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by Joseph J. Buff,[IMAGE]2002

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[IMAGE] The Advanced SEAL Delivery System (ASDS) minisub enables a major step forward in Special Warfare mission tasking. This shirtsleeves-environment SEAL “taxi” is a shock-hardened autonomous combatant vessel deployable from a host nuclear submarine. The present article will explore additional possible roles and uses for this versatile new subcompact warship, on the premise that expanding the envelope of naval submarine capabilities enhances global peace maintenance (conventional deterrence), and improves low-casualty and low-collateral-damage peace restoration (warfighting). As discussed below, a) the dry hyperbaric lock-in/lock-out chamber of the ASDS amounts to an important and flexible undersea Ocean Interface, and b) the potential of the ASDS to carry materiel internally and/or in external harnesses makes the minisub itself a stealthy interface between a submerged SSN/SSBN/SSGN and a friendly surface unit or shore facility or enemy target objective.

Summary of ASDS Specifications
Open-source information on the ASDS is available in references [1], [2], and [3]. [1] and [2] include an external artist’s rendering; [3] includes a schematic of the internal layout.

The ASDS is manufactured by Northrop Grumman and subcontractors. The ASDS is 65 feet long, 8 feet wide, and displaces 55 tons surfaced -- the vessel is road- and air- transportable. It is battery powered, with a single propeller screw, and has a reported range of 125 nautical miles at 8 knots. The on-board O2 renewer and CO2 scrubber give an underwater hotel endurance of several days. The ASDS is equipped with rudder and bow and stern planes, bow and stern anchors, retractable auxiliary side thrusters, and variable ballast tanks.

The ASDS has no sail, but is provided with a non-hull penetrating optical periscope, and a separate radio/ESM mast, which retract by folding back atop the hull. The vessel has forward- and side-looking sonars, and both GPS and inertial navigation gear.

The internal layout consists of a two-man Control Compartment forward, a central hyperbaric lock-in/lock-out sphere, and a rear Transport Compartment. The two-man crew and the SEAL passengers enter the ASDS from the host sub through a mating trunk leading to the bottom hatch of the lock-in/lock-out chamber -- the chamber also has a top hatch for use while surfaced (freeboard is low) or for emergency escape. Once on station the SEAL swimmers depart the ASDS through the bottom hatch, after the hyperbaric chamber’s atmospheric pressure has been equalized with that of the local seawater at the depth of the keel -- the ASDS need not be shallow to perform this evolution, although air compressor capabilities, and diver physiological limits and decompression tables, would affect the mission profile. The crucial advantage of the ASDS is that the SEALs remain relaxed and dry, and avoid loss of body heat, until they drop into the water through the hatch. The capacity of the Transport Compartment is approximately eight men with their equipment.

While the operating depth and crush depth limits of the ASDS are classified, note that an Italian minisub intended for combat swimmer transport, the 3gst9, is reportedly capable of diving below 2000 feet [4].

The ASDS as Combat DSRV
Increased emphasis on littoral warfare heightens the chance that a disabled submarine (DISSUB) might be forced to bottom on a continental shelf in hostile waters, or be rendered immobile above its crush depth in a zone of enemy sea-surface denial or control. The DISSUB might result from enemy action, or from an operational casualty -- situations might arise in which the emergency diesel is functional but snorkeling would invite immediate attack. Capture of the DISSUB and/or its crew by the enemy would represent a major intelligence and propaganda coup for the aggressor regime. Rescue of the crew, removal or destruction of crypto gear and other sensitive materials, and retrieval or safe demolition of the DISSUB itself, would be extremely high priorities for American naval forces.

In this adverse scenario, the nearest Deep Submergence Rescue Vehicle (DSRV) may be several days away, and the two DSRVs extant at the time of this writing are not specifically designed for use in combat [2]. Deployment of a new modular rescue system, which stages from a (vulnerable) surface ship, may also be impossible in wartime [5]. In such an emergency an ASDS, possibly already in-theater mounted on a host fast-attack or guided-missile sub, could act as a field-expedient stealthy rescue vessel, shuttling between the DISSUB and the mini’s parent SSN/SSGN.

