Taking Submarine Design and Operations into the 21st Century
Mr. Armstrong is a member of the Royal Institute of Naval Architects, Chartered Engineer. He Worked for the UK MoD for six years as a member of the Royal Corps of Naval Constructors and spent two years as the naval architect for a large Royal Navy Nuclear Submarine Project Team. At present he is involved with naval stealth technology programmes for the Royal Navy and other navies.
The investment that has been made into the design of submarines over the last 50 year since World War II has achieved significant improvements and benefits to their operational performance which was optimised for their Cold War NATO hunter killer role. At the same time, the slow evolutionary design process produced safe and gradual changes to the hull and on board systems, building on the accepted practices of previous successful designs. Only the adoption of nuclear steam plant in the 1950s stands out as a revolutionary step change in submarine design. Now accepted as the standard choice for large submarine main propulsion, at the time, the move to nuclear power was difficult and not universally accepted as the way forward.
One area of submarine design which has suffered from the evolutionary process is the weapon handling and discharge system (WHOS). With the need for new submarines to carry and launch existing weapons and for new weapon designs to be carried and launched by in-service submarines, WHDS has existed in a design loop. Although more sophisticated, a modern, conventionally armed SSN has less firepower today than a submarine 50 years ago when comparing their displacements. With the development of submarine launched cruise and anti-ship missiles, decoys and remotely operated underwater vehicles (ROVs), and the move towards a wider range of operational roles, the demands on space within the weapon stowage compartment can result in only a handful of heavyweight torpedoes being carried to sea. To overcome this constraint a new design of WHDS for submarines has been invented and patented. Called Magnum, the invention places two rings of weapon canisters at either end of the pressure hull which are able to revolve around the outside of the hull inside a standoff secondary hull, allowing weapons to be fired fore and aft. Sketch 1 below shows a typical installation.
For a submarine similar to a SSN688 class, it would be feasible to fit 20 weapon canisters in each ring with each canister holding two weapons, creating a total weapon carrying capability equivalent to 80 heavyweight torpedoes. With eight doors available to the forward ring, and four doors in the after ring, each revealing the complete face of a canister carrying two weapons, then with every door opened, the submarine would have 24 weapons ready for immediate launch.
The Weapons Canister
There are two types of weapon canisters; the enclosed pressure resistant version and the open pallet version. Both versions will be designed to a modular arrangement with common power and data interfaces. Sketch 2 illustrates each type.
The enclosed canister will be designed to carry, side by side, two weapons equivalent to a heavyweight torpedo (Mk 48) or similar, such as a cruise missile (Tomahawk) or anti-ship missile (Sub-Harpoon). The diameter of the weapon could be as great as 30 inches although a range of smaller diameters would be equally acceptable by varying the internal sleeve diameter. It is therefore possible to carry existing 21 and 26 inch diameter weapons through the selection of the appropriate sleeve diameter, hence Magnum does not make the existing stock of in-service weapons obsolete. Within one ring of 20 canisters it would be feasible to carry 20 different pairs of weapons each with a different diameter. The weapons would be isolated inside their launch sleeves which in tum would be shock protected inside the canister. The canister would travel on a suspension system within the ring frame which in tum would be shock mounted to the pressure hull Power would be fed to the weapons and internal canister electronics through an induction loop system charging an internal battery at the rear of the canister and the data link would operate either through close proximity blue/green laser optics or RF frequency transmission. The canister would be designed with an external shell from a composite material such as carbon fibre or aramid fibre reinforced plastic, while the internal weapon sleeves would be designed to be pressure resistant to protect the weapon from the hydrostatic pressures of deep diving. It is envisaged that the weapons would be stowed in a neutral. inert environment until required for launch. A gaseous medium would be drawn out and pumped into a small holding tank as the canister is flooded up immediately prior to launch.
The front of the canister would have two petal leaf door arrangements which would open outwards as the weapon is launched. The opening mechanism could be mechanical, operated from a small internal hydraulic accumulator, they could be opened by pressurising the canister from a normal below ambient internal pressure to a slightly higher pressure thereby pushing out the door sections or a frangible cover could be incorporated, designed to allow the emerging weapon to push through. Thus the weapons would be protected from shock, handling damage and a corrosive environment. The canister itself would be corrosion resistant and non-magnetic. The selected one shot launch system could be designed to eliminate all noise transients until the weapon was running.
The method of launching the weapon will now be selected by the weapon manufacturer who will be totally responsible for the internal design and arrangement of the canister. the only constraint being the overall modular, pre-specified external size and shape of the canister, its weight and common power and data links. The advantage of Magnum is that the launch system for each type of weapon can vary and the selected system will only need to be a one shot design. Possible launch systems, before the weapon’s own propulsion system takes over, could be; a separate short lived boost motor (propeller driven or solid fuel rocket) pushing the weapon out from behind, a hydraulic ram operated from an accumulator, an air bag arrangement inflated via a gas generator or through a simple swim out arrangement.
