The air defence system of Great Britain relies on a reporting service, fighter aircraft and anti-aircraft guns and balloon barrages
OPERATIONS ROOM AT FIGHTER COMMAND. Here information gleaned from the various reporting systems is plotted on the large scale map on the centre table. The controller, seated in the gallery, overlooks the map and by means of radio telephone he is able to guide fighters into visual contact with the enemy.
THE air defence system in Great Britain is the most highly developed organization of its kind in the world. Briefly it relies upon: first, a reporting service; second, fighter aircraft; and third, anti-aircraft guns and balloon barrages.
Britain’s secret weapon, “radiolocation”, is one-half of the reporting system; the Royal Observer Corps is the other. Radiolocation warns of enemy approach; the duty of the R.O.C. is to keep track of the movements of the enemy over this country. This information is required for two main purposes, first to enable fighters to be sent to the right position to intercept the raiders, and second, to enable air-raid warnings to be issued to districts in advance of the direction in which the raiders are proceeding.
The general system is that of forming a network of observer posts over the area to be observed and to connect the posts by direct telephone to centres where track is kept of aircrafts’ movements.
In charge of each post there is a head observer; an observer group officer with three assistants is responsible for the supervision of each group of posts.
A centre controller with one assistant looks after the administration, training and efficiency of his crews; about twenty-five men are on duty in a centre at a time. A centre with all its associated posts is known as an observer group and there is an area commandant with two deputies who take charge of each seven observer groups. The headquarters of the Royal Observer Corps is at Fighter Command.
In each fighter group operations room there is an R.O.C. liaison officer always on duty who is in direct telephone communication with every observer centre within the fighter group area. He receives regular reports from each one of these centres regarding all events that have been reported by posts, such as types of aircraft seen, the dropping of bombs and flares, aircraft in distress and so on. He passes these immediately to the operations officer or to the intelligence officer in the fighter group operations room.
At each centre there is a centre room, with a plotting table on which is fitted a squared map, round which are seated the plotters, each with a small switchboard connecting him by direct fine to three posts. Each plotter has a tray containing distinctive counters required for plotting tracks and indicating the number and height of any aircraft reported. Walking round the table is the table supervisor watching the tracking, receiving reports from the plotters of information, other than numbered square plots, given by posts. He has a supply of various lettered and numbered symbols, from which he can select the appropriate ones which have to be placed against the leading plotting counter of each separate track.
KEEPING TRACK OF ENEMY RAIDERS. Plotters at a Royal Observer Corps centre during the Battle of Britain. They are each connected by telephone to three observer posts, and each has a tray of counters for plotting the tracks of enemy aircraft. Overlooking the table, on a raised dias, are the duty crew controller, and the tellers who pass and receive information from adjacent centres.
Overlooking the centre table, on a raised dais, are the duty crew controller, the “teller” passing information to R.A.F. operations rooms, another “teller” passing to and receiving from adjacent centres various items of information, telephonists connected both to the Observer Corps liaison officer at fighter group and to the liaison staff in sector operations room, and the “recorder” who keeps a record of the tracks appearing on the centre table. On this dais is also an alarm officer giving information to selected factories and aerodromes of the movements of enemy aircraft which pass within range.
In order to indicate how the whole system operates let us assume that a centre in a coastal observer group has received information that an enemy raid is approaching over the sea. The raid is allotted a symbol which the table supervisor immediately places in a small move-able stand on the edge of the centre table at the position at which the enemy aircraft are expected to cross the coast.
The plotters connected to the coastal posts warn the crews what to expect. The two men of each post are scanning the sky, No. 1 has set the height bar of one of the instruments used to the height given from the centre. One of the two makes use of binoculars. When the enemy aircraft are seen No. 1 estimates their height and readjusts the height bar, if necessary. He then adjusts the sights until they are on the aircraft and says “On”. No. 2 looks at the pointer of the instrument and reports to the centre the appropriate square, the number of aircraft, their height and their direction of flight. After passing his message, he amplifies it later by specifying the type of aircraft and any other information which may be of value. No. 1 keeps his sights on the aircraft and says “On” at frequent intervals. No. 2 reports appropriate squares as the pointer of the instrument moves over them.
