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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.
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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.
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