Since the TURTLE, Navies have explored power and energy systems aimed at extending the submerged endurance of submarines. These efforts include advanced battery technlogy, “closed cycle” engines, fuel cells, and nuclear power. The current potential market for superior performance in new and backfit submarine construction has prompted extensive foreign R&D on these Air Independent Propulsion (AlP) technologies. With reduced new construction there is a potential market for retrofitting existing conventional submarines for improved endurance and for use by Third World countries. The AlP technologies are also applicable to swimmer delivery vehicles and autonomous or unmanned underwater vehicles. This review presents the historical background of these technologies and briefly discusses several recent applications.
The requirement to increase the submerged endurance of submarines was recognized in World War I, and has long been a goal of designers and operators alike. The idea for an air independent diesel engine started as early as 1901 in Germany. AlP is the production of power by the conversion of energy through combustion or chemical reaction without the use of air. Professor H.G. Walter began experiments in 1937 using hydrogen peroxide as an oxygen source. Damiler Benz (in an independent effort from Walter’s) started the modification of several 1400 to 1500 horsepower diesel engines• for closed cycle operation in 1938. The Third Reich was the first to operate an AlP equipped submarine in 1940. Today this work is readily recognized in the submarine technology community as the Walter cycle turbine engine submarines.
There has been a renewed interest in AlP in the last decade for the application to the conventional and midget submarine. The latter includes manned and unmanned submersibles, most notably swimmer delivery vehicles, remotely operated vehicles, and tourist submersibles. This interest has been driven by the limitations of batteries and nuclear power, iml the potential for new business in new construction and overhaul. Companies, shipyards and countries are competing for the export market. A demonstrated submarine AlP capability may determine the successful bidder in a competitive environment; e.g., Kockums versus HDW for the Australian submarine contract. Today there is similar posturing for the potential Canadian submarine business. Sager in 1990 projected a market potential of conventional submarine construc-tion world wide for the decade of the ’90s to be 100. Many of the countries he surveyed do not need the virtually unlimited sub-merged time as provided by the SSN, or cannot use nuclear power. Today Sager’s forecast appears very optimistic due to the decline in world economics, and the realization of the cost of a submarine program versus the cost of a more defensive maritime posture. Baker reported in 1991 that new submarines and new technologies will not proliferate at the rate predicted in many popular press treatments. The current geopolitical environment includes a focus on limited intensity conflict and regional warfare. This environment and the world economics have increased the interest in swimmer delivery vehicles and the midget submarine because of lower cost and tactical impact. This may be an incentive to Third World countries to buy or develop AlP systems.
At present we are experiencing glasnost and learning more each day about Soviet technology much like we did after 1945 in Germany. However, don’t expect a similar type of resurgence of AlP interest or development as happened in 1945 due to this new access to Soviet technology. Since the advent of nuclear power, the requirements for an AlP system have changed, and the Soviet AlP technology may not be as revolutionary as the German technology was in 1945.
The following sections review the several AlP developments noting the status of development or availability of the candidate systems and associated technologies. The Swedish Stirling engine system now has been demonstrated operationally for three years. The success has resulted in a contract for three A19, GOTLAND class submarines with a fully integrated Stirling engine AIP system. Other AlP systems are well along in the development process but national economics (e.g., Germany and Dutch) have slowed most if not all of these projects.
This background will help document the chronology and history of AlP technology. This historical summary is especially interest· ing when comparing today’s developments with the high relative power levels and the high number of development projects in Germany from 1938 to 1945. Many of these old systems4 are similar to today’s without the benefit of advanced materials, sensors and computer control systems.
The German development included both closed cycle reciprocating engines and turbines; Kreislauf and Walter systems. The reciprocating engine projects included at least six known test beds ranging to 1500 horsepower. These engines used the Kreislauf engine modifications and auxiliary components to change from surface or snorkel operation to air independent or closed cycle operation. Cryogenic oxygen, LOX, was stored internally or as a high pressure gas (6000 psi) externally. External hydrogen peroxide storage was also in development. The peroxide was 80 percent concentration and was catalytically decomposed in an auxiliary component, or was direcdy injected (tried but never adopted for diesel engines). The Walter turbine design was selected for the higher horsepower and higher speed applications.
A prototype submarine, the V80, was completed in 1940, but AlP development proceeded slowly and it was not until 1943 that much work was done on the Type 17. The Type 26 was to be the main operational design but development was then too late for the Germans to complete and produce in sufficient numbers. They concentrated instead on the Type 21; the Walter designed hull for high submerged speed and conventional propulsion machinery.’
Walter’s laboratory in Keil was inspected in 1945 and a complete submarine turbine propulsion unit of about 2000 horsepower was found intact with a partially completed unit for 6260 horsepower and the necessary ancillary test rigs, workshops and laboratories. This inspection was significant in learning the development status of applications using hydrogen peroxide which included submarine and torpedo propulsion, launching ofVls, and jet propulsion engines. It was also learned that much of the original German work on the snorkel had taken place there.
The valuable experience gained from these early German AlP efforts was transferred to the U.S., UK and the Soviets at the end of the war. The U.S . continued engine development at the Navy’s Annapolis laboratory after the war until the mid 1950s.6 Six new power plants were in development at that time, including the first nuclear power system.
The Kreislauf diesel engine using hydrogen peroxide was also under development by the Japanese during World War D. By December 1944 they were building a hydrogen peroxide turbine powered “manned suicide torpedo”, the KAITEN 2 and 4. A similar design was also being build using liquid oxygen.