[Joe Buff / JoeBuff.Com] The ASDS would need to bring its mating trunk or an equivalent docking collar, to connect to the DISSUB’s own escape trunk(s), but the DISSUB would not require latching pylons (used by the host for ASDS transport). If current ASDS mating trunk design does not permit this evolution, perhaps an appropriate “rescue” docking collar might be developed, or adapted from those on existing DSRVs. The ability to mate with the escape trunk of a DISSUB skewed to odd angles would be an important capability of the docking hardware.

Note that in relatively shallow water, the DISSUB’s crew could make conventional escapes while wearing Steinke hoods or the new Mark 10 full-body survival suits, but rather than ascending all the way to the surface in hostile waters they could pop right into the ASDS hyperbaric chamber. The batch of DISSUB escapees could then be decompressed at a controlled rate while avoiding hypothermia. Furthermore, the hyperbaric chamber could be used to temporarily treat crewmen suffering from decompression sickness -- the ASDS could bring medical corpsmen to the scene. Unlike a conventional hyperbaric chamber used for diver treatment, the ASDS lock-in/lock-out sphere does not possess an air lock. A host submarine’s own escape trunk might, in an emergency, serve this purpose, allowing corpsmen and supplies to enter and exit the high-pressure environment at will.

As with other rescue systems, a number of shuttle trips would be needed to evacuate the entire surviving crew, along with code books and/or other (small) sensitive items not readily destroyed within the DISSUB. (The use of more than one ASDS, or of an ASDS to supplement an on-site DSRV, would quicken the operation.) The passenger capacity of the ASDS might be increased in an emergency, by overcrowding the Transport Compartment and cramming additional personnel into the hyperbaric sphere and even the Control Compartment, while adjusting buoyancy with the variable ballast tanks. Lifting capacity could be further increased by jettisoning the anchors.

Finally, engineering personnel from the mini’s host sub, and/or SEALs, might enter the DISSUB to consult on environmental protection, and on hulk salvage or demolition, with the DISSUB’s senior officers, under guidance from higher command authorities maintained via two-way covert connectivity. If instead the DISSUB were neutrally buoyant but had suffered a mobility kill, an underwater tow cable might be rigged by divers, or by the ASDS or an unmanned undersea vehicle (UUV) equipped with robotic grapnels. Conceivably, SEALs equipped with mixed-gas rebreathers might be tasked to protect the DISSUB from enemy salvage divers or combat boarders, by staging from an ASDS and operating from inside and/or outside the DISSUB hull. In the most grim case, where the DISSUB had been destroyed by enemy fire and the crew had perished, protection of the wreck and retrieval of the mortal remains would be of strategic importance.

Note that the use of the ASDS allows the host submarine to lurk at a safe distance from the DISSUB, whose general position may be known to the enemy -- the ASDS might zigzag and take dog-legs on its way to and from the host. The cruising endurance of the ASDS creates an uncertainty as to the location of the parent which exceeds the range of typical heavyweight ASW torpedoes, and even exceeds by roughly an order of magnitude the lethal radius of a 1 megaton underwater blast [6]. Especially if time constraints allow for relatively lengthy transits between the DISSUB and the host (with battery charges as needed), this helps protect the parent sub from barrage attacks both conventional and nuclear.

Undersea Replenishment, Undersea “Bos’n’s Chair”
An ASDS could shuttle between two submerged submarines, neither of which is disabled. This would permit transfer of anything small enough and light enough to be accommodated within the minisub. Information, such as intelligence reports or software upgrades, can be transmitted by secure radio, laser, and/or acoustic links, but personnel, special gear, food, medical supplies, and spare parts, all must be physically transported. So long as at least one of the full-size submarines that make the rendezvous carries an ASDS, and docking collars are compatible, the task is fairly straightforward. The ability to carry some eight SEALs with equipment suggests the cargo carrying capacity of the ASDS is at least one ton.