The other two of the three posts which are connected to the same plotter at the centre overhear these reports and when possible obtain a correct height which they pass to the plotter at the centre. As the raid proceeds, the plotter at the centre may order one post to cease reporting and another nearer one to carry on. When the raid has passed out of range of the original posts, adjacent posts will have picked it up and the next plotter at the centre will continue the tracking, and so on.
As soon as a plotter receives a plot he places a counter on the appropriate square. Alongside he places a numeral counter to indicate the number of aircraft and another to indicate their height in thousands of feet. The table supervisor ensures that the raid symbol is started off at the head of the raid when first plotted and that it is turned so that it can easily be seen by those on the dais.
Immediately the first plotting counter is placed on the table the tellers on the dais begin to “tell” to the R.A.F. operations rooms concerned. In addition, information is passed over the liaison lines regarding the type of aircraft or any other information of importance. The recorder makes a pencil record of the track on a small scale chart and the alarm controller watches the track carefully so as to be ready to warn particular areas.
The duty controller supervises the work of the whole crew and answers any special queries from the R.O.C. liaison officer in the fighter group operations room. As the raid crosses the centre table, the inter-centre teller informs the appropriate adjacent centre of its movements so that all are ready in the adjacent centre to carry on the track when it crosses the observer group boundary.
The tracking by night or in cloudy weather can be continued with almost equal accuracy, but the type, number and height cannot of course be given. Not only can the R.A.F. authorities responsible for interception and the issue of air raid warnings see the picture but the other services and civilian departments interested can also be kept informed.
DUTY CREW AT AN OBSERVER POST. Observer posts are manned night and day by two men. No. 1 ascertains the height of the aircraft by means of the instrument shown above, while No. 2 makes reports to the centre as to the number of aircraft, their height, and the direction in which they are flying.
The work of the Corps referred to above may appear to be simple, and easy to apply, but in reality it is much more complicated than has hitherto appeared. In the example given, one enemy raid came in over the coast and was tracked and passed from centre to centre without difficulty. The problem is entirely different when four or five hundred enemy aircraft fly in, split up into small formations to attack different targets and at the same time are being intercepted by our fighter squadrons. It is no longer a simple matter of keeping track of a raid flying on a steady course: “dog fights” are taking place at altitudes where the aircraft can be heard but not seen. Squadrons are wheeling, diving, climbing, and the difficulties of the observers at the posts and the centres are enormously increased. Yet every endeavour must be made to maintain the tracks of the enemy, and in order that confusion shall not arise the tracks of our own aircraft must also be plotted. At night when enemy aircraft are streaming in large numbers across the country singly but close to one another the difficulty of keeping separate tracks of each can be realized.
It is not permissible even yet to discuss the principles of “radiolocation”. Suffice it to say that, as the word implies, it is a radio means of giving observers accurate information of the movement of aircraft. By this new and highly intricate method of detection it is possible for skilled controllers on the ground to guide defending fighters to within visual distance of attacking bombers.
Information gleaned by means of the reporting system is plotted in operations rooms at Fighter Command, fighter groups and stations. Operations rooms are furnished with a large scale map on a table, around which stand telephone operators with their combined headphones and microphones. They are permanently in touch with units of the reporting system, and by means of movable counters they transfer the information received by telephone diagrammatically to the map. The counters are designed to reveal the positions, numbers, heights and courses of aircraft. The officer in control of operations sits at his desk on a raised platform giving him a good view of the map.
