the V1 'buzz bomb'

A German V1 flying bomb at Duxford Imperial War Museum UK

The Fieseler Fi 103/FZG-76 (Vergeltungswaffe-1, V-1), known as the Flying bomb, Buzz bomb or Doodlebug, was the first modern guided missile used in wartime and the forerunner of today's cruise missile.

The name Vergeltungswaffe, meaning "reprisal weapon", was coined by German propaganda minister Goebbels to signify reprisals against the Allies for the bombing of the Fatherland. FZG is an abbreviation of Flakzielgerät (anti-aircraft cannon aiming device), a misleading name.

The V-1 was developed by the German Luftwaffe during the Second World War and was used operationally between June 1944 and March 1945. It was used to attack targets in south-eastern England and Belgium, chiefly the cities of London and Antwerp. V-1s were launched from "ski-jump" launch sites along the French (Pas-de-Calais) and Dutch coasts until the sites were overrun by Allied forces. A small number were air launched from German aircraft over the North Sea. The V-1 was later complemented by the more sophisticated V-2 rocket. The last V-1 attack struck British soil on March 29, 1945, two days after the final V-2 attack.

The V-1 and V-2 "Vengeance weapons" added a new terror to an already terrible war—robot missiles. Once launched, these weapons flew without human intervention to strike distant targets. The V-1 was essentially a small pilotless aircraft with minimal guidance and a large warhead. The V-2 ballistic missile was also unmanned but of a far greater technological sophistication. Both weapons could hit a city-sized target but accuracy greater than this was rare. The V-weapons failed to turn the tide of war but they did force the Allies to devote large amounts of time and resources in fighting and defending against them.

The V-1 is thought by some to have been the first cruise missile used in combat, but this distinction actually belongs to the Kettering "Bug" Aerial Torpedo, a small propeller-driven Allied flying bomb of WWI.

Description

The V-1 was designed jointly by Robert Lusser of the Fieseler company and Fritz Gosslau from the Argus engine works as the Fi 103. It was powered by an Argus pulse jet engine providing 2.9 kN (660 lbf) of thrust for a top speed of 630 km/h (390 mph) and a range of around 250 km (150 miles), which was later increased to 400 km (250 miles). It was 7.9 meters (26 feet) long and 5.3 meters (17 feet) in span and weighed 2180 kilograms (4,800 pounds). It flew at an altitude of 100 to 1000 meters (300 to 3,000 feet). It carried an 850-kilogram (1,870-pound) warhead. The missile was a relatively simple device with a fuselage constructed mainly of sheet metal, and could be assembled in around 50 man-hours.

Pulsejets are the simplest jet engines, little more than a one-way air intake valve, combustion chamber and fuel system. The V-1 was often called the buzz bomb because of the characteristic buzzing sound of its engine. This also led to the "doodlebug" name, after an Austrian insect. The characteristic sound comes from the series of ignition pulses which gives the engine its name. Fuel is squirted into the combustion chamber as air is forced in from the front through shutters that form the one-way valve. When the right fuel-air mixture is obtained, the engine fires, closing the shutters and blasting the expanding gases out the tailpipe. As pressure drops in the chamber, the shutters are forced open and the process repeats itself, approximately 100 times per second. As the V-1 entered its terminal dive the engine would cut out, and people along the bomb's flight path would listen intently for the silence that heralded the V-1's impact. As the pulsejet requires a high volume of incoming air, the V-1 was launched from a ramp through the use of a solid fuel booster.

Guidance system

Final dive of a V-1 over London

The guidance system was very crude in construction but sophisticated in conception (and had a few flaws in execution). Once clear of the launching pad, an autopilot was engaged. It regulated height and speed together, using a weighted pendulum system to get fore and aft feedback linking these and the device's attitude to control its pitch (damped by a gyromagnetic compass, which it also stabilized). There was a more sophisticated interaction between yaw, roll, and other sensors: a gyromagnetic compass (set by swinging in a hangar before launch) gave feedback to control each of pitch and roll, but it was angled away from the horizontal so that controlling these degrees of freedom interacted (the gyroscope stayed trued up by feedback from the magnetic field, and from the fore and aft pendulum mentioned before). This interaction meant that rudder control was sufficient without any separate banking mechanism.

There was a small propeller on the nose, connected to a long screw thread going back inside the missile. On this thread was a washer, and at the back end of the thread were two electrical contacts. As the missile flew, the airflow turned the propeller and hence the threaded shaft; the washer would be wound along the shaft as it turned. When it reached the electrical contacts it would make a circuit, which energised a solenoid attached to a small guillotine. This guillotine would cut through the elevator control cable which would in turn put the sprung-elevator into the fully-down position, putting the V-1 into a sudden dive.

