The tactical antisubmarine warfare challenge is more complex and less predictable than ever. Torpedo defense—countering the weapon rather than the shooter—isn't new, but it's more important than ever.
It is 1939. Germany announces it will erect an underwater wall of steel around Great Britain—a wall of torpedo-equipped submarines—and starve the country into submission. Coordinated groups of German submarines called wolf-packs exact a devastating toll on shipping across the Atlantic through the first years of the war. Often they lie in wait near harbor entrances, and just a few miles off the U.S. and European coasts, the ocean floor is littered with the poisoned fruits of their efforts.
It is 1939 again. Contrary to a widely held misperception, the end of the Cold War did not end the underwater threat to surface forces. They actually are at greater risk today, and will be for the foreseeable future. We are operating more and more in the submariner's backyard, and his ability to hide in those critical areas has outstripped our ability to find him. Sixty years later, the littoral antisubmarine challenge has scarcely changed, and much of what we learned about open-ocean ASW in the Cold War is irrelevant.
The tactical challenge is more complex as well. We used to face one potential adversary; now there are a dozen. One of them, China, has detailed war plans involving the United States and has threatened that U.S. ships would be attacked should we attempt to repeat the 1996 presence mission off Taiwan. The wide variety of submarines on the world market—30 different models with a range of features—their weapons, and their tactics confound any efforts to build a "one size fits all" defense.
Torpedo defense—negating the inbound weapon instead of the firing platform—is not new, but it has gained greater importance in the past 15 years. This focus on countering the "arrow" rather than the "archer" parallels a similar shift in antiair warfare (AAW), and meaningful lessons can be extracted from our AAW experience for application to the growing danger from below.
The Current (Poor) State of the Art
If a submarine cannot be held at arm's length, we must contend with its weapons. Torpedo defense methods fall loosely into two categories. Nonreactive measures are taken prior to torpedo launch. They include maneuvers (such as evasive steering and sinuous course clocks) and decoys to complicate the submarine's fire-control problem. Reactive torpedo defense encompasses measures taken once the torpedo has been detected.
For surface ships, reactive torpedo defense has consisted mainly of special high-speed maneuvers to avoid the inbound threat. The problem is, these maneuvers are both threat specific and time critical. Execute the wrong one (evasion for a straight-running torpedo when what you actually have is an inbound wake homer, for example) and you probably will get hit. Pick the right tactic but execute it early or late and the ship also is at grave risk. Learning of an inbound threat is a particular problem for ships without sonar, such as aircraft carriers and logistics and amphibious ships, but even if these ships could detect the threat, their maneuvering characteristics generally preclude successful implementation. The most mission critical units are therefore deaf and crippled, ripe prey for the modern Wolfpack—or lone wolf, for that matter.
Most of the significant Cold War advances in the detection, classification, and localization phases of ASW do not readily provide target range information. They focus on passive acoustic techniques. Range is important, because it gives essential data on available reaction time and optimal timing of a response. Could you imagine attempting close-in AAW with no range information? That is the current state of torpedo defense.
Finding the target is hard; killing it is harder. Underwater fire control is far more difficult than that for close-in self-defense against missiles. First, information transmission paths in water are chaotic—thermoclines, salinity gradients, wake effects, and reflections and reverberation complicate the process of determining a contact's position dramatically. Second, data rates are constrained by the underwater environment. Propagation speeds (RF/infrared in air versus acoustic in water) are nearly 200,000 times slower in water, so contact information received by a ship will be much older. Of course, underwater threats move more slowly than airborne threats, but what matters is the threat's ability to maneuver/close relative to the rate at which new information reporting the change can arrive and be assimilated. Table 1 compares the ratio for a Mach 3 missile (a very challenging air threat) to that for a 40-knot torpedo (a typical speed for some threats of primary concern).
Consider the implications. Referenced to the speed at which new data and the rates of change of those data can be learned, the 40-knot torpedo is 4,500 times faster than the Mach 3 missile (.012 / .000269). Now, one probably need not (or cannot) process incoming information as quickly as RF or infrared radiation can provide it, but even if the defender can use information on an inbound missile at only 1/1,000 of the rate at which it can arrive, the 40knot torpedo still is effectively five times faster than the Mach 3 missile. Of course, what does this matter, if the ship has no effective counter for the threat?