Round the room are indicators showing the number of aircraft available for operations, and the “state of readiness” of squadrons. A word of explanation is perhaps necessary here. Squadrons must have their rest periods sometime in the twenty-four hours, and to provide for these and at the same time to allow for sufficient aircraft with crews to be available for operations, a system has been adopted in which the following abbreviations are used: “Released” means that a squadron is off duty till a certain time; “Available,” that a squadron is ready to come to stand-by at short notice; “Readiness,” that a squadron is able to come to stand-by in about five minutes, though in practice it often happens that a squadron in readiness is ordered to take off immediately; and “Stand-by,” that a squadron is able to take off within the shortest possible time.
At the controller’s side is a microphone, by means of which he can talk to the fighter squadrons, whether on the ground or in the air. Also there are telephones to headquarters as well as to adjoining groups and stations. In most operations rooms there will also be found liaison officers whose duty it is to keep the anti-aircraft defences accurately and quickly informed of the movements of both enemy and friendly aircraft, of the height of balloon barrages.
Let us imagine ourselves to be actual onlookers standing beside the controller at a fighter station, when suddenly one of the telephonists near the map slides a small frame containing counters of various colours with numbers on each, to a particular part of the map. If we were in one of the more senior headquarters action would be taken on this to give certain districts an air raid warning. We at a fighter station, however, are engrossed with the fact that this is a daylight raid approaching the area for whose defence the station is responsible.
A glance at the squadron state of readiness board shows the controller which squadrons are most readily available, and he at once warns them to stand by, or even to take off immediately, thus allowing them to climb towards the incoming enemy. The enemy’s height is shown on the operations table, and a quick calculation into which wind speeds, distances, speed and rate of climb of our fighters enter, shows when and where interception is likely to occur.
The squadron in the most advanced state of readiness already has its aeroplanes to the leeward side of the aerodrome, ready to take off into wind, with engines warmed up at regular intervals — depending on the time of year. The pilots are near their machines, resting in shelters, with flying kit on, ready to leap aboard their aircraft at a second’s notice.
Now our fighters are off the ground and climbing hard. Information is sent to the fighter leader by radio telephone from the operations room telling him of the enemy’s movements, the movements of other fighter squadrons, and the course to fly to intercept the enemy. The fighter leader controls his formation by radio telephone and it is thus possible to plot his position by direction-finding wireless, and, at the same time, to indicate his course also, from moment to moment, on the operations room map.
The draughtsman is busy transferring information from the main map to his tracing; the anti-aircraft officers are telephoning to their batteries; and other squadrons are being brought to stand-by in case it is necessary to reinforce those already in the air. Soon other raids begin to reveal their nearness on the map. We see that the original raid has suddenly turned back towards Northern France before it penetrated inland. It was only a small raid of, say, three aircraft, but coming in now from another direction the counters indicate a formation of at least thirty-six. Here is something for our fighters to get their teeth into, and the controller decides to leave the small formation alone, and to concentrate on the larger one. From the times of their previously plotted positions the speed of the larger formation can be estimated and another quick calculation by the controller gives him the course which his fighters should steer to intercept.
Every one’s attention is now riveted on the counters on the table, it is like a race game played on a dining-room table. The enemy counters alter course, another lightning calculation and the controller’s voice gives the fighter leader another change of course. The counters indicating our fighters get close to the enemy — surely they must be in sight by now — no, a glance at the weather board shows scattered cloud. The fighter leader reports that he has been joined by a second, and yet a third squadron. The controller sighs in relief; he has been able to concentrate a superior force of fighters at the right time, place and height to meet the enemy. At last “enemy sighted” comes through and then a set of crisp, simple orders from the fighter leaders to their flight and section leaders. A wealth of' experience, knowledge of his pilots, tactics, lightning appreciation of the enemy’s position and his own, and positions of the sun and clouds influence these orders. After the fight the leaders inform their units and are given courses to steer to bring them back to their home aerodromes.
If we were to pay a visit to a typical searchlight position we would see a curious device mounted on a four-wheeled trailer with a couple of men operating it. It is a sound locator, and consists of a number of sound-reflecting elements like very large motor car headlight reflectors mounted so as to swing up and down and also round in a circle, so that it can be aimed at any point in the heavens. Sound waves entering a locator are reflected from the inner surfaces and become concentrated at a point in the centre of the back of the device, whence they are drawn off by telephones placed on the heads of the operators.
