Too few Americans fully appreciate the continuing and emerging vital roles of U.S. Navy nuclear submarines, and that is most unfortunate because each one is so important to our national security. To achive the myriad missions our country demands of them and their crews in the 21st century, including various taskings of the sort that Joe Buff has discussed in his prolific writings for years, subs need to be relatively large and fast as undersea warships go, and consequently expensive. Some pundits have raised the issue of whether America should buy a larger number of cheaper diesel boats instead. A more robust response for America’s unique global-reach strategic interests is the Tango Bravo feasibility study now underway, searching hard for “Technology Breakthroughs” (hence the name) that might make nuclear submarines be less costly to build and man, while they also become even more effective than now. Within this broader context, Joe Buff has carefully looked at the question of submarine size in isolation. He offers compelling observations, both technical and practical, to help demonstrate that –- everything else being equal –- smaller alone is often not better.

Joe's analysis was aided by his previous work in thinking about and writing of the world of submarining. His sources of information were in the public arena and it was his interest, ingenuity and common sense which has made him a knowledgeable commentator on issues of undersea science, strategy and operations. He has done that not only in his several novels but in the pages of THE SUBMARINE REVIEW, a professional magazine for the submarine community. As Editor of that magazine I have asked Joe to write about some subjects and his own initiative has led him to investigate and comment on other substantive issues. Our readership has responded positively to those efforts.

It is particularly appropriate that those interested in general military matters have the benefit of Joe Buff's insights.

--Captain James C. Hay, USN (Ret.) Editor, THE SUBMARINE REVIEW

Introduction:

The extreme quiet of a diesel sub on batteries is well known, although according to some Silent Service practitioners the decibel difference relative to a modern nuclear submarine moving at quiet tactical speeds is somewhat overrated. Air independent propulsion (AIP) systems have been developed or proposed that would augment the diesel’s traditional engine-generator-motor set and battery bank to enhance the “indiscretion ratio” of these boats, i.e. improve their non-snorkeling submerged endurance. In addition, the smaller size of diesel and diesel-AIP boats (here collectively denoted SSK) could be seen as an advantage in littoral (shallow water and/or near-shore) warfare vice a nuclear-powered fast attack sub (SSN).

This article will examine the relatively low displacement (weight and size) of representative modern SSKs compared to Western SSNs, and will help show that the smallness of “enemy” SSKs can be a significant weakness in real combat operations against the U.S., UK, and our allies. (Other reasons for the U.S. Navy to choose good SSN designs over cheaper SSKs will be discussed in later Parts of this multi-part article, including lessons to be learned from the Royal Navy’s budget-strapped decision to go from a mixed SSN/SSK fleet to an all-SSN fleet.)

Note first one fundamental fact: Since all submarines while submerged (main ballast tanks flooded) are by their nature neutrally buoyant, anything that adds weight without reducing safe operating depth (i.e., thinning the pressure hull) forces the pressure-hull envelope to increase buoyancy in the only way that it can, by displacing more water -– it has to get bigger. Otherwise, once all variable ballast tanks were pumped or blown dry, the sub would sink like a stone until it either hit the bottom, or passed through crush depth and imploded, whichever came first. And a bigger hull, for the same propulsion-system power output, means a slower vessel, causing both strategic and tactical disadvantages. More propulsor power to solve these problems, in turn, means more (heavier) propulsion machinery and thus an even bigger hull -– a vicious circle in which the SSN always beats the SSK, because its nuclear reactor has much greater power density than any diesel-AIP could ever achieve.

The present writer will here, in part, take a view as futurist. Some of the following discussion would apply over the next ten to fifteen years, as advanced off-board sensors and remote combat vehicles become operational with our nuclear-powered SSN fleets, while other countries acquire more SSKs.

Surfaced Displacement Comparison

Consider the following data on surfaced displacement (weight) in tons:

SSK	SSN
Russian Improved Kilo 2,350(a)	USS Seawolf (SSN 21) 7,467
German Klasse 212A 1,370(b)	USS Miami (688-I) 6,300
Swedish Type A-19 1,384(c)	UK Astute Class(d) 6,690
Notes: (a) no AIP. (b) Fuel cell AIP. (c) Stirling cycle AIP. (d) in service 2006.

