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No 420/1 impact fuze for Hedgehog ASW , Britain , WW2

pzgr40

Well-Known Member
Premium Member
At the start of WW2 the hunt for U-boats was a difficult and often unsuccesful event. The U-boat had to be visually found, and depth charges were rolled off the deck while sailing over the last known position of the enemy submarine. An improvement was the use of the K-gun that allowed a larger radius of error while sailing over the enemy submarine, projecting the depth charges towards the last assumed position of the submarine. However, the fuzes of these depthcharges either had factory set explosion depths, either a small range of depths that could be chosen with large intervals. Killing a submarine was more a matter of luck than good marksmanship.

Sonar proved to be an important improvement, allowing allied ships to actively engage enemy submarines. However, Sonar contact was lost as soon as the ship sailed over the enemy submarine to engage it with depth charges, so a solution had to be found to maintain sonar contatct and also launch depthcharges at the enemy submarine at the same time.
In 1942 this solution came in the form of a new weapon; the Hedgehog. This consisted of six rows of four spigot launched depth charges, of which the spigots were placed in such an array that the 24 depth charges landed in a circular pattern with a diameter of 30 meter, 230 meter in front of the ship after launch. This allowed the attacking ship to turn port- or starboard after launching to maintain sonar contatct with the enemy submarine.

Another improvement was that the fuze used in this spigot mortar was not a hydrostatic or time fuze that would explode at a set moment -independent if anything was in the vincinity-, but a contatct fuze. So, a water column meant a hit was scored.
The Hedgehog spigot mortar in combination with the Sonar proved to be a deadly combination for the German U-boats, the kill ratio improved from one killed submarine per 60,5 attacks (without sonar and normal depth charges) to one kill per 5,7 attacks with Sonar and the Hedgehog; A nearly 11 fold improvement! This meant the times that Germans U-boats freely roamed the Atlantic ocean were over.

The Hedgehog spigot bomb weighed 65 Lb (29 Kg) and was 7” (178mm) in diameter. It was filled with 30 Lb (14kg) TNT, or 35Lb (16kg) Torpex). A direct hit with a Hedgehog on a U-boat was deadly, no U-boat is known to have survived a direct hit.

The Hedgehog was used by the British, Canadian, US and Dutch navy . The Dutch navy used it up to the 1980’s.

The Hedgehog was fuzed with the No.420/1 impact fuze, placed in the nose of the Hedgehog.

Description and functioning of the No.240/1 impact fuze:
The fuze is nearly completely brass made, exept for the zinc booster housing and the steel arming vane and the threaded rod for the arming vane. Picture 02, 03, 04 & 05 show the fuze in safe position.
In rest, the threaded rod (1) of the arming vane (2) presses on the weight cylinder (3), also forcing the spring (4) loaded firing pin sleeve (5) and the spring loaded firing pin (6) in the lower position. The firing pin is held in position by three balls (7), pushed inward by a hole in the base of the the weight cylinder that fits over the balls in the firing pin sleeve. The firing pin in lower position also locks the detonator (8) from moving in line with the firing cap.
Four sping loaded radial locking pins (9) are placed around the firing pin sleeve at 90 degrees each.
A setback weight (10) with a shear wire is placed around the top part of the fuze cover, locking the arming vane from rotation, keeping the fuze safe.

Upon firing the setback weight moves downward, breaking the shear wire, releasing the arming vane. During the travel through the air the vane will not rotate, as the smal vanes of the arming wire do not deliver enough force to rotate. Upon impact on the water (picture 06, fuze in position as it travels through the water), the vane starts rotating until it has unwound completely. The weight cylinder, firing pin sleeve and firing pin move upward, forced by the spring under the firing pin sleeve. The four radial cylinders move inward , blocking the firing pin sleve from moving down again. As the firing pin moves upward, the spring loaded safety shutter rotates in line with the firing pin. In this position, the firing pin is kept locked only by the weight cylinder keeping the steel balls in inward position. Upon straight impact, the weight cylinder moves straight forward by inertia (pict.07 -purple), upon graze , the weight cyliner rotates in the direction of contact (pict 07 - blue). This releases the steel balls to move outward, and allows the spring loaded firing pin to move into the firing cap, which ignites the booster and main charge.

The Fuze No.420/1 is the British designation for this fuze , the US designation was Mk156 or Mk158.
The difference is that the Mk158 has a copper shear wire, the Mk156 does not have a shaer wire as it is used in the 7,2” rocket , which does not generate sufficient setback to arm the fuze.

Regards, DJH
 

Attachments

  • Pict.01 - Hedgehog wapon.jpg
    Pict.01 - Hedgehog wapon.jpg
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  • Pict.02 - Fuze No.420-1 - safe-  backside.jpg
    Pict.02 - Fuze No.420-1 - safe- backside.jpg
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  • Pict.03 - Fuze No.420-1 - safe -cutaway side.jpg
    Pict.03 - Fuze No.420-1 - safe -cutaway side.jpg
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  • Pict.04 -  Fuze No.420-1- safe - fuze detail.jpg
    Pict.04 - Fuze No.420-1- safe - fuze detail.jpg
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  • Pict.05 -  Fuze No.420-1 - setback sleeve detail.jpg
    Pict.05 - Fuze No.420-1 - setback sleeve detail.jpg
    189.5 KB · Views: 46
  • Pict.06 -  Fuze No.420-1 - armed - cutaway side.jpg
    Pict.06 - Fuze No.420-1 - armed - cutaway side.jpg
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  • Pict.07 - impact fuze functioning.jpg
    Pict.07 - impact fuze functioning.jpg
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