It is expected that existing weapons would require minor modifications for their deployment in a Magnum canister, although future weapons would be designed from the outset for long term canister stowage and operations. This may mean that existing weapons would have a shorter maintenance cycle requiring the regular removal and testing of the canister and its weapons in a similar manner to air-to-air missiles fitted to the wings of fighter aircraft. Self checking systems in the caniter, monitoring the health of the weapon and its internal support equipment, may help to evaluate the need for maintenance and periodic service by reporting back faults to the submarine’s main command and control system.
The canister would be designed to be overall neutrally buoyant, seawater would replace the weapon after launch thus eliminating the need for a dedicated weapon compensating tank. Any small variations would be accommodated in the submarine’s main trim and compensating system.
A typical launch scenario would be:
1. A preparation to launch signal is passed from submarine command and control to the selected canister, or canisters, along with a fire control solution, which is constantly being updated from submarine sensors.
2. The weapon communicates its readiness status and on board three axis position data to allow confirmation of the range and bearing to target.
3. The signal to launch is given, the outer hull door is opened and confirmed. The canister is pressurised to slightly higher than the ambient water pressure allowing the segmented hatch at the front to open, followed by weapon launch.
If the weapon is wire guided then the wire will reel out from the inside of the front of the canister. This wil1 mean that the outer hull door must remain open and the ring cannot turn until the wire has been cut. However, even with the ring stationary there will be the potential for another 15 weapons which can also be launched, the other ring will still remain operational.
After several years in service, when traditional submarine launched weapons (modified for their Magnum canister role) have become obsolete, new submarine launched weapons will have the ability to be designed without many of the constraints of the present system. The need to have 21 inch or 26 inch diameters with circular cross sections and launched by water ram discharge would be relaxed. They could have larger or smaller diameters, have a triangular cross section (if such a shape was desirable), fixed wings or over-sized control surfaces, be shorter in length or have a tandem configuration. It may be possible to carry an anti-aircraft missile system to deal with the threat from maritime patrol aircraft and dipping sonar helicopters. The forward ring would carry mainly offensive weapons such as heavyweight torpedoes, anti-ship missiles and land attack weapons, while the aft ring could carry defensive weapons such as countermeasures, decoys, mines, ROVs and even rocket propelled anti-torpedo darts. However the ability to carry any weapon in either ring would be possible and desirable, a range of weapons in each ring would provide operational redundancy.
The canister would have small wheels attached to a suspension system to allow it to move inside the Magnum ring frame. Doors would be positioned on the top of the casing flat which, when opened, would allow empty canisters to be removed and full canisters to be inserted by being lowered into the ring frame. Two large, high torque slow moving hydraulic motors would be used to drive the canisters around the ring frame in 18 degree increments. Magnum eliminates the need to have a pressure hull hatch open to the sea for loading, thus allowing rapid reloading evolutions to take place at sea alongside a submarine tender or a Military Sealift Command vessel with the appropriate hydraulic crane and lifting frame. Canisters could be loaded onto a C-130 transport aircraft, carried on a truck or slung under a helicopter and taken to a submarine at short notice anywhere in the world.
The open canister would be designed to be a pallet to carry a variety of stores such as mines, special forces equipment, seabed sensor packages and remotely operated vehicles. The range of operations carried out by the ROVs could involve stealthy mine hunting and clearance duties, decoy activities, provision of a bistatic sonar platform, support to special forces with covert intelligence gathering activities, and peacetime support to search and rescue and environmental surveys. The variety of equipment described above would either be dropped from the pallets through keel doors located underneath the ring or accessed via the loading doors in the casing. With ROVs it is also envisaged that they will also be able to be captured by clamps built into the pallet once they have returned to the submarine and manoeuvred beneath the keel doors.
The Magnum design will therefore allow submarines to deploy remote mine hunting systems using submersible unmanned vehicles ahead of an expeditionary naval force allowing littoral and coastal minefields to be breached without the presence of MCM surface vessels revealing such an operation is in progress. It will also be possible to fly out a special forces team, along with their combat gear stowed onto pallets, anywhere in the world in a transport plane and/or heavy lift helicopter to quickly deploy in a pre-positioned waiting submarine.
