Production of the 42cm granate introduced a filling step where molten TNT is poured into the granate. During cool down the TNT shrinks but starts to solidify from the the outsides in. This can cause a solid outside while the core is still molten and fluid. If you continue a open cavity/ space will be created, in the core of the material. To avoid this you have to press the molten material into a mould, shrinkage is compensated by the pressing together. Of course a tricky business and needs a good process control. Also dimension plays a role the bigger the more difficult to control and this is why big charges are devided into several smaller charges, We know that pre -war production of 42cm were of a better quality but war production, possible made under different standards process condition and "fresher" caused several expolsion of the granate while still in the barrel. I think that during war production the control on the creation of those cavities was less and firing caused an internal movement or fraction causing the charge to explode in the barrel.
Any additional information, thoughs or discussion welcome.
Regards, Harry
-- Edited by Haverba on Friday 17th of September 2021 07:08:29 AM
-- Edited by Haverba on Friday 17th of September 2021 07:48:22 AM
-- Edited by Haverba on Friday 17th of September 2021 07:49:20 AM
Possibly of more concern is the formation of bubbles in the shell filling when it was poured into the shell casing.
A bubble in the TNT or whatever the filling is can act as a point ignition source by the gas inside the bubble heating
by compression as the shell accelerates. Bubble formation is usually controlled by being careful with filling temperatures and filling rates.
Molten TNT is a high viscosity liquid similar to honey so a great deal of care is required to fill the shell casing without producing bubbles in the liquid.
Although applying pressure externally to the filling will help close up cracks and shrinkage it will do nothing to bubbles except for a temporary volume reduction.
The French had major problems with quality control in their shell production for the 75mm Mle 1897 guns in the first two years of WW1.
I've read that they lost more Mle 1897s to barrel explosions until 1916 than they lost to German action.
As you said, the Germans had issues with the 42cm projectiles during the war. The French also had issues with large projectiles - the 520mm
railway howitzer projectiles were very problematic. One of the two 520mm howitzers was destroyed by a barrel explosion in 1918 during initial testing
and the other in 1942 when the Germans used it in the Siege of Leningrad.
Excellent information thank you very much. If i understand it correctly it is the mechanism of a bubble heated up by compression, an adiabatic process cause an ignition
I used to work in the plastic industry and we had similar problems with injection moulding. After injecting molten plastic into a cooled tool, shrinkage can cause cavities, especially in the thicker parts. To compenstate this effect, during cool down a so called holding pressure is kept on the solidifying plastic in the tool. If the venting of the tool is not working and no air can be pressed out/ escape from of the tool, a pocket of air is pressed together causing burnt plastic surfaces, we call it "the Diesel effect. Harry
-- Edited by Haverba on Friday 17th of September 2021 10:49:36 AM
Just serached in Das Gerät der Artillerie vor, in und nach dem Weltkrieg- Das Gerät der schweren Artillerie. H. Schirmer were he clearly states that no explaination was found.
Justrow in Die Dicke Berta und der Krieg Justrow mention that the detonators were prodcuced not according pre war regulation and standards and that tolerances in dimensions can possible explain
Als inpurities in the TNT with the crafmanship of the workers is mentioned, in the pre war production no problems were found
Due to etensive use the first zone in the barrel is burnt out, there is a small zone with no riflling. So the granate moves smooth through this first zone till it hits the remaining rifling, resulting in shock.
Regards,
Harry
-- Edited by Haverba on Friday 17th of September 2021 02:51:59 PM
The last series of images appear to be a breech explosion rather than a barrel detonation. Certainly the first image a couple of posts back
was a classic barrel detonation with the barrel broken off.
I was looking at explosive filling technology and noted that these days US Army standards require every projectile to be X-rayed after filling
to ensure that there are no voids or bubbles in the explosive filling. This technology wasn't available in WW1 - early X-ray machines were in limited medical use.
