Carel Prins was general manager for design and engineering at RDM Submarines ill Rotterdam until 2000, when he retired from the company. He holds a masters in mechanical engineering and a Ph.D. in nuclear power engineering. He is vice-chairman of the Kursk Foundation.
Hans Ort served in the RNLN until 1982. Twenty years were spent in and with the submarine service, with two seagoing commands as a highlight, followed by supervising the building of the Zwaardvis-class submarines. Before making flag rank, there were commands of a frigate and a supplier, both sailing for a 6-month period in SNFL. After leaving the navy he spent ten years in industry, dealing with submarine building.
Network-centric warfare at sea is an important force multiplier, linking the capabilities of surface, underwater and airborne units. Even as these new integrated operations are developed it is worthwhile to take a closer look at some excellent submarines that have the capability to operate independently as well as in an integrated network. And these need not necessarily be nuclear submarines. There is a family of submarine designs that found its origin in the experimental ALBACORE and, for instance in the Netherlands, evolved as a line of ocean going diesel electric submarines. The Royal Netherlands Navy has operated submarines since the beginning of the 20th century. The development of new submarines for the RNLN has always been based on a strong relationship between the Navy and industry. This has created a number of centers of excellence, complimentary to the navies’ own technical capabilities, collaborating in the design of boats to mission requirements for operations in the waters of the North Sea and the Atlantic, but also for the tropical environments in the Far East.
Today the RNLN operates four Walrus class diesel submarines. These ocean going submarines that were delivered to the RNLN between 1988 and 1994 started their development at a time when the Cold War was still dictating their requirements. For patrols in the Northern Atlantic long range, high autonomy and deep diving rated high, as did the need for an advanced combat system and potent weapons. The Walrus combat system is one of the first fully integrated sensor and weapons control systems. The weapons selected for the boats were the Mark 48 torpedo and the Sub-Harpoon missile. With a total capacity for 22 heavy weapons that compares well with some nuclear submarines.
The Walrus Heritage
When the RNLN made their plans for the submarines to follow the first post WW II Dutch designed triple hull Dolfijn class in the fifties, the staff requirement called for nuclear boats. So the focus was on submarine development in the US. The experimental ALBACORE was as much the revolutionary turning point for the RNLN as it was for the US Navy. The design of the operational Barbel class was adopted to lay out new submarines for the Nether-lands. These became the Zwaardvis class and were initially intended to be built as a stepping stone towards nuclear submarines. But eventually it became clear that the price tag was more than anticipated and when the US denied the Netherlands the submarine reactor technology it was decided to abandon the original staff requirements for nuclear submarines. Future development was thereby limited to diesel electric submarines, designed as SSK’s.
The similarity between the Barbel and the Zwaardvis classes is evident. Both have the partly double hull construction with a single hull parallel mid-body, both sporting a UD ratio of about 7.7. The 2640 ton boat has three diesel generators to ensure a low indiscretion ratio. The six torpedo launching tubes are of Dutch design and incorporate a pneumatic-mechanical positive ejection system. The weapons are Mark 37 Mod C torpedoes. The Zwaardvis class, therefore, is capable of handling Otto fueled torpedoes. Two watertight bulkheads divide the boat into three compartments. The torpedo room takes up the forward compartment with a storage capacity of20 weapons including the weapons in the tubes. Propulsion and auxiliary equipment are located in the aft compartment. A double armature main electric motor gives the boat a 20 knot submerged speed. The 8.4 meter diameter midsection is dedicated to the accommodation distributed over two decks with the battery compartment below. The central control room is located directly below the sail. The steering position for steering both course and depth faces forward and has two redundant positions. Diving controls are on the port side of the central control room, leaving starbciard for the sensor-and weapons control. Diesel generators and propulsion are controlled from a separate control room located in the engine room aft.
The sensor suite includes a medium frequency circular array sonar in the bow, a passive ranging sonar, active sonar and an HF intercept sonar. The boats received a Signal digital target tracking and fire control console during mid life modernization.
