Operation Diver

Operation Diver was the British codename for their countermeasures against the V-1 flying bomb campaign launched by the German Luftwaffe in 1944 against London and other parts of Britain. The V-1 was known by the codename "Diver".

Diver Plan
The "Diver Plan" was prepared in early 1944 following the first reports of the weapon in April 1943 and the discovery of its planned launch sites in late 1943. The plan had to be flexible enough to cover both the expected assault on Britain and the needs of the invasion of Europe. When the German attack began, on the sixth day after the landings on the beaches of Normandy, the message "Diver, Diver, Diver" put the plan into action. Defences that had been guarding the embarkation ports for the invasion were redeployed against the V-1.

Defences
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, 2,500 shells to bring down a 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 (such as the 40mm Bofors guns) 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 (roughly 30 GHz frequency) gun laying radars based on the cavity magnetron and the development of the proximity fuze 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 (supplanting the previous electro-machanical Kerrison Predictor) 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.

Part of the area which the "Diver"s had to cover was given over for fighter operations. Several squadrons were put onto anti-Diver operations. Most fighter aircraft were too slow to catch a V-1 unless they had a 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 were explosive projectiles, which meant that detonating the warhead could destroy the 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 World War II 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 still provide sufficient thrust for 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 hit the warhead.

Aircraft


When the attacks began in mid-June 1944 there were fewer than 30 Hawker Tempests in No. 150 Wing RAF to defend against them. Few other aircraft had the low-altitude speed to be effective. Early attempts to intercept V-1s often failed but techniques were rapidly developed. These included the hair-raising method of using the airflow over an interceptor's wing to raise one wing of the Doodlebug, by sliding the 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; North American 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 de Havilland Mosquito. Specially modified P-47M Thunderbolts (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 &mdash; at night the V-1's engine could be heard from 16 km or more away, and the exhaust plume was like a beacon. Wing Commander Roland Beamont had the 20mm cannons on his Tempest harmonised at 300 yards (275 m). This was so successful all other aircraft in 150 Wing were thus modified.

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 in July 1944 to fight the V-1s, had ample speed but suffered from unreliable armament and accounted for only 13.

Squadrons and units engaged in anti-Diver operations

 * 363rd Fighter Group, P-51B (Mustang)
 * No. 315 Squadron, Mustang III
 * No. 129 Squadron, Mustang III
 * No. 3 Squadron RAF, Hawker Tempest V, RAF Newchurch
 * No. 486 Squadron RAF, Hawker Tempest V, RAF Newchurch
 * Fighter Interception Unit, Tempest V, RAF Newchurch
 * No. 56 Squadron RAF, Tempest V, RAF Newchurch
 * No. 501 Squadron RAF, Tempest V, RAF Westhampnett, Manston, Bradwell Bay, Hunsdon
 * No. 274 Squadron RAF, Tempest V, RAF Merston, RAF Detling, Gatwick, West Malling
 * No. 222 Squadron RAF, Tempest V, Gilze-Rijen (The Netherlands)
 * No. 96 Squadron RAF, Mosquito XIII, RAF Ford and RAF Odiham
 * No. 91 Squadron RAF, Spitfire XIV, RAF West Malling
 * No. 322 Squadron RAF, Spitfire XIV, RAF West Malling

Technological advancements
By mid-August 1944, the threat was all but overcome&mdash;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 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 rose to 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 were shot down. The rate increased from one V-1 for every 2,500 shells fired to one for every hundred.

A deception concerning the V-1 was also 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 witness accounts and many lives may have been saved by the resulting tendency of future V-1 bombs to fall short.

End of operations
In September 1944, Duncan Sandys announced that the "Battle of London" against the bomb was effectively over.