There are three doors - the rear (inner) door, the cap (outer door) and the shutter. The shutter is the door on the outer hull, which conforms to the streamlined shape. The terms are all British, and I don't know what the German terms are.
http://www.uboatarchive.net/U-570BritishReport.htm Page 16.
This will tell you more about torpedo tubes than you ever wanted to know.
http://www.maritime.org/fleetsub/tubes/index.htm