V-1 flying bomb

Rear view of V-1 in IWM Duxford, showing launch ramp section

The Argus pulsejet's major components included the nacelle, fuel jets, flap valve grid, mixing chamber venturi, tail pipe and spark plug. Compressed air forced gasoline, from the 640 liter fuel tank, through the fuel jets, consisting of three banks of atomizers with three nozzles each. Argus' pressurized fuel system negated the need for a fuel pump. These nine atomizing nozzles were in front of the air inlet valve system where it mixed with air before entering the chamber. A throttle valve, connected to altitude and ram pressure instruments, controlled fuel flow. Schmidt's spring-controlled flap valve system provided an efficient straight path for incoming air. The flaps momentarily closed after each explosion, the resultant gas was partially compressed by the venturis, and the tapered tail pipe further compressed the exhaust gases creating thrust. The operation proceeded at a rate of 42 cycles per second.[17][18][19]

Beginning in January 1941, the V-1's pulsejet engine was also tested on a variety of craft, including automobiles[20] and an experimental attack boat known as the "Tornado". The unsuccessful prototype was a version of a Sprengboot, in which a boat loaded with explosives was steered towards a target ship and the pilot would leap out of the back at the last moment. The Tornado was assembled from surplus seaplane hulls connected in catamaran fashion with a small pilot cabin on the crossbeams. The Tornado prototype was a noisy underperformer and was abandoned in favour of more conventional piston-engine craft.

The engine made its first flight aboard a Gotha Go 145 on 30 April 1941.[20]

A V-1 on display in the Musée de l'Armée, Paris

A reconstructed starting ramp for V-1 flying bombs, Historical Technical Museum, Peenemünde (2009)

The V-1 guidance system used a simple autopilot developed by Askania in Berlin to regulate altitude and airspeed.[15] A pair of gyroscopes controlled yaw and pitch, while azimuth was maintained by a magnetic compass. Altitude was maintained by a barometric device.[21] Two spherical tanks contained compressed air at 900 pounds per square inch, that drove the gyros, operated the pneumatic servo-motors controlling the rudder and elevator, and pressurized the fuel system.[22][23][19]

The magnetic compass was located near the front of the V1, within a wooden sphere. Just before launch, the V1 was suspended inside the Compass Swinging Building (Richthaus). There the compass was corrected for magnetic variance and magnetic deviation.[24][25]

The RLM at first planned to use a radio control system with the V-1 for precision attacks, but the government decided instead to use the missile against London.[26] Some flying bombs were equipped with a basic radio transmitter operating in the range of 340–450 kHz. Once over the channel, the radio would be switched on by the vane counter, and a 400-foot aerial deployed. A coded Morse signal, unique to each V1 site, transmitted the route, and impact zone once the radio stopped transmitting.[27][28]

An odometer driven by a vane anemometer on the nose determined when the target area had been reached, accurate enough for area bombing. Before launch, it was set to count backwards from a value that would reach zero upon arrival at the target in the prevailing wind conditions. As the missile flew, the airflow turned the propeller, and every 30 rotations of the propeller counted down one number on the odometer. This odometer triggered the arming of the warhead after about 60 km (37 mi).[29] When the count reached zero, two detonating bolts were fired. Two spoilers on the elevator were released, the linkage between the elevator and servo was jammed, and a guillotine device cut off the control hoses to the rudder servo, setting the rudder in neutral. These actions put the V-1 into a steep dive.[30][31] While this was originally intended to be a power dive, in practice the dive caused the fuel flow to cease, which stopped the engine. The sudden silence after the buzzing alerted listeners of the impending impact.[12][19][32][33]

Initially, V-1s landed within a circle 19 miles (31 kilometres) in diameter, but by the end of the war, accuracy had been improved to about 7 miles (11 kilometres), which was comparable to the V-2 rocket.[34]

The warhead consisted of 850 kg of Amatol, 52A+ high-grade blast-effective explosive with three fuses. An electrical fuse could be triggered by nose or belly impact. Another fuse was a slow-acting mechanical fuse allowing deeper penetration into the ground, regardless of the altitude. The third fuse was a delayed action fuse, set to go off two hours after launch.[35][36]

The purpose of the third fuse was to avoid the risk of this secret weapon being examined by the British. It was too short to be any sort of booby trap, but was instead meant to destroy the weapon if a soft landing had not triggered the impact fuses. These fusing systems were very reliable, and there were almost no dud V-1s recovered.[37][38]