The percentage of total displacement dedicated to combat sensors and systems, weapons loadout and other stores, plus crew habitability tends to be similar for both SSKs and SSNs: approximately 13% or 14% according to published references. Thus it can be said that undersea warfighting payload (defined here as the sum of these components of weight) may be, in absolute number of tons, 2.5 to 5 times as large for an SSN as for an SSK: between 185 and 320 tons for representative diesel or diesel-AIP boats, vice from 800 to 1000 tons for the SSNs. Furthermore, the reserve buoyancy (taken as submerged displacement minus surfaced displacement) of the SSN designs averages 2.3 times that of the SSKs. Why does any of this matter?

Warfighting Effectiveness: It seems inarguable that SSNs possess substantial advantages over SSKs (whether the latter are augmented with AIP systems or not), regarding a) rapid stealthy transit to and from the theater of operations, and b) continued rapid submerged movement during tactics in the OPAREA. The top quiet speeds of Seawolf and Virginia equal or exceed the absolute maximum speeds of any SSKs! But the following additional capabilities are also needed for a submarine to complete its assigned mission tasking successfully:

1. Sensors and systems. Active and passive sonars and signal processors and display consoles. Radio, radar, laser, acoustic, and other communications/connectivity equipment, and electronic support measures (ESM) signals interception gear. Target motion analyzers, other weaponry controls, various computers and data storage capacity, and navigation systems.

2. Weapons and Vehicles Loadout. Torpedoes, missiles (anti-shipping, and land attack), and mines. Decoys and countermeasures. Unmanned undersea vehicles (UUVs), and unmanned aerial vehicles (UAVs). Remote-control combat vehicles (Manta?). Special operations minisubs (Advanced SEAL Delivery System) -- plus accomodation and physical fitness provided for commandos. Counter-mine reconnaissance and removal gear (LMRS prototype).

3. Crew. Battlestations and section watchstanders. Approach and Fire Control Coordination talent, command infrastructure. Operators of C4I consoles, remote vehicle control/downlink consoles, sensors, navigation, engineering, and weapons systems. Maintenance and damage control workers throughout the boat, including on-board data administrators and systems operators. Mess management/crew comfort personnel. Note that increased automation to reduce crew size presents a serious conundrum: there are more things requiring constant maintenance that might fail at a critical moment (the automation equipment itself) yet fewer skilled people (crew) available to perform preventive maintenance and make emergency repairs!

A submarine with smaller payload will perforce have less capacity in at least one, and almost certainly in all three of the above crucial areas.

Crew size determines and limits the boat’s ability to sustain prolonged combat action in a complex high-threat environment. A diesel boat with a crew of two dozen (German, Swedish) or fifty (Russian, Chinese) may be less expensive to operate than a nuclear boat with a crew of well over one hundred, but during lengthy battlespace preparation and domination phases, a manpower advantage of up to five-to-one may prove decisive. The larger SSN crew will be able to “out-think and outfight the other guy,” if only by being able to outlast him.

Firepower is crucial to deter or destroy a military opponent. Representative diesel torpedo-room loadouts are under 20 units. For SSNs, loadouts can range from 26 for Los Angeles-class boats through 38 for the Astute-class and the Virginias, to 50 for Seawolfs. (Late Los Angeles-class vessels, and the Virginias, also have a separate 12-weapon vertical launch system for Tomahawk cruise missiles.) In a fast-paced littoral melee, during which anti-torpedo defenses may come to play a significant role, sustained rates of offensive fire become important. The guy who runs low on ammo first, or who runs out altogether, is at a severe disadvantage. To the degree that UUVs and UAVs, mine countermeasures, and other off-board sensors and vehicles take up space and weight, there is less room for warshot torpedoes, missiles (including undersea-launched anti-aircraft missiles, e.g. Polyphem), and mines. Thus if SSN and SSK carry equal numbers of non-warhead-bearing devices that are launched through the torpedo tubes, the SSN’s advantage in raw killing power is even greater than total loadout figures would suggest.