Why hard kill? There is one common aspect of the available nonreactive and reactive methods. They are all soft-kill techniques, meaning they do not destroy the inbound threat. They merely attempt to prevent it from hitting the ship, and they bring along their own set of vulnerabilities. Towed decoys inhibit maneuverability. Decoys are very threat specific, and signal-processing advances in homing torpedoes will continue to degrade their already limited effectiveness. Depending on the type used, a launched decoy may interfere with the ship's sensors. Besides, no decoy can distract a straight- or pattern-running torpedo armed with a contact fuse, and it is this very low-technology weapon that has been the choice in every successful antiship torpedo engagement since the end of World War II. No ship's commanding officer would accept having the SLQ-32, chaff, and infrared decoys as the only counters to an inbound missile in a hostile fire zone, so it is startling that the community appears to have acquiesced to this limited capability against underwater threats.
The hardest near-term threat. The wake-homing torpedo is an extraordinary challenge. First, the acoustic environment is chaotic. Vortices, bubbles, and the proximity of the surface complicate obtaining good fire control (and terminal guidance) data. Second, terminal guidance is based on calculating a predicted intercept point. If the target is maneuvering and range rate and bearing rate information is available only sporadically (or is old), the prediction will be off and the intercept will not occur. The kill mechanism must cope with that challenge, either through near instantaneous intercept or through sophisticated composite tracking.
So, a multifarious threat, short reaction times, a vexing environment, and very capable weapons define the current torpedo defense problem. How do we solve it?
Underwater AAW—Path to an Integrated Capability
In AAW, great emphasis has been placed on the continuum of the problem and on the need for an integrated, layered defense. The Standard missile/Rolling Airframe missile/Close-in Weapon System AAW suite on today's cruisers and destroyers can trace its lineage to the hard lessons learned off Okinawa in 1943, where surface forces had their first taste of stream raids of antiship cruise missiles. The elements developed over the past 60 years for cone with an airborne threat apply directly to blunting the growing danger from below—robust detection, classification, and localization, both hard and soft kill weapons, a layered defense, and an integrated control system tying it all together.
This last requirement—for an integrated control system—has been a stumbling block. The first serious, modern effort began in 1984 with the National Surface Ship Torpedo Defense (SSTD) program. Its aim was to provide non-ASW ships with a counter for the wake-homing torpedo, and the system was to have included a (passive only) cueing sensor, decoys, and an anti-torpedo torpedo. The first SSTD system failed its operational evaluation (OPEVAL) and was not released to the fleets, but the program did result in one enhancement to the fleet's torpedo defense posture—the Multisensor Torpedo Recognition and Alertment Processor. The Naval Undersea Warfare Center in Newport, Rhode Island, is working on an outgrowth of that system, but it is passive acoustic only, so the war fighter still would have no range information.
One interesting option for providing range data is derived from the diverse fields of bioengineering and hardrock mining. Termed electrohydraulic effects (EHE), the technology uses spark gap projectors to create underwater electrical pulses and a very large, compressive shock front. Depending on the power level, these focused pressure waves (FPW) can crush kidney stones or fracture rock. FPW could be a highly effective torpedo hardkill technique, and at lower power outputs the arrays can be monostatic, bistatic, or multistatic sonar sources to help provide both bearing and range information.
Acoustics may not be the only means for obtaining warning and range information. Submarine commanders routinely come to periscope depth to visually confirm their targets, and although a periscope is difficult to detect, a recent Proceedings article reported on successes of an Automatic Radar Periscope Detection and Discrimination (ARPDD) initiative. The radar's ability to find and classify periscopes with a very acceptable false-alarm rate might provide another opportunity to obtain a composite, multisensor detection and track on a potential threat. Integrating an above-water sensor in an underwater sensor network would add a new dimension to undersea warfare. If nothing else, ARPDD could provide cueing to get decoys in the water in time to complicate the submarine's target solution or to confuse an inbound torpedo.
Hard-kill options. The Navy has considered a variety of counter-weapons for torpedoes-towed explosive arrays, precision launched munitions, antitorpedo torpedoes (ATT), directed-energy weapons, and supercavitating supersonic projectiles (SC-SSPs). The latter three have demonstrated sufficient potential to warrant further development.