This was intended to be a power dive, but the abrupt negative-G (or perhaps simply the angle of the descent) caused the fuel flow to cease which stopped the engine. As there was a belly fuse as well as a nose fuse, there was still usually an explosion, although not always with the device buried deep enough to increase the effect of the blast. Sometimes the sudden dive system would fail and the missile would coast in on a flat trajectory; this led to a rumour that there were two versions, which was not so.

At the launch site the engineers would preset the starting position of the washer on the shaft according to the known distance to the target and an estimate of the headwind. It sounds very rough-and-ready but it was accurate enough.

Operation and effectiveness

V-1 in flight

The first test flight of the wonder weapon V-1 was in late 1941 or early 1942 at Peenemünde. Early guidance and stabilisation problems were finally resolved by a daring test flight by Hanna Reitsch, in a V-1 modified for manned operation. The data she brought back after fighting the unwieldy V-1 down to a successful landing enabled the engineers to devise the stabilisation system described above. The idea of a piloted V-1 as a suicide weapon sprang from this mission.

The first offensive launch was from June 12 to June 13, 1944. The Allies had previously organized a heavy series of air attacks (Operation Crossbow) on the launch sites (beginning in December 1943) and now also attacked the V-1s in flight (see Countermeasures below). Because of a combination of defensive measures, mechanical unreliability, and guidance errors, only a quarter successfully hit their targets.

Once the Allies had captured or destroyed the sites that were the principal launch points of V-1s aimed at England, the Germans switched to missile launches aimed at strategic points in the Low Countries, primarily the port of Antwerp.

Although most V-1s were launched from static sites on land, the Luftwaffe did, from July 1944 to January 1945, launch a number of V-1s from Heinkel He 111 aircraft flying over the North Sea. This would also have been the launch method for the proposed piloted version of the weapon, and is how the very earliest experimental versions of the V-1 were tested. Late in the war, several piloted V-1s were built; known as Reichenbergs, they were never used in combat. It was also hoped to use the Arado Ar 234 jet bomber to deploy V-1s, either by towing them aloft, or by launching them from a "piggy back" position atop the aircraft. Neither Ar 234 concept was employed before the end of the war.

Almost 30,000 V-1s were manufactured. Approximately 10,000 were fired at England up to March 29, 1945. Of these, about 7,000 were "hits" in the sense that they landed somewhere in England. A little more than half of those (3,876) landed in the Greater London area.

An almost equal number were downed by the combination of fighters and barrage balloons. When the V-1 raids began, the only effective direct defence was interception by a handful of very high-performance fighter aircraft, especially the Hawker Tempest. The British were able to redirect V-1s aimed at London to less populated areas east of the city by sending false impact reports via the German espionage network in Britain, which was actually controlled by the British.

In the London area, roughly 5,500 persons died as a result of V-1 attacks, with some 16,000 more persons injured.

Intelligence reports

The codename Flak Ziel Gerät 76 was somewhat successful in disguising the true nature of this device, and it was some time before references to FZG 76 were tied to the V83 pilotless aircraft (an experimental V-1) which had crashed on Bornholm in the Baltic, and to reports from agents of a flying bomb capable of being used against London. Initially British experts were skeptical of the V-1 because they had considered only solid fuel rockets as a means of propulsion, which put the stated range of 130 miles (209 km) out of the question. However when other types of engine were considered they relented, and by the time German scientists had achieved the needed accuracy to deploy the V-1 as a weapon, British intelligence had a very accurate characterisation of it.

A deception concerning the V-1 was played on the Germans using double agents. MI5 (by way of the famed Double Cross System) had these agents provide Germany with damage reports for the June 1944 V-1 attacks which implied that on average the bombs were travelling too far, while not contradicting the evidence presumed to be available to German planners from photographic reconnaissance of London. In fact the bombs had been seeded with radio-transmitting samples to confirm their range, but the results from these samples were ignored in favour of the false eyewitness accounts, and many lives may have been saved by the resulting tendency of future V-1 bombs to fall short of built-up areas.

Countermeasures

A Spitfire using its wingtip to topple the gyros of a V-1 Flying Bomb

The British defence against the V-1 was codenamed Operation Diver. Anti-aircraft guns were 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 the ever-changing approach tracks of the missiles which were in turn influenced by the Allies' advance through Europe.

Anti-aircraft gunners found that such small, fast-moving targets were difficult to hit. At first, it took, on average, 2500 shells to bring down a single V-1. The average altitude of the V-1, between 2,000 and 3,000 feet (610 and 915 m), was in a narrow band between the optimum engagement heights for light and heavy anti-aircraft weapons. These low heights defeated the rate of traverse of the standard British QF 3.7 inch mobile gun, and static gun installations with faster traverses had to be built at great cost. The development of centimetric gun laying radars based on the cavity magnetron and the development of the proximity fuse helped to neutralise the advantages of speed and size which the V-1 possessed. In 1944 Bell Labs started delivery of an anti-aircraft predictor fire-control system based around an analogue computer just in time for use in this campaign.

