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Diesel-AIPs: Low Displacement as a Weakness
by Joseph J. Buff, 2002

Photo Courtesy: Walter P. Noonan

The extreme quiet of a diesel sub on batteries is well known. Air independent propulsion (AIP) systems have been developed or proposed that would augment the diesel’s engine-generator-motor set and battery bank to enhance the “indiscretion ratio” of these boats, i.e. improve their non-snorkeling submerged endurance. 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 of representative modern SSKs compared to Western SSNs, and will argue that said feature of “enemy” SSKs is a significant weakness in real combat operations against the U.S., UK, and our allies.

The present writer in part takes 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 hunter/killer fleets. This article will end with comments on anti-SSK strategy suggested by their low-displacement disadvantages.

Surfaced Displacement Comparison
Consider the following data [1] on surfaced displacement in tons:

SSK	SSN
Russian Improved Kilo 2,350(a)	USS Seawolf (SSN 21) 7,460
German Klasse 212 1,360(b)	USS Miami (688-I) 6,300
Swedish Type A-19 1,380(c)	UK Astute Class(d) 5,900

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% [2]. 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 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 NSSN equal or exceed the maximum speeds of SSKs [1]. But the following additional capabilities are also needed for a submarine to complete its assigned missions and tasks successfully:

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

2. Weapons and Vehicles Loadout. Torpedoes, missiles (both 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 transport (Advanced SEAL Delivery System). Counter-mine reconnaissance and removal gear (NMRS, LMRS).

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.

[Joe Buff / JoeBuff.Com] A submarine with smaller payload will perforce have less capacity in at least one, and almost certainly in all three of the above critical 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) [1] may be less expensive to maintain and operate than a nuclear boat with a crew of well over one hundred [1], but during lengthy battlespace preparation and domination phases, a manpower advantage of up to five-to-one may prove decisive. The larger crew will be able to “outthink 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 loadouts are under 20 units [1]. For SSNs, loadouts can range from 24 for Los Angeles-class boats through 36 for the Astute-class, to 38 for NSSNs and about 50 for Seawolves [1]. 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 (which add weight even if worn externally). 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 correlograms, and sophisticated piloting aids such as high-resolution gravimeters, 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 the battle. Furthermore, powerful active sonars require large electrical supplies that may drain a diesel’s battery banks unacceptably -- an SSN has unlimited generator capacity, though at the cost of greater noise. And size matters, too. The larger beam and length of an SSN (X2 relative to SSKs is representative [1]) provides a 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 seawater into the boat at a rate independent of displacement and reserve buoyancy. Clearly, a larger boat has more time, before the ability to surface is completely lost, during which to control and repair damage resulting from or causing flooding. In addition, a larger boat (SSN) can be subdivided more readily into watertight compartments. Internal pressure bulkheads are very heavy. The German Klasse 212 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 larger boat has an advantage here.

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 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 hull -- otherwise it would just 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 [1], a significant fire hazard. Point 4 above is worth elaboration. It has been argued [2] that SSKs can be designed with the shielding and insulation needed for survivability, given that nuclear submarines have indeed 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.

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) environment. This culture exists in the U.S. and UK for SSNs (and SSBNs). 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 fire/explosion or battery fire/explosion may immediately kill the SSK and its entire crew. In contrast, equipment and training exist to combat and 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 involve a lengthy transit and repeated snorkeling.)

2. Attacks against mid-ocean lines of communication (SLOCs), i.e. anti-shipping operations and commerce raiding. (This still requires a lengthy transit with high risk of detection.)

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 [1], 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 comes to the fore.

[Joe Buff / JoeBuff.Com] 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, an amount of money invested in one extremely capable boat (SSN) is better militarily than the same amount invested in several less capable boats (SSKs). When equipped with UUVs and UAVs, along with advanced mine and counter-mine capabilities and combatant minisubs, the SSN 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 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 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 argument is particularly true for submarines, where coordination among a submerged flotilla is extremely 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.

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/remote 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 if not militarily.

3. Sortie the SSKs but have them lurk in hiding as a threat and a 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. Attack enemy SSKs at the dock with missiles, and/or with special operations forces. (These are all 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 cooperating with airborne and surface weapons platforms and their active sonars. Also locate the enemy by LIDAR blue-green laser ASW detectors [3], portable/temporary SOSUS nets [4], MAD, and thermal and wake anomaly effects. Maintain connectivity with UUVs by high-bits-per-second wireless underwater acoustic means, and do so from below periscope depth with surface and air units via sonobuoy-sized transceiver relay nodes [5]. Find bottomed SSKs using NMRS and LMRS, and prosecute them mercilessly.

3. As in 1 and 2, seek out the SSKs wherever they may be. Give them not a moment’s peace. Deny them access to ports 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, bioluminescence detection [6]) and advances in sonar signal processing will make it harder and harder for a diesel-AIP to use one traditional infiltration tactic, namely hiding under or in the wake of a surface vessel. LIDAR scanners may soon 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. It can be expected than in any war or declared zone of exclusion, merchant ships on which to ply this tactic will be scarce indeed in any case.

The greatest threat presented by an SSK may therefore be a WMD (weapons of mass destruction) mission while “Allied” defenses are lulled in peacetime. Vigilance in USW by carrier battle groups on maneuvers, diligence in HUMINT and ELINT regarding enemy intentions and SSK fleet readiness and movements, and constant IUSS surveillance for suspicious diesel signatures on the high seas, 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 maneuver constantly while avoiding detection, and must be required to fight 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 may be an important disadvantage to an aggressor nation dependent on such vessels. Tactics to exploit this weakness and deter/defeat aggression would include forcing a prolonged and continuous fast-paced mobile battle for seaspace domination, in which the SSKs’ fuels, weapons loadout, and crew are worked to exhaustion and their sources of replenishment are neutralized. Blue Force nuclear attack subs, with their larger payload capacity, unlimited high-speed cruising and electrical supply, and enhanced survivability -- busily deploying advanced combat sensors and systems, special operations teams, and off-board littoral fighting vehicles and probes -- 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.

REFERENCES

1. Combat Fleets of the World 1998-1999, by A. D. Baker III. Naval Institute Press, Annapolis, 1998.

2. Concepts in Submarine Design, by R. Burcher and L. Rydill. Cambridge University Press, Cambridge and New York, 1994.

3. “Foreign ASW Technology Developments,” by J. R. Benedict, Jr. The Submarine Review, July, 1995, page 88.

4. “Submarine Force Plans and Programs: Preparing for the Challenges of the 21st Century,” by RADM D. A. Jones, USN. The Submarine Review, October, 1995, page 31.

5. “Update on UUV Technology,” by F. M. Cancilliere. The Submarine Review, July, 1995, pages 79-80.

6. Anti-Submarine Warfare, by J. R. Hill. Second edition, Naval Institute Press, Annapolis, reprinted 1993, page 43.

Originally published in the January 1999 issue of THE SUBMARINE REVIEW, a quarterly publicatication of the Naval Submarine League, PO Box 1146, Annandale, VA, 22003. Posted here with permission of the Naval Submarine League.

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