When stealth must be maintained at all costs, during peace or war or quasi-war, and/or the receiving submarine’s mission prevents it leaving station -- perhaps due to one-of-a-kind equipment or uniquely qualified riders -- such an evolution achieves clandestine undersea replenishment in the forward OPAREA. Alternatively, so that the special equipment/riders remain on station, they could be transferred to the arriving submarine in the ASDS, while the original submarine departs for “conventional” replenishment, satisfies crew quality-of-life considerations, and transports the ASDS for its next assignment.

Covert Ambulance, Cargo Lighter, or Liberty Boat
Instead of shuttling between two submerged submarines, an ASDS might travel back and forth between a submerged submarine and a surface unit or shore installation. (With a flotation cradle to reduce its depth-to-keel, the ASDS could operate from the well deck of Navy amphibious ships [2].) Situations might arise in which a) enemy surveillance and/or the threat of attack discourages a friendly submarine from surfacing, yet b) it is desirable for people or equipment to enter or leave the sub. As above, if personnel and/or cargo are transferred in the minisub as an intermediate step the enemy does not learn the submerged host’s exact location, and can not even tell what type of submarine (let alone what class) might be involved. The use of one or more ASDSs for ship-to-shore and shore-to-ship transfers can even disguise the number and the nationality (U.S. Navy, Royal Navy, etc.) of full-size submarines present. Such an evolution could in fact be used as a deception or diversion, to imply the presence of a parent sub when none were really there. (Conversely, a surfaced host with a conformal minisub hangar inside an Ocean Interface hull module -- a latter day “bat cave” -- could deny OPFORCE recon of evidence whether an ASDS were embarked.)

Because of its limited dimensions and good quieting (battery propulsion), the ASDS might perform these ship/shore transfers covertly, perhaps surfacing and submerging under a pier or awning to avoid detection, or entering and leaving through an opening below the waterline in an innocent-looking merchant ship serving as a “milch cow.” (The small height, length, and beam of the minisub, and its four retractable side thrusters, allow it to operate in very shallow, restricted waters.) This stratagem harnesses further the counterintelligence potential inherent in the technology, making the ASDS more survivable while it preserves the protection of the deep for its much higher-value parent(s). Finally, with the ASDS and its hyperbaric sphere as supplemental decontamination points, shore transfers become possible which separate the host sub and its crew from the surface effects of weapons of mass destruction (WMD).

Stealth Landing Craft or Ship Takedown Platform
The discussion above regarding covert personnel and cargo transfer also applies to direct combat operations. By surfacing against a pier or seawall, or beaching in suitable terrain and surf conditions, an ASDS can deploy and/or retrieve through its top hatch a Marine infantry squad or other non-swimmer landing party -- the minisub amounts to a submersible landing craft.

By deploying from an ASDS alongside a targeted oil rig or ship -- perhaps one seized by terrorists or one involved in smuggling contraband -- a SEAL takedown squad can achieve complete surprise while arriving in peak physical condition. The SEALs could dive out through the ASDS bottom hatch, or exit “dry” through the top hatch while a surfaced minisub hugs the objective. Again, the ASDS is more stealthy that its nuclear powered host sub, more shallow-capable, and in extremis more “expendable.” The ASDS also has better speed and endurance than the Mark VIII SEAL Delivery Vehicle (SDV).

Especially for the dry-exit option, the target’s speed and the local sea state would need to be within the ASDS operating envelope. Perhaps friendly surface units might create a diversion, or perform maneuvers, to force a targeted ship to slow sufficiently. Instead, the SEALs themselves might deploy from the ASDS bottom hatch special netting to tangle propellers, or lay a line of sea anchor “snares” that simulate an engineering casualty on the target.

The ASDS might be fitted with a waterproof skirt to be erected around the top hatch to prevent swamping, and might shelter in the lee of the objective, during foul weather -- the vessel could also use a flotation collar to increase its limited freeboard, and to enter very shallow water. (Recall such skirts were used on battle tanks during World War Two amphibious operations.) In this manner released hostages, prisoners, and any wounded could be moved to the ASDS without the use of scuba or free diving, or even needing to swim. Evacuation would thus be accomplished without reliance on surface craft or helos -- some tactical situations might necessitate such hasty covert egress.

