A torpedo is basically a missile designed to travel though water instead of, or in addition to, air, to blow up a target in the water, most often a submarine, but sometimes, a surface warship. They can be deployed from fixed wing aircraft, fighter jets, submarines and warships.
The U.S. Navy began using the 324mm (12.75 inch) Mk 54 LWT [Light Weight Torpedoes] in 2004 and since 2010 has been upgrading these with a Mk 54 Mod 2 LWT upgrade kit that consists of a more sensitive sonar (to seek out a submarine) and more powerful computer to interpret what the sonar hears. This upgrade takes advantage of the many advances in electronic and computer components over the two decades since the current Mk 54 was first developed. Many components for the Mk 54 came from the older Mk 50, which ceased production in the mid-1990s. Since then, old Mk 50s have been cannibalized for parts but that supply is running out. Rather than just build more of these older components, new and improved components were designed and, as with this Mod 2 kit, used to upgrade existing Mk 54s and equip new ones.
The Mk 54 is carried mainly by aircraft and helicopters as well as many American and foreign surface ships. The LWT has replaced depth charges as the main weapon against submarines. The Mk 54 is particularly effective when used by aircraft equipped to seek out submarines. Patrol aircraft can carry up to eight lightweight torpedoes, while helicopters can carry up to three, but usually just one or two. The Mk 54 weighs about 340 kg (750 pounds) and has a warhead containing 45 kg (100 pounds) of explosives. Its guidance system has been deliberately designed to work well in shallow coastal waters, where ships are believed most likely to encounter subs. Until 1991, when the Cold War ended and the Russian nuclear sub fleet disappeared, the emphasis was on fighting subs on the high seas where the water was much deeper.
There are several upgrades available for the Mk 54. For example, to make the Mk 54 more effective on patrol aircraft, the U.S. Navy developed glide kits. Putting wings on torpedoes is all about the concern at the growing use of anti-aircraft missiles by submarines. To deal with that problem, the Navy sought to equip some Mk 54 torpedoes, which are normally dropped into the water at a low altitude by P-3 or P-8 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 6,300 meters (20,000 feet), which is outside the range of submarine-launched anti-aircraft missiles. When dropped the torpedo can glide for 10-15 kilometers. When down to about 100 meters (300 feet) altitude, the glide kit is jettisoned and the torpedo enters the water to seek out the sub. Normally, aircraft have to descend to under 330 meters (a thousand feet) to launch the torpedo. This takes time and fuel as well as putting stress on the aircraft.
Many subs have sensors sensitive enough to detect low flying helicopters (the main target for their anti-aircraft missiles) and aircraft. Patrol aircraft are more effective if they 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.Mk 54 LWTs cost about a million dollars each and are a cheaper and somewhat less capable replacement for the Cold War era high tech Mk 50 and the old reliable Mk 46. The Mk 54 is a more cost-effective alternative to the three million dollars 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 but could go deep (560 meters beneath the surface) to kill Russian nuclear subs. Alas, when the Mk 50 finally became available in the late 90s, the high-seas Russian nuclear subs were gone and the typical target was now a quieter diesel-electric sub in shallow coastal waters. In response to that 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 3.25 meter (ten foot) long Mk 54 is a bit of a hybrid, created to save money and be more capable against quieter subs operating in shallower water. The Mk 54 has a range of about 10 kilometers and a top speed of about 72 kilometers an hour. It has 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 explosives in the warhead.
From here.Since the 1960s some 25,000 of the older 230 kg Mk 46 torpedoes were manufactured and many are still in use, even though the last Mk 46 upgrade was in 1979. Capable of reaching targets 370 meters deep, the Mk 46 received several upgrades that kept it worth retaining. A few thousand Mk 54s have been produced so far. 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.