Gardening with Bomber Command

Information on aerial mine laying and technical details of the mine is comprehensive, the following article is brief and hopefully detailed enough to provide you with some insight of the mining campaign.

Mining Campaign

Bomber Command squadrons strategically placed mines in the North Atlantic waters to reduce German shipping/naval routes, immobilize harbors/ports, and hinder shipping traffic in rivers that feed factories/cities. Bomber Command code named all the waterways using a vegetable name, therefore you have “Gardening” the planting of mines. 

Not only did the German’s defend their cities and factories from Allied bombing but they also had to commit more then 20,000 men to minesweeping activities along the coast of Europe. Constructing anti-aircraft batteries, and have maneuverable flak cannons at key locations along the coastline of northern Europe was a necessity. 

Bomber Command Minefields/Vegetable Gardens

There are four maps to display the minefields, each with a corresponding list of numbers/geographic names and vegetables for reference. The different colors are only used to distinguish the minefields from each other. Size and location are approximate. (click hyperlink to see map)

Kattegat Channel/Baltic Sea 1-27 

North Sea/The Netherlands/Germany/Denmark/Norway/ 28-38

North Coast France/The Netherlands 38-51

Atlantic Ocean/France/English Channel 51-65

The Mine

Two ground crew maneuver a mine in preparation for loading.

 When we think of a “Mine” the visualization is the steel ball with the horns protruding from its shell, bobbing in the sea or submerged below the surface. These would be tethered by a cable attached to a weight on the sea floor, awaiting detonation by passing vessel. The aerial mine dropped from a bomber was considerably different in appearance, and purposely sank to the ocean floor awaiting non-contact detonation.

The aircraft mine is cylindrical shaped, approximately 17 inch’s in diameter and 9 feet long including the wood or metal fairing. The weight averaged 1,500 lbs. (680 kg) of which 750 lbs. (340 kg) of explosive (amatol or minol). A small parachute attached at the end would facilitate vertical entry into the water, and the nose fairing would prevent deflection during its flight through the air and would disintegrate upon impact with the water. 

Two devices prevented accidental arming of the mine while in transport; a safety fork prevents the closing of the arming switch while aboard the aircraft, manual removal of the fork was optional just prior to jettisoning the mine from the aircraft.

There was also a safety switch fitted with a soluble plug and operated hydrostatically which kept the mine in a safe condition both before laying and until a predetermined period had elapsed once submerged below 5 fathoms/30 feet of water. The main circuits of the mine are held open and the detonator isolated from the battery until the closing of the switch.

Schematic of Air Dropped Mine.

Types of Mines

Acoustic

Sound signatures are created by all ships in water; combination of engines running, and propellers turning all have distinctive sounds. Manufacturers of mines knew the horse power of engine(s) and size of screws required to power vessels. Therefore, a microphone placed in the mine could home in on sound signatures of propeller and engine noise of passing ships, differentiating whether the vessel was large or small then detonate on the pre-selected signature.

Magnetic

Magnetic fields are created during construction of a steel ship; electrical cables are routinely strung through the perimeter of the hull to diminish the magnetic field, but not eliminated. Once again the magnetic mine is finely tuned with an electrical device to identify the strength of the magnetic field (vessel) passing over and will detonate according to the pre-set.

As the war progressed so did the evolution of the mine, engineers were able to incorporate properties from either and manufacture an “Acoustic-Magnetic” or “Magnetic-Acoustic” mine.

Period Delay Mechanism

The firing circuit is held open by the “PDM” until all the operations for which the mechanism has been set have been worked off, then on the next actuation of the mine the relay closes and the circuit fires the mine. The maximum permissible setting of the mechanism is 12, i.e., the mine will not fire until it has actuated 13 times. The PDM was useful for foiling the German minesweeper as it escorted vessels through a minefield.

Mine-Laying

It was not a matter of the pilot flying directly to the intended area at any altitude or speed then releasing ordnance and turning around. There were other variables to deal with; high or low tide, depth of the water, direction of approach, and enemy defences. 

Aircrews had guidelines to follow when assigned a “Gardening” operation. Once the target was determined there would be a flight path selected for the group of bombers participating in the nights vegetable planting. It is likely bombing raids are taking place on the same night, an aircrew with a couple of mines did not want to get mixed in with a squadron headed to an over land target.

Diagram of 3 Lancaster's on a mining "Op" utilizing pinpoint visualization.

 An aircraft(s) from the Pathfinder Squadron assists the group of bombers using “Gee” leading their flight route, then dropping  a colored marker at a desirable visual pinpoint of land. The navigator would inform the pilot the intended target was nearing, then the pilot would make alterations to his altitude and speed, ideally the aircraft speed should be 180 mph, and altitude at 1500 feet. The aircraft would now have to make a straight 12 mile timed run at this altitude, and speed, any faster than 200 mph and the parachute could be torn off.

The mines (usually 2) are released between 3 and 5 second intervals (depending on type of a/c), their trajectory is curved; gravity, a/c speed, resistance of air, the nose fairing and parachute were all factors in planting a mine that will function. The aircraft would continue in a straight line for another few minutes to confuse the enemy as to where the mines were jettisoned, completing the drop a crew would head for home; however no mining mission was ever this straight forward.