Stealthy Platform for Advanced Deployable System (ADS)
The Advanced Deployable System (ADS) is a portable and expendable passive acoustic undersea surveillance system designed for littoral use [7,8]. The ADS battery-powered hydrophone lines can be emplaced from a Towed Deployment Vehicle (TDV) streamed from a surface unit. A shore installation provides the equipment needed for operation, for communications, and for monitoring the sonar data. But shore installations are vulnerable to detection, and therefore to countermeasures or attack. Strategic situations might arise in which a clandestine ADS adds value. It is conceivable that versions of the ADS might be deployed and even monitored from an ASDS.

As one possible arrangement, the necessary operator consoles and power sources could be placed within the Transport Compartment, and reels of (thin) hydrophone line could be played out through the bottom hatch. In deep water a submerged buoy could bring waterproof ADS jacks and plugs up to a more convenient depth, for access by divers or by technicians simply reaching down into the water from the equalized lock-in/lock-out sphere. A special adapter for the hatch rim might be provided to permit opening and closing the hatch without breaking these connections. The ASDS could then anchor while submerged within the limits of its test depth, perhaps masked by topography or a wreck, allowing the ADS operators to perform their duties in a clandestine but shirtsleeves environment. Depending on the length and quality of the ADS hydrophone line(s), and on local sound propagation characteristics, detection capabilities might significantly exceed those of the ASDS’s own sonar suite (which does not include a towed array). In this manner the host -- while concealed by terrain or poor sonar conditions -- could launch weapons at a hostile contact based solely on targeting data from the ADS and the mini. Alternatively, the host might be cued to get in trail of the target.

Portable batteries might supplement the amp-hours available from the ASDS ultra-high-power-density silver zinc battery bank. Such ADS power packs could be emplaced on the bottom rather than carried in the minisub. They might be borne in an external harness, similar to the mine laying harness of the German Type 212 diesel/fuel cell submarine [9]. “Renewable” power sources might instead derive energy from local wave action, currents, and tidal flow, or from water temperature differences at different depths [10] -- the potential availability of this “free” energy is one advantage of operations in the littorals. For that matter, multiple reels of hydrophone line might also be carried externally, making the ADS/ASDS scheme somewhat less weight- and space-critical. Furthermore, an ASDS might be rigged to tow the TDV while submerged, so as to deploy a “conventional” ADS clandestinely.

[Joe Buff / JoeBuff.Com] Regardless of the specific hardware used, various ADS data-transfer amplifier/relay options exist, including sequential combinations of a) an automated transmitter or a non-radiating data recorder emplaced by an ASDS which then departs, b) relay onward of “live” information by ADS operators in an ASDS remaining on-site, and c) transmittal of data and/or a tactical assessment from the host nuclear submarine to a battlegroup commander and/or theater or higher command authorities. Information could be sent from the ASDS to its host in real time via undersea secure acoustic link, or by trailing a (lengthy) fiber optic wire between the parent and the mini. Radio or laser-burst contact could be established when it was safe for the ASDS and/or the host to raise a mast. The parent sub or the mini might deploy a delayed-action radio buoy well away from the ADS, for stealth.

For better transmission performance than achievable with a buoy, the ASDS itself could break EMCON (radio silence) at a distance from its deep-running host, to pass on (and/or receive) urgent data while masking the host’s location. Note that this evolution could apply to messages other than ADS data, giving an ASDS-equipped host what amounts to a super-capable lower-observable all weather stand off transceiver vehicle.

