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Underwater Stealth: Mine Countermeasures' Ace in the Hole

Stealth is a game played superbly by the Air Force’s F-117 Nighthawk fighter and B-2 Spirit bomber aircraft in attacking heavily-defended targets. Although these airplanes are not completely invisible to radar at close range, their small detection radii, combined with careful mission planning, allows them to slip through gaps in air defense systems1. Some years ago, the U.S. Navy also entered the stealth game with the demonstration ship Sea Shadow2, whose technologies are now being incorporated into the design of the DD(X) destroyer and other surface combatants. Stealth technologies reduce ship susceptibility to detection and tracking by acoustic, hydrodynamic, and electromagnetic field sensors, both above and below water. Underwater stealth is especially important in defeating the threat posed by naval influence mines.

Mines are dangerous. Since 1950, naval mines have inflicted three times the number of ship causalities on the U.S. Fleet than all other threats combined3. During Operation Desert Storm (1991), USS Tripoli (LPH-10) was heavily damaged by an Iraqi contact mine, while an Italian magnetic-acoustic influence mine, the MANTA, attacked USS Princeton (CG-59)4. As recently as Operation Iraqi Freedom (2003), coalition navies were faced with the possibility of a significant naval mine-clearing operation, which was obviated when the weapons were captured before they could be deployed by Iraqi forces5.

Mines are cheap and can be easily manufactured or bought on the international weapons market. They are difficult and time consuming to find and neutralize, and they can be deployed covertly by an adversary without directly confronting the threat of U.S. naval forces. Casualties from a naval minefield can cost the lives of Sailors, delay or alter the outcome of a conflict, prevent rapid reconstitution of naval capabilities, damage local economies, and adversely influence foreign and domestic politics.

Effective U.S. naval mine-clearing capabilities are a major enabler for the “10-30-30” war fighting concept established by Defense Secretary Donald H. Rumsfeld6. Under his plan, the services would have 10 days to deploy a major force anywhere in the world, 30 days to fight and decisively win the war, and then 30 days to be ready to fight again. Neutralizing a minefield sufficiently to allow forcible entry or strike operations in only 10 to 15 days is a major challenge, even with an armada of underwater, surface, and airborne mine countermeasure (MCM) platforms. A ship casualty caused by a single mine that was missed during clearing operations could adversely impact all phases of the “10-30-30” strategy.

The Rules

The mine countermeasures game is complicated. The risk of losing a ship or submarine to a mine is very scenario dependent, and it is sensitive to such parameters as:

  • The density of mines in the field (determined by number and spacing)
  • The availability of mine hunting and sweeping platforms in theater and their effectiveness in the specific operational environment
  • Mission plans and their time constraints
  • The required length and width of safe-transit lanes (“Q” routes) and the area needed to conduct operations
  • The likelihood of friendly combatants to actuate a mine during their transit through the field
  • The vulnerability of friendly vessels to damage from a detonation

Although the absolute effectiveness of mine-clearing operations and their impact on the overall mission depends heavily on these factors, the general relationship of combatant losses to MCM tactics and technologies has a well-defined character irrespective of scenario details.

Off board MCM systems are very effective in detecting and neutralizing mines moored in the water column, especially in comparison to their performance against bottom mines. In addition, onboard mine hunting sonar equipment is being developed that can quickly and reliably detect moored contact mines with sufficient warning time to allow a naval vessel to avoid them. In contrast, the buried or partially-buried multi-influence bottom mine is a very imposing threat and difficult to defend against.

Naval combatant susceptibility to bottom influence mines has a parabolic dependence on the level of MCM effort expended before the first attempt to transit the field. Figure 1 shows hypothetical examples of this parabolic relationship for dense, medium-dense, and sparse minefields. The horizontal axis represents the amount of MCM effort measured in platform-days, that is, the sum of the number of days each MCM platform (helicopters, unmanned underwater and surface vehicles, etc.) devotes to the clearance operation. Although the absolute scales on the axes of the graph and the relative vertical separation between the three curves will depend on specific scenarios, the trends shown in the figure apply to any minefield.

The MCM “Game Strategy”

All good defenses are layered, and MCM is no exception. The first and best defense against mines is to prevent their manufacture, transport, and deployment. But because of tactical or political constraints, many of them will slip through and be deployed against U.S. and allied vessels. The second defensive layer includes detecting mines by hunting, destroying them with explosive charges, and decoying them with influence sweeping. However, there is always a chance that one or two will be missed due to mission time constraints, unfavorable environmental conditions, equipment malfunctions, operator error, or poor planning. The final burden of defense then falls on underwater signature stealth, to hide a vessel from attack by a mine or to blind it with a jamming signal.

The MCM effectiveness curves show several important characteristics of mine-clearing operations that can be used to plan a strong defensive strategy. First, the time constraints of the “10-30-30” war-fighting concept will limit the best possible MCM effort to some maximum value. As demonstrated by the vertical line in Figure 1, the risk to combatants would then vary depending on the density of the mines encountered. The first and best MCM strategy is to prevent mines from being laid, or to keep a sparse minefield from becoming dense by denying enemy forces the opportunity to deploy the weapons.

Losing a ship to a minefield or accepting casualties to its crew is unacceptable. The sinking of a single naval platform or its receiving mission-abort damage could easily extend the conflict beyond the 30 day war-fighting phase of “10-30-30”, and could also delay the second 30 day phase of preparing for the next war. Therefore, a low to very-low risk level is required for transiting combatants. The intersection of the horizontal line in Figure 1 with the asymptotic portion of the effectiveness curves shows that to achieve a low-risk condition could require a significant, or even unachievable, MCM time-line, depending on the mine density. The diminishing returns of the MCM effectiveness curves (flattening at higher levels of MCM effort) are caused by the resource-intensive process of removing the last one or two mines from the field; a characteristic of all mine-clearing scenarios. It takes only one missed $10,000 mine to sink a $2 billion ship.