The Ring and Drive System
The ring frame would be manufactured from a high strength tubular steel to create a cage and rail system for the canisters and stores pallets to ride in. Within the frame there would be a drive collar free to rotate and driven by two high torque, low noise, slow moving hydraulic motors mounted 180 degrees from each other inside the pressure hull operating through shafts and sealed hull penetrations. One alone would be sufficient to rotate the drive collar. The canisters and pallets would be loaded into the ring frame through the top loading hatch and slotted into the drive collar while their wheel/suspension systems would then lock onto the frame rails. Thus the ring frame provides the main shock protection structure and the rails for the canisters to move in, and the drive collar securely locates each canister within the ring frame and provides the means for rotating the canisters. Sketch 3 below illustrates this arrangement through several regularly spaced shock mounts which would also serve to allow slow movement in the pressure hull, arising from contraction at deep diving depths, to be accommodated. A significant advantage of Magnum is the large reduction of pressure hull penetrations with the elimination of the torpedo tubes and a weapon compensating tank. It also allows the submarine to be reloaded without the danger of a pressure hull torpedo loading hatch open to the water. This means that both Magnum rings could be reloaded at the same time while alongside a submarine tender at sea.
Maintenance of the hydraulic motors and their control systems could be carried out afloat. If all the canisters are lifted out, the free flood ring space around the hull can be inspected by divers and cleaned out using high pressure water jetting equipment. It would also be possible to make the ring space watertight and pump it out to allow visual inspection and maintenance in the dry.
The Outer Doors
The outer doors would be simple flaps operated by hydraulics which would open to reveal the complete front of the canister with its twin segmented end caps. It is possible for the doors to be manufactured from a composite fibre reinforced plastic to reduce their weight, improve their dimensional tolerances and make them inherently damped thus making no noise when they are being opened. The same design strategy and materials can be employed on the top loading hatches and keel doors.
With the Magnum rings positioned at the ends of the pressure hull, the forward outer doors will be positioned further aft from the main bow sonar than with the present torpedo tube arrangement, thus allowing a larger bow sonar to be fitted with a greater spatial coverage as well as allowing a smoother flow regime to be maintained over the bow area and for a distance aft thereby reducing self noise.
The Data/Power Links
To allow the canisters and pallets to rotate within the ring frames, no simple hardwire connection will be possible, however early studies have shown that high speed data links could be achieved through the use of laser. Short range RF transmission are also possible and a communication system could involve redundancy and safety cross checks as the onboard canisters computer and the main submarine command and control system (SCCS) transmit information back and forth.
The SCCS will be in communication with every weapon canister and stores pallet and will be displaying their operational readiness. If the tactical situation should ·suddenly change with the submarine in a land attack arrangement, for example by the detection of an enemy submarine, then the CO can rapidly demand a change in the submarine’s capabilities which the secs will initiate by quickly turning both rings to present the number and type of weapons best able to meet the changing tactical circumstances.
In addition to significantly improving the operational performance of a submarine with reduced procurement and in-service maintenance costs, Magnum also improves safety for the crew by removing the danger of carrying explosives and toxic fuels inside the pressure hull. There will be a reduction in the number of pressure hull penetrations as well as the elimination of the torpedo tubes with their requirement for inner and outer tube door inter-locks. A twin hull design will also offer greater protection for the crew against pressure hull penetration arising out of the impact from a shaped charge weapon.
Overall Submarine Design Factors
Direct handling of weapons by the crew, either through loading or via the racks within the weapon handling compartment (torpedo room), will no longer be required, while the complicated internal hydraulic and air conditioning systems in way of the weapon handling compartment will be simplified with the deletion of this space. The number of crew members therefore can be made smaller, thereby reducing demands even further on internal pressure hull volume.
Thus the shape of the submarine will create a length to beam ratio approaching the optimum as the overall diameter increases and the length of the pressure hull decreases with the reduced demand on internal volume. This will result in an improved hydrodynamic form which will allow the submarine to go faster for the same installed shaft horsepower or, for the same operational speeds, allow the nuclear reactor greater longevity before (or even it) requiring refueling and also reduce coolant pump and reactor flow noise.
The space between the outer secondary hull and the pressure hull can be used efficiently to stow additional equipment, some of which at present may be inside the pressure hull. Certain tanks, such as fuel, hydraulic and fresh water. could be located in this space, the large sonar blister arrays fitted to the vessel’s flanks could now be built in flush with the outer hull surface, while a proportion of the main ballast tank requirement could be located along the hull with their banks of air bottles. The twin hull design would also allow interacting twin coatings of acoustic cladding materials to be applied thus allowing for even greater reduction of radiated noise and target strength signatures.
USS ALEXANDER HAMILTON (SSBN 617)
October 22-24, 1999, Groton, CT. Contact:
Ralph A. Kennedy
89 Laurelwood Road
Groton, CT 06340