Adjuvant Sensor Platform
With its optical periscope, ESM mast, and sonars, the manned ASDS can serve as a kind of brilliant UUV, one with considerable range, endurance, and autonomy. This role could be performed while the ASDS transits on its primary mission, including the time it loiters pending recovery of its debarked Special Warfare passengers. With its mast up, at least intermittently, the ASDS could monitor high-baud-rate friendly message traffic for its deeply submerged host -- including Tactical Tomahawk targeting data, or e-mail familygrams. The ASDS could also gather SIGINT (signal intelligence) from closer in shore than the host and/or from a different relative bearing to the enemy emitters. Alternatively, instead of SEAL passengers the ASDS Transport Compartment might carry a load of deployable sensors, or the space and weight capacity might be used to store enemy artifacts (defuzed mine arming packages?) retrieved by on-board divers. In general, with its own capable sensor/connectivity suite, the ASDS could serve as an agile covert scouting craft for one or more parent submarines.

It is conceivable that the ASDS could be equipped to control a Long-term Mine Reconnaissance System UUV via the LMRS’s standard autonomous acoustic link, once the LMRS is launched from a larger host sub. In this manner the reach into the littorals, and concomitant risk-taking, are “delegated” from parent to ASDS to LMRS in turn. Each platform might relay up-to-the-minute hydrographic and bathythermometric data, as well as minefield and other tactical data, to the next unit off shore, enabling the larger platforms to “move in” more safely and covertly.

As an extra node from which to detect and localize enemy targets, the ASDS can aid the parent sub to triangulate and smartly derive a useable firing solution. (Again, real time communications would be achieved by low-probability-of-intercept digital acoustic link, or perhaps by a fiber optic wire. ASDS positional data would be obtained from its inertial navigation system, initialized at the host and updated when feasible by GPS and/or HF sonar bottom terrain orienteering.) An ASDS’s bearings-only passive sonar data could substitute for an additional TMA leg by the parent sub. Target range could be refined quickly using the different arrival times at the two friendly vessels of a single enemy acoustic transient. The ASDS instead of the parent might go active, or (as above, with the ADS) might be the sole source of hostile contact tracking for its host. This targeting augmentation allows the host sub to Attack Effectively First [11], and/or evade the enemy successfully, as the ROEs (rules of engagement) require. If weapons were launched, precautions would be needed to protect the ASDS from both friendly and enemy fire. An ASDS crewman might drop standard 3-inch and 6-inch torpedo countermeasures through the bottom hatch of the pressurized hyperbaric sphere, assuming an ASDS were not fitted with regular signal ejectors. In extremis the ASDS might mimic a full-size SSN behaviorally and/or acoustically, to serve as a decoy to protect its parent ship, or to deceive or intimidate an enemy. (The flank speed of the ASDS might roughly equal the maximum practical speed of an SSN with external dorsal load -- an ASDS, DSRV, or Dry-Deck Shelter.)

Arming the ASDS
As designed, the ASDS is unarmed. It might be possible to equip it with offensive and defensive weapons in one or more of several ways. An external neutrally buoyant mine carrying harness might be fitted, like that of the German Type 212s mentioned above. An external weapons canister might be fitted to an ASDS, similar to the Magnum system discussed in [12]. These steps would bring the ASDS capabilities closer to that of the proposed off-board combatant UUV, Manta [13]. As a field expedient, small mines might simply be rolled out of the ASDS bottom hatch once pressure had been equalized.

Were the host also fitted with a Magnum or Ocean Interface, the parent might be able to reload some types of weapons from the mini while submerged, with the aid of divers using adjustable buoyancy bladders and tethers to shift the units. The ASDS equipped with a Magnum (or temporary external cradles) could thus become a submersible weapons carrier for its host, shuttling between the SSN/SSGN and a sub tender or milch cow or pier, enhancing undersea replenishment capabilities.

Arming the ASDS externally might increase its own (and the mated host sub’s) flow noise, and impair speed and maneuverability, but it would allow the ASDS to defend itself when going in harm’s way. It might become standard practice to release the ASDS when the host commences undersea combat, to be free of the added hydrodynamic drag created by the piggybacked minisub. Pre-battle separation could also be made if the ASDS were carried internally. An armed ASDS would then become an SSN’s “escort fighting vessel” as well as adjuvant sensor platform, passing “flag signals” back and forth acoustically. (Imagine sending “Engage the enemy more closely” on covert digital HF sonar.) At a minimum, the provision in the mini’s hyperbaric sphere of a few torpedo countermeasures, and (during wartime) small mines with brief time delays, would improve ASDS survivability.

