Every once in a while in a sub movie when one sub is being chased by another sub, someone will say “They’ve opened the outer doors!” or “Their torpedo tubes are open!”.
How on earth can another sub detect that?
Opening the outer doors is a mechanical action, which produces transients (sounds). Some transients are recognizable. That said, this is mostly a Hollywood invention.
IANAS (I am not a seaman), but here is what I’ve heard/read:
When the torpedo tubes outer doors are opened, seawater flows into the tube. Apparently, this makes a distictive enough (and loud enough) sound that the other sub can recognize it.
I think they have to flood the torpedo tubes first, to ensure that they can launch the torpedo correctly. If they popped open the door just as the torpedo launched, I guess the inrushing water could prevent the torpedo from leaving the tube.
BCE
I bet that would be bad.:eek:
To be correct, you first flood the tube, then open the outer doors. You probably wouldn’t be able to open the outer doors against sea pressure if the tube wasn’t flooded first to equalize the pressure. So there is no “inrush of seawater” that is of concern.
But to answer the OP, the mechanical action of the doors can be picked up by any trained sonar operator within range.
Yes.
Not only would opening the doors to a dry tube be hard to do, it would be damaging to the doors if you succeeded, and even harder on the torpedo, which doesn’t much like getting slammed back in the tube’s breach door by the onrushing water. At 100 feet deep, the suddenly applied force to the nose of the torpedo would be in excess of 8500 lbf[sup]*[/sup], if you just slammed the muzzle door open. Not real healthy for the weapon.
True, so far as it goes. Note that “in range” means “real damned close,” for things like tube doors. These are precise pieces of equipment, and not very noisy. Flooding tubes is noisier, IME.
As I said, earlier: It’s mostly Hollywood puffery.
[sup]*[/sup]Some assumptions were made, IRT the area & shape of the nose of the torpedo, rate of flooding, as so on. If you only consider the diameter of the fish, and assume flooding of the tube progresses as a solid ‘plug’ of water, then force applied to the fish goes up significantly.
Basically, it’s done the same way in which the Starship Enterprise is able to detect when an alien civilization they’ve never met before is charging weapons and locking on. 
Ex-bubblehead, checking in. Adding to what Tranq said. The most noise is by the rush of water entering the tube when flooded, before the doors open. The doors are hydraulically opened, and are basically quiet at battle station ranges. To avoid detection of “opening outer doors” a la The Hunt for Red October, most CO’s will flood and open tubes before getting into stalking range <6-8000 yds. As Tranquilis also mentioned, “damned close” is less than 4000 yds, with almost assured destruction of both boats if they have decent firing geometry on each other and fire within that envelope.