Ken, et al:

You wrote: "......(perhaps to keep the noise down)....."

With all the noise of the noise of the impulse air pressure entering the
torpedo tube, who's worrying about the "Plate" coming to an abrupt stop in the tube, or it being forced back into the tube against the breech door after the "eel" left? In addition to those noises, when the G7a and G7e starting mechanisms were tripped, more noise occured. A torpedo-firing
U-boat generally fired on the surface, and later mostly at periscope depth, due to ASW fears, so I don't understand the concern for noise during a firing evolution.

You wrote: "There were also other things inside a U-boat (compressors, etc.) that raised the pressure inside the boat........"

The air compressors did not increase the pressure in the boat, on the contrary, they in effect, lowered the pressure when operating to charge the air flasks.

I may be wrong, but I believe that the "expulsion heads" used air pressure to eject their contents into the sea, mostly at or near periscope depth, in order to conserve air. This required venting inboard also.
Routine servicing of the G7a units required the topping-off of the "eel's"
air flasks, and when the charging line was removed from the torpedo, a small amout of air escaped into the room. I don't know if U-boats used
compressed air to shift seawater in their ballast tanks (like the U.S. fleet
boats), but that would be another source of air pressure in the boat., after venting the tanks. How did the diving officer ascertain that the boat was "tight" during its dive? The fleet boats bled high-pressure air in the boat and watched an altimeter located on the diving stand to verify if the boat was 'tight'.....

If the conning-tower hatch flew open when the first person attempted to
open it, then that was merely stupidity. All that is required for venting that accumulated pressure in the boat, is to "crack" the hatch by controlling how many turns are used on the handwheel. It can be accomplished easily every time by experienced submariners.

You wrote: "I dont know if the pressurre differential......."

All a submariner has to do is "pop" his ears, to keep equalized, similar to
equalizing in an airliner or shallow-water diving.

You wrote: "In one case a "mine ejector" was used........"

Why not eject the "hot run" the same way as firing a regular torpedo?

The G7a and G7e torpedoes started their propulsion systems immediately at launch. This was accomplished by a "tripping latch"
positioned on top near the afterbody. The torpedo had to be loaded into the tube until it came into contact with the retractable torpedo stop bolt in the top of the tube. The "plate" was positioned against the tail of the torpedo and contacted the inner part of the breech door. This configuration kept the the torpedo aligned with the retractable tripping mechanism until it was launched.

The G7e as well, had to be serviced periodically. The continuous
generation of hydrogen from its batteries, even at rest or during a charging evolution, required venting into the boat's atmosphere. An
accumulation of more than 4% hydrogen in any part of the boat would
result in an explosion; so that's when the air compressor is lit off.

When viewing "DAS BOOT", it appears that the torpedomen have a liesurely existence. This far from the truth; they are the busiest people on the boat either durng torpedo firings or when it's time to "routine" their "eels."

Maybe some knowledgeable forum contributor can help me out here.

Regards,

ROBERT M.