SnakeDoc, et al:

The torpedo piston has guides that travel in machined grooves on the inside wall of the torpedo tube. Toward the breech end, these grooves taper until they disappear near the muzzle end of the tube, limiting the travel of the piston; this allows the
impulse air to remain inside the boat. By opening the tube vent, the impulse air is
now bled off, allowing the piston to return to the breech end. You wrote. "During whole launch..." Do you mean during a salvo launch? Was there a torpedoman shortage, necessitating the waiting until the salvo was complete to vent and flood
the tubes?

The U.S. boats used a "poppet-valve" mechanism to capture the impulse air after a
launch. There was a roller assembly that came in contact with the outer skin of the torpedo and was linked to the poppet valve itself. When the torpedo was launched, the roller followed the contour of the torpedo's after body, permitting the valve to
open, allowing the impulse air and some seawater to drain into the bilges. This venting operation had to be timed (3 to 5 seconds at periscope depth) just right or excessive seawater would enter the boat. This was the only "automatic feature" the
boat had, there were no "automatic gears."

Later,

ROBERT M.