Those are sometimes called "stealth" torps. They run quiet, and slow, so they don't want the noisy launch procedure. But they also don't do 50 knots. On the other end of the scale there are the rocket torpedoes, really missiles that fly underwater at speeds over 200 knots. You might hear them coming but won't be able to do much about them.

https://en.wikipedia.org/wiki/VA-111_Shkval

And there are even anti-aircraft torpedoes in development. They are a total gamechanger.

https://en.wikipedia.org/wiki/IDAS_(missile)

Actually, latter versions of the Mark 48 will do this, running at variable speed. Slow launch, creep towards the target, and dogleg even, then run up to 50+ knots once it's close enough to be heard. With wire guidance, the sub could essentially drive the torpedo to where they wanted it before violent speed and maneuvering would likely cut the wires.

This was depicted somewhat famously in the book Red Storm Rising. By all accounts it was accurate.

Shkval is a really special case and not the supertorpedo some made it out to be. It's a countermeasure really, and it's range is far too short to use against other submarines. It's a last resort weapon, when someone's already found you.

IDAS seems really fascinating to me though, like the "Stinger SAM mast" concepts I'd sometimes hear about. IDAS seems more practical and safe to me though. Coming up to periscope depth to take a potshot at a helo sounds like a risky maneuver to me. IDAS would give better options.

Manpad-type missiles like the British Blowpipe have been mounted on submarine masks.

A major threat to a submarine is all the helicopters and patrol aircraft with their dipping sonars etc. There are some really great pics taken from periscopes of opposition helicopters during the cold war. Nothing you'd want to see in a hot war neither.

It seems good to throw a little missile at the helicopter you've just spotted approaching, to give them something to evade and worry about as you make your escape?

A helo would have to be much more careful if it thought it might be shot at by a missile, so some deterrence effect. OTOH if you shot at it and missed it would active dip right on top of you and then launch some LWTs, and you'd be blasted.

If they could leave an autonomous missile pod floating just below the surface, with a submarine-like ESM mast, that would be more interesting. You couldn't get in dip range to verify subsurface contacts without being in MANPAD (BOTPAD?) range. Really the same idea as autonomous mines, which have a magnificent power of area denial.

The current tactical solution to people taking potshots at helicopters is flying in pairs. Whoever gets shot at evades. Whoever doesn't get shot at shoots at wherever the shot came from. The Navy doesn't use this tactic much because the ocean is relatively free of people hiding in apartment buildings and bushes with RPGs but they could quickly adopt it if needed at the expense of effectively halving the size of their helicopter fleet.

I think if I just dodged a short range missile launched from the mast of a submarine, I’d drop a torpedo immediately to force the sub to dive.

There are now wire-guided AA-missiles that are launched from torpedo tubes while the sub is underwater.

Come on, everyone who has played Dangerous Waters enough knows that the best defence against maritime patrol aircraft is for the captain to boldly surface the boat and take the pesky plane down with a shoulder-launched missile ;)