The Applied Research Lab at Penn State has been working for several years with the Ordnance Laboratory at Indian Head, Maryland, to develop a 6.25-inch-diameter antitorpedo torpedo. Only the third U.S. torpedo fielded with digital processing, its guidance and control section uses a single array for guidance, homing, and fusing, and the multimode seeker and processors give the ATT an onboard composite track of the target. The ATT also makes use of neural nets and fuzzy logic to recognize the threat type and adapt its behavior in flight. Prototypes have successfully intercepted straight, pattern, and wakehoming torpedoes.
One very promising technology for an inner layer of hard-kill torpedo defense is FPW-EHE, and at least one important study has recommended consideration of directed-energy technology as a means to improve underwater defense. The technology has proved feasible in mining equipment for hard-rock drilling in flooded mineshafts and in ore crushers.
FPW-EHE brings a unique element to shipboard combat systems—the same arrays used for hard kill can serve as the sensor source. Because detection by the array defines the sound path to the target, the fire-control solution is obtained automatically. In addition, the "interceptor" travels to the target at the speed of sound; there is no time problem. Using electric spark arrays, EHE essentially has an unlimited magazine capacity. There also are commercial flywheel energy storage systems that could supply the array. The benefits, especially in view of the decision to make DD-21 all electric, present opportunities for autonomously sustainable combat power.
Supercavitating-supersonic projectiles (SC-SSPs) have been studied for about 17 years. At supercavitating speeds, a body becomes surrounded by a lowdensity vapor bubble. This reduces drag dramatically, permitting supersonic "flight" under water. The range of SC-SSPs probably will fall inside the envelope that could be protected by FPW-EHE, but that makes them a natural third layer of defense. Employed in short bursts of multiple rounds, the high-speed projectiles could populate a predicted intercept volume fast enough to obviate the need for terminal guidance.
A layered, integrated system. Systems integration will be a key to fielding a credible capability, and a blueprint for approaching that challenge can be found in the AAW self-defense realm. The Ship Self-Defense System now being installed in large amphibious ships uses a local-areanetwork (LAN) architecture and a set of LAN access units to tie together legacy air and surface search radars, electronic surveillance sensors, and weapons. The result is a fully integrated composite picture of automatically fused and correlated data. Data rates are higher and track integrity is improved dramatically, resulting in markedly higher kill probabilities. The same approach could be used for a torpedo defense capability.
Why Aren't We There Yet?
Simply put, torpedoes aren't as "sexy" or ominous as ballistic missiles. To be sure, the missile threat is real, and it must be countered, but our intense scrutiny of the skies distracts us from the underwater threat. Miserly support led in part to the failure of the national SSTD program—there just was not enough time or money dedicated to solving the engineering problems or to testing the system. We should learn from that mistake, but the emphasis in Navy research, development, test, and evaluation funding suggests otherwise. Figure 1 shows the funding plan for three mission areas. Of note, the fiscal year 2000 budget for surface ship torpedo defense is less than $5 million, and the funding is zeroed out for fiscal year 2001.
The solutions to filling our capability gap are easy to articulate but difficult to implement, given the current skyward focus and dwindling defense dollars:
- Put all torpedo defense programs under a single resource sponsor and a single program manager. That is the only way to break down the monumental stovepipes that have crippled the efforts to date.
- Range is essential. Build a detection, classification, and localization system for non-ASW ships that gives them a fighting chance to detect and survive a torpedo attack.
- Integrate all available sensors to give torpedo defense the same composite track capability the Ship SelfDefense System provides amphibious ships for close-in antiair warfare.
- Field the antitorpedo torpedo as soon as possible.
- Make a long-term, stable commitment to developing both focused pressure waves/electrohydraulic effects and supercavitating-supersonic projectiles. There is at least one torpedo on the world market that can travel at speeds of up to 200 knots. There is no counterweapon available.
- Technology developers must start early on the integration effort. Given the complexity of today's systems, integration is a key enabling technology.
The effort will appear expensive, but compared to recent air defense programs it is cheap insurance—and the alternatives are not pleasant to contemplate.
Captain Vining, a 1973 graduate of the U.S. Naval Academy and a nuclear-trained surface warfare officer, retired in 1999 after 26 years of service. While on active duty, he served in nuclear-powered cruisers and carriers, completed a command tour in a Knox (FF-1052)-class frigate, and was assigned as Assistant Chief of Staff for Surface Warfare Programs at Commander, Operational Test and Evaluation Force. A licensed engineer, Captain Vining now works at Noesis, Inc., a small engineering firm providing support to the Naval Sea Systems Command and the Office of Naval Research.