The minefields were often located in enemy channels, harbors, ports, and inland waterways, all along the Atlantic coast, North and Baltic Seas. The field was often narrow, ranging from 1000 feet to a mile across, if the tide was out and the mine was dropped out of the zone there was a chance the enemy could retrieve it for examination. Furthermore, planting the mine in an area not covered by a minesweeper would be considered a waste.

Aircraft mines were in short supply, in the event a crew did not lay the ‘Vegetable” then it was brought back to the base and used again. Landing with mines was not dangerous as the previously mentioned safety measures prevented detonation, even if the a/c crashed. Crews were not to drop them outside swept channels in enemy territory as it could jeopardize friendly naval operations on the enemy coast. If a crew had to drop a mine due to unforeseen circumstances then it was to be in deep water, and charted.

Flak cannons on a barge, Kiel Harbor.

 Flying at an altitude of approximately 1500 feet the biggest fear for the crew was the enemy defenses; besides permanent emplacements, a variety of flak cannons could be mobile on the water or along the coastline.  Heavy machine guns to rifle fire could also bring down a bomber. Along with searchlights, and the Luftwaffe night-fighters, there was another threat, the Sperrbrecher 

The Sperrbrecher and Mine Sweeping

The Sperrbrecher (combination minesweeper/flak cannons) were a fleet of fifty ships, if you were to include converted merchant ships (5000 tons or more) then a couple of hundred mine sweeping vessels. These ships had 20 mm or 37 mm cannons positioned on high platforms at bow and stern, others would have 105 mm cannons or 88 mm flak cannons, the RAF referred to these as “Heavy Flak Ships”.

Sperrbrecher (Pathmaker) 27 "Saar" 115 meters in length, 7500 tons, and a speed of 14 knots - one diesel engine/one shaft. The weapons included two 105 mm and two 37 mm flak cannons, along with fifteen 20 mm MG's. 

Escorting ships/U-boats in and out of minefields the Sperrbrecher was equipped with a VES-System - an immense magnetic field generator that would transmit a powerful magnetic pulse in hopes of detonating the mine 500 yards ahead of it, or reproduce sound waves with a form of mechanical hammer or a tuning fork. In some circumstances it may only actuate the mine according to the preset then the following ship could detonate it, or worse the Sperrbrecher could be the victim. Vulnerable to aerial attacks many Sperrbrechers would have barrage balloons attached, roughly 50% of the Sperrbrechers would be destroyed by mining operations.

Detonation

Once a target vessel has caused the closing of internal contacts, a fraction of a second later the mine has been transformed into a incandescent gas bubble, resulting in a primary pressure wave/pulse which travels at the speed of sound in all directions. If the seabed on which the mine was sitting is hard sand, or bedrock then the primary pulse will be reflected and strike the target simultaneously.

The pressure from this pulse/wave is massive and constitutes the most destructive factor of the explosion, upon contacting the ship it subjects the hull to an extensive shock, buckling or ripping apart plates, and damaging internal structures within the vessel, ultimately its goal to sink the ship.

The destructive incandescent gas bubble escaping to the surface.

 There may be a number of secondary pulses caused by contraction and expansion of the gas bubble as it rises to the surface. Once reaching the surface the bubble will create a “mound” or “plume” which can reach an altitude of 300 feet, damaging any target immediately over it. An anti-detonation device prevented explosions of other mines in close proximity to the exploding mine. 

Aircraft and Mine Load Maximums

 Halifax - 2

Hampden - 1

Lancaster - 6

Manchester - 4

Stirling - 7

Wellington - 2

Interesting Facts

Amatol is a highly explosive material, a mixture of TNT and ammonium nitrate, mixture ratios range from 80 percent ammonium nitrate/20 percent TNT, down to 50/50.

German seaplanes lay the first magnetic mines in November 1939 off the east coast of England and drop one on tidal flats in the Thames Estuary. The British defuse and discover its secrets, an essential stride in mining warfare.

A British laid mine off the Friesian Islands sinks a German vessel carrying scientists who specialized in counter-mining devices.

Mines laid by Bomber Command 47,307

Thanks to the following People/Reference Books/Websites

Schematic of Air dropped Mine; Matt Lacroix, www.bombercrew.com

Peter Clare

Roger Griffiths

Frederick Haldimann

Jan Linzmaier

Handbook of “A” MARK I – V MINES 1943. Operational Aspect – Construction of Mine – Mine Attachments.

Handbook of Operational Data for Independent Mines/Firing Systems 1945. (Approximately 160 pages of text and diagrams - KEW).

Chapter Five “VEGETABLES I HAVE KNOWN OR AN INTELLIGENCE OFFICER'S GUIDE TO GARDENING” (From W E Jones ‘Bomber Intelligence, 103 150 166 170 Squadrons Operations & Techniques ‘42-’45’, Midland Counties Publications, 1983)

DIE DEUTSCHE KRIEGSMARINE 1935-1945 Vol I-IV / Breyer,S. & Elfrath, U. & Koop, G.; Weltbild Verlag 1994 (Volume I-III: German/English) T. Riepe  A. Kyd-Rebenburg

Michael Emmerich - www.german-navy.de