The double hull contains the weight compensation tank and fuel tanks. With the spacious design of the boat, ample vertical stability is a bonus. Furthermore, the submarine has high quality habitability with a large galley, two messes, a wardroom and a bunk for each member of the crew of 76 officers and ratings. Such quality living conditions are important for ocean going patrols of long duration. The construction yard was the Rotterdam Dockyard Company (ROM). The lead boat of a class of four, Zwaardvis was commissioned in 1972 and the last decommissioned in 1995.
At the end of the 70’s a contract for the supply of submarines for Taiwan was concluded. The 4 boat contract, which ran in parallel with the development work for the new generation RNLN submarines, was given to the Wilton Feijenoord yard, a sister yard of ROM. The submarines, the Sea Dragon class, were to be a copy of the Zwaardvis class regarding the platform design. This made a speedy delivery possible because no time consuming new pressure hull design was necessary. Within this pre-defined hull a more advanced sensor and weapons control system-supplied by Signal-was projected, as well as modem control and monitoring systems for platform machinery. Two boats were built and delivered in 1988; political pressure prevented the construction of the next two.
When plans for the new generation RNLN boats, the Walrus class, were developed, a number of specific goals were set. The diving depth should be substantially increased, provisions made for a mix of advanced weapons, the sensor suite should be extended and accompanied by multi functional operating consoles. Furthennore, the platform systems would have to be fully automated. Although the general arrangement is comparable to the Zwaardvis, these requirements resulted in a step change of the well-proven design.
For the much increased diving depth a new steel was developed, comparable to HY I 00, with improved toughness to withstand high shock loads. After full-size testing of a segment of the pressure hull the extensive stress analysis software was validated giving the new design a high level of confidence. The integration of the US designed torpedo tubes-the Mark 56 tubes instead of a Dutch design, a consequence of the greater launching depth and the choice of new weapons, the Mark 48 torpedo and Sub-Harpoon-posed an interesting challenge to the designers to reconcile the allowable pressure loads on the tubes with the construction of the water transfer tank and spherical front bulkhead of the pressure hull. The advanced weapons allowed going from six to only four tubes in conjunction with two Mark 19 air turbine ejection pumps.
The sonar sensors consist of a medium frequency circular array sonar, a passive ranger, a flank array, an HF intercept, an active array and a low frequency towed array. The last is of the clip-on type. Thin line arrays were not available at the time and the winch of a standard RTAS proved too bulky to be fitted. For the Gipsy sensor and weapons control system, Signal (now a Thales company) developed one of the first multi function operating consoles-Walrus has seven-that allow many sonar, navigation or fire control operations to be performed in parallel, using all and any of the consoles. These consoles are arranged on the starboard side of the central control room.
The communication system covers a wide range of frequencies and includes antennae on hoistable masts as well as a floating antenna. An ESM mast, radar mast, snort mast and two periscopes complete the mast arrangement.
The periscopes are positioned above a raised central platform in the middle of the control room. This elevated position gives the commanding officer an overview of all activities. He supervises combat system operations on his right, while he can see the single steering position on his left-looking forward on the front bulk-head-and further aft to his left he sees two stations for platform control.
The helmsman can steer the boat manually, but also has an autopilot to his disposition. He can feed instructions into the system to perform maneuvers in a certain sequence, which will then be executed automatically. The rudders of Walrus, as opposed to the cruciform rudders of Zwaardvis, are arranged in X-form. Besides giving a high maneuverability to the boat the arrangement has built-in redundancy: only two of the rudders are needed to steer course and depth. The rudders can also be steered manually using push buttons as a further redundancy.
The engine room, distribution switchboards and main electric motor room are found in the aft section of the submarine. The Integrated Monitoring and Control System (IMCS). supplied by Imtech in the Netherlands, includes full automation of all propulsion and auxiliary equipment. The IMCS has three layers of independent redundant control, including hard wire direct control of emergency functions. This makes for unmanned machinery spaces, which was a first in submarine design. The result is a reduction of the crew from 76 to 50. Platform control and monitoring requires only two operating consoles in the central control room. There is one video display for monitoring and control of the submarine propulsion and one for diving control. The video displays show mimics of all systems on board and individual motors an valves can be started, respectively opened, by tracker ball control. An NBCD panel is located above the video displays. Including the helmsman, the platform is in the hands of three operators. One important task of the IMCS is the automatic starting and stopping procedure for snorting operations. With a single instruction on the control panel, the preparation for snorting and the starting of the three diesel genera-tors, placed abreast in the engine room delivering 2850 kW, is effected. This cuts down substantially on the time to change from electric sailing at periscope depth to actual charging of the battery. IMCS takes less than a minute, thus substantially reducing the time when the submarine is indiscrete.