ELEMENTS OF A SOUND LOCATOR . The instrument swings round a point X. One man listens out on mirrors A and B and swings the locator round until he gets maximum sound. A second man operates mirrors C and D and swings them up and down.
The elements of a locator are grouped as shown in the diagram shown here. The device swings round the point X. One man listens out on A and B mirrors and swings the instrument round till he gets the sound loudest in A and b. As he is listening to the engine of a rapidly moving aircraft he may have to keep moving the arm A-B round continually to keep the sound loud in his telephones, so that he follows the course of the aircraft by its sound.
A second man operates the mirrors C and D and swings them up and down. It is clear then that by reading off the positions on scales to which the locator is pointing the direction of an aircraft in the sky can be ascertained. By calculating where this direction line crosses that from another locator which is aimed at the same aircraft, its actual position may be known.
High performance of modern aircraft has rendered the operation of sound locators increasingly difficult. With radiolocation, however, the directing of search-lights and anti-aircraft gunfire is becoming increasingly quick and accurate.
Consider an aircraft at 20,000 feet approaching at a speed of 240 miles per hour. From 20,000 feet sound takes about seventeen seconds to reach the ground and as in that time the aircraft has travelled forward 1,994 yards, or a little over a mile it follows that the sound locator will be directed at a point that distance behind the aeroplane. This, of course, has to be allowed for in giving directions to searchlights or guns.
Few people in England are unfamiliar with the appearance of searchlight beams. The light is produced by means of what is known as an electric arc-lamp, which, briefly, has the advantage of producing a brilliant centre of light. Instead of being dissipated in all directions this light is concentrated by means of an optically designed reflector and shot into the sky as a single beam from a source of some millions of candle-power. Searchlights are nearly all mobile and, with their self-contained generating sets, can be quickly moved from place to place.
The searchlights can be turned round and brought up and down so that the beam can be directed at any point in the sky. The scales on the two axes of the light are marked in degrees as are those on the sound locator, so that any readings on one can rapidly be applied to the other. The sound locator operator simply passes his readings, corrected to allow for enemy speed, to the searchlight crews, who set the information on their scales and switch on. If all goes well the beam immediately illuminates the aircraft.
This simple explanation may perhaps make it clear as to how the problem of sound location increases when the sky is full of aircraft, making it very difficult for the operators to keep track of any one individual aeroplane. There are also the various interruptions caused by anti-aircraft barrage, high winds, and any local noises near the sound locator.
The antidote to the searchlight from the bomber’s point of view is to paint the raiding aircraft with black, lustreless paint, to shroud the engine exhaust system so that no red hot pipes or flames are visible and to cut down the cockpit and interior lighting to the bare minimum necessary for navigation and flying. The light likely to be shown by or reflected from a bomber is thus reduced to something very small indeed. Night glasses may assist in distinguishing it, however, and, again, searchlights may illuminate the enemy sufficiently to enable one of our night fighters to identify and attack it. Various attempts to overcome this difficulty of seeing our bombers from the ground have been tried by the Germans, and light blue, pink, orange and red searchlights have been seen on various occasions over enemy territory.
The actual A.A. gun positions consist of well camouflaged and well protected emplacements spaced some distance apart with attendant range finders, a predictor and telephones. There is an officer in charge who plots the incoming raids on a large map on the operations table, on information which is continually coming in by telephone. When enemy raiders have approached near enough the searchlights switch on and the battery observers begin to watch through special night glasses for signs of the enemy.
