South Korea has developed a missile, that is fired from a ships VLS (Vertical Launch System), and carries a lightweight torpedo out to 19 kilometers. When the missile reaches the approximate position of the submarine, the torpedo is released, and floats to the water via a parachute. Once in the water, the torpedo uses its own sensors to find and attack the submarine.
There are already lightweight torpedoes launched from the air that glide up to 20 kilometers before entering the water. The U.S. Navy has two glide kits for air-launched anti-submarine torpedoes (Fish Hawk and LongShot).
Putting wings on torpedoes is all about U.S. Navy concern about the growing use of anti-aircraft missiles by submarines. To deal with that problem, it wants to equip some Mk-54 torpedoes, that are normally dropped into the water at a low altitude, by P-3 patrol aircraft, with an add on glide kit. These systems consist of wings, control flaps, a flight control computer, battery and GPS for navigation. The kit allows a torpedo to be released at 20,000 feet, which is outside the range of submarine launched anti-aircraft missiles, and glide, for 10-15 kilometers, down to about 300 feet altitude, where the glide kit would be jettisoned, and the torpedo would enter the water and seek out the sub. Normally, the P-3 has to descend to under a thousand feet to launch the torpedo. This takes time, and puts stress on the aircraft. All existing P-3s are quite old, and it will be a few years before a replacement is ready, so reducing stress on the current ones is a major issue. Indeed, it may be the main issue for introducing the glide kit. The P-3 stress problem is rather larger than the number of subs out there equipped with anti-aircraft missile systems. These systems have been around for years, and many are basically shoulder fired type missiles adapted for launch from a water-proof container that is released by a submerged sub.
There are other reasons for the glide kits, and VLS launched torpedoes. Many subs have sensors that are sensitive enough to detect low flying helicopters (the main target for the subs anti-aircraft missiles) and aircraft. The P-3 is also more effective if it can stay at high altitude all the time. Moreover, the glide kit is easy to build, since it can use items already used for smart bombs (JDAM) and earlier glide kits.
A P-3 usually carries eight torpedoes. The Mk54 is a 12.75 inch weapon, weighing about 700 pounds and with a warhead containing a hundred pounds of explosives. Its guidance system has been designed to work well in shallow coastal waters.
There are several other light weight torpedoes that could use the glide kits, or be installed in missiles like the South Korean system. The French-Italian MU90 lightweight torpedo already has orders for nearly a thousand of the 669 pound, 9.4 foot long, 12.7 inch weapon. The MU90 has a maximum speed of over 90 kilometers an hour (with a max range of 12 kilometers), and a minimum speed of 52 kilometers an hour (for a max range of 25 kilometers). It can operate at depths of over 3,000 feet. The MU90 apparently does a very good job with being stealthy (not alerting the target sub that it was coming), and being good at defeating countermeasures. The MU90 uses sonar and an acoustic sensor for finding its target, and its warhead can penetrate the hulls of all subs currently in service.
One of the MU90s main competitors is the American Mk 54 lightweight torpedo, which entered production four years ago. Costing about the same as the Mu90 (about a million dollars each), the Mk 54 is a cheaper, and somewhat less capable replacement for the Cold War era high tech Mk 50 and the old reliable Mk 46. The 750 pound Mk 54 is a more cost effective alternative to the three million dollar Mk 50, which was in development for over two decades. The Mk 50 was difficult to build because it was meant to be a "smart" torpedo that was light enough to be carried by helicopters, and could go deep to kill Russian nuclear subs. But when the Mk 50 finally became available in the late 90s, the typical target was a quieter diesel-electric sub in shallow coastal waters. So the Mk 54 was developed, using cheaper, off-the-shelf, electronic components, some technology from the Mk 50 and larger Mk 48, as well as the simpler, but not deep diving, frame and propulsion systems of the older Mk 46 lightweight torpedo. Thus the ten foot long Mk 54 is a bit of a hybrid, created to save money, and also be more capable against quieter subs operating in shallower water. The Mk 54 has a range of about 10,000 meters and a top speed of about 72 kilometers an hour. It has a built in sonar that can search for the target sub, as well as acoustic sensors (listening devices to pick up any sounds a sub might make). The Mk 54 also has an onboard computer and a data file of underwater noises and search tactics, which are used as it tries to find its target, and keep after it until it can hit the sub and destroy it with the hundred pounds of explosives in the warhead.
In the last 40 years, some 25,000 of the older Mk 46 torpedoes were made, and at least a few thousand Mk 54s will be manufactured. Mk 50s are kept in inventory to deal with the few hostile nuclear subs that are still out there, although the Mk 54 also has a capability of going deep, just not as deep as the more expensive Mk 50. The MU90 is seen as a better value than the Mk 54, if only because it is a more recent design, and costs the same.