Because of the great diving depth, and inter-cool system provides the cooling of all equipment. The fresh water circuit itself is cooled in a sea water heat exchanger. Employing an inter-cool system reduces the number of pressure hull penetrations for added safety. At the same time the system effectively reduces corrosion problems, especially important in tropical waters.
The increased performance has made the boat a bit longer than the Zwaardvis class. Habitability of the Walrus class has been kept at a high level. The displacement is 2800 ton, maintaining the Zwaardvis hull diameter but adding a few frames. Furthermore Walrus has a good growth margin for future upgrades.
The Cold War dictated the naval staff target for the Zwaardvis and Walrus class boats. Nowadays the question is often asked whether the reduced threat downgrades the functional requirements for a new submarine with corresponding lower investment costs. Frequently this notion then dictates the navy budget, reflecting the desire to reap the peace dividends. The mission of the RNLN was ASW in collaboration with other NATO forces in the Northern Atlantic. The surface fleet, the patrol aircraft and the submarines each had their role. In the meantime the specifications of new frigates are much more oriented towards air defense, although they have maintained a multipurpose capability. The Walrus class boats still have a long life and it is not anywhere near the time of making decisions on replacements. The RNLN obviously has to be regarded as an element of NA TO collaboration or at least as operating in conjunction with its allies and Walrus, being a versatile, relatively low cost weapon, is very effective in playing in multiple roles.
In a conference’ organized by MIT, “Antisubmarine Warfare after the Cold War,” in ’97 it was concluded, from the US Navy point of view, that the new threat is more from the proliferation of modem non-nuclear submarines. “No other individual platform compares to a modem submarine, whether nuclear or non-nuclear, in its ability to combine a potent offensive punch with the ability to evade counterattack by opposing forces.” The conference proceedings also mention that only a small effort is needed to run an effective submarine service, like the 400 officers and enlisted men who run the RNLN submarine service consisting of 4 Walrus class boats, a SIM tender and the submarine school.
Whether considering the submarine as part of a post Cold War ASW task force, or as the challenger to it, the modem submarine with a well trained crew has a high return on investment. As a case in point, Walrus has shown on many occasions during NATO exercises to be able to break through the protective screen of a carrier group without being detected. For both purposes, it is worthwhile to have modem submarines with advanced sensor and weapon capabilities, that are difficult to detect.
The present day role of diesel submarines can be illustrated by looking at three small navies and their submarine mission characteristics.
1. For the Netherlands, joined operations in a NA TO context are basic to their existence, as it has been since WW II. This is particularly true for submarines. For that reason there is a role for a submarine, suited for a variety of missions: defense as well as attack, ocean as well as inshore. NA TO missions take RNLN submarines to operating areas as far away from base as the Mediterranean or the Gulf. This way they provide their contribution to the defense umbrella of NA TO of which the Netherlands is an integral part. It follows that bilateral collaboration with one or more of the members of NA TO is as effective. In this sense the navies of the UK and the US are frequent partners. The submarines may also be sent across the Atlantic to patrol the Caribbean waters. The ocean going capability of the Walrus class, so needed for their previous Cold War patrols in the Northern Atlantic, stands them in good stead. Jn short, the present day missions of the RNLN submarines are as follows. Peace enforcing and peace keeping missions. In support of US efforts during the Balkan wars Walrus class boats pa-trolled the Adriatic and recently operated in the Gulf as part of Enduring Freedom. Counter drugs operations consisted of surveillance of radio and radar emissions, reporting on ship movements. In peacetime the deployment of the submarines to train surface units and the submarines themselves remain at a high level of preparedness. Increasingly important become their deployment in brown water operations, not so much close to the Dutch shores on the continental shelf, but in covert operations in hostile waters. All this without forgetting their war time operations against surface units and other submarines. Quite a versatile mission, for which the Walrus class is very well suited.
