The U.S. Navy recently ordered the first 40 (of 890) upgrade kits for its Mk 54 lightweight anti-submarine torpedoes (LWT). The new Mod 2 LWT upgrade 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 technology over the decade since the Mk 54 tech was developed.
The Mk-54 is carried mainly by aircraft but also by many American and foreign surface ships and 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 often just one). The Mk 54 is a 324mm (12.75 inch) weapon, weighing about 340 kg (750 pounds), and with 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 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.
The Mod 2 upgrade is only one of many for the Mk 54To make the Mk-54 more effective on patrol aircraft, the U.S. Navy has also developed glide kits. Putting wings on torpedoes is all about 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 (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 6,300 meters (20,000 feet), which is outside the range of submarine launched anti-aircraft missiles, and 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 puts stress on the aircraft.
Many subs have sensors that are sensitive enough to detect low flying helicopters (the main target for the subs anti-aircraft missiles) and aircraft. Patrol aircraft are 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.
The Mk 54 lightweight torpedo entered production nine years ago. Costing 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 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 3.25 meter (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 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.
In the last 40 years some 25,000 of the older Mk 46 torpedoes were manufactured. 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.