Oh, a submarine "rarely" goes above 30 knots? You know this, how exactly? Because you're a nuke sub expert? Alfas were well known to have burst speeds of >40 knots, BTW. In any case, by way of an example the USS San Francisco was routinely transiting the Pacific Ocean well in excess of 30 knots when it collided with a seamount. This was not a drill, not a high speed test, not an evasion from a detector. These subs normally transit open oceans at max speed or near max speed simply because they can. Fuel is not an issue, and out in the open ocean during peacetime, detection and tracking is not an issue. The sub collided with a known underwater feature due to the incompetence of its crew and the proper punishments were meted out, but the point is that the claim that subs rarely go above 30 knots is categorically false.Anyway your number are a bit off Submarine speed is rarely goes above 30 knot=0.92km/sec and the rocket speed is not Mach 0.85 but Mach 0.95=(1163km/hr)/60=19.5km/sec So distance of 50km can be reached in 2-1/2 sec. So the submarine move about 2.3km which standard range of torpedo with 50 knot speed and 1 hr endurance
I the first generation missile, CY-1 some years ago, but this analysis observes that CY-1 “obviously could not meet the Chinese Navy’s requirements.” The article does not claim definitive information on the characteristics of the Yu-8, but offers the following projections. The missile is expected to be about five meters in length, and to weigh less than 700 kilograms. The range is said to be approximately thirty kilometers at a speed of Mach .9 to .95. The torpedo part of the weapon is estimated to be 324 millimeters in width and capable of detection (both passive and active) over 1.1 to 2.5 kilometers.
So anyway, according to your math, mach 0.95 is apparently "1163km/hr=19.km/sec" with 50km being reached in "2-1/2sec". ROFLMAO clearly you forgot or conveniently dropped an additional 60 in your conversion. The ACTUAL speed in m/s of a mach 0.95 weapon is 323 m/s, meaning it takes 2 minutes and 35 seconds to reach 50km, in which time a sub will have traveled 2.32km. The search range of a Yu-8 is apparently 2.5km which means the sub is already near the edge of the torpedo's detection range when it gets dropped into the water, and if the torpedo's seeker doesn't acquire the submarine within ~11 seconds the torpedo will not be able to acquire the submarine at all. I'm not saying it's impossible, but this is certainly not the ideal situation to put a torpedo in for it to do its job. Far better is the instant drop from a helo right on top of the sub being detected, so that the chance the sub can evade the attack is about as close to zero as you can get.