Target detection and situational awareness are vital warfighting attributes supported by good C4I, connectivity hardware, and sensor suites. Once more, a larger displacement is desirable. As computer systems become miniaturized, more and more tasks are found for computers to perform. Increasingly sophisticated sonar capabilities such as wide aperture array instant target ranging, and complicated navigation and ship-control aids such as high-resolution gravimeters and computer-assisted autopilots, take up space and weight. A boat with 2.5 to 5 times the payload for such equipment is 2.5 to 5 times as capable to win a battle, even one against multiple simultaneous threats. Furthermore, powerful active sonars require large electrical supplies that may drain a diesel’s silent battery banks and fuel cells unacceptably -- an SSN has unlimited generator capacity, at the cost of (reportedly) only negligibly greater noise. And sheer physical dimensions matter, too. The larger beam and length of an SSN (X2 relative to SSKs is representative) provides a sonar bow sphere with four times the surface area, and a wide aperture array with twice the aperture. This can be especially critical at times such as littoral melees when towed arrays are not deployed.

Survivability: A successful submarine design must not only be able to put weapons repeatedly on target, it must be able to avoid or overcome damage due to enemy near misses and direct hits. A larger-displacement boat has the edge in several ways:

1. Flooding: A leak of a given cross sectional area at a given depth (pressure) will admit tons of seawater into the boat at a rate that cares nothing for displacement or reserve buoyancy. Clearly, a larger boat thus has more time, before the ability to surface is completely lost, during which to control and repair damage causing (and also resulting from) the flooding. In addition, a larger boat (SSN) can be subdivided more readily into watertight compartments. Internal pressure bulkheads are very heavy. The German Klasse 212A design, for instance, has no internal subdivision against flooding.

2. Shock Isolation: Shock isolation and quieting gear work hand in hand. They take up space and weight. Distancing from the outer hull is an important means to protect crew and sensitive equipment from blast concussion. A large boat has an advantage.

3. Hull Thickness: To withstand a given pressure, everything else being equal, the thickness of the hull must be proportional to the beam. Thus, obviously, a large SSN needs a thicker hull to withstand the same test depth as a small SSK. However, some warhead effects (including shaped-charge torpedo warheads and directed energy weapons) act locally, in which case a thicker hull gives added protection just like tank armor. By virtue of its smaller size/displacement, the SSK in fact is forced to carry a thinner, more vulnerable hull -– otherwise it would sink to the bottom and stay there.

4. Volatile/Hazardous Substances: An SSN’s nuclear reactor contains dangerous materials. However, modern AIP designs do as well. Air independent systems, whether based on internal or external combustion or fuel cells, require on-board supplies of liquid oxygen, liquid hydrogen, and/or high-test peroxide. These are highly flammable and/or explosive. In addition, high-power-density batteries can operate at temperatures up to 1000 degrees centigrade, (vastly higher than an SSN reactor’s core), presenting a significant fire hazard on a small boat.

Point 4 is worth elaboration. It has been argued that SSKs can be designed with the shielding and insulation needed for survivability, since nuclear submarines have been built (at least in some countries) with an outstanding record of reactor operating safety. However, three counter-arguments can be made:

1. Shielding and insulation require considerable weight. If an SSK design becomes weight-critical, safety may be compromised, perhaps unknowingly until the vessel enters battle or suffers a lethal accident at sea.

2. Decades of experience and tradition may be required to assure ongoing safe handling of volatile substances in a combat or near combat (Cold War-like) environment. This culture exists in the U.S. and UK for SSNs (and SSBNs, and the new SSGN conversions). It is unclear whether Admiral Rickover’s legacy of quality control and personal accountability can possibly be replicated by aggressor nations (actual or hypothetical) for their current or planned AIP-equipped SSK fleets.

