, the fire-control officer gave the command "Fire!"; the shells for each gun were pulled out of their individual fuse-setters, where their burst had been timed under the control of the Director; immediately and automatically rammed home and the breeches closed. To the Army's regret, the guns were not fired but if they had been the shells would have traveled on a path converging with that of the plane, and would have exploded within lethal range of it. Officers who had seen the set-up used against a towed target said that instead of an occasional hit, the target sleeve was invariably torn to ribbons by a salvo of four guns.

     How the Electrical Director works can be explained by starting with the artilleryman's problem of aiming a gun at a fixed visible target, say a captive balloon. The gunner must know the direction and distance of the target, and also the muzzle velocity of the shell, the direction and velocity of the wind, and the density of the air. Using ballistic tables, he finds a direction and elevation to point the gun, and the setting of the fuse. These tables, converted into electrical terms for a particular gun, are built into the Electrical Director, so that if its telescopes and range finder are pointed at a fixed target, the battery is automatically aimed in the right direction.
 

     If however the target is an airplane in flight, consideration must be given to the distance and course which it will travel during the several seconds while the shell itself is in flight. As the telescopes follow the plane, the Electrical Director gathers information from which it predicts just where its target will be when the projectile reaches it, assuming that the plane flies a straight course as precision bombers must. The Director then selects, with the aid of its built-in ballistic tables, a direction and angle of fire, and a fuse setting, which bring the shell to its rendezvous with the plane and explode it there.
 

     In the art of ranging on a moving object, the Electrical Director represents a considerable advance in several particulars. For one, it has solved the difficult problem of compensating for the errors introduced by the human element. There is a natural tendency for the observers to permit the target to stray from the cross-hairs in the telescopes. Realizing that this has occurred, the observer attempts a quick recovery which may be interpreted as a change in the speed of the target. If this information were used, the gun would quickly change its lead to fit the false rate of speed. Engineers of the Bell Telephone Laboratories have long been familiar with electrical networks for telephone systems, so they invented a brand new one by which the momentary fluctuations are averaged out, and the Electrical Director considers only the smooth rate reported over a period of several seconds.
 

     Another characteristic of electrical apparatus which is particularly fortunate in wartime is the possibility of building it to high accuracy with ordinary shop personnel and production methods.
 

     Strangely enough, this Electrical Director originated in a dream. One night in the middle of 1940, while the Low Countries were being invaded, Dr. David B. Parkinson awoke with the impression that he had been a member of an anti-aircraft gun crew, which was bringing down a plane with every shot. Somehow he knew that in the gun control mechanism was an electrical device somewhat like something he had been working on in Bell Telephone Laboratories. Next morning he talked to his associate, Dr. Clarence A. Lovell; neither of them knew anything about guns, ballistics or fire control, but
the problem caught their fancy. It has always been a basic principle in Bell Laboratories that a promising idea should be
explored, so along with their other work Parkinson and Lovell studied the problem and how telephone techniques could be applied to it. A possible solution, in memorandum form, was communicated to the Navy and the Army; the Anti-aircraft Artillery Board promptly endorsed the venture and the O.S.R.D. agreed to finance it. A week before Pearl Harbor, a model had been delivered to the Army for test.

     That model had been visualized by Lovell, Parkinson and their research associates, and the design had grown in the hands of skilled instrument makers working under direct supervision. When production had been authorized by the Army, it was necessary to translate the model into designs which would fit into the Army's field requirements and which could be made by Western Electric shop facilities. Apparatus development engineers, already collaborating on the project, undertook a large amount of mechanical and electrical design looking toward smaller size, greater accessibility and ruggedness, and the solution of certain :functional difficulties.
 

     Western Electric Company, manufacturing organization of the Bell Telephone System, builds these Directors. A satisfactory device of this kind requires not only good engineering design but also the application of the techniques of manufacturing necessary to produce a product which will stand up in field service. Here the long experience of the Western Electric Company in making telephone equipment for reliable performance in the Bell System has been applied to making these Directors dependable units in field service.

     No small part of the credit for the final achievement is due to Western Electric's subcontractors who shouldered those jobs for which they had facilities. Four companies played a leading part: International Harvester and Liquid Carbonic undertook to make the tracker; Teletype Corporation the altitude converter - a device which interprets the indications of an optical height finder; and Union Special Machine Company made some of the extremely accurate gearing.

     It was necessary that the Army, in adopting a new device such as this, should provide for adequate training of its people in handling the equipment. Here, the Laboratories were able to assist the Ordnance Department by providing training for about 350 Army people. Instructors in this school have been engineers drawn from the Laboratories staff.
 

     An impressive feature of the show was the presence of some 300 draftsmen, scientists, clerks, engineers and mathematicians of the Laboratories. Many of them for the first time saw a full-scale demonstration of the apparatus of the apparatus on which they had worked for three years. Congratulating them on their achievement, General Campbell said: "You have performed nobly, as part of the little-known force of technical workers in the Army and out, who have produced for your fighting men an arsenal such as no army in the world has at its command. I want you to know that the safe return of your husbands, brothers and sweethearts depends on no small measure on the weapons such as this splendid one which you have designed".
 