V1 on Walter catapult ramp at Éperlecque

Ground-launched V-1s were propelled up an inclined launch ramp by an apparatus known as a Dampferzeuger ("steam generator"), in which steam was generated when hydrogen peroxide (T-Stoff) was mixed with sodium permanganate (Z-Stoff).[39][40] Designed by Hellmuth Walter Kommanditgesellschaft, the WR 2.3 Schlitzrohrschleuder consisted of a small gas generator trailer, where the T-Stoff and Z-Stoff combined, generating high-pressure steam that was fed into a tube within the launch rail box. A piston in the tube, connected underneath the missile, was propelled forward by the steam. This enabled the missile to become airborne with a strong enough air-flow allowing the pulse-jet engine to operate. The launch rail was 49 m (160 ft) long, consisting of 8 modular sections 6 m long, and a muzzle brake. Production of the Walter catapult began in January 1944.[41][42]

The Walter catapult accelerated the V-1 to a launch speed of 200 mph, well above the needed minimum operational speed of 150 mph. The V-1 made British landfall at 340 mph, but accelerated to 400 mph over London, as its 150 gallons of fuel burned off.[12]

On 18 June 1943, Hermann Göring decided on launching the V-1, using the Walter catapult, in both large launch bunkers, called Wasserwerk, and lighter installations, called the Stellungsystem. The Wasserwerk bunker measured 215m long, 36m wide, and 10m high. Four were initially to be built: Wasserwerk Desvres, Wasserwerk St. Pol, Wasserwerk Valognes, and Wasserwerk Cherbourg. Stellungsystem-I was to be operated by Flak Regiment 155(W), with 4 launch battalions, each having 4 launchers, and located in the Pas-de-Calais region. Stellungsystem-II, with 32 sites, was to act as a reserve unit. Stellungsystem-I and II had nine batteries manned by February 1944. Stellungsystem-III, operated by FR 255(W), was to be organized in the spring of 1944, and located between Rouen and Caen. The Stellungsystem locations included distinctive catapult walls pointed towards London, several "J"-shaped stowage buildings referred to as "ski" buildings, and a compass correction building. In the spring of 1944, Oberst Schmalschläger had developed a more simplified launching site, called Einsatz Stellungen. Less conspicuous, 80 launch sites and 16 support sites were located from Calais to Normandy. Each site took only 2 weeks to construct, using 40 men, and the Walter catapult only took 7–8 days to erect, when the time was ready to make it operational.[41]

Once near the launch ramp, the wing spar and wings were attached and the missile was slid off the loading trolley, Zubringerwagen, onto the launch ramp. The ramp catapult was powered by the Dampferzeuger trolley. The pulse-jet engine was started by the Anlassgerät, which provided compressed air for the engine intake,and electrical connection to the engine spark plug, and autopilot. The Bosch spark plug was only needed to start the engine, while residual flame ignited further mixtures of gasoline and air, and the engine would be at full power after 7 seconds. The catapult would then accelerate the bomb above its stall speed of 200 mph, and ensuring sufficient ram air.[43][44][45]

A battery of static QF 3.7-inch guns on railway-sleeper platforms at Hastings on the south coast of England, July 1944

The British defence against German long-range weapons was known by the codename Crossbow with Operation Diver covering countermeasures to the V-1. Anti-aircraft guns of the Royal Artillery and RAF Regiment redeployed in several movements: first in mid-June 1944 from positions on the North Downs to the south coast of England, then a cordon closing the Thames Estuary to attacks from the east. In September 1944, a new linear defence line was formed on the coast of East Anglia, and finally in December there was a further layout along the Lincolnshire–Yorkshire coast. The deployments were prompted by changes to the approach tracks of the V-1 as launch sites were overrun by the Allies' advance.

On the first night of sustained bombardment, the anti-aircraft crews around Croydon were jubilant—suddenly they were downing unprecedented numbers of German bombers; most of their targets burst into flames and fell when their engines cut out. There was great disappointment when the truth was announced. Anti-aircraft gunners soon found that such small fast-moving targets were, in fact, very difficult to hit. The cruising altitude of the V-1, between 600 to 900 m (2,000 to 3,000 ft), was just above the effective range of light anti-aircraft guns, and just below the optimum engagement height of heavier guns.