Hiding in Plain Sight
During anti-submarine operations, biologics may be misclassified as possible enemy submarines (POSSUBs), and may draw fire [14]. In some circumstances, an ASDS exposed to detection by the enemy may seek to be misclassified as a “CERTWHALE,” a definite biologic, and thus hope to avoid drawing fire.

By virtue of its size and shape, the ASDS has the potential to mimic a cetacean. Given the minisub’s quiet battery propulsion (i.e., minimal self-noise tonals), it has the option to emit whale sounds via active sonar or purpose-fitted hydrophones, enhancing its disguise as a large biologic. The crew might maneuver, as to course, speed, and depth variations, in a manner similar to the behavior of some whale species known to be endemic to the target area at the applicable time of year. With periscope and antenna masts folded back, the ASDS might breach at appropriate intervals and expel compressed air to aggressively imitate a whale spouting. Hull coatings might be developed to match the active sonar, radar, and passive infrared characteristics of whale hide and blubber.

While such antics might compromise inherent ASDS stealth, mission tasks could arise that require the minisub to subject itself (or its wake or flow noise) to almost certain detection by enemy lookouts or sensors. The ASDS might still be attacked while attempting this proposed cetaceous subterfuge, but the vessel is combat shock hardened, and such blending with the natural environment could fool or confuse the opposition for long enough, enabling greater survivability and tactical flexibility. The ASDS might attempt to mingle with a pod of genuine whales, to infiltrate hostile waters more safely.

The introduction of inhabited and uninhabited adjuvant vehicles, deployed from an SSN or SSBN or SSGN, is changing the nature of undersea warfare, as is the concept of the USS Jimmy Carter’s Ocean Interface hull module. The NMRS and LMRS (as well as high-frequency low-probability-of-intercept active sonar) have brought the Silent Service a long way from the “Hell’s Bells” mine detection gear of World War Two. Unmanned aerial vehicles (and high data rate antenna masts) are raising the “eye height” available in a submarine’s control room from that of an optical periscope to that of an observation aircraft or reconnaissance satellite. Clearly, the potential versatility of the ASDS Special Warfare minisub, and the ability of the basic platform to be “up-capabled,” make it a particularly cost effective force multiplier for the full-size naval submarine fleet.


  • 1. Combat Fleets of the World 1998-1999, by A. D. Baker III. Naval Institute Press, Annapolis, 1998, page 1051.
  • 2. “The ASDS Advantage,” by Leslie West. Sea Power magazine, July, 1998, pages 39 - 42.
  • 3. “Advanced SEAL Delivery System.” Undersea Warfare, Spring, 1999, Special Pullout Section, page 11.
  • 4. “Bring Back the Midgets!” by Tom Hunter. The Submarine Review, April, 1994, page 57.
  • 5. “Submarine Rescue -- Past, Present, and Future.” Undersea Warfare, Fall, 1999, pages 26 - 28.
  • 6. Anti-Submarine Warfare and Superpower Strategic Stability, by Donald C. Daniel. University of Illinois Press, Urbana, 1986, page 21.
  • 7. “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.
  • 8. “New Littoral Undersea Surveillance System Completes Successful Sea Test.” Undersea Warfare, Fall, 1999, pages 29 - 30.
  • 9. Baker, op. cit., page 257.
  • 10. Atlas of the Oceans, edited by John Pernetta. Rand McNally, London, pages 110 - 111.
  • 11. Fleet Tactics, by CAPT Wayne P. Hughes, Jr., USN(Ret.). Naval Institute Press, Annapolis, 1986, page 38.
  • 12. “Project Magnum: Sub Design in 21st Century,” by Harold J. Armstrong. The Submarine Review, January, 1999, pages 96 - 105.
  • 13. Baker, op. cit., page 1005.
  • 14. Anti-Submarine Warfare, by W. J. R. Gardner. Brassey’s (UK) Ltd., London, 1996, page 52.

    Originally published in the January 2000 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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