2. Taiwan has two Sea Dragon class submarines, the modernized Zwaardvis design of the 80’s. Taiwan’s submarine mission to defend itself has to consider the shallow waters of the Strait of Taiwan as well as the deeper waters at its entrances and to the east of the island. Submarines can deny the use of the Strait. The Sea Dragons perform well for the ROC Navy. A require-ment for more boats has existed for some time and submarines similar to the Sea Dragon or Walrus class would be desirable. These would add greatly to the capabilities of the ROC Navy. The close proximity to mainland airbases necessitates the submarines have a high autonomy, staying out at sea and hiding from enemy units while at the same time presenting a formidable threat to these units. Low signatures and deep diving capabilities are important.
Surveillance of movements of enemy shipping requires a quiet platform that has the size to carry an extensive sensor suite. Denying the use of the Strait mandates a substantial weapon load of mixed composition. These should consist of torpedoes, missiles and mines, giving the submarines a high hit capability, while staying undetected.
At the same time the submarines’ stealth will tie down a great number of enemy units and because of their long range the threat area can be substantially extended.
3. South Korea has been building up their Submarine Force for quite some time. The first batches of type 209 are followed now by a series of AIP capable type 214 boats. It is expected that the next step will be the acquisition of ocean going submarines. Also the ROK Navy has to consider patrols in both shallow and deep water. Historical fear of occupation and the independence the ROK has now enjoyed for decades has resulted in long term planning to establish the means for a strong defense. Their staged submarine acquisition plan has been laid out for a long time and has been executed with some stops and starts as dictated by the economy, but without deviating from their plans. And the requirement for ocean going boats was already made clear long ago.
The submarines of the upcoming KSS lil program shall be capable of performing in a forward defense strategy and be the equal of any submarine threat in the area. They provide a platform for advanced weapons, maybe including land attack missiles. Furthermore, littoral operations in hostile waters will be included in the mission profile. The deep water areas should be a place to hide a submarine from air attacks. The boats have to be deep diving and AIP capable.
Submarine deployment is different for the three navies: operating as part of an alliance in the Dutch case versus the fulfillment of an independent national defense strategy for the others. The functional requirements, however, come very close. And the relatively small navies of the three countries have the same need for boats that can be called big, as diesel submarines go.
AIP Option and Operational Aspects
Extended submerged endurance is a performance multiplier of any diesel submarine. The various submarine designers have followed more than one way to provide AIP. In Germany, operations in the Baltic have set the functional requirements for AIP. Much effort has been put in the development of Fuel Cell technology, and the type 212 boats, now becoming available for the German Navy, have been fitted with a system delivering 300 kW. The system runs on pure hydrogen stored in canisters containing a metal hydride to which the hydrogen is adsorbed. In Sweden, 75 kW Stirling engines burning sulfur free diesel have been in use on RSN submarines for some time including the present A-19 class. The French version is a high pressure boiler burning methanol and supplying steam to a fully enclosed high r.p.m. steam turbine cycle called the MESMA system.
The system pursued by RDM in the Netherlands is the Closed Cycle Diesel (CCD). The original idea stems from the 40’s when the German Navy already experimented with it. Removing exhaust gas-primarily C02-at greater depths has become possible through the use of a compact rotating absorber, developed in the UK in the late 70’s. Based on this technology, ROM started to engineer a submarine CCD system in the mid-80’s. The AIP development was part of an RDM program for a new submarine design, the Moray. The very flexible design concept-in the 1600 to 2200 ton range-offered great adaptability to a variety of platform and combat system requirements, for ocean and inshore missions. One of these requirements is to be AIP capable. Detailed integration studies and designs were made for the Moray submarine and a 400 kW CCD test facility was constructed. In 1993 a prototype was tested in a collaboration of the German Thyssen Nord See Werke and RDM, on the ex-U-I submarine of the German Navy. This boat had been previously used to test a Fuel Cell system and was now converted for CCD. Trials demonstrated the viability of the CCD as a submarine system during sea trials of one month in the Baltic. Most importantly, the noise attenuation measures taken proved that the radiated noise as registered on the noise range was no more than that of the Fuel Cell AIP. With so many AIP options, what was the reason for ROM to choose the CCD? There are a number of aspects to consider.
