In the trenchesU.S. Navy SEALs receive new airlock mini-sub

Published 12 February 2010

The U.S. Special Operations Command wants to replace the standard “wet” Swimmer Delivery Vehicles (SDVs) with a new and enlarged pocket submarine; a mini-sub with a proper pressure hull and an airlock would allow attacking frogmen to travel dry to their target

Interior of the S301 looking toward the forward viewing port // Source:

The U.S. Navy SEALs are to get a new and enlarged pocket submarine which will allow them to travel most of the way to an objective inside in the dry and then exit through an airlock before swimming on for their final approach.

Lewis Page writes that the SEALs and comparable elite forces such as the British SBS have long used “wet” Swimmer Delivery Vehicles (SDVs) to approach an objective below the waves, allowing them to move faster and further than an unaided frogman can. With a normal SDV, the trooper is not inside a pressure hull — he is immersed in the surrounding water. This can be a problem, as hours spent unmoving in cold water can sap the strength of even the strongest soldier. Hence the desire for a mini-sub with a proper pressure hull and an airlock, allowing attacking frogmen to travel dry.

For many years, the SEALs were known to be working on this via the so-called Advanced Swimmer Delivery System (ASDS). Pages notes that the prototype ASDS was dogged by technical snags and never reached an acceptable standard of performance. It was also harshly criticized by the U.S. Government Accountability Office (GAO).

The final blow for the ASDS came when it caught fire while stored ashore and was gutted. This left the SEALs reliant on their trusty 1970s-vintage open-water Mark 8 SDVs. Tthere was also a Mark 9, capable of firing torpedoes, but this was retired in the 1990s. In any event, the Mark 9 seems to have been intended for torpedoing ships at anchor or in harbor).

Page writes that following the ASDS disaster it appeared that the U.S. special-operations community would henceforth content itself with a new replacement open-to-the-sea SDV, somewhat modernized, under the name Shallow Water Combat Submersible, but this is not the case.

This week the U.S. Special Operations Command announced it intends next month to lease an S301 submersible from Virginia company Submergence Group, which also offers a two-man research sub. The S301 is to be delivered to the SEAL units at Pearl Harbor in Hawaii, formerly the home of the ill-fated ASDS, and will “be used by field units for doctrinal, operational, and organizational purposes” — with “operational” being the most interesting word.

Submergence Group themselves are fairly tight-lipped about their S301’s performance, but they do specify that it can carry two pilots and 6 swimmers — or underwater robots, inflatable boats, etc. The SEALs also state that in their opinion the S301 is a “lock out” boat capable of unloading the six passengers in a single airlock cycle. They also believe that it can cruise at better than 5 knots and travel at least 10 nautical miles on a single li-ion battery charge.

Page writes that one of the most critical factors affecting the design of the SDVs and then ASDS was the fact that the short-ranging battery minisubs are carried into range of their objectives aboard full-size U.S. Navy nuclear submarines. With the SDV, a large “Dry Deck Shelter” airlock hangar is fitted to the sub’s hull to hold the minisub and allow its crew of SEALs to board.

The ASDS was too large to fit into a cramped hangar, and was intended to dock with the sub directly: it was this which proved its downfall, as it tended to suffer serious damage if the carrying sub went at all fast.

“Whether the S301 is intended to operate from a mothership is unclear,” Page writes. It seems to be significantly bigger than an SDV, so dry-hangar operations are probably out: on the other hand there is no indication of its being able to mate with a full-size sub’s escape hatch. It seems likely that the S301 will move over long distances by other means, probably using a surface support ship.

The U.S. Navy’s SSGN underwater special-ops mothershipsOhio class ICBM subs stripped of their nukes following arms-reduction treaties, and refitted with conventional cruise missiles plus accommodation for a force of frogman-commandos - will probably have to rub along with SDVs for a while yet.