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Playing the Ace

Underwater stealth for ships is achieved through the application of signature- reduction technologies. Influence mines actuate on the mechanical or electromagnetic energy generated by a ship or submarine’s hull, machinery, or electrical equipment. Ships and ship systems generate acoustic and seismic signatures, hydrodynamic (pressure) signals, static magnetic and electric fields, and electromagnetic (alternating electric and magnetic) fields. An influence mine’s firing logic combines outputs from its sensors to:

  • Reduce environmental background noise
  • Classify the target
  • Localize the target to maximize lethality of the attack
  • Identify and reject signals from minesweeping systems or other false target sources.

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The Swedish Visby-class corvette is a composite hull (carbon fiber reinforced plastic) combatant that has both above and underwater stealth design features. The design of the Visby minimizes the optical and infrared signature, above water acoustic and hydroacoustic signature, underwater electrical potential and magnetic signature, pressure signature, radar cross section, and actively emitted signals.

Minimizing a naval vessel’s underwater signatures makes each of these four tasks much more difficult for the mine, and sufficient stealth can actually render it ineffective.

Like stealth aircraft flying against an air defense system, reducing underwater signatures can shorten an influence mine’s actuation radius to the point where it is no longer a threat. For example, water depths deeper than a bottom mine’s attack range need not be immediately cleared, along with the buffer zones along the edges of transit lanes and maneuvering areas. In addition, vessels employing underwater stealth technologies would have a reduced probability of actuating any residual mines that might have been left after clearing.

Decreasing the attack radii of deployed influence mines is analogous to reducing the effective density of the field. For example, if 100 mines have been deployed in an area but only 25 can detect the transiting targets due to the latter’s low signatures, then the effective density of the field has been reduced by 75 percent. As shown by the parabolic mine-clearing curves in Figure 2, underwater signature mitigation can reduce the effective mine density, and improve the efficiency of MCM operations by lowering the time needed to achieve a low risk condition significantly. Eventually, all mines will have to be removed from the field before naval and commercial ships not equipped with stealth technologies can transit the area, but this can be accomplished after the time-constrained forced-entry or strike phase of the operation has been completed.

A second way underwater stealth improves MCM effectiveness is to increase the efficiency of minesweeping. The firing thresholds of mines are typically set so that for actuation, the target needs to be close enough for a detonation to result in a high kill probability. Depending on the scenario, setting the firing threshold too high – requiring a larger signature – could result in a catastrophic failure of the minefield, allowing all ships to pass safely. But if ship signatures are reduced and the minefield planner does not likewise lower his actuation thresholds, then the firing radius of his mines will be smaller. On the other hand, if the minefield planner decreases the actuation thresholds to maintain the same damage radii with quieter ship signatures, the more sensitive firing criteria will make the mines easier to sweep. In either case the risk to transiting ships is reduced, and can be viewed once again as equivalent to lowering of the mine density curves of Figure 2.

An ace in the hole does not guarantee a winning hand, especially if it is played poorly. It has also been suggested that artificially enhancing the amplitude of a vessel’s underwater signatures would reduce the threat of influence mines by causing them to detonate while the target is still outside the warhead’s damage range. However, the firing logic found in modern multi-influence weapons easily prevents this from occurring. Thus, deliberate signature amplification would raise the effective density of the field by increasing the actuation ranges and threat from those mines that were previously rendered ineffective using underwater stealth techniques, while providing little protection to follow-on traffic (see Figure 3). Risking a $2 billion manned combatant to sweep a minefield instead of a helicopter or unmanned surface vehicle is not a good bet.

Ironically, a submarine, the quintessential stealth naval vessel, cannot use all the mine-clearing tools available to surface ships. To remain undetected, pre-cursor sweeping before transiting a minefield is generally not an option for submarines. Even if unmanned underwater sweep systems were available, their successful use in detonating mines would immediately give away a submarine’s approximate location or reveal its intended lane of transit. A submarine must rely solely on hunting mines, avoiding them, and if necessary, covert – non-explosive – neutralization.

Removing sweeping from the submarine’s mine-clearing toolbox raises the minefield’s effective density in comparison to an equivalent surface ship scenario. For the reasons discussed, all mines may not be detected during hunting operations. In addition, losing the benefit of pre-cursor sweeping increases the mine hunting time-line necessary to reduce the submarine’s risk to an acceptable level. Therefore, more mines will remain in the field – yielding a higher effective density – for a submarine, hunting-only scenario than in the equivalent case for surface ships that includes sweeping (Figure 4) As a result, a submarine requires more underwater stealth and higher levels of signature reduction than a surface ship to survive similar types of minefields.

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The Sea Shadow program began in the mid-1980s. Its purpose is to explore a variety of new technologies for surface ships, including ship control, structures, automation for reduced manning, seakeeping and signature control.

Stacking the Deck

Unlike poker, warfare is not a game that needs to be played fairly. The lives of Sailors and the wellbeing of the nation are at stake. Overwhelming military force must be used to win conflicts quickly and within the clearly defined “10-30-30” time constraints. Underwater stealth can enable combatants to achieve surprise and conduct their strike-warfare missions quickly and with low risk of mine damage, even with minimal MCM efforts.

New signature reduction technologies could make naval combatants as invulnerable to mine threats as the F-117 fighter and B-2 bomber are to air defenses. However, as with aircraft, achieving stealth must be a primary design objective from the very beginning, because incorporating the means to quiet underwater signatures cannot be an afterthought while building ships and submarines.

All aspects of mechanical and electrical ship systems must be considered in designing a vessel with quiet underwater signatures. These include hull shape and internal structure, material properties, propulsion and auxiliary machinery, electrical systems, payloads, sensors, and active signature-compensation systems. A true stealth ship would have a very small mine risk curve similar to that shown in Figure 5. Like its more familiar aircraft counterparts, the stealth ship may not be completely invisible to influence mines at extremely close ranges; but combined with careful mission planning, it could slip in and out of minefields with near impunity.

The design and construction of a strike-capable “Underwater Stealth Ship” would benefit U.S. naval warfare in several ways. First, a squadron of these combatants could be used against objectives that are heavily defended by mines with the same military advantages realized by stealth aircraft against robust air defenses. Second, all the aspects of a vessel’s structure, systems, and individual components that contribute to its underwater signatures would be identified, their relative importance quantified, and silencing methods developed. Simple and inexpensive changes to designs could be immediately incorporated into new construction. Also, the development of new technologies to provide revolutionary, low signature levels would be accelerated in the process. Incorporation of these new technologies into the design of all future naval vessels could then concentrate on reducing system costs and ship impact. The force-multiplying payoffs and the technology development process of the F-117 and B-2 aircraft exemplify the right way to play the game of underwater stealth.

Dr. Holmes is a Senior Scientist in the Underwater Electromagnetic Signature & Technology Division (CODE 75) in the Carderock Division of the Naval Surface Warfare Center in West Bethesda, Md.

(left to right) Fig. 1 Mine countermeasure effectiveness and the relationship between combatant risk and mine-clearing effort.

Fig. 2 The impact of underwater stealth on mine-clearing effectiveness.

Fig. 3 The impact of deliberate signature amplification on risk of loosing a combatant.

Fig. 4 The impact of omitting sweeping from submarine mine-clearing operations.

Fig. 5 Risk of a minefield to a stealth ship.

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