MSubs S301 Dry Swimmer Delivery Vehicle: 2008

The conventional method of delivering special forces covertly to the seashore uses a swimmer delivery vehicle which is fully flooded and requires all to wear diving equipment. This can be an extremely arduous experience, especially in cold water. They are delivered as near as possible using a nuclear submarine fitted with a dry deck shelter (DDS). This is around 2.5m diameter and has a floodable compartment around 7m long sealed off by a full diameter hydraulically operated door. The current UK version is slightly larger than the US version, designed by Electric Boat in the 60's. For current (2020) US and UK special forces these are US designed MK8 SDVs built in Panama City in Florida, now rather long in the tooth. They are powered by silver zinc batteries which are eye-wateringly expensive and have a useful life of around 5 cycles. In the real world they have been technically surpassed by the lithium battery some time ago.

At the start of a mission, the special forces pass up from the nuclear submarine into the chamber which is at this point completely dry. The vehicle is parked slightly to one side for access. After boarding, the chamber operator floods the chamber and the door is opened with the sub hovering at around 20m. The SDV reverses out and sets off towards the target. The maximum range is kind of a secret but at the same time obfuscated by exaggerated claims of past users. The practical reality is that the round trip should be about 50 miles or less. Upon arrival the vehicle is parked, switched off, on the sea bed where it remains for the duration of the mission.

It has long been a desire to make this journey in the comparative comfort of a dry vehicle kept at normal atmospheric pressure. Northrop Grumman were given the task of designing a dry vehicle that would take 6 swimmers and fit into a current DDS. The report they eventually delivered concluded it was not possible. I was given the task to take a second look to see if it was actually possible. The result was S301 which carries 6 swimmers, and 2 crew and fits in an American DDS with minor mods to internal equipment. It has been reported that the total cost of the fully operating S301 was less than the cost of the Northrop Grumman study which said it was not possible.

The success of S301 has completely changed the US special forces strategy regarding swimmer delivery.

As can be deduced from the pictures, there is a forward compartment for the pilot and co-pilot, entered through the opening and fold back of the bow viewport, then a pressure bulkhead with a hatch leading to the lock-out compartment containing reasonably comfortable seating for 6 swimmers. The swimmers exit from an aft hatch pointing down at 45 degrees. The exit trunk keeps the water below floor level.

This very light inward opening lockout hatch was designed by John Ramsay and I one evening on a few pieces of A4 over warm English beer in the Royal Oak, Meavy, Devon. Although I say it myself, it is quite inspired and is now the standard pattern for all two-way hatches at MSubs as can be seen on their website.

The batteries are external and were to be the same lithium units used on the Northrop Grumman Advanced Seal Delivery System (ASDS). S301 would use 4, ASDS had 14. The prototype was initially fitted with lead acid and then commercial lithium Iron Phosphate as used in a Segway. The pods were therefore the same ID and length as the ASDS pods.

Main propulsion was by a pair of standard 15kW oil-filled brushless DC motors, from Fischer Panda in Germany, specially adapted for this service. These low-speed direct drive units are extremely efficient and I have used them now on all JFD rescue subs since 2012 and the Triton DV24 Tourist sub built in Barcelona (2020).

Although the customer said at the start that classification was not required, I have been here before. So, just in case, we built and tested the hull as if it was being built under survey with all the 3rd party QA documentation. When the boat as launched and it turned out classification would be required, it was not an impossible task to gain post-build approval through DNV-GL.

S301 was completed in 2008 and was sent first to Virginia Beach then to Pearl Harbour, Oahu, home of the ASDS. However, ASDS had just burnt to a crisp due to a battery fire during charging. This was in the early days of battery management with the exciting but powerful lithium cobalt chemistry. This disaster, while bad for the ASDS project was greatly to the advantage of MSubs. S301 was the only dry swimmer delivery vehicle in town and therefore received far more attention than it might have. Fortunately, the boat behaved itself perfectly and we were able to continue development in extremely agreeable surroundings.

In partnership with Lockheed Martin, the sub was further developed in West Palm Beach with lithium batteries and a long, streamlined tail. At this point the US special forces were now very interested indeed in the prospect of a compact dry swimmer delivery vehicle.