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Diesels Just Blowing Smoke?
by Joseph J. Buff, [IMAGE]Copyright 2007 by U.S. Naval Institute PROCEEDINGS

This article first appeared in the U.S. Naval Institute's magazine Proceedings

Photo Courtesy: Walter P. Noonan
[IMAGE] Should the Navy shift back to diesel-powered submarines as it re-shapes its forces to meet the challenge of conducting military operations in coastal areas?

The debate is roiling the waters in the submarine community. The question is whether the Navy should rely on large, nuclear fast-attack and guided-missile submarines for conducting undersea operations in littoral waters or begin acquiring smaller, diesel-powered subs to do the job. Proponents of diesel subs argue they would be able to penetrate closer to shore than the nuclear boats and would be more maneuverable, quieter, less vulnerable -- and far cheaper, to boot.

As such, they contend, the ideal structure for the U.S. submarine force in this age of littoral operations would be a so-called "high-low mix" that combines high-capability nuclear attack subs and guided-missile subs -- SSNs and SSGNs -- for blue-water conflict with a supplemental fleet of diesels for coastal operations. The diesel subs would be forward-based, to help overcome their limitations of range and mobility. And they would be equipped with state-of-the-art, air-independent propulsion systems (AIPs), which would carry oxygen in some form as an additional fuel, enabling the diesel boats to remain submerged without snorkeling for weeks at a time. (Diesel AIPs are known as SSIs, differentiating them from purely diesel-electric powered hunter-killer subs, the SSKs.)

So far, the discussion in Navy professional journals has been dominated by proponents of diesel subs. But many of the arguments put forward in favor of diesel boats for the U.S. Navy's particular needs either are myths or have been significantly exaggerated. There is a persuasive case that the nuclear-powered sub fleet can do the job better than diesels, at less overall cost. It is more wise to focus attention, not on acquiring new American diesel boats, but on developing the Chief of Naval Operations' concept of "the 1,000-ship Navy" as a global maritime security force that includes SSKs and SSIs of other cooperating countries.

Closer Encounters

Proponents of acquiring diesel subs argue that because an SSI is smaller and requires less sea clearance than a nuclear boat, it can penetrate far closer to shore without risk of bumping its nose, dragging its tail, or breaking the surface unintentionally. In reality, however, the differences aren't that great. The typical SSGN is only about 25 feet higher than a diesel-propelled SSI, and for the Virginia-class SSN the disparity is only some 15 feet. Moreover, the question of how much clearance is acceptable for a particular class of sub depends more on a vessel's ship-handling and stability than on the size of the boat itself. Submariners say the key to operating safely in littoral areas with a large sub is simply to move slowly.

What is more, the ability of nuclear boats to operate closer to shore can be improved by equipping them with minisized unmanned undersea vehicles or autonomous undersea vehicles (UUVs or AUVs). They can be used as remote robot sensor probes, enabling crew members to scout ahead and combine their survey of on-the-spot conditions with satellite data on local sea characteristics.

Contentions that nuclear-powered subs are less maneuverable than diesels are similarly flawed. Virginia-class subs are equipped with a new computer-controlled autopilot and hovering system that enables them to maintain a specific depth to within one-tenth of a foot and to remain perfectly level in any but the roughest seas. Thus, they can penetrate close to shore wherever the contours of the sea floor permit. Ohio-class SSGNs -- onetime SSBNs that have been converted into SSGNs -- also are very stable. In their former incarnations, they had to be able to fire sub-launched ballistic missiles (SLBMs) over thousands of miles with pinpoint accuracy and recover quickly from each jolt to be ready for another possible launch. As the Ohio-class subs are overhauled, they receive hovering and trim systems enhancements.

Turning radius is another false issue. While it's true that an SSI typically is only half as long as an SSN or SSGN, ship length is not a major factor in maneuvering horizontally. And if a nuclear sub needs to wriggle into very narrow corners, it can deploy its manned and unmanned minivehicles, which are far smaller than any SSI.

It is also a misconception that submerged, non-snorkeling SSIs can run at their top speeds for long periods. When a diesel submarine accelerates to sprint speed, it draws power from its regular batteries. Once the batteries go flat, the AIP equipment of an SSI's propulsion system permits cruising at only a few knots. If the captain wants to use the AIP system to recharge his batteries, his speed is even more restricted for quite some time. The reality is that it limits the range of the SSI, making it easier for an enemy nuclear sub to pursue and destroy the diesel. If the pursuing nuclear boat gets into trouble, its superior mobility and its wider array of available countermeasures (and burgeoning arsenal of "stand-and-fight" weapons such as sub-launched anti-air Sidewinders and anti-torpedo torpedoes) will enable it to defend itself if necessary, withdraw to deep water and later repenetrate the littoral area at a more opportune time. While an SSI must use its fuels carefully and recharge its batteries frequently, a nuclear boat can recharge its minivehicles (its UUVs and AUVs) indefinitely.

Indeed, equipping a nuclear submarine with off-board probes inherently solves the problem of covert command and control among several SSIs: there is a single control room where all human tactical decisions are made, face-to-face. Submariners say modeling simulations have shown that one nuclear sub with two or three autonomous probes can patrol a length of seacoast several hundred miles long.

