Norway is buying $145 million worth of British Stingray lightweight (589 pound) torpedoes for use by helicopters, patrol aircraft and surface ships. The manufacturer does not discuss the price of individual torpedoes, because the Stingray has had budget problems throughout its three decades of development and use. Stingrays probably cost over $1 million each. The 8 foot long, 12.75 inch diameter Stingray is designed to be dropped from the air, and has a range, once in the water, of 11 kilometers. It can dive to 800 meters and has a 99 pound shaped charge warhead, to penetrate the sturdy hull of deep diving submarines using its own sonar to find the target.
The two main competitors of the Stingray are the MU90 and Mk 54. The MU90 is a French-Italian effort, a 669 pound, 9.4 foot long, 12.75 inch torpedo that entered service two years ago. The MU90 was built to replace a lot of the thousands of elderly U.S. Mk46 lightweight torpedoes still in use. 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 did 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.
The American Mk 54 lightweight torpedo, which entered production five 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 kilometers 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 will be 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.