I always thought that in addition to the torpedoes main engines there were maneuvering thrusters too, constantly firing short bursts, steering the torpedes to their targets ---True. But given that we retrospectively know (from TWOK onwards) that there is an opaque casing.I always liked the way the torpedos looked there....more like energy rather than a casing being fired.
It's possible that the torpedoes are energy weapons and the physical casings we see are sabots used while the torpedoes are inside the ship, inside the launch tube. They are vaporized when the energy torpedo undergoes ignition a couple meters outside the launch tube.