Let it be said from the very beginning that the nuclear propelled submarine is the ultimate underwater vehicle, in particular when it comes to sustained mobility and endurance in the stealthy ocean depths. No other underwater vehicle can even come close to any of these performances, whatever sophistication of conventional plants are developed, including any non-nuclear Air Independent Propulsion (A.IP) systems, whatever energy converter and energy storage they may use.
Although the deep ocean depth constitutes the classical environment for deep diving and fast SSNs, today’s submarine warfare is also a matter of operations in the so-called littorals. From a U.S. Navy perspective, these littorals can be virtually anywhere in the world and they may have to be reached covertly and in a hurry, an ideal task indeed for the large SSN.
However, one can perhaps see a certain paradox here, in that the large SSN is indeed unrivalled for the transit but it may be less ideal for at least some operations upon arrival. This potential problem, however, is not the subject of this paper. It is mentioned here merely to point out that for a country like Sweden, the littorals are nearby and Sweden can therefore make very good use of non-nuclear submarines, particularly so if they are fitted with AIP.
Seen in the historic perspective, once the art of submerging in a controlled fashion was ensured for the early primitive boats, efforts to improve underwater endurance became a high priority. These efforts were significantly accelerated during various conflicts involving submarines. During WWII for instance, advancements of the airborne radar effectively, and forever, drove submarines away from the surface. The weaknesses of submarines requiring periods of surface running to charge the batteries with air consuming diesel engines were exploited to the fullest, with quite staggering losses as the result. Attempts to lower the catastrophic casualty rates by introducing innovative designs were certainly done. However, as is well known they came too late to have any influence on the outcome of the conflict.
One design, and perhaps the most well-known, aimed at presenting a smaller target when recharging, by arranging the diesel engine air induction through a mast, hoistable from the submerged submarine. Other efforts were made to increase the submerged endurance by improving battery capacity and also by installing more of them.
Although these measures meant improvements in battery technology and larger boats, there were also initiatives for other and less weight and volume consuming ways of providing AIP, namely to carry certain reactants and process these in a suitable energy converter into power for sustained underwater running.
It is these kinds of concepts which today are coming to full operational maturity in order to augment the combat efficiency and the survivability of modern non-nuclear submarines. The Swedish concept which deploys Stirling cycle heat engines as energy converters reached this maturity in 1989 and is now a standard feature in the Swedish submarines of A19 Gotland class. The system is fitted in the compartment just aft of the pressure tight bulkhead which divides the hull into two compartments. The upper level contains the engine-generator modules whereas the oxygen tanks are fitted in the lower level. The installation is capable of providing several hundred hours of low speed submerged running, more than four times the energy stored in the ordinary battery.
AIP systems, whether currently in use or under development have one thing in common; they significantly increase the submerged endurance which was previously entirely decided by the size of battery installation.
As long as the submerged endurance was purely depending on the battery, it was natural to focus development efforts to improve the specific energy content of the battery itself (or in some cases shift to other battery types than the common lead-acid type). Consequently, such developments have very successfully been carried out and the post-WWII years have seen quite dramatic improvements in this area. Today a state-of-the-art lead-acid battery cell will yield more than twice the energy than a cell of the same weight 50 years ago.
Further increases in battery energy density are possible, but one can suspect that the efforts to do so will be increasingly difficult and expensive the closer one comes to any technical limit. In these circumstances the most obvious solution might be just to install more battery to achieve better endurance. This, however, will quickly drive boat size to unacceptable levels, hence the search for an alternative and smaller power system to provide energy for the submerged running, i.e., a system of much higher energy density than even the most modem lead-acid battery. This search which started already during WWII carried on quite strongly and reached a fundamental milestone when nuclear propulsion for submarines came of age with NAUTILUS.
Sweden was one of several countries which modeled their post WWII first and second generation submarines on the German latewar Type XXI, a submarine with substantially more and better batteries than previous types and therefore with very good underwater performance. A third post WWII Swedish submarine generation was developed for the Swedish Navy in the early 1960s. The development included investigations and tests to explore whether an AIP system could be included in that design. The technology studies for that purpose were based on previous foreign trials with diesel engines run in a closed cycle. This required a system in which the exhausts were scrubbed of COi and recirculated to the induction side where fresh oxygen was injected to make up a combustible mixture. The oxygen bad to be carried onboard, as for instance high test peroxide.
The Swedish program reached the stage of full scale testing in a land based facility but was eventually terminated because of uncertainties in technology as well as costs. The submarine project was then established as a pure conventional design of which five were delivered by Kockums between 1968 and 1972.