Barrage balloons were also deployed against the missiles, but the leading edges of the V-1's wings were equipped with balloon cable cutters and fewer than 300 V-1s are known to have been destroyed by hitting cables.

Fighter defences had also been mobilized as part of Operation Diver. Most fighter aircraft were too slow to catch a V-1 unless they had a useful height advantage. Even when intercepted, the V-1 was difficult to bring down. Machine gun bullets had little effect on the sheet steel structure, and 20 mm cannon shells had a shorter range, which meant that detonating the warhead could destroy the intercepting fighter as well.

The V-1 was also nearly immune to conventional air-combat techniques because of its design, which eliminated the primary "one-shot stop" points of pilot, life-support and complex engine. A single hit on the pilot or oxygen system can force an abort or cause the destruction of a normal plane, but there is no pilot in a cruise missile. The reciprocating engines of WWII aircraft and the turbojet engines of today's fighters are also vulnerable, as a tiny nick in a quarter-inch oil line or one small shell fragment can destroy such engines. However, the Argus pulsejet could be shot full of holes and continue to provide sufficient thrust for continued flight. The only vulnerable point was the valve array at the front of the engine and the only one-shot stop points on the V-1 were the bomb detonators and the line from the fuel tank, three very small targets buried inside the fuselage. An explosive shell from a fighter's cannon or anti-aircraft artillery was the most effective weapon, if it could be detonated in a direct hit on the warhead itself.

When the attacks began in mid-June of 1944 there were fewer than 30 Tempests in 150 Wing to defend against them. Few other aircraft had the low-altitude performance to be effective. Initial attempts to intercept V-1s were often unsuccessful but interdiction techniques were rapidly developed. These included the hair-raising but effective method of using the airflow over an interceptor's wing to raise one wing of the Doodlebug, by sliding the interceptor's wingtip under the V-1's wing and bringing it to within six inches (15 cm) of the lower surface. Done properly, the airflow would tip the V-1's wing up, overriding the buzz bomb's gyros and sending it into an out of control dive. At least three V-1s were destroyed this way.

The Tempest wing was built up to over 100 aircraft by September; P-51 Mustangs and Griffon-engined Spitfire XIVs were polished and tuned to make them almost fast enough, and during the short summer nights the Tempests shared defensive duty with Mosquitoes. Specially modified P-47Ms (half their fuel tanks, half their 0.5in {12.7 mm} machine guns, all external fittings, and all their armour plate removed) were also pressed into service against the V-1 menace. There was no need for radar — at night the V-1's engine could be heard from 16 km (10 miles) or more away, and the exhaust plume was like a beacon.

In daylight, V-1 chases were chaotic and often unsuccessful until a special defence zone between London and the coast was declared in which only the fastest fighters were permitted. Between June and mid-August 1944, the handful of Tempests shot down 638 flying bombs. One Tempest pilot, Squadron Leader Joseph Berry of No. 501 (Tempest) Squadron, downed fifty-nine V-1s, and Wing Commander Roland Beamont destroyed 31.

Next most successful was the Mosquito (428), Spitfire XIV (303), and Mustang, (232). All other types combined added 158. The still-experimental jet-powered Gloster Meteor, which was rushed half-ready into service to fight the V-1s, had ample speed but suffered from a readily-jammed cannon and accounted for only 13.

By mid-August 1944, the threat was all but overcome—not by aircraft, but by the sudden arrival of two enormously effective electronic aids for anti-aircraft guns, both developed in the USA by the MIT Rad Lab: radar-based automatic gunlaying (using, among others, the SCR-584 radar), and above all, the proximity fuze. Both of these had been requested by AA Command and arrived in numbers, starting in June 1944, just as the guns reached their free-firing positions on the coast.

Seventeen per cent of all flying bombs entering the coastal 'gun belt' were destroyed by guns in the first week on the coast. This kill rate rose week on week to reach 60 per cent by 23 August and 74 per cent in the last week of the month, when on one extraordinary day 82 per cent of all targets available to the guns fell. The kill rate increased from one V-1 for every 2,500 shells fired to one for every hundred.

After the war

After the war, the armed forces of both the United States and the Soviet Union experimented with the V-1 in an assortment of scenarios. The most successful was a U.S. Navy experiment to mount V-1s on submarines. This was called the KGW-1 Loon, which was an adaptation of the U.S. Army's JB-2 Doodle Bug. The JB-2, built by Republic Aviation (airframe) and Ford Aerospace (pulsejet engine), was reverse-engineered by inspection of V-1 wreckage found in England and was first flight-tested less than four months after the first V-1 attack. While the first flights were from Eglin AAF, Florida, extensive testing was also done at Wendover AAF in Utah, launching only a few hundred feet from the sheds where the first atomic bombs were being built. The JB-2 was intended as a weapon in the invasion of Japan, which was prevented by the atomic bombing of Hiroshima and Nagasaki. Following the war, testing at Wendover continued, including comparison tests between the original German missile and the American copy.