I believe the Germans used GUN-COTTON 36 for several reasons : it was cheap, easy to make ,did not require exotic or expensive or rare materials to make but more importantly it was a very stable explosive to conditions of heat,cold and humidity, all the things you would find inside a u-boat. The Germans were very afraid of an internal explosion from a torpedo so the torp. did not have a detonator screwed in untill the EEL was ready to be loaded into a tube.While the torp. was in storage the front of the warhead had only a cap or a plug to fit into the threaded hole,what this was made of I do not know but as a diver I dove on the U-85,352 and 853 and all the torps that I have seen these where gone ( rotten away ? ) , maybe wood etc. Another big reason for this type of explosive is for the job it had to do,ripping open and dislodging large chunks of metal plates and bulkheads requires an explosive with a lower "BRISANCE ",or shattering effect of the explosive.