A.A. Battery
The problem the battery has to solve is that of trying to hit an object roaring through the air on a course, speed and height, all of which may be altered at any moment at the whim of the pilot, so as soon as the enemy aircraft is sighted by the battery, the officer in charge has to utilize what information he has, or can obtain, as to course, speed and height in order to calculate the point in space at which to fire his guns, so that the shell and the aircraft shall, if possible, arrive at that point simultaneously. One can no more than assume that the enemy will follow a particular course. If he is not being fired on, or if he is approaching an important objective, however, it is probable that he will continue at constant speed on a straight and level course.
There is then the flight of the shell to consider. It naturally takes time to reach the place where it is designed to meet the aircraft, and during its flight it is subject to effects of wind and temperature. On the assessment of these factors depends the fuse setting, which determines the time between the firing and the bursting of the shell. If all the multitudinous calculations thus involved had to be done without some quick mechanical means, the aircraft would be well out of range before the gun could be fired. The predictor delivers the answer not only at once, but also continuously while it is directed on the enemy.
Balloon barrages also play an important part in protecting important objectives on land and even out at sea. The balloons are made of special fabric and are filled with hydrogen gas which, being lighter than air, causes the balloon when inflated to rise, drawing with it the mooring cable. It is this cable which constitutes the danger to aircraft. Contact with it is almost certain to damage the aeroplane sufficiently to bring it down, either crashing or in a forced landing.
The balloon is so designed that if there is a wind, increased lift is obtained, the balloon acting as a kite. It is therefore possible to fly a barrage at greater heights on reasonably windy than on calm days.
Captive balloons are prone to other weather effects such as thunderstorms, when they may act as excellent lightning conductors. If struck by lightning, the balloon is usually set on fire and becomes a total loss. Snow may freeze on the top surface of the balloon and reduce performance owing to the increased weight.
To a casual observer it might appear that owing to the spaces between the balloons in the barrages, there would be plenty of room for aircraft to turn so as to avoid coming in contact with the cables. It is true that on days when there is no cloud, the balloons may be visible to an enemy pilot and he might dive into the barrage to launch his attack, but once within the barrage he might find it extremely difficult to avoid the not easily discernible balloon cables as turns at high
speed with fast modern monoplanes cannot be carried out in a small space.
But on days when there is a layer of cloud, the balloons will not be visible and it would be a foolhardy act indeed to fly into the barrage area, for it would be impossible to see the balloon cables until too late to avoid them. In these conditions, or at night, it is the knowledge that the balloon barrage exists at a given locality that prevents the pilot going down so that he can sight his target on the ground. In other words, the barrage has affected the pilot’s morale.
This brings us to the second reason for the barrage, and that is to prevent the enemy coming lower than a given height — that is, the top of the barrage. Thus low flying attacks on targets such as cities and war factories are prevented.
The maximum height to which the barrage can be raised cannot, of course, be divulged, but the greater the height, the greater the weight of cable which has to be lifted. All sorts of tricks can be played with it to outwit and mislead the enemy, such as placing balloons at varying heights, moving balloons up and down during the alert periods, and changing the concentration of balloons.
The enemy has tried out many devices to overcome the danger of hitting balloon cables. One idea which the Germans experimented with was a device to fend off the balloon cable. It was fixed as shown in Fig. 8, but detracted considerably from the aircraft’s performance. Britain had also experimented with various cutting devices on the leading edge of the aircraft’s planes.
The weapons of attack used by aircraft consist of bombs, torpedoes, mines, and fire with shell-firing guns and machine guns. Bombs have high explosive or incendiary fillings. The high explosive bomb consists of metal body filled with explosive. The effect is designed to be produced either by disruptive effect of blast, which is caused by the sudden release of a large quantity of gases under high pressure when explosion occurs, or through the damage caused by fragments.
Britain’s Big Bombs
The blast of a bomb has most effect in a confined space, and thus it is essential for the bomb to penetrate well into the target if maximum disruptive effect is desired. Britain has produced the largest and most effective bomb yet used in this war. It weighs 8,000 lb., is about seven and a half feet long and two and a half feet in diameter. No open space could be found in Britain for testing it, so widespread is the effect of its blast.