The U.S. Navy operationally demonstrated a diesel engine system using hydrogen peroxide and diesel fuel on a dry swimmer delivery vehicle, the USS X-1, commissioned in 1956.7 In 1958 she was classified “Out-of-Service, In-Reserve” due to a hydrogen peroxide leak and explosion. She was later refitted with a conventional diesel electric system and served as a research vessel until 1973 when she was retired.
The UK continued the Walter cycle turbine development and commissioned two submarines in 1956 and 1958, the EXCALI-BUR and the EXPLORER (commonly called the “exploders”). These systems were experimental and were removed from service in 1963 due to a continuing problem with hydrogen peroxide leaks and fires. Most of the AlP work both in the U.S. and the UK was terminated to focus all development on the nuclear program.
The Soviets also experimented with the German technology and may have modified several M class submarines (250 ton) with a Kreislauf cycle diesel using liquid oxygen in the early 1950s. This system recycled engine exhaust gases into the engine after removing the carbon dioxide with a soda lime absorber. This development continued and several of the QUEBEC class (750 ton) were originally fitted with this system on the center shaft. These submarines (lead ship launched in 1954) were called cigarette lighters or Zippos because of the explosions attributed to the AlP systems. The Soviet interest in AlP continued in parallel with nuclear propulsion development into the 1960s. The WHALE, a 1500 ton, 250 foot long submarine was outfitted with a Walter cycle hydrogen peroxide system from 1956 to 1961. There is speculation that AlP demonstrations were planned for a WHISKEY, ZULU, FOXTROT, the JULIE1T cruise missile submarine, and the KIL0.9 10 AlP was also being considered to augment nuclear systems as power boosters for high speed sprint operations. Lack of hard evidence indicates these demonstrations were not very successful.
During the 1960s there were several U.S. reviews of power systems which provided the guidance and direction for R&D efforts of advanced non-nuclear and non-weapon (torpedo) systems; deeper diving submarines and submersibles. For 25 years the recommendations of these efforts received very little funding due to the focus on nuclear power. The exception was the development in the 1970s and 1980s of the closed Brayton cycle engine with a Li/SF6 combustor. The laboratory development of this type molten lithium system was first reported by Phillips Research Laboratories around 1970 in the Netherlands.
The interest in non-nuclear power systems was renewed in the 1970s in the UK, Germany, Italy, Japan and Sweden in response to requirements in both the commercial (offshore oil) and subma-rine requirements. In 1970, Riccardo, a British company, worked on a 36 horsepower Perkins diesel engine as a diver power package. 11 In 1972 the Japanese company Hitachi worked on a recycle Perkins engine. Several U.S. patents were granted during the period from 1960 to 1980 but many did not have any experi-mental follow up. S.S.O.S. of Italy was granted a U.S. patent12 in 1981 and subsequently demonstrated this approach in their Phoenix Project. Their PH 1350 vehicle used a 100kW closed cycle diesel engine operating on diesel fuel, oxygen and recycled exhaust. The foreign military requirements were for small coastal patrol submarines for which nuclear power was not available or affordable. Also recognized was the growing potential market for submarine export sales as reported in the U.S. Naval Institute Proceedings. 13
It is not possible to recommend or suggest a preferred AlP technology considering the variety of applications and the varying stages of development. The candidate systems HQI noted below are either in early stages of development or have been eliminated as not having serious potential.
Many developments were previously noted to have roots in Germany. Today’s developments do not include any references on the German hydrogen peroxide systems for oxidant storage so important in the 1940s and 1950s. The safety problems appear to have eliminated the use of high test peroxide, HTP, >65 percent concentration, as a viable oxidant candidate.
Proton Exchange Membrane (PEMl Fuel Ce)l. This type of fuel cell is capable of operation on reformed methanol or sulfur free diesel fuel, thereby considerably reducing the weight and volume required for energy storage. They can also be operated on air during surface transit, thereby saving oxygen for submerged operations. The results of the fuel cell prototyping effort in Germany are significant. The high cost, and the risk in power scaling for total propulsion power, are current limitations. The technology has considerable promise.
Stirlin& Eneine. Two V4-275R engines are currently complet-ing the third year of an operational evaluation aboard the Swedish submarine NACKEN. This engine design has also been demon-strated at high pressure and venting exhaust overboard reduces the size, weight and power requirements of an exhaust gas manage-ment system. A bubble trail is likely at very shallow depths. Multiple engines are required for higher power levels due to the limited available engine power, 75 kW. High structurbome noise, inherent to reciprocating engines and compressors, requires dampening for quiet operation.
Closed Cycle Diesel. These systems have been the subject of considerable and successful development for power levels to 500 kW. The system derives significant benefit from the advanced state of development of diesel engine technology and the variety of engines available. The engines have high structurbome noise levels. The major limitation is the large, noisy, and power intensive exhaust processing system for scrubbing and pressurizing for onboard storage or overboard discharge.
The submarine or submersible application of these candidate AlP systems is determined by the three major design factors. These are in addition to the mission requirements: system safety, stealth, cost, technology risk, etc. The design factors are:
- The creative integration of the oxidant and fuel storage weight and volume into the submarine (e.g., using hydrides for both hydrogen storage .arul ballast).
- The power capability of the energy converter or multiples thereof (suitable for auxiliary or total propulsion power; a surveillance or trailing mission requirement).
- The exhaust system requirements (size, weight, power, etc.).
The potential of AlP use in the near tenn is high for midget submarines and submersibles. The potential for use on submarines is less for auxiliary power (> 200 leW) and low for total propul-sion power ( > 500 kW). The potential of the hybrid or auxiliary power concept and backfitting is the highest potential.