First, the Soviet Shkval torpedo was developed in the 1980′s

Second, supercavitaion

1) Blinds your own sonar

2) Makes a noise loud enough to be heard a couple of hundred miles away by modern passive sonars (warship propellors are designed NOT to caviate for that very reason). Since a sub’s primary value is stealth, the idea is nonsensical

3) Objects moving at high speed in any medium are predictable – it takes a lot of energy and time to change the course (Newton’s Laws of Motion). So anyone who wants to intercept it, just tracks the noise signal for a brief period, gos to where the computer says the sub si alikely to be and voila. And because you are blind (see item 1), you don’t even know he’s waiting for you.

Oh, and if you apply too much force trying to maneuver, you break the vehicle. In a sub, there aern’t any ejection seats, everybody dies.

Outside of that, it’s a great idea.