Zygoma:
Word. I'm an ex-submariner myself, served in the USN for five years, four on a sub.
So all the factors leading into torpedo launch depth are basically as follows:
1. "failure" depth of the: torpedo tube (to the inside of the boat), breech door (inner door to the inside of the boat), the torpedo itself.
2. The amount of pressure your torpedo launch system can generate (versus water pressure on the outside of the boat). If you can't overcome sea pressure, you can't "launch" a torpedo, it would have to find its own way out. AFAIK, WWII u-boats didn't have "swim-out" torpedos, they had to be launched.
For condition #1, the torpedo tubes on these boats were made of brass, I think, which would not give it superior resistance to excessive water pressure. They were not made to launch deeper than periscope depth based on their (then) current combat doctrine.
(oops,correction: made of bronze)
http://www.ubootwaffe.pl/component/o...337/view,items
At the end it mentions specifically the breech doors being the weakest link as far as pressure.