The altitude and speed were more than the rate of traverse of the standard British QF 3.7-inch mobile gun could cope with. The static version of the QF 3.7-inch, designed for use on a permanent, concrete platform, had a faster traverse. The cost and delay of installing new permanent platforms for the guns was fortunately found to be unnecessary, a temporary platform devised by the Royal Electrical and Mechanical Engineers and made from railway sleepers and rails was found to be adequate for the static guns, making them considerably easier to re-deploy as the V-1 threat changed.[57][lower-alpha 5]

The development of the proximity fuze and of centimetric, 3 gigahertz frequency gun-laying radars based on the cavity magnetron helped to counter the V-1's high speed and small size. In 1944, Bell Labs started delivery of an anti-aircraft predictor fire-control system based on an analogue computer, just in time for the Allied invasion of Europe.

These electronic aids arrived in quantity from June 1944, just as the guns reached their firing positions on the coast. Seventeen percent of all flying bombs entering the coastal "gun belt" were destroyed by guns in their first week on the coast. This rose to 60 percent by 23 August and 74 percent in the last week of the month, when on one day 82 percent were shot down. The rate improved from thousands of shells for every one V-1 destroyed to one for every 100. This mostly ended the V-1 threat.[58] As General Frederick Pile put it in an April 5, 1946 article in the London Times: "It was the proximity fuse which made possible the 100 percent successes that A.A. Command was obtaining regularly in the early months of last year...American scientists...gave us the final answer to the flying bomb."[59]

Eventually about 2,000 barrage balloons were deployed, in the hope that V-1s would be destroyed when they struck the balloons' tethering cables. The leading edges of the V-1's wings were fitted with Kuto cable cutters, and fewer than 300 V-1s are known to have been brought down by barrage balloons.[60][61]

The Defence Committee expressed some doubt as to the ability of the Royal Observer Corps to adequately deal with the new threat, but the ROC's Commandant Air Commodore Finlay Crerar assured the committee that the ROC could again rise to the occasion and prove its alertness and flexibility. He oversaw plans for handling the new threat, codenamed by the RAF and ROC as "Operation Totter".

Observers at the coast post of Dymchurch identified the very first of these weapons and within seconds of their report the anti-aircraft defences were in action. This new weapon gave the ROC much additional work both at posts and operations rooms. Eventually RAF controllers actually took their radio equipment to the two closest ROC operations rooms at Horsham and Maidstone, and vectored fighters direct from the ROC's plotting tables. The critics who had said that the Corps would be unable to handle the fast-flying jet aircraft were answered when these aircraft on their first operation were actually controlled entirely by using ROC information both on the coast and at inland.

The average speed of V-1s was 550 km/h (340 mph) and their average altitude was 1,000 m (3,300 ft) to 1,200 m (3,900 ft). Fighter aircraft required excellent low altitude performance to intercept them and enough firepower to ensure that they were destroyed in the air (ideally, also from a sufficient distance, to avoid being damaged by the strong blast) rather than the V-1 crashing to earth and detonating. Most aircraft were too slow to catch a V-1 unless they had a height advantage, allowing them to gain speed by diving on their target.

When V-1 attacks began in mid-June 1944, the only aircraft with the low-altitude speed to be effective against it was the Hawker Tempest. Fewer than 30 Tempests were available. They were assigned to No. 150 Wing RAF. Early attempts to intercept and destroy V-1s often failed, but improved techniques soon emerged. These included using the airflow over an interceptor's wing to raise one wing of the V-1, by sliding the wingtip to within 6 in (15 cm) of the lower surface of the V-1's wing. If properly executed, this manoeuvre would tip the V-1's wing up, over-riding the gyro and sending the V-1 into an out-of-control dive. At least sixteen V-1s were destroyed this way (the first by a P-51 piloted by Major R. E. Turner of 356th Fighter Squadron on 18 June).[62]

The Tempest fleet was built up to over 100 aircraft by September, and during the short summer nights the Tempests shared defensive duty with twin-engined de Havilland Mosquitos. Specially modified Republic P-47M Thunderbolts were also pressed into service against the V-1s; they had boosted engines (2,800 hp) and had half their 0.5-inch (13 mm) machine guns and half their fuel tanks, all external fittings and all their armour plate removed to reduce weight. In addition, North American P-51 Mustangs and Griffon-engined Supermarine Spitfire Mk XIVs were tuned to make them fast enough, At night airborne radar was not needed, as the V-1 engine could be heard from 10 mi (16 km) away or more and the exhaust plume was visible from a long distance. Wing Commander Roland Beamont had the 20 mm cannon on his Tempest adjusted to converge at 300 yd (270 m) ahead. This was so successful that all other aircraft in 150 Wing were thus modified.

The anti-V-1 sorties by fighters were known as "Diver patrols" (after "Diver", the codename used by the Royal Observer Corps for V-1 sightings). Attacking a V-1 was dangerous: machine guns had little effect on the V-1's sheet steel structure, and if a cannon shell detonated the warhead, the explosion could destroy the attacker.

