Gardening with Bomber Command
The following article is brief but hopefully detailed enough to provide you with some insight of the mining campaign, the information on technical details of the mine is comprehensive and touched lightly on detail. I am open to constructive criticism on the article, should you have any comments then feel free to contact the web master. Fellow researchers and sources of information I would like to thank are listed at bottom of article.
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)
When we think of a “Mine” the visualization is the steel ball with the horns protruding from its shell, bobbing in the water or submerged just below sea level 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). An anti-detonation device prevented explosions of other mines in close proximity to the exploding mine. 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.
Sound signatures are created by all ships in water; combination of engines running, and propellers turning all have distinctive sounds. Manufacturers of mines knew what horse power of engine(s) and size of screws were 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 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.
Aircrews had guidelines to follow when assigned to a “Gardening” operation, it was not a matter of flying directly to the intended area at any altitude or speed the pilot desired then releasing your ordnance and turning around. There were other variables to deal with also; high or low tide, depth of the water, direction of approach, and then you have the enemy to deal with.
Once the target was selected there was a flight path selected for the group of bombers participating in the nights “Gardening” operation. There would likely be bombing raids taking place on the same night and an aircrew with a couple of mines did not want to get mixed in with a squadron headed to an over land target.
An aircraft(s) from the Pathfinder Squadron would assist the group of bombers using “Gee” leading their flight route then dropping a colored marker at a desirable visual pinpoint of land. Once the navigator informed the pilot the intended target was nearing 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 Sea, and Baltic Sea. 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.
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 50 ships (100 including converted merchant ships - 5000 tons or more) among 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”.
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 ballons attached, roughly 50% of the Sperrbrechers would be destroyed by mining operations.
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.
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.
Thanks to the following People/Reference Books/Websites