At that time another technology was already under investigation as a future potential submarine power generation system, namely a system utilizing fuel cells. These devices convert energy in a direct chemical process between two reactants, normally oxygen and hydrogen. Again the system bad reached an advanced testing stage in a land based facility but again the program had to be terminated because of uncertainties in technology and costs. The levels of ambition in both these programs were high; the respective installation was aimed at providing power at all running modes, i.e. the traditional diesel/battery system was to be completely replaced.
Modern AIP Concepts
Towards the end of the 1970s, the ambition had been reduced and the add on concept was identified. In such a concept the AIP system was to be configured in a separate autonomous hull module which could be inserted into existing submarines and new construction projects alike. The add on module would constitute a compact storage of significant amounts of energy and a conversion system to augment and complement the battery in order to extend the submerged endurance at low patrol speeds.
From a technical point of view the system would provide an alternative power source to the battery for running at silent speeds and consequently, from a tactical point of view, it bad to display the same low noise signatures. It was assessed that stretching an existing submarine by 15 to 20 percent to accommodate the AIP module would not have any notable impact on the original performance, particularly in view of the much better-a factor of 4 to 5-submerged endurance which would be the result.
Obviously, when incorporating a module in a new submarine design all proper provisions could be taken from the outset of design work.
The Stirling Solution
Studies to identify the most suitable energy converter for the Swedish system were completed in the early 1980s. Given the usual constraints in available resources and a desired target time for introduction into naval service of the new system, the studies conclusively pointed to the Stirling cycle beat engine as being the best candidate.
Most elements of the engine itself were at that time defined under other programs and the principles for heat creation by combusting fuel and pure oxygen at an overpressure-a key feature of the underwater engine-were established. Additionally, the high efficiency of the Stirling engine, the efficiency in storing oxygen as liquefied oxygen (LOX) and utilization of fuel oil as the fuel promised an installation of high energy density. Furthermore, the prospects of achieving excellent balancing of the rotating parts and the mode of continuous combustion, all indicated that stringent noise emission requirements also could be met.
The development program for the full system was commenced in 1982 with a series of rig testing which eventually produced the power unit, i.e. the engine with its over pressure combustion chamber and the electrical generator together with appropriate control systems
In parallel, studies and various testing to establish safe handling procedure and storage arrangements for the LOX were conducted. Since the first system was to be retrofitted, although as a permanent installation, to an existing submarine, the added hull module containing the system bad to be totally autonomous and weight compensated to fit inside the original submarine trim polygon.
The Battery Boat Dilemma
Advance in battery technology, together with opportunity to carry more battery have in some case stretched the submerged endurance of battery submarine towards the 100 hour mark. The recharge must then commence. However, the interval between recharging will normally be less because of the tactical wisdom of avoiding complete discharge in order to retain a tactical reserve of around 50 percent.
The fitting of an additional energy supply for the submerged running, but as a much denser package than bulky batteries, is what AIP in this context is all about.
A normal AIP installation of this kind will give the submarine commanding officer several hundred hours submerged at low speed running during the patrol from this system alone. And on top of that, another hundred hours from a fully charged battery.
A theoretical and stereotype mode of utilizing this capability is for the AIP submarine to start patrolling in his dedicated area on the AIP system and with the battery fully charged. The AIP running will not permit any battery discharge. Oxygen is of course consumed instead, up to a point-let’s say a day or two-when a target is engaged requiring power flexibility, hence the AIP plant is shut down and the battery is engaged until the target is eliminated. The submarine then go back to the AIP mode until the next target opportunity. And so on, until the oxygen is consumed. The rate of battery discharge is slowed down and the submarine has been in the operational area for many days; it has eliminated a number of targets and has remained air-independent and stealthy during the whole period.
The Swedish System
The major elements of the Swedish system are the Stirling engine generator sets, the LOX storage and handling system,auxiliaries and the control system. The fuel storage and handling system is integrated with the bunkering and tankage system for the diesel engines. The Stirling engine is the energy converter in the AIP plant. It converts beat from combustion of oxygen and fuel into mechanical work through a thermodynamic cycle carrying the name of the person, Robert Stirling, who was first with its practical application. Characteristic for this cycle in its ideal shape are the four steps:
1-2 isothermal compression (on the cold side)
2-3 constant volume displacement (from cold to hot side)
3-4 isothermal expansion (of the heated working gas)
4-1 Constant volume displacement (from bot to cold side)
The working gas, i.e. the gas contained inside the engine and the beater, is helium. The heat collection part is located inside a separate combustion chamber to collect heat for the cycle, the heat being created by continuous combustion in the chamber of fuel and oxygen. The cycle creates movements of the pistons which in turn rotate a crank-shaft which then drives the electrical generator to provide the electrical power. The actual engine has four cylinders and pistons. The cylinder pressure curve is sinusoidal and smooth and the engine is furthermore meticulously balanced and fitted to a double elastic mounting arrangement. Consequently, the resulting vibration levels and noise signatures are extremely low.