3. An oxygen or hydrogen or hydrogen peroxide fire/explosion may immediately kill the SSK and its entire crew. In contrast, equipment and training exist to contain radiological hazards from a limited reactor accident -- shielding and redundancy are important components of the displacement of a nuclear submarine. If both SSK and SSN have casualties related to their air independent fuel systems, the SSN may be much better able to repair itself and keep on fighting.

Strategy Implications: An aggressor might seek to use its SSKs in one or more of several ways.

1. Acts of terror or war against Blue Force (U.S., UK, etc.) coastal population centers and military or industrial installations. (This would potentially involve an extremely lengthy transit, probably exceeding submerged AIP endurance, thus requiring snorkeling to run the noisy diesel engines and pull in fresh air for the crew.)

2. Attacks against sea lines of communication (SLOCs) in mid-ocean or at choke points, i.e. anti-shipping operations and commerce raiding or attacks upon warships. (This often requires a lengthy transit with high risk of detection via acoustic and advanced non-acoustic ASW sensors –- see below.)

3. Defense of the aggressor’s own local seaspace, to prevent Blue Force amphibious operations and/or land strikes that would bring down the in-power “evil” political regime.

In these three missions, SSKs have two apparent advantages. First, they cost perhaps one fourth or one fifth as much as a nuclear attack sub, so an aggressor can purchase many more of them for the same money. Second, to ultimately defeat that aggressor nation, however/wherever hostilities begin, we must eventually dominate their littoral, the home waters of their SSKs -- and this is where their propulsion systems perform optimally, and where their difficulty of detection is at its best.

But if the arguments earlier in this discussion are accepted overall, then an SSN penetrating enemy waterspace has several counterbalancing strengths. Perhaps most critical is the classic one of concentration of forces. That is, a given amount of money invested in one extremely capable boat (SSN) is better militarily than the same amount invested in several separate less capable boats (SSKs). Besides the military concentration-of-forces edge, the SSN also achieves a balance-sheet superiority: Much “fixed-overhead expense” is saved since only one of everything is needed instead of lots of copies of everything to fit out the bigger squadron of smaller hulls.

The SSN, when equipped with UUVs and UAVs along with advanced mine and counter-mine capabilities and combatant minisubs, can indirectly reach into the shallowest waters to seek and destroy the enemy SSKs one by one. Clearly, a remotely controlled “probe” launched from an off-shore SSN is much smaller and quieter than even the best imagineable SSK design, and it is also much cheaper and more expendable than the diesel-AIP boat lurking in the littoral. The apparent four or five to one advantage in numbers of the SSK for the same money is turned on its head, to become an up to five to one advantage in concentrated fighting power (payload weight) for the SSN. This general observation is particularly true for “emerging nation” submarines, where close-combat coordination among a submerged flotilla is infamously difficult. However, for this perspective to continue to hold true as the number of SSKs in the world constantly increases, clearly an adequately-sized SSN fleet is vital; otherwise, eventually, the SSKs can win by dint of sheer numbers.

Once the aggressor’s SSK fleet has been contained in its home waters, the enemy has at least three remaining options:

1. Keep its SSKs in-harbor as a force-in-being, representing a threat to any invasion by Blue Forces.

2. Actively engage Blue Force SSNs and their offboard fighting vehicles, in the littoral and out in deeper water, in hopes of inflicting sufficient losses to force a withdrawal or stalemate, at least politically/psychologically if not militarily.

3. Sortie the SSKs but have them lurk in hiding as a threat and deterrent, akin to SSBN tactics. Perhaps seek to refuel/reprovision them clandestinely at sea, or in harbors of nations friendly to the aggressor.

Tactics to counter these three options, respectively, would include:

1. Mine enemy harbor mouths. Also attack enemy SSKs at the dock with missiles, bombers, and/or special ops forces. (These are missions for which modern SSNs are ideal if not essential.)

