Hedgehog

Hedgehog (Mk 10/11) and (Mk 15)

In their war against the U-boats, the British developed an ungainly looking, multi-barreled weapon called the Hedgehog in January 1942 that fired a barrage of 24 -7.2" Mk 6 bombs (called projector charges), each weighing 75 pounds, in an oval pattern over a wide area. Before development of this weapon, submarine-killers employing depth charges had to pass directly over a submerged submarine. The Hedgehog was the first stand-off submarine killing weapon. The bomb was not a rocket, but was propelled up to 250 yards by an impulse charge of smokeless powder. The explosive charge consisted of 30 pounds of TNT, or 35 pounds of Torpex. Destroyer Escorts routinely carried 240 missiles which was enough for 10 launches. A reload took about 3 minutes. The missiles are loaded on cylindrical bars called spigots, six are attached to each cradle, with four cradles in the projector. These cradles are interconnected and can be swung about a fore-and-aft axis by means of a roll-correction gear assembly mounted on a gun-train indicator pedestal. This movement is limited, but allows enough movement to compensate for roll of the ship and to aid in leading the target. The charges are electrically fired by a ripple switch, firing the missiles in pairs every 0.10 seconds, with the highest trajectories being fired first so that all 24 hit the water at about the same time, this minimizing the submariner's reaction time. The Mark 10 and Mark 11 were almost identical, differing only in the shape of their pattern and range. The Mark 10 fired an elliptical pattern measuring about 140 x 120 feet to a range of 200 yards. The Mark 11 fired a circular pattern measuring 200 feet in diameter out to a range of about 188 yards.	Mark 10/11 Front Mark 10/11 Rear
National Archives #80G427432 80G701378 80G701381
Detonation of a spread of hedgehogs that appear to have struck a shallow bottom and detonated. Courtesy of the Royal Navy
Mark 15 Hedgehog was a "trainable" version that that adjusted to sloped decks, variable seas, etc. The Mark 15's pattern was the same as the Mark 11's, i.e. a circular pattern measuring 200 feet in diameter out to a range of 188 yards. The platform for the Mark 15 was adapted from the standard 40mm Bofors quadruple mount. Firing could be accomplished remotely from the plotting room.	Mark 15 Front Mark 15 Rear
In later ASW adaptations, both the Mark 10/11 and the Mark 15s were mounted on the same vessel. The two Mk 10/11s were trained on the outer limits of the Mk 15's range, thus extending the coverage area in front of the ship. In this photo, each of the Gearing-class destroyers has fired their Mk 10/11 to achieve overlapping coverage of an area.	Mk 10/11 and Mk 15 mounts
HISTORY The major ASW deficiency of a depth charge attack was the blind time. The first blind time was due to the sonar design during WW2. The sonar had a narrow "spotlight" or conical-shaped forward-looking beam. The sonar's cone-shaped beam, combined with the transducer's limited downward tilt capability, caused the DEs to lose contact when the target was approximately 100-150 yards, or three times it's depth. The second blind time occurred because of the time required for the depth charges to sink to the target depth, particularly since depth charges were normally launched off the stern of the ship. See Depth Charges for a detailed description of the DC lag time and countermeasures. The total time from loss of sonar contact, movement to the last known target position, and the sink time required for the depth charges, was several minutes. This usually allowed experienced submariners sufficient time to move out of the danger area. The British Hedgehog was designed to eliminate much of this blind time. Upon sonar target acquisition, the Hedgehog launched 24 missiles, each containing 30-35 pounds of explosive, 250 yards forward of the ship. The missiles landed in the general area of the submarine in an elliptical pattern measuring 195 feet wide by 168 feet deep. The missiles were impact-fuzed and detonated upon striking the target. If there was no hit, the missiles did not detonate, which allowed the DE to maintain sonar contact for a follow-up attack.
The Mark 6 Hedgehog was similar to the 7.2" rocket fired from the Mark 20 and 21 "Mousetrap" rocket launcher. It had tail fins and a shroud, but did not contain a rocket motor. Instead, the tail was hollow (see above) and contained a small black powder propelling charge that propelled the missile for about 8 seconds. It entered the water at a velocity of 144 feet per second at an angle of 58 degrees. The diameter was 7.2" and the overall length was 3' 10.5". The contact fuze armed after 15 feet of subsurface travel after entering the water. A direct hit was necessary to detonate the missile. The sink rate on the hedgehog was 24 feet per second.