During operation it is run at a constant speed of 2000 rpm and can develop up to 75 kW. A total system of four units could easily support even a large submarine at slow speeds and including the hotel load.
The combustion chamber is an integral part of the engine unit although the combustion is external to the engine itself, the created heat being transferred to the working gas inside the engine across the heater pipes connected to the cylinder tops.
There are two prominent features of the combustion chamber arrangement. One is the technique to control the combustion temperature, given that the reactants provided are fuel and pure oxygen and the other provides the ability to discharge the combustion products-carbon dioxide, water and some excess oxygen-straight overboard against the diving pressure. The combustion flame temperature is controlled by diluting the incoming pure oxygen to a mixture suitable to provide a gas temperature of 1800C (and an average temperature of 750C at the heater tube walls). The diluting substance is the combustion gas itself, part of which is being recirculated for this purpose and injected into the incoming oxygen. Recirculation is achieved without moving parts but rather through the creation of a static pressure drop at the points of the inrushing oxygen, which will bring parts of the combustion products to that point.
The overboard discharge of the exhaust is achieved by conducting the combustion at an over pressure corresponding to a certain diving depth. On the reactant side, this is facilitated by allowing and controlling an over pressure in the oxygen supply tank. The fuel is injected by traditional fuel oil pumps.
The combustion chamber itself is a pressure vessel on top of the engine unit and the exhaust discharge line ends in a non-return valve set to the maximum diving pressure and a discharge disperser into the outgoing system cooling water flow.
LOX is a daily industrial commodity in many countries and techniques, technologies and procedures for its storage and handling are well established. However, the inclusion of such storage etc. into a military submarine with mission times of several weeks requires specific considerations. Firstly, the thermal insulation needs to be superb to avoid losses caused by heat leakage and secondly, it has to be structurally aligned with safety requirements for the submarine as a whole. The typical Swedish installation comprises two tanks, each of stainless steel and with outer and inner structures separated by high vacuum multi layer insulation. The tanks are fitted inboard, resiliently connected to the hull structure for shock protection. The inside tank pressures are kept at a constant level to allow for direct supply to the combustion chamber inlets. This holding pressure is obtained by evaporating LOX and feeding it to the top of the tanks. Oxygen to the combustion chamber is taken from the tank bottom as LOX and brought into gaseous phase in an evaporator. Heat for this process is taken from the Stirling engine fresh water cooling system.
Swedish AIP Status
As a result of the pioneering efforts in this particular AIP technology in Sweden, the Swedish Navy is currently the only western navy to routinely operate any kind of a complete AIP system in non-nuclear military submarines.
The complete installation in the submarine NACKEN in 1989 and a number of successful patrols to follow, paved the way for the incorporation of this capability into the three submarines of A19 Godand class.
These submarines were contracted in the early 1990s. The lead submarine conducted acceptance sea trials and was delivered to the fleet in mid 1996 and the second of the class commenced sea trials in July this year after being launched only five months earlier. The third unit will be launched in September {Ed. Note: After THE SUBMARINE REVIEW goes to print.] and goes on sea acceptance trails early 1997.
The control of the AIP system is integrated, as a small panel, with the overall propulsion control console. The starting and shutting down of the system is a push-button operation. The console contains all means for controlling and monitoring the entire propulsion plant, i.e. diesel-generator sets, the main propulsion motor and the AIP system. It also provides monitoring of the main battery as well as control and monitoring of all valves associated with the propulsion plant. Indeed conditions, etc. of all platform systems can be called up on the screens.
These truly state-of-the-art submarines with their unique propulsion system will be in service with the fleet for the next 25 to 30 years. The current AIP system and its capability is presently fully defined.
Naturally, there are also ways identified by which further enhancements can be achieved. These would range from parametric changes to the engine itself and the system to installation trade offs between the conventional plant and the AIP plant. In all cases the result will yield further improvements of the submerged endurance, which is most certainly the way ahead for non-nuclear submarines.
It is also with great interest one is looking forward to the introduction in operational submarines of AIP systems using alternative energy conversion devices, reportedly in 1999 (France) and 2003 (Germany). The pioneering work in the AIP field conducted in Sweden currently forms the very peak of a long and proud submarine tradition in that country. The momentum of the development is considerable and a long lasting competitiveness is projected.