A Spitfire using its wingtip to "topple" a V-1 flying bomb

In daylight, V-1 chases were chaotic and often unsuccessful until a special defence zone was declared between London and the coast, in which only the fastest fighters were permitted. The first interception of a V-1 was by F/L J. G. Musgrave with a No. 605 Squadron RAF Mosquito night fighter on the night of 14/15 June 1944. As daylight grew stronger after the night attack, a Spitfire was seen to follow closely behind a V-1 over Chislehurst and Lewisham. Between June and 5 September 1944, a handful of 150 Wing Tempests shot down 638 flying bombs,[63] with No. 3 Squadron RAF alone claiming 305. One Tempest pilot, Squadron Leader Joseph Berry (501 Squadron), shot down 59 V-1s, the Belgian ace Squadron Leader Remy Van Lierde (164 Squadron) destroyed 44 (with a further nine shared) and W/C Roland Beamont destroyed 31. A Dutch pilot in 322 Squadron, Jan Leendert Plesman, son of KLM president Albert Plesman, managed to destroy 12 in 1944, flying a Spitfire.[64]

The next most successful interceptors were the Mosquito (623 victories),[65] Spitfire XIV (303),[lower-alpha 6] and Mustang (232). All other types combined added 158. Even though it was not fully operational, the jet-powered Gloster Meteor was rushed into service with No. 616 Squadron RAF to fight the V-1s. It had ample speed but its cannons were prone to jamming, and it shot down only 13 V-1s.[67]

In late 1944 a radar-equipped Vickers Wellington bomber was modified for use by the RAF's Fighter Interception Unit as an airborne early warning and control aircraft.[68] Flying at an altitude of 100 ft (30 m) over the North Sea at night, it directed Mosquito and Beaufighters charged with intercepting He 111s from Dutch airbases that sought to launch V-1s from the air.[69]

The first bomb disposal officer to defuse an unexploded V-1 was John Pilkington Hudson in 1944.[70]

To adjust and correct settings in the V-1 guidance system, the Germans needed to know where the V-1s were impacting. Therefore, German intelligence was requested to obtain this impact data from their agents in Britain. However, all German agents in Britain had been turned, and were acting as double agents under British control.

Aftermath of a V-1 bombing, London, 1944

On 16 June 1944, British double agent Garbo (Juan Pujol) was requested by his German controllers to give information on the sites and times of V-1 impacts, with similar requests made to the other German agents in Britain, Brutus (Roman Czerniawski) and Tate (Wulf Schmidt). If given this data, the Germans would be able to adjust their aim and correct any shortfall. However, there was no plausible reason why the double agents could not supply accurate data; the impacts would be common knowledge amongst Londoners and very likely reported in the press, which the Germans had ready access to through the neutral nations. In addition, as John Cecil Masterman, chairman of the Twenty Committee, commented, "If, for example, St Paul's Cathedral were hit, it was useless and harmful to report that the bomb had descended upon a cinema in Islington, since the truth would inevitably get through to Germany ..."[71]

While the British decided how to react, Pujol played for time. On 18 June it was decided that the double agents would report the damage caused by V-1s fairly accurately and minimise the effect they had on civilian morale. It was also decided that Pujol should avoid giving the times of impacts, and should mostly report on those which occurred in the north west of London, to give the impression to the Germans that they were overshooting the target area.[72]

While Pujol downplayed the extent of V-1 damage, trouble came from Ostro, an Abwehr agent in Lisbon who pretended to have agents reporting from London. He told the Germans that London had been devastated and had been mostly evacuated as a result of enormous casualties. The Germans could not perform aerial reconnaissance of London, and believed his damage reports in preference to Pujol's. They thought that the Allies would make every effort to destroy the V-1 launch sites in France. They also accepted Ostro's impact reports. Due to Ultra, however, the Allies read his messages and adjusted for them.[73]

Max Wachtel

A certain number of the V-1s fired had been fitted with radio transmitters, which had clearly demonstrated a tendency for the V-1 to fall short. Oberst Max Wachtel, commander of Flak Regiment 155 (W), which was responsible for the V-1 offensive, compared the data gathered by the transmitters with the reports obtained through the double agents. He concluded, when faced with the discrepancy between the two sets of data, that there must be a fault with the radio transmitters, as he had been assured that the agents were completely reliable. It was later calculated that if Wachtel had disregarded the agents' reports and relied on the radio data, he would have made the correct adjustments to the V-1's guidance, and casualties might have increased by 50 percent or more.[74][75]