Information Department
A.T.& T. Company
195 Broadway, New York

M9 GUN DIRECTOR VS. ROBOT BOMBS

     How the Electrical M9 Gun Director, having made a reputation for bringing down German planes, was called on to help overcome the menace of the buzz-bombs was the story brought back by Clarence A. Lovell and R. R. Rough of Bell Telephone Laboratories from a recent trip to England and France.
 

     American batteries equipped with 90 mm. guns and M9 Directors were regularly bringing down 76 per cent of all the buzz-bombs which passed through their area. In Normandy, planes were brought down with anywhere from 12 to 36 shots, and in one case, three shots fired from only a single gun bagged the Nazis. The M9 was developed by Bell Laboratories and built by Western Electric.

     "When I arrived in England early in July", said Dr. Lovell, "The British were in the process of revising and strengthening their anti-robot defenses. With the earlier set-up, airplanes had the first go at the robots. Only the fastest pursuit planes could over-take them; but in spite of that, the planes were bringing down about 35 per cent of all those which crossed the coast line. Heavy A.A. guns took care of 10 per cent, lighter guns and balloons of a few more, so that about half reached their target - London. While my party was in France, the heavy guns were moved forward to the coast line, with most of the fighter planes behind them; a score of American battalions equipped with our Electrical Directors were mixed in with the British; and then the gunners went to town. On one typical day, 143 flying bombs reached the coast line. Artillery bagged 65, the R.A.F. got 35, the balloons brought down 17, and only 23 got through. Out of 7 that came across in a single flight, the guns got five. When you consider that the target is traveling at 350 to 450 miles per hour and is only 17 feet wide, and that the range was on the average as far as from the Battery to the Empire State Building, that is really some shooting. Some nights when the firing was heavy, the gun barrels glowed cherry-red."
 

     The procedure, according to Dr. Lovell, was this. Almost immediately after a bomb left the launching site, its position was plotted on a big chart, much like the ones set up in the United States for aircraft spotting. In a few seconds, its course was evident and word was flashed to any aircraft off-shore that might be near enough to be useful. Batteries in its path were also alerted, picked up the target on their Director telescopes and as soon as it came within range, they opened fire. Sometimes the bomb exploded on the first salvo, sometimes it got past; if so, the patrolling planes in the rear area went after it. Sometimes it was "wounded" - the control mechanism was knocked out, and the bomb circled crazily or dashed off to sea again. One dramatic deed was still being talked of in a British battery which Dr. Lovell visited. A British pilot had winged his bird, which started on a long glide directly for that battery. The pilot sped up alongside, maneuvered his wing under the bomb's wing, tipped it up and "rode it off" to a course which carried it clear of the battery, when he let go. The bomb exploded harmlessly in a field. Said one of the British girls who was "manning" their director, "I grabbed my helmet and, as I ran, wondered where to put that helmet so it would do the most good ."
 

     To study the anti-aircraft batteries in France, Dr. Lovell and his companion, R. R. Rough, crossed in a ship, landed with their bedding rolls in a barge, and rode a truck some miles inland to a headquarters. While they were bedding down, a German plane crashed and burned a few hundred yards away. Next day they were introduced to an officer - a former entomologist - whose assignment was to investigate and catalog every enemy plane brought down. From him they learned that from D-day to the end of June the heavy A.A. artillery brought down a quarter of all the planes they engaged. The score for July, while incomplete, was practically as good. All this shooting was at night since the Allied air cover was so complete that the Germans did no daytime flying. Usually the target was hit in anywhere from 8 to 36 shots although in one classic example, when the battery was not alerted in time, only one gun fired, and in three rounds brought down the target. All of this heavy artillery was equipped with Electrical Directors developed by Bell Telephone Laboratories, New York.
 

     While in France, Dr. Lovell had an opportunity to see a robot launching site. It had been badly mauled by our air force but it was quite evident that a launching was quite an event. The flying bomb was brought into a non-magnetic room, where the compass was set for the course to be flown. It was then rolled out on a car and placed on the launching ways - rails about 8 ft. apart and 60 feet long. Some sort of rocket was used to bring the car and bomb up to a speed where the bomb would take off under its own power. A stabilizing device would make it climb to a predetermined altitude and the compass would direct it on its course. The heavy concrete protection for personnel with narrow slots covered by heavy glass, and blast marks here and there, showed that launching was attended by real danger to the launchers.

     On their return to England, the engineers revisited the anti-robot batteries, and visited certain military laboratories. They saw a mechanical director captured from the Germans which impressed them as of competent design and workmanship. They were told it gave "excellent" results under certain conditions, but lacked the flexibility of the Electrical M9 Director.

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