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Dmitry Yudo aka Overlord, jack of all trades
David Lister aka Listy, Freelancer and Volunteer

Sunday, June 24, 2018

Tandem HEAT warhead

In the years prior to the Second World War the Swiss inventor Henry Mohaupt started work on the Munroe effect, and is credited with developing the hollow charge anti-tank projectile. Although this is a bit of a generalization as there were many people thinking along these lines and may have also held the claim. The British Army embraced the idea of the HEAT projectile and began to work with it, resulting in a 2" No. 68 anti-tank grenade. The Germans were not far behind however by now with the war in full swing the British began to see themselves as having a technical advantage over the Germans with this technology.
The No2 68 Grenade was fired from a standard cup discharger on a SMLE.
To this end the British began to work on HEAT projectiles for a quite a few applications. In the spring of 1942 a requirement was issued for hollow charge rounds for all field guns, and both the 3.7" Mountain Gun in service with British and Commonwealth forces in Burma, and the new 95mm Close Support howitzer. The latter was a new infantry gun made from a section of 3.7" AA gun barrel, a modified breech from a 25-pounder and the recoil mechanism from a 6-pounder. As it had the same calibre as the 3.7" howitzer, and would also be a howitzer, the British could not use their conventional naming system. To avoid confusion they metricized it and made the calibre 1mm bigger, giving it the name of Ordnance QF 95mm Howitzer. The same solution was used in later years for the 77mm on the A34 Comet.

A Churchill Mk.V. The tank mounted versions of the 95mm had a counterweight on the muzzle. If you look closely the bottom of the counter weight is flattened. Something quite a lot of people miss.
The 95mm came in two varieties. One fired a fixed round and one a split round. The fixed round version was used in tanks such as the Churchill Mk.V or the Centaur. The split round was designed as an infantry gun but would never see service. It would however be fitted to the Alecto self-propelled gun when it entered its brief service life.
Alecto's in Germany, armed with 95mm's.
The shells designed for both guns met a simple problem, the hollow charge warhead had to be detonated some distance from the targets surface to allow the jet to form, today we call this standoff. In these shells the standoff was calculated at 62.48mm. The fuse had to be at the front of the shell to impact first and trigger the detonation. However, the detonation needed to start at the base of the shell. In between the two was the shaped charge, which needed a hollow cavity to allow the jet to form. In modern shells this is achieved by micro-electronics and a wire, but such technology was not available at the time.

The two options open to the designers were a really long mechanical striker that ran down the centre of the round, or a detonating train. The latter was an explosive line that carried the explosion from the fuse down to the desired location.

At first the simplest option was chosen, the long striker. However, the width had to be limited to avoid interfering with the hollow charge, this meant it was extremely thin and would not function fast enough to maintain the standoff. A detonating train would be very complex and difficult to manufacture to survive even limited muzzle velocities.

In July 1942 a new idea was tried. This involved two hollow charges facing towards each other. The top one facing backwards (towards the base of the shell) was smaller and had a shallow cone angle. This was triggered by a simple direct-action fuse, and it would then fire its jet along the length of the projectile and hit a pellet which initiated the detention of the main hollow charge. Trials were held with a 6.35mm hollow charge, which could trigger the pellet at a range of 1ft. It was found this initial charge could be fired fast enough to avoid the smaller shaped charge being moved out of alignment by the impact of the projectile on the target. This type of charge has been on occasion described as a 'Spitback' fuse, although that is not an official designation.
3.7" howitzer in action in Burma in 1944
When a 3.7" howitzer was used to fire these experimental rounds the penetration of the main charge was found to be very poor, far below expectations. It was suspected that the revolution of the shell was causing problems, and this was confirmed when a test shell was spun up to 12,000rpm when fired. To get around the issue of spin the cone angle was changed from 80 degrees to just 45 degrees and the standoff distance lowered. At this point the requirement for field guns to be able to fire hollow charge shells was dropped but remained for the 3.7" and 95mm.

In the final design the nose charge was aimed to fire down a hollow tube that started at the apex of the 0.08" brass liner. At the base of the hollow tube was the pellet to cause initiation of the main charge. Twenty rounds were manufactured for trials and penetrated 90mm of IT80 plate, sloped at 30 degrees, or 77mm at 44 degrees. A trial production run of 2,000 rounds was ordered, with one small change, the liner was switched to steel. Of these 200 rounds were fired in August 1943 and achieved 110mm of penetration at 30 degrees. Not a single round was a dud, and all functioned perfectly. 
95mm HEAT round, in its fixed version for use in tanks.
 With all in hand main production was started by a different manufacturer of the 95mm HEAT rounds. Immediately problems started as proof testing failed. After fitting the fuses to empty 40mm rounds, and firing them through mild steel plate, which triggered the fuse, it was found that alignments were off due to the new manufacturer forgetting to use gauging properly during the assembly process. Equally the assembly quality was bad with some shells having their nose caps just placed on the round and not pushed home before being welded. The defect with the alignment of the fuse was fixed around May 1944.

In June of the same year a suggestion was made to change the No.233 fuse for a No.243 fuse. This would allow the use of the round on soft targets. However, trials showed that if the round was fired under 2,000 yards it would be a dud.

Problems with the shells continued when a further trial in the first half of 1944 fired 99 shells filled with an inert filling, but a live fuse. It was found that the filling was leaking into the hollow tube that the nose fuse was aimed to fire down. This would absorb the jet and prevent the main charge from detonating.

There is surprisingly little evidence of what happened to the 95mm rounds during 1944, there is no evidence that they were issued in Europe. As the rounds were listed as 'not required for service' in October 1944, it seems unlikely that they were ever issued. My best speculation is that the manufacturing problems persisted and that few, if any, rounds passed proof stages. Thus, the entire project was abandoned.

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