Let us consider for a moment the types of high explosive bombs now in use. First of all we have anti-personnel bombs designed to break up when the explosion occurs into the greatest possible number of fragments big enough to produce casualties over a large area. Then there are “general purpose” bombs, which strike a balance between blast and fragment effect. These bombs are not designed to stand up to being dropped on very hard targets, such as the armoured deck of a battleship, and for this purpose we have semi-armour piercing and armour piercing bombs.
INSIDE A BOMB STORE. The R.A.F. keep their bombs safely stored away underground. When they are required they are loaded into trolleys and hauled to the aircraft. The men seen above are getting a consignment of 500-lb. bombs out of store ready for a big raid on objectives in Germany.
The first of these have heavy bodies which disintegrate into large fragments. They can penetrate light armour plate and multi-storied buildings, and can be relied upon to reach their maximum depth of penetration into the target without breaking up. The armour piercer is, of course, designed to penetrate well into heavily armoured vessels when dropped from a great enough height.
Among other recent types are the special bombs which have been produced for use against submarines. They contain large charges of high explosive, and, like the depth charge, give an intense pressure wave in water which will cause the hull of a submarine to leak if the explosion is close enough to it.
Incendiary bombs usually depend on phosphorus or magnesium compounds suitably ignited for their effect. They are of various sizes and for convenience are carried in special containers fitted to the bomb cells of the aeroplane, from which they can be scattered over large areas. There are also gas bombs which consist of easily breakable containers filled with gas in liquid form. It was always laid down that poison gas would never be dropped by the R.A.F. unless the enemy first made use of it against us.
Now a word as to the fusing of bombs. The fuse is a device by which the explosive charge in the bomb is made to explode. The explosion may be desired instantaneously when the bomb hits the ground, or after very short intervals of time from small fractions of a second up to several seconds or after long periods, running into days or even weeks. The fuses are fitted either in the nose or tail of the bomb, sometimes in both places, depending on the type of bomb. Nose fusing is usually chosen for instantaneous action and tail fusing for delay functioning. Experiments carried out against specially constructed targets, and results of experience in operations help the Air Staff to decide on the type of bomb and fusing best suited to the particular type of target to be attacked.
Magnetic mines dropped from aircraft do not have cables and sinkers, or anchors attached to them; they remain lying on the bottom of the sea in which they are dropped. They have to be rendered insensitive to the impact of the water when dropped, but to become extremely sensitive afterwards.
Another weapon which is used by aircraft is the torpedo. Perhaps of all air weapons the torpedo has proved the most startlingly effective in this war. The most striking examples were afforded by the attacks on the Italian fleet at Taranto and Matapan, the disablement of the Bismarck, and the sinking of the Prince of Wales and Repulse by the Japanese.
The torpedo dropped by aircraft is very similar to the ship’s torpedo; it is slightly more robust as it has to stand being dropped into the sea from an aircraft perhaps flying at 250 miles per hour at a low height of up to 150 feet. It runs a distance considerably shorter than the ship’s torpedo, at a speed of up to forty-five land miles per hour. Its use is a tremendous test of the courage of the crew of the aircraft as release has to be close to the target. The dropping aircraft has to take rapid evasive action after releasing the torpedo so as to avoid the heavy concentration of fire from the ship’s guns.
Qualified pilots of the Fleet Air Arm undergo a course of intensive training in the technique of torpedo attack at the Royal Naval Air-Torpedo School somewhere along the coasts of Britain. All their training is carried out under actual battle conditions, and attacks on the target practice ship are made with real torpedoes fitted with dummy heads. These torpedo heads contain a buoyant apparatus so that the torpedoes can rise to the surface after running their course.
LOADING A BEAUFORT TORPEDO BOMBER. Torpedoes dropped from the air are very similar to those used by ships, but are slightly more robust so as to stand up to the impact of being dropped into the sea by fast-flying aircraft. Here, an aircraft torpedo is seen being loaded into the bomb cell of a Bristol Beaufort.