Safety. The Fuel Cell operates on hydrogen and oxygen as fuel and oxidant. Storage of both on a submarine requires very sophisticated handling and control systems. The German submarines have that at a high cost. Methanol, as used in the MESMA system, is a toxic fuel. Both the Stirling engine and the CCD use common diesel fuel. Where the Stirling requires absolutely sulfur free fuel, the CCD has the logistic advantage of standard diesel fuel.
Noise. The Fuel Cell itself has no moving parts and is per se noiseless. Even with its auxiliary equipment, the Fuel Cell system is very silent. The Stirling engine and the MESMA system have reciprocating and rotating equipment and are noisier than the Fuel Cell. The diesel engine in the CCD system constitutes an important equipment noise source, but attenuation of structure borne noise (triple flexible mounts) and airborne noise (acoustic enclosure) have proven that the radiated noise is at the level of the Fuel Cell system. As proven on the ex-U-1, the radiated noise signature is a neutral discriminator.
Efficiency. Because the efficiency of a Fuel Cell system is higher than that of a diesel or Stirling engine and the energy content of hydrogen is much greater than that of diesel fuel, the required weight of fuel and oxidizer is lowest for the Fuel Cell by a large margin. The problem, however, is that the safest way to store hydrogen is to adsorb it to a metal hydride. Hydride can only contain hydrogen to a maximum of two percent of the hydride weight. (Reformers of diesel fuel to produce hydrogen are not yet commercially available. Besides, they are bulky and lower the system efficiency.) An AIP plant is usually placed in a closed compartment or plug. To keep such a plug neutrally buoyant, the Fuel Cell plug has to be much larger than required for the plant volume alone. So, for example, 2 for a 3000 ton submarine, the Fuel Cell plug would be 44 percent longer to compensate for the extra weight in comparison with a plug for a CCD AIP system. In this case both plugs contain all consumables, weight compensation tanks and the plant. To put it differently, given the same size plug containing either a CCD AIP or a Fuel Cell AIP, the first would give the submarine in the example a 20 day sub-merged endurance at 4 kn., whereas the Fuel Cell AIP provides for 15 days.
Operational. Diesel engines for any desired output are easily obtainable. Stirling engines suffer in efficiency when engines are scaled up well over 75 kW. The CCD system lends itself very well for up scaling. Fuel Cell systems are modular by nature and higher power ratings should be possible to achieve. Why is more power needed than for supplying the hotel load and a speed of 4 knots? A speed of 4 knots is good for surveillance patrols, in a confined area. In a submerged intercept maneuver, however, own speed and that of the contact should not differ too much. Otherwise the intended intercept cannot be achieved. For a submarine with a maximum AIP speed of 4 knots, the chance to intercept an oncoming target sailing at, say 15 knots, is only nine percent of that of a submarine with a 10 knots AIP speed.
Price. The CCD is by far the cheapest system. In cost of plant as well as overall submarine integration costs. It should be pointed out that the maintenance and training costs because of commonality with the conventional submarine plant are modest as well.
Considering these criteria, ROM decided to develop a CCD system. While the cost involved-development cost and price to a customer-had much weight, the operational aspects were paramount. RDM’s decision was not only based on export considerations. The parent navy has to count its pennies too.
The Walrus platform is a good base for future development. The growth margin and ample vertical stability allow for introduction of new technology.
The partly double hull construction provides space to accommodate canisters or pods for advanced decoys or UUV launchers, for instance such encapsulating techniques as the Broaching Universal Buoyant Launcher (BUBL). Even if the outer envelope of the boat is to be increased, locally or overall, the pressure hull can stay untouched which would be a great cost saver, both as a refit or for new construction boats.
The platform could be fitted with the communication systems and antennae to become a link in network-centric operations, especially when a UUV launching capability is added.
The Walrus class could be fitted with an AIP plug. To give the boat the endurance of the example above, i.e., 20 days at 4 knots, the plug would be about 9 meters long.
The Walrus is a big boat in comparison with many other diesel submarines. Small navies are relatively poor navies, but sometimes are better off with such a substantial versatile submarine, either when they are to be serious players in an alliance or solitary operators facing the threat of larger navies. In this context Small Navies need Big Boats, getting a Lot at Modest Expense.