2. As in 1, but also use to the maximum the SSN’s superior sensor capabilities, weapons loadout, and warfighting endurance in a battle of mobility. Harass the SSKs constantly, and maintain a high rate of exchange of ordnance, non-reusable sensors, and expendable countermeasures. Do this by network-centric warfare cooperation with friendly airborne and surface weapons platforms, and their active and passive sonars. Also locate the enemy by LIDAR blue-green laser ASW detectors, LASH underwater color and shape anomaly detectors, portable/temporary SOSUS-like hydrophone grids, magnetic anomaly detection, and thermal, chemical, and wake anomaly effects. Maintain connectivity with UUVs by high-bits-per-second wireless underwater covert acoustic means, and do so from below periscope depth with surface and air units via sonobuoy-sized transceiver relay nodes and impending breathrough “comms at depth and speed” systems. Find bottomed SSKs using off-board probes, and prosecute them mercilessly.

3. As in 1 and 2, seek out the SSKs wherever they may be. Ideally, start by having SSNs in-theater before hostilities begin, and trail each SSK from port as it sorties. (Again, a large enough SSN fleet is essential to doing so with adequate effect.) Give the SSKs not a moment’s peace. Deny them access to bases and tenders for replenishment, and sink or take down their milch cows. Deny the diesel crews their sleep and ruin their ability to think straight. Make every SSK mission a one-way mission. Localize, demoralize, and destroy.

The advent of undersea photonics (LIDAR, LASH, bioluminescence detection) and advances in sonar signal processing will make it harder and harder for a diesel or diesel-AIP to use one traditional infiltration tactic, namely hiding under or in the wake of a surface vessel. LIDAR scanners may permit “delousing” simply by looking under the keel. And the tonals generated by SSK diesel engines and/or near-surface screw cavitation can presumably be picked out of other noise by an alert escort’s or helo’s sonar watch, when properly equipped and trained. It can be expected than in any shooting war, or declared zone of exclusion, merchant ships upon which SSKs could ply this tactic will be scarce in any case.

One major threat presented by an SSK may be a weapons of mass destruction mission while “Allied” defenses are lulled in peacetime. Vigilance in undersea warfare by carrier battle groups on maneuvers, diligence in HUMINT and ELINT regarding enemy intentions and their SSK fleet readiness and movements, and constant acoustic and non-acoustic surveillance for suspicious diesel signatures on the high seas as well as in the friendly homeland littorals, will all give some protection. Once more, numbers of SSNs on deployment are crucial.

The WMD-laden SSK may be on a suicide mission as well. It is always wise for Blue Force commanders to assume enemy vessels are manned by determined opponents who will fight to the death in performance of their perceived duty. But for suicide forces, deterrence by the surety of mortal peril is simply not enough. A guaranteed hard kill is necessary, i.e. PK of virtually 100% for the defensive system overall. The discussion above about low displacement disadvantages and counter-tactics would still apply. The SSK must be forced to do the impossible: maneuver constantly while avoiding detection, fighting its way through a multi-layered active defense before reaching any high-value targets -- all while lacking sustained high-speed submerged endurance and without a large combat weapons/systems payload.

Conclusion: The small size of representative diesel-AIP submarine designs can be an important drawback to an aggressor nation dependent on such vessels. Tactics to exploit this weakness and deter/defeat aggression would include forcing a prolonged and continuous battle for seaspace dominance, in which the SSKs’ fuels, weapons loadout, and crew are worked to exhaustion and their sources of replenishment are neutralized. Blue Force nuclear powered fast-attack subs, with their much larger payload capacity, unlimited high-speed cruising, infinite electrical supply, and enhanced survivability -- busily employing/deploying advanced combat sensors and systems, large special operations teams, and off-board littoral probes and fighting vehicles -- will help assure the “good guys” remain fully combat effective until, with the lowest possible casualties and least collateral damage, victory and peace are finally achieved. The lower cost of an SSK compared to an SSN is thus a red herring: The several SSKs one can purchase for the price of one good SSN are fundamentally unable to make up in numbers for what they lack as a group in overall warfighting quality.

JoeBuff.Com / Joe Buff Inc. Joe Buff, President Dutchess County, New York E-Mail readermail@JoeBuff.Com