The policy of diverting V-1 impacts away from central London was initially controversial. The War Cabinet refused to authorise a measure that would increase casualties in any area, even if it reduced casualties elsewhere by greater amounts. It was thought that Churchill would reverse this decision later (he was then away at a conference); but the delay in starting the reports to Germans might be fatal to the deception. So Sir Findlater Stewart of Home Defence Executive took responsibility for starting the deception programme immediately, and his action was approved by Churchill when he returned.[76]

Both British (80 AA Brigade) and US Army anti-aircraft batteries (30th AAA Group) were sent to Antwerp together with a searchlight regiment. The zone of command under the 21st Army Group was called "Antwerp-X" and given the object of protecting an area with a radius of 7,000 yards covering the city and dock area.[85] Initially attacks came from the south-east, accordingly a screen of observers and searchlights was deployed along the attack azimuth, behind which were three rows of batteries with additional searchlights.[86]

US units deployed SCR-584 radar units controlling four 90mm guns per battery using an M9 director to electrically control the battery guns.[87] Backup for the American guns was automatic 40mm batteries, which were not effective against V-1s.

British gun batteries were each equipped with eight QF 3.7-inch AA gun and two radar units, preferably the US SCR-584 with M9 director as it was more accurate than the British system.[88] Backup for the British guns was also automatic 40mm batteries.

The radar was effective from 28,000 yards, the M9 director predicted the target location position based on course, height and speed which combined with the gun, shell and fuse characteristics predicted an impact position, adjusted each gun and fired the shell.[89]

In November attacks began from the north-east and additional batteries were deployed along the new azimuths, including the 184th AAA Battalion (United States) brought from Paris. Additional radar units and observers were deployed up to 40 miles from Antwerp to give early warning of V-1 bombs approaching.[90] The introduction of the VT fuse in January 1945 improved the effectiveness of the guns and reduced ammunition consumption.[91]

From October 1944 to March 1945, 4,883 V-1s were detected. Of these, only 4.5% fell into the designated protected area.[92] The effectiveness of the anti-aircraft defence meant that only 211 got through the defences; however, those that fell within the area caused damage and loss of life.

After reverse-engineering captured V-1s in 1946, the French began producing copies for use as target drones, starting in 1951. These were called the ARSAERO CT 10 and were smaller than the V-1. The CT 10 could be ground-launched using solid rocket boosters or air-launched from a LeO 45 bomber. More than 400 were produced, some of which were exported to the UK, Sweden, and Italy.[93]

The Soviet Union captured V-1s when they overran the Blizna test range in Poland, as well as from the Mittelwerk.[94] The 10Kh was their copy of the V-1, later called Izdeliye 10.[94] Initial tests began in March 1945 at a test range in Tashkent,[94] with further launches from ground sites and from aircraft of improved versions continuing into the late 1940s. The inaccuracy of the guidance system when compared with new methods such as beam-riding and TV guidance saw development end in the early 1950s.

The Soviets also worked on a piloted attack aircraft based on the Argus pulsejet engine of the V-1, which began as a German project, the Junkers EF 126 Lilli,[95] in the latter stages of the war. The Soviet development of the Lilli ended in 1946 after a crash that killed the test pilot.[94]

A KGW-1 being fired from USS Cusk in 1951

The United States reverse-engineered the V-1 in 1944 from salvaged parts recovered in England during June. By 8 September, the first of thirteen complete prototype Republic-Ford JB-2 Loons, was assembled at Republic Aviation. The United States JB-2 was different from the German V-1 in only the smallest of dimensions, with only the forward pulsejet support pylon visibly differing in shape from the original German pilotless ordnance design. The wing span was only 2.5 in (64 mm) wider and the length was extended less than 2 ft (0.61 m). The difference gave the JB-2 60.7 square feet (5.64 m²) of wing area versus 55 square feet (5.1 m²) for the V-1.[96]

A navalised version, designated KGW-1, was developed to be launched from LSTs as well as escort carriers (CVEs) and long-range 4-engine reconnaissance aircraft. Waterproof carriers for the KGW-1 were developed for launches of the missile from surfaced submarines. Both the USAAF JB-2 and Navy KGW-1 were put into production and were planned to be used in the Allied invasion of Japan (Operation Downfall). However, the surrender of Japan obviated the need for its use.[96] After the end of the war, the JB-2/KGW-1 played a significant role in the development of more advanced surface-to-surface tactical missile systems such as the MGM-1 Matador and later MGM-13 Mace.