Running Silent

The question of quiet -- and ultimately of how vulnerable a submarine is likely to be -- is a complex one. Granted, an SSI sitting motionless on the bottom is temporarily difficult to locate, but only temporarily, and its sonars and weapons will be degraded by terrain. Furthermore, U.S. submariners report that, taking a snapshot in time during a live engagement, Virginia-class SSNs and Ohio-classSSGNs are tactically quiet at speeds up to the "dash" speed of an SSI, which is barely 20 knots.

But an engagement between an SSN and an SSI doesn't occur in an abstract instant. It unfolds more like a campaign -- the cat-and-mouse hunt often plays out over several days. Drills against capable SSKs and SSIs during the past few years have taught U.S. sub captains to not rush into the exercise area boldly, but rather to play a waiting game. Unlike a nuclear sub, which can maintain ultraquiet until the food for its crew runs out, smaller SSKs and SSIs soon need to run environmental-control machinery, bilge pumps, and other auxiliary equipment, all of which produce acoustic signatures that will give the vessels away to a nuclear sub's vastly superior passive sonars (see below). Additionally, exotic modes of extremely high- and low-frequency active sonars show tremendous promise at first-detection against SSIs lurking in littorals.

The new Virginia-class SSN, optimized to operate in littoral waters, is superbly quiet, and its non-acoustic signatures have been reduced to an absolute minimum. The converted Ohio-class guided missile subs began their lives as hard-to-find SSBNs, and their stealth has been enhanced during the conversion process. When these boats are provided with a wide array of satellite data on local sea characteristics -- detailed well down into the water column -- and equipped with UUVs and AUVs to be used as remote off-board sensor probes, their captains will know their shallow-water battlespaces even better than their adversaries do.

That said, stealthiness is not everything when it comes to undersea warfare. Over the years, submarines often have had to break their quiet temporarily, either to make their presence known and send an enemy running or to create a diversion. Sometimes -- such as during a commando-launching operation or an electronic intelligence mission -- it is all but impossible to maintain quiet. Virtually every real-world littoral submarine tasking involves temporary spikes in signatures, which violate constant and total invisibility; this is particularly true during any combat ops.

Moreover, the seas just off coastal areas typically contain high levels of background noise, both steady and intermittent, from passing cargo vessels, from oil and natural gas drilling, and even from the vibrations of heavy industry on nearby land. Wind, rain, and waves from bad weather can also create noise. A properly equipped submarine can exploit this clutter to mask sounds that might radiate outward as the boat performs individual tasks. Thus, enemy listening grids pose less of a danger than might appear. The same is true of static minefields, since the sub's job is mainly to locate, map, and negotiate through them carefully herself, not clear them like a minesweeper for heavy transit by other forces. Indeed, submarine stealth and own-ship protection depend not only on sensor quality and internal noise levels, but on the training given the officers and crew and the doctrine that they follow.

In short, an expensive nuclear submarine, if properly handled, and especially working in tandem with nearby U.S. ships and aircraft, is not unacceptably vulnerable to less expensive enemy SSKs and SSIs. Breakthoughs in "comms at speed and depth" can only enhance the nuclear submarine's potency as a member of that networked American team. The purpose of sea power in wartime is to inflict loss on an opponent by taking calculated risks. An overly casualty-averse posture invites defeat.

Diesel Disadvantages

The diesel sub's smaller size has some minuses as well as pluses. No matter how sophisticated the boat's design -- and the surface displacement of a typical SSI is only one-third to one-fifth of the Virginia's -- it does not have enough space to contain the weapons, AUVs, sonars, electronics, and special operations passengers that a nuclear sub can carry. In general, a sub's ability to keep tabs on what is going on around it increases more than proportionally to its weight.

For example, six sonar consoles fed by larger on-hull arrays, with more advanced signal processor algorithms running on faster computers, yield more than three times the usable knowledge of two displays, smaller arrays, simpler signal processors, and weaker computers distributed onto each of three separate subs. This is especially true in the case of the latest sophisticated techniques needed to hunt for enemy SSIs.

Further, forward-deploying of American diesel subs poses something of a gamble -- particularly if they are deployed in modest numbers, as the proponents' plan recommends. Because SSIs have limited mobility, they are essentially restricted to the theater to which they are sent. (A Swedish SSI leased by the U.S. Navy recently had to be transported from Sweden to San Diego as cargo on a surface ship.) If tensions or shooting were to break out in a theater in which our SSIs were not already deployed, they might not arrive until it was too late to do any good. By contrast, SSNs and SSGNs cover the globe.

There also is the question of cost. While it is true that building one nuclear sub can be three to five times as expensive as one diesel boat, this is not the calculation that really matters. The most accurate measure is to compare the cost of designing and building a new diesel sub class in adequate numbers with the cost of developing and building the UUVs and AUVs needed to equip existing nuclear sub classes in adequate numbers for use in littoral areas. By that computation, the nuclear submarines come out way ahead, and represent a better investment for the money.

The bottom line: the best strategy to follow in shaping the U.S. Navy's Submarine Force for littoral warfare is to choose nuclear vessels that are equipped with UUVs and AUVs rather than turning to diesel boats. While the danger of loss is always there, an SSN or SSGN can outfight or outflank an enemy SSI wolf pack, no matter how well-commanded those hostile boats might be. To level the playing field, by deploying SSIs instead of SSNs, and to ignore the opportunities of the CNO's 1,000-ship Navy, will save neither